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Doktorarbeit / Dissertation, 2012
II REVIEW OF LITERATURE
III MATERIALS AND METHODS
IV EXPERIMENTAL RESULTS
LIST OF TABLES
1. Details of the parents used in the study
2. Details of Cross Combinations
3. Parental study using SSR primers
4. Analysis of Variance of RBD for different biometrical traits
5. Analysis of variance of RBD for different physiological traits
6. Analysis of variance for combining ability for different biometrical traits in parents and hybrids
7. Analysis of variance for combining ability for different physiological traits in parents and hybrids
8. Variability parameters for different traits
9. Mean performance of lines for different biometrical traits
10. Mean performance of lines for different physiological traits
11. Mean performance of testers for different biometrical traits
12. Mean performance of testers for different physiological traits
13. Mean performance of hybrids for different biometrical traits
14. Mean performance of hybrids for different physiological traits
15. Genetic components for different traits
16. General combining ability effects of lines and testers for different biometrical traits
17. General Combining ability effects of lines and testers for different physiological traits
18. Specific combining ability effects of hybrids for different biometrical traits
19. Specific combining ability effects of hybrids for different physiological traits
20. Heterosis (Per cent) for Days to 50 per cent flowering, Plant height and Productive tillers per plant
21. Heterosis (Per cent) for Panicles per plant, Panicle length and Spikelet fertility
22. Heterosis (Per cent) for Filled grains per panicle, Hundred grain weight, Harvest Index and Single plant yield
23. Heterosis (Per cent) for Proline content, SPAD Chlorophyll meter reading and Chlorophyll Stability Index
24. Heterosis (Per cent) for Relative water content, Biomass yield and Dry root weight
25. Heterosis (Per cent) for Dry shoot weight, R: S Ratio and Root length
26. Proportional contribution of lines, testers and their interaction variance (per cent) for different biometrical traits
27. Proportional contribution of lines, testers and their interaction variance (per cent) for different physiological traits
28. Phenotypic correlation coefficients between single plant yield and component characters
29. Direct and indirect effects of different characters on single plant yield
LIST OF FIGURES
1. Heritability and Genetic advance for single plant yield and its components
2. Mean performance of parents for single plant yield
3. Mean Performance of hybrids for single plant yield
4. Magnitude of additive and dominance genetic variance for different biometrical traits
5. Magnitude of additive and dominance genetic variance for different physiological traits
6. General combining ability effects of parents for single plant yield
7. Specific combining ability effects of hybrids for single plant yield
8. Range of heterobeltiosis for different traits
9. Range of standard heterosis for different traits
10. Proportional contribution of lines, testers and line x tester interaction
11. Direct effect of different traits on single plant yield
12. Dendrogram of 21 genotypes derived from UPGMA cluster analysis using Jaccard coefficient based on 26 polymorphic SSR markers
LIST OF PLATES
1. Crossing Block
2a. Evaluation of F1 under aerobic condition
2b. Evaluation of F1 under aerobic condition
3a. Hybrid recommended for heterosis breeding
3b. Hybrid recommended for heterosis breeding
4. Hybrid recommended for recombination breeding
5. Gel depicting polymorphism among the parents for RM 278
6. Gel depicting polymorphism among the parents for RM 279
7. Gel depicting polymorphism among the parents for RM 206
8. Gel depicting polymorphism among the parents for RM 324
Rice (Oryza sativa L.), a member of poaceae family, is one of the world’s most important food crops, feeding more than half of the world’s population. It is the most diversified crop due to its adaptation to wide range of geographical and climatic regimes. According to estimates from the United States Department of Agriculture, the average world rice productivity in 1960 was 1.84 tonnes per hectare and in 2009 it was at 3.59 tonnes per hectare with a production of 452 million tonnes whereas the rice area is projected to increase by almost one million hectare from 154.4 million hectares in 2007-08 to 155.3 million hectares in 2008-09. The world population, particularly in the rice consuming countries is increasing at a faster rate. By the year 2025, about 756 million tonnes of paddy, which is 70 per cent, more than the current production, will be needed to meet the growing demand (Tuong and Bowman, 2002). Crop improvement in rice depends on the magnitude of genetic variability and the extent to which the desirable genes are heritable.
Rice is the staple food for over 70 per cent of Asians, the majority of whom are living below the poverty line. More than 90 per cent of the world’s rice is produced and consumed in Asia (Barker et al., 1999) and rice production must be increased by an estimated 56 per cent over the next 30 years to keep up with population growth and income-induced demand for food in most Asian countries where about 75 per cent of total rice production comes from irrigated lowlands (Maclean et al., 2002).
India has the largest area under rice crop (about 45 million ha) and ranks second in production next to China with a production of 84.5 million tonnes in an area of 5.60 million hectares whereas, Tamil Nadu ranks 12th in rice production and second in productivity and grown in an area of 2.6 million hectare with a production of 8.19 million tonnes and productivity of 3.2 tonnes per hectare during 2009-2010. Rice contributes 43 per cent of total food grain production and 46 per cent is irrigated; 35 per cent is rainfed medium and low land; 12 per cent in sunderban rainfed upland and the remaining three per cent falls under deep water cultivation. Almost 25 per cent of the world’s rice is grown under rainfed lowlands and frequently affected by uneven rainfall distribution. Another 13 per cent of the rice area under cultivation is always subjected to water stress during the growing season.(Bouman et al., 2007).
However, even though rice is an important food source for many millions of people, it is also the single largest user of water, requiring two to three times more water input (rain, irrigation) per unit of grain produced than the major cereal crops, such as wheat and maize.
More irrigated land is devoted to rice than to any other crop. With the growing population, increased urbanization and environmental degradation, the supply of fresh water for all human activities is depleting and the situation is getting rapidly worse. For example, it has been estimated that by 2025, 15 million ha of irrigated rice will suffer ‘physical water scarcity’, and most of the 22 million ha of irrigated dry season rice grown in South and Southeast Asia will suffer ‘economic water scarcity’ (Tuong and Bouman, 2002). Most of the world’s rice production comes from irrigated and rainfed lowland rice fields. Therefore, the development of new rice cultivation techniques and cultivars are required to reduce water consumption in rice production systems.
Various field techniques to save irrigation water have been explored.
They include direct seeding, keeping the soil saturated and alternate wetting and drying system (AWD) in lowland fields. Bouman and Tuong (2001) reported that, compared with continuously flooded conditions, small yield reductions (0 to 6 per cent) occurred under saturated conditions, but larger reductions (10 to 40 per cent) occurred under AWD, when soil water potential (SWP) during dry phase reached values between −10 and −40 kPa. Therefore, in order to sustain and to increase the rice production to meet the future demands with limited water supplies, there is a need to genetically alter the basic water requirements of rice through breeding techniques (Vijayakumar et al., 2006). Aerobic rice is one such option to decrease water requirements in rice production.
A new water-saving technology is called aerobic rice system (Bouman 2001; Bouman et al., 2005). In aerobic rice system, fields remain unsaturated throughout the growing season, as in wheat or maize cultivation. Water can be supplied by surface irrigation (e.g. flush or furrow irrigation) or by sprinklers, but in both cases, the goal is to keep the soil wet but not flooded or saturated. Using this technology, farmers can actually reduce the irrigation water requirement upto 50 per cent and can obtain yields of 4.5 to 6.5 t/ha (Naoki Matsuo et al., 2010). This new concept of “Aerobic Rice” combines the characteristics of both upland and high yielding lowland varieties with less water requirement and high response to inputs. Traditional upland rice varieties are grown this way, but they have been selected to give stable but low yields in adverse environments where water availability is very low. On the other hand, high-yielding lowland rice grown under aerobic conditions shows greater potential to save water, but at a severe yield penalty. The distinguishing future of aerobic production system is that crops are direct seeded in free draining; non puddled soils where no standing water is maintained in the field and roots grow mainly in aerobic environment (Atlin et al., 2005). Maintaining the production and reducing the water use by rice, is a complex trait (Lafitte and Bennet , 2002). Since aerobic rice is targeted at water-short areas, socio-economic comparisons must include water-short lowland rice and other upland crops. The development of high-yielding aerobic rice is still in its infancy and germplasm still need to be improved and appropriate management technologies developed. New varieties that are high yielding and responsive to inputs in aerobic conditions must be developed if the concept of growing rice like an irrigated upland crop is to be successful
Many studies of aerobic rice systems have focused mainly on comparing agronomic traits such as shoot growth, yield and yield components of aerobic rice with those observed under flooded paddy conditions (Bouman et al., 2005; Yang et al., 2005; Peng et al.,. 2006). This may be because the concept of aerobic rice is quite new and the difference in soil water conditions between aerobic rice system and temporary water stress conditions is not yet appreciated. Furthermore, there are no studies which have compared the agronomical, morphological and physiological traits between the two situations.
The success of plant breeding programme depends to a greater extent on the knowledge of the genetic architecture of the population and selection of appropriate breeding method for the improvement of traits of interest. It is essential to estimate the various types of gene action for the selection of appropriate breeding procedure to improve the quantitative and qualitative characters (Banumathy et al., 2003). Understanding the target environment is critical for the success of drought tolerance research. Plants possess several morphological and physiological adaptations to overcome the deleterious effects of water stress. Drought tolerance is quantitatively inherited with a complex physiological reaction; thus its genetic basis has received limited attention and development of drought tolerant varieties has been slow (Nadarajan and Muthuramu, 2005). Therefore, it is essential to understand the effects of the prevailing drought stress in the target environment on both yield and drought tolerant traits in order to undertake the genetic improvement of aerobic rice in this region. Most of the traits are quantitative in nature; hence it is necessary to know the inheritance of these traits. Genetic information about the combining ability of parents and hybrids and nature of gene action involved in the inheritance of a trait would be of immense value to plant breeders in the choice of parents and to identify potential crosses of practical use.
Significant yield advantage gained through the adoption and spread of hybrid rice technology had helped China to add about 350 million tonnes of extra rice to its food basket during 1976-1998 and enabled it to divert some of their rice areas to other commercial crops (Anon, 1998). Hybrid rice technology had also shown increased yield, farmer profitability and better adaptability to stress environments such as water scarce and aerobic conditions. Development of rice hybrids with high yield potential for aerobic conditions would be one of the exciting researches to be carried out to overcome the existing water crisis in India. Breeding strategies based on selection of hybrids require expected level of heterosis as well as the specific combining ability.
In breeding high yielding varieties of crop plants, the breeders often face with the problems of selecting parents and crosses. Combining ability analysis is one of the powerful tools available to estimate the combining ability effects and aids in selecting the desirable parents and crosses for the exploitation of heterosis. The Line x Tester analysis provides information about general combining ability (gca) of parents and specific combining ability (sca) effects of crosses and is helpful in estimating various types of gene actions. Zhang et al. (2002) studied the heterosis and combining ability of hybrid rice. The genetic improvement of rice for aerobic environments has not been understood well and major efforts in this front are lacking.
Therefore, the present investigation was carried out to identify best combining parents and hybrids suitable for aerobic cultivation. Keeping the above points in consideration, the present investigation has been formulated with the following objectives;
1. To study the genetics, including physiological, morphological and molecular characteristics of drought tolerance, yield and yield components involving aerobic cultivars
2. Comparison of hybrids with the promising checks under non puddled condition
3. To assess the gca of parents and sca of hybrids
4. To estimate the heterosis for physiological, yield and yield component traits
5. To study the interrelationship between drought tolerant traits under aerobic condition and to partition the total effects into direct and indirect effects
6. To identify best hybrids for optimum yield coupled with drought tolerant traits and
7. To assess the polymorphism of the parents through molecular markers.
Rice with the widest adaptation is being cultivated in almost all the types of soils as well as climatic conditions. However, about half of the rice area in the world and 56 per cent of rice in India, do not have sufficient moisture during its growth phase mainly due to inadequate rainfall and less water efficient irrigation system which ultimately cause enormous loss in crop yield (Anbumalarmathi and Nadarajan, 2008).
In India upland rice is grown traditionally in aerobic soils under rainfed conditions with minimal inputs and thus produces very low yields (Atlin and Lafitte, 2002). Food security of this continent depends heavily on rice production from conventional transplanted system, which is under threat as decreasing freshwater availability may lead to water scarcity in 15 million ha of Asia’ s irrigated rice by 2025. The per capita availability of water in most of the basins in India, rice growing regions in particular, also predicted to decline below water scarcity line by 2025. Hence, it is imperative to develop alternate rice production systems, which will save water, while maintaining the productivity. Growing rice under aerobic conditions as in the case of other crops will reduce the water requirement by atleast 30 per cent (Sheeba et al., 2005).
The slogan ‘Rice is Life’ is more appropriate for India as this crop plays a vital role in our National food security and is a means of livelihood for millions of rural household (Misra et al.,, 2004). Therefore, in order to sustain and to increase the rice production to meet the future demands with limited water supplies, there is a need to alter genetically the basic water requirements of rice through breeding techniques (Vijayakumar et al., 2006).
The challenge is to develop novel technologies and production systems that will allow rice production to be maintained or increased in the face of declining water availability. Aerobic rice is an emerging agronomical production system that uses less water than conventional flooded rice (Tuong et al., 2005). The aerobic rice system uses rice varieties capable of responding well to reduce water inputs in non-puddled and non-saturated soils (Bouman and Tuong, 2001; Atlin et al., 2006; Peng et al., 2006; Luo, 2010). The crop can be either rainfed or irrigated, depending on rainfall distribution, soil type and subsurface hydrology. Thus, there is some overlapping in the definitions of traditional upland rice and aerobic rice (Bouman et al., 2007). Rainfed rice in fertile uplands using high-yielding varieties and ample rainfall can be regarded as aerobic rice because the specific requirement is high production under aerobic soil conditions, whereas non-irrigated rice with lower yield expectations should be regarded as upland rice.
2.1. DROUGHT AND DROUGHT TOLERANCE IN RICE
Among the abiotic stresses, drought is a serious limiting factor that reduces rice production and yield stability in rainfed ecosystems. Conventional breeding for drought tolerance is slow in attaining progress due to poor understanding of genetic control of drought tolerance.
A large portion of the world’s poor farm is rainfed system where the water supply is unpredictable and drought is common. In Asia, about 50 per cent of the rice land is rainfed and, although rice yields in irrigated systems have doubled and tripled over the past 30 years, only modest gains have occurred in rainfed rice ecosystems. In part, this is because of the difficulty in improving rice varieties for environments that are heterogenous and variable and in part because there has been little effort to breed rice for drought tolerance (Anon, 2003).
A phenomenal enhancement in productivity of rice, wheat and maize has been accomplished and has been well documented as green revolution across the world. In complete contrast to this scenario, the increase in productivity of crops in drought prone habitat has been meager, especially in rice, a crop grown under diverse moisture stress (Shashidhar, 2008).
When rice populations are subjected to drying soil conditions, genotypes are very clearly separated into those that wilt and dry readily and those that maintain a measure of turgor and viability as stress continues (De Datta et al., 1988). Furthermore, genotypes differ in their recovery upon rehydration and the level of genotypic recovery is closely related to its hydration status prior to recovery (Malabuyoc et al., 1985). Drought tolerance in terms of these responses is most likely dependent mainly on one or more of the following components:
- Moderate water use through reduced leaf area and shorter growth duration
- The ability of the roots to exploit deep soil moisture to provide for evapotranspirational demand
- The capacity for osmotic adjustment which allows to retain turgor and protect meristems from extreme dessication and
- The control over non-stomatal water loss from leaves.
Published results (Turner, 1986; Yu et al., 1995) indicated that traditional upland cultivars generally tend to excel in root growth and soil moisture extraction capacity while lacking in osmotic adjustment. These cultivars usually develop severe leaf hydration and leaf rolling as soon as soil moisture is depleted. When plant water deficit develops, they tend to conserve moisture and leaf water status by stomatal closure. Stomatal closure in these cultivars may be induced by either lack of osmotic adjustment or by a hormonal root signal (Davies et al., 2002). On the other hand, low land improved cultivars tend to lack root depth, but they generally excel in osmotic adjustment. Such cultivars retain turgidity and leaf gas exchange to lower leaf water potential than the former cultivars. However, without sufficient deep root development, they are bound to severely desiccate once the top soil is dried.
It can be speculated that under upland situations with deep soil moisture there have been a selective advantage to deep and thick root systems, which served to maintain high leaf water status and dehydration avoidance. Under such conditions, deep roots have evolved in adapted materials. Osmotic adjustment did not evolve under such conditions because plants were usually avoiding severe water deficit. The capacity for osmotic adjustment may have evolved where leaf tissue water status was often reduced by water deficit, such as in lowland rice where deep rooting is often deterred by the subsoil compaction. These different modes of response to drought stress require validation and further research to suggest clues to desirable breeding strategies with respect to the different rice environments.
2.2. NEED FOR AEROBIC RICE AND ITS IMPORTANCE
Food security in Asia is challenged by increasing food demand and threatened by declining water availability. More than 75 per cent of the rice supply comes from 79 million ha of irrigated land. Thus, Asia’s present and future food security depends largely on the irrigated rice production system. However, the water-use efficiency of rice is low and growing rice requires large amounts of water. In Asia, irrigated agriculture accounts for 90 per cent of total diverted freshwater and more than 50 per cent of this is used to irrigate rice. Until recently, this amount of water has been taken for granted, but now the global “water crisis” threatens the sustainability of irrigated rice production. The available amount of water for irrigation is becoming scarce (Gleick, 1993; Postel, 1997). The reasons for this are diverse and location-specific, but include decreasing quality (chemical pollution, salinization), decreasing resources (e.g., falling groundwater tables, silting of reservoirs) and increased competition from other sectors such as urban and industrial users. Because of the increasing scarcity of water, the cost of its use and resource development are increasing as well. The conventional flooding technique is very high water consuming. Brown et al. (1978) have indicated that 48 per cent (570 mm) of the applied irrigation water (1180 mm) is lost through evapotranspiration (ET). The remainder is lost due to runoff and infiltration. Water represents a major and necessary production cost for rice growers. Therefore, farmers and researchers alike are looking for ways to decrease water use in rice production and increase its use efficiency.
A fundamental approach to reduce water inputs in rice is to grow the crop like an irrigated upland crop such as wheat or maize. Instead of trying to reduce water input in lowland fields, the concept of having the field flooded or saturated is abandoned altogether. Upland crops are grown in nonpuddled aerobic soil without standing water. Irrigation is applied to bring the soil water content in the root zone up to field capacity after it has reached a certain lower threshold e.g., halfway between field capacity and wilting point (Borell et al., 1997).
The amount of irrigation water should match evaporation from the soil and transpiration by the crop. Since, it is not possible to apply irrigation water to the root zone only, some of it is lost by deep percolation and is unavailable for uptake by the crop. Typical field application efficiencies vary from 60–70 per cent using surface irrigation (e.g., flash or furrow irrigation) to more than 90 per cent using sprinkler or drip irrigation (Jha and Singh 1997).
De Datta et al. (1973) tried growing rice like an upland crop using furrow irrigation in the dry season of 1971 at IRRI. Using the high-yielding lowland variety IR20, total water (irrigation plus rainfall) savings were 56 per cent and irrigation water savings 78 per cent compared with growing the crop under flooded conditions. However, yield decreased from 7.9 to 3.4 t ha–1. Upland cultivars that performed equally well under flooded and dryland irrigation were used, but their yields of around 5 t ha–1 were much lower than those of lowland cultivars. Studies on non-flooded irrigated rice using sprinkler irrigation were conducted in the United States in Texas and Louisiana (Westcott and Vines, 1986 and McCauley, 1990). Experiments used commercial rice cultivars under lowland cultivation.
Irrigation water requirements were 20–50 per cent less than in flooded rice, depending on soil type, rainfall and water management. The highest yielding cultivars (producing 7–8 t ha–1 under flooded conditions) however had yield reductions of 20–30 per cent compared with flooded rice. The most drought-resistant cultivars produced the same under both conditions, but their yields were much lower (5–6 t ha–1). Under economic conditions prevalent in the US, the adoption of irrigated dryland rice (using existing cultivars) was not economically attractive. New varieties must be developed if growing rice like an irrigated upland crop is to be successful.
Lowland cultivars have been selected to give high yields under continuously flooded lowland conditions. They generally suffer a yield loss when the soil water content drops below saturation. Upland varieties have been developed to give stable though low yields in adverse environments where rainfall is low, irrigation is absent, soils are poor or toxic, weed pressure is high and farmers are too poor to supply high inputs.
Therefore, IRRI recently coined the term “aerobic rice” to refer to high-yielding rice grown in nonpuddled aerobic soil. This aerobic rice, which can be rainfed or irrigated, should be responsive to high inputs and should tolerate flooding. It has to combine characteristics of both upland and high yielding lowland varieties.
2.2.1. Aerobic rice breeding and its improvement
Water in irrigated rice production has been taken for granted for centuries, but the “looming water crisis” may change the way rice is produced in the future. Water-saving irrigation technologies that were investigated in the early 1970s, such as saturated soil culture and alternate wetting and drying, are receiving renewed attention from researchers. These technologies reduce water input, though mostly at the expense of some yield loss.
Traditional upland rice crops are grown in unbunded, unflooded fields, where soil conditions in the root zone remain aerobic through most of the growing season. Farmers usually treat upland rice as a subsistence crop, investing little on inputs beyond family labour. Because upland rice varieties are grown without irrigation in unsaturated soils, they are considered to be drought tolerant. However, upland rice yields under traditional systems are low, averaging one to two tonnes per ha in most rice growing regions. Intensification of management of these systems with currently available germplasm is difficult because most traditional upland rice varieties are tall, low tillering and prone to lodging when grown under conditions of favorable moisture and high soil fertility.The development of aerobic rice varieties is probably the most ambitious challenge of all (Lafitte, 2002).
However, little progress has been made in the breeding for aerobic rice, because the fundamental mechanisms of drought tolerance and yield contributing characters of aerobic rice are poorly understood. Development of high yielding aerobic rice is still in its infancy and germplasm still needs to be improved and appropriate breeding and management technologies developed. Hence, improvement of rice yield under aerobic systems will be made possible only through the incorporation of drought tolerance and yield contributing traits in breeding programme.
A number of physiological and morphological traits have been proposed to improve the performance of aerobic rice challenged by limited water and extensive input conditions. There is strong evidence that high input aerobic systems require specially bred cultivars that differ from both conventional upland varieties and elite irrigated varieties. Thus, aerobic rice varieties need to be more tolerant to drought stress, particularly at the sensitive reproductive stage, than most irrigated varieties and aerobic rice breeding programs must emphasize drought tolerance. Optimization of aerobic systems will likely require the development of a new cultivar type combining moderate drought tolerance, high rates of tillering, high harvest index and lodging resistance (Atlin, 2003).
The China Agricultural University (CAU) started its systematic aerobic rice breeding programme in the early 1980s. This programme is based on the genetic recombination of lowland and upland varieties from different eco-geographic origins. Han Dao 113 and Han Dao 58 are currently the most extensively grown aerobic rice varieties (Wang Huaqi et al., 2002). Yield and drought tolerance of an aerobic rice is a complex trait, expression of which depends on action and interaction of different morphological, physiological and biochemical characters. The identification of genes that are responsible for morphological and physiological traits and their locations on chromosomes have not been possible, but their inheritance pattern and nature of gene action have been reported. Polygenic inheritance of root characters is reported by Ekanayake et al. (1985). Though some more reports in this regard for other traits are available, further investigation is the need of the hour to have better understanding of genetic control of morphological and physiological traits contributing to drought tolerance and higher yield under aerobic conditions.
Recently, a new class of upland adapted cultivars with improved lodging resistance, harvest index and input responsiveness has been developed by breeding programmes in China, Brazil and Philippines. These varieties combine some of the yield potential-enhancing traits of lowland high-yield varieties adapted to low input systems. IRRI recently coined the term "Aerobic Rice" referred to high yielding rice grown in non-puddled aerobic soil. It is commercially grown in Brazil, China, Philippines and rice-wheat belt of India (Haryana, Punjab and Uttar Pradesh). The distinguishing feature of aerobic production system is that crops are direct seeded in free-draining, non-puddled soils where no standing water layer is maintained in the field and roots grow in a mainly aerobic environment (Bouman, 2001 and Atlin et al., 2005).
Traditionally aerobic rice is grown in rainfed uplands with low or no inputs with declining water availability for agricultural use, aerobic rice cultivation is expected to expand into the irrigated, intensive and high productivity cropping systems (Vijayakumar et al., 2006).
Aerobic rice production also constitutes a separate target environment from traditional upland rice based systems. Water inputs in aerobic rice was more than 50 per cent lower (470-650mm), water productivity 64-88 per cent higher, gross returns 28-44 per cent lower and labour use 55 per cent lower compared with lowland rice. Thus aerobic rice, which can be rainfed or irrigated, should be responsive to high inputs and should tolerate flooding. It has combined characteristics of both upland and high yielding low land varieties. Breeders have to respond to the challenge of breeding varieties that perform well under non-permanently flooded conditions.
Development of high yielding aerobic rice cultivars will considerably improve the rice production in rainfed and water scarce low land conditions. However, little progress has been made in breeding for aerobic rice; because the fundamental mechanisms of drought tolerance and yield contributing characters of aerobic rice are poorly understood. Development of high yielding aerobic rice is still in its infancy and germplasm still needs to be improved and appropriate breeding and management technologies developed. Hence, improvement of rice yield under aerobic systems will be made possible only through the incorporation of drought tolerant and yield contributing traits in breeding programme.
Atlin et al. (2005) reported, the rice cultivars for aerobic system need to combine high biomass production, harvest index and lodging resistance with moderate drought tolerance, particularly at the sensitive stage, because aerobic system depends on direct seeding in dry soil, without accumulation of standing water, vigorous early growth is also needed to compete with weeds and to root deeply to avoid early season drought. Recent research has shown that drought tolerance and high-spikelet fertility under reproductive stage stress in aerobic adapted germplasm are strongly associated with short duration and minimal flowering delay under stress.
Aerobic production systems based on high-yield, input responsive upland cultivars have already been developed to replace conventional irrigated lowland rice production on the water short plains of north eastern China, producing yields of 4.5 to 6.5 t/ha, with substantial water savings relative to conventional irrigated production (Bouman et al., 2005).
Atlin and Lafitte (2002) reported, the yield in high input aerobic systems is currently about 30 per cent lower than conventional irrigation management and at least 100 per cent greater than yield in conventional low-input upland systems, which average to 2 t/ ha because of the lower yield of aerobic systems. Direct selection under high-input aerobic management from the earliest stages of a breeding programme is therefore likely to be required to maximize selection response. Early screening of breeding lines under high-fertility aerobic management is particularly important in selecting for lodging resistance, a key trait for aerobic cultivars.
2.3. GENETICS OF DROUGHT TOLERANCE AND YIELD
Yield and Drought tolerance are a complex trait, expression of which depends on action and interaction of different morphological (earliness, reduced leaf area, reduced tillering, efficient rooting system, leaf rolling and stability in yield), physiological (reduced transpiration, high water use efficiency, stomatal closure and osmotic adjustment) and biochemical (accumulation of proline, polyamine, trehalose, etc., increased nitrate reductase activity and increased storage of carbohydrate) characters. Very little is known about the genetic mechanisms that condition these characters. Very little is known about the genetic mechanisms that condition these characters. The genetic improvement of rice for aerobic (non-flooded) has not been understood well and major efforts on this front are lacking (Vijaykumar et al., 2006).
The long root and high root number are controlled by dominant alleles and thick root tip by recessive alleles (Armenta-Soto et al., 1983). The identification of genes that are responsible for morphological and physiological traits and their locations on chromosomes have not been possible, but their inheritance pattern and nature of gene action have been reported. Polygenic inheritance of root characters is reported by Ekanayake et al. (1985). However, leaf rolling has shown monogenic inheritance (Singh and Mackill, 1991).
The genetic basis is still very narrow and only a few successful aerobic rice varieties exist. Further efforts need to be directed at increasing the yield potential and looking at broad biotic (diseases, weed) and abiotic (drought, micronutrient deficiency) stress tolerances. Besides yield, grain quality should be a key focus of attention since this will determine consumer acceptability (Wang Huaqi et al., 2002).
Though some more reports in this regard for other traits are available, further investigation is the need of the hour to have better understanding of genetic control of morphological and physiological traits contributing to drought tolerance and higher yield under aerobic conditions
2.4. DROUGHT TOLERANT TRAITS
Kumar and Kujur (2003) clearly indicated that maintenance of higher plant water status under drought plays a key role in stabilizing the various plant processes and yield. A strong relationship between leaf water potential under drought was observed with delay in flowering and membrane stability. The maintenance of higher leaf water potential under drought was achieved by higher root to shoot ratio. A strong relationship between delay in flowering with spikelet sterility and grain yield was observed under drought conditions. The higher partitioning of dry matter into root helps in water extraction from deeper soil layers in drying soil and maintains higher water potential.
Pantuwan et al. (2001) also reported that delay in flowering is a strong indication of drought susceptibility in rice. The delay in flowering, high leaf water status and higher root to shoot ratio, photosynthetic stability, lower leaf temperature and higher membrane stability under drought contribute significantly to flowering stage drought tolerance in rice. These traits can be used as indirect selection criteria to improve the grain yield stability of rice under drought.
Generally more early varieties were of good performance and adapted to dry conditions. Earliness may be used as a suitable criterion for selecting improved varieties. Cultivars with early or medium heading dates performed better under aerobic condition. Genotypes of aerobic rice showing low moisture at harvesting time performed very well (Russo Salvatore, 1996).
Ravindra Kumar et al. (2004) reported that well - developed root system helped in maintaining high plant water status along with a short delay in flowering which ultimately reflected in yield stability under drought conditions.
Breeding for a large root system contributes towards enhanced drought tolerance in rice crop. Maximum root length, root dry weight and root volume were recorded to be highly heritable. Higher expected genetic advance as per cent of mean was obtained for root volume, root dry weight, total root number, shoot dry weight and number of tillers. Hence, good response to selection for these characters could be anticipated (Adnan Kanbar et al., 2004).
Among the several factors contributing to enhance tolerance to drought, roots were the main organ for plant water uptake. Maximum root length and root dry weight were good indicators of drought tolerance in upland rice (Nguyen et al., 1997 and Mane et al., 2003). Even though grain yield under stress is the primary trait for selection of breeding programme for drought prone environment, low heritability of yield necessitates an alternative approach such as selection for secondary traits. Yield improvements in water limited environments could be achieved by identifying and selecting the secondary traits that contribute drought tolerance. The effectiveness of selection for secondary traits to improve yield under water limited conditions has been demonstrated in sorghum (Tuinstra et al., 1998), maize (Chapman and Edmeades, 1999) and wheat (Richards et al., 2000).
2.5. ROLE OF SECONDARY TRAITS AND PUTATIVE TRAITS FOR DROUGHT TOLERANCE
Yield improvement in water limited environments could be achieved by identifying and selecting secondary and putative traits that contribute drought tolerance. An idea on the extent of association between traits conferring drought tolerance will be much helpful to decide upon the traits to be given importance in selection for drought tolerance. A positive association between traits warrants the simultaneous improvement of both the traits while restricting selection to any one of the associated traits. On the other hand, a negative relationship between two traits necessitates equal weightage to be given on both the traits during selection process. Relative importance of drought tolerance attributes may be decided based on highly correlated trait with grain yield and with other major mechanisms of drought tolerance.
2.5.1. Secondary traits
Even though grain yield under stress is the primary trait for selection of breeding programme for drought prone environment, low heritability of yield necessitates an alternative approach such as selection for secondary traits. Secondary traits are plant characteristics that are associated with yield under stress and provide additional information about how yield will change under drought. Carefully selected secondary traits through indirect selection could be helpful in improving selection response.
Yield improvements in water limited environments could be achieved by identifying and selecting the secondary traits that contribute drought tolerance (Ganapathy and Ganesh, 2008). The effectiveness of selection for secondary traits to improve yield under water limiting environments has been successfully demonstrated for water use efficiency in wheat (Sanguineti et al., 2007), stay greenness in sorghum (Harris et al., 2007) and anthesis-silking interval in maize (Monneveux et al., 2006 and Liu et al., 2010).
Kumar et al. (2008) reported that most of the secondary traits have moderate to high heritabilities under stress indicating the possibility of incorporating them into breeding program.
For a secondary trait to be useful in a breeding programme, it has to pass five tests.
- It must be genetically correlated with grain yield in the predominant stress situations that occur in the target environment.
- It should not be affected very much by environment, that is, it should be highly heritable in the screening system.
- There must be variation among lines for the trait.
- It should not be associated with poor yields in the unstressed environment and
- It must be possible to measure the trait rapidly and economically.
220.127.116.11. Days to 50 per cent flowering
According to Russo Salvatore (1996), early varieties were of good performance and adapted to dry conditions. Earliness might be used as a suitable criterion for selecting improved varieties. Cultivars with early or medium heading dates performed better under aerobic conditions. Very long duration varieties showed significant yield reduction compared with the top-yielding varieties which indicated a poor adaptability to dry conditions.
When rice experiences a water deficit before flowering, a delay usually occurs in flowering time. Lines with a longer delay will tend to produce less number of grains, even if the water stress is relieved later. Pantuwan et al. (2001) and Kumar and Kujur (2003) reported that delay in flowering due to drought was a strong indicator of drought susceptibility.
Rice is extraordinary sensitive to water deficit from 12 days before 50 per cent flowering to about seven days after flowering. Selection for days to 50 per cent flowering as an effective way to improve drought tolerance was earlier reported by Michael Gomez et al. (2003) and Yogameenakshi et al. (2003). Delay in flowering due to stress was also reported by Imanywoha et al. (2004). Flowering delay due to moisture stress was a strong indicator of drought susceptibility (Sheeba et al., 2005).
Drought tolerant varieties exhibited a slight reduction in days to flowering under severe stress relative to the irrigated low land environment, whereas for all other cultivar classes, flowering was delayed by two weeks or more as a result of severe stress (Atlin et al., 2005).
All traits except days to 50 per cent flowering expressed moderate to high indirect effects via leaf drying and dry root weight (Sheeba et al., 2010).
The proportional contribution to total genetic variance by the testers was found to be higher for days to 50 per cent flowering, plant height, number of filled grains per panicle, spikelet fertility, grain yield, harvest index and total dry matter production (Malarvizhi et al., 2010).
Negative heterosis is desirable for days to 50 per cent flowering because
this will make the hybrids to mature earlier as compared to parents. Almost all the crosses had either equal or early flowering than the standard variety, Sarjoo – 52 (Tiwari et al., 2011)
18.104.22.168. Plant height
Plant height is an important trait because
it is often associated with lodging of plants under unfavourble situations. The plant height
increased gradually from tillering to flowering and
remained almost constant till maturity. Peng et al. (1999)
and Zhang et al. (2004)
reported that increasing plant height would allow greater
biomass production and yield potential.
In general, moisture stress resulted in reduced plant height and susceptible lowland types were more sensitive than upland varieties (Basu Raychaudhuri and
Das Gupta, 1981). Tomar and Prasad (1996) reported that plant height in rice was associated with thick and deep root system. Plant height was slightly lowered in aerobic condition than the standard (flooded types) culture. The lower amount of water supplied using the aerobic method reduced plant growth and prolonged the cycle duration according to different cultivars (Russo Salvatore and Salvalaio Iside, 1998).
The superior plant type in terms of height and tiller number for yield in aerobic systems was not yet known, so a range of plant types was being evaluated. Varieties for aerobic rice cultivation should have lodging resistance, an ability to partition plant matter into grain and higher nutrient responsiveness (Castaneda et al., 2002). Atlin et al. (2005) reported the medium-height aerobic-adapted cultivars consistently yield high and were lodging resistance.
In spite of water stress at tillering prolonged vegetative period, reduced plant height, tiller number, leaf length and induced leaf rolling, the data showed that these genotypes were earlier in heading, remained tall in height, having more tillers/plant and they were able to recover after the water stress condition was terminated, having smaller leaf canopy to minimize transpiration rate, have good drought score from 1 - 3 and desirable flag leaf area which contribute by the higher proportion of carbohydrate to grain filling after heading (Abd Allah et al., 2010). Upland cultivars were in general considerably taller than the lowland cultivars at seedling stage (Sanusan et al., 2010).
22.214.171.124. Number of productive tillers per plant
Tiller production increased sharply from active tillering
to panicle initiation stage and declined gradually towards
flowering and remained almost constant during ripening
phase in all rice cultivars (Yoshida, 1981). Number of panicles per unit land area, the dominant yield component that influenced grain yield, linearly decreased with the number of tillers per square meter (Cruz et al., 1986). The number of panicles was significantly lower in plants, which are exposed to aerobic condition at vegetative stage than the plants under flooding. In clay loam soil, the number of panicles was relatively high in aerobic condition only at reproductive stage, due to the production of second-generation tillers at grain filling stage (Nieuwenhuis et al., 2002). More number of productive tillers is one of the important traits to increase the yield under aerobic system of rice production (Malarvizhi et al., 2010).
More panicle bearing tillers per plant is believed to be closely associated with high grain yield per plant resulting high productivity. Significantly high number of productive tillers per plant was identified in the cross combinations (Tiwari et al., 2011).
126.96.36.199. Panicle number per plant
Number of panicles per unit area was determined during the period up to about 10 days after maximum tiller number was reached (Murata and Matsushima, 1975). Grain yield in cereals was
highly dependent upon the number of productive tillers produced by each
plant constituting an important morpho-physiological trait for grain yield in rice (Tao et al., 2006) and
dependant on the environmental conditions during tiller bud initiation and subsequent
developmental stages (Fageria, 2007).
Gravois and Helms (1992) reported that optimum rice yield could not be attained without optimum panicle density of uniform maturity. Similarly, specific absorption rate of N per root dry weight during grain filling stage was the most important factor for achieving high rice productivity (Osaki et al., 1995).
Fageria and Baligar (2003) reported that application of N in adequate amount accounted for about 91 per cent variation in panicles m-2, about 75 per cent variation in spikelet sterility and about 73 per cent variation in 1000 grain weight.
With lower soil water potentials, the elongation of internodes, the number
of panicle and the crop growth rate were reduced in comparison to flooded conditions (Lu et al., 2000). Number of panicles was significantly lower in plants exposed to aerobic condition at vegetative stage than the plants under flooded situations (Nieuwenhuis et al., 2002).
Aerobic varieties had significantly more panicles than the traditional upland varieties, but 100-150 panicles per meter fewer than irrigated lowland types. Selection for increased panicle number might be a promising avenue for increasing aerobic rice grain yield (Atlin et al., 2005). Adequate N supply throughout the growth cycle of rice plant was one of the main strategies to increase rice grain yield (Fageria, 2007). Reduction in panicle number was observed in rice under water stress situations (Ichwantori et al., 1999). Similar results were registered with reduction in tiller production as well as mean panicle weight with lower water supply situation in drip irrigation system (Vanitha, 2008).
It was also observed that the N application could improve the number of panicles (Paikaray et al., 2001; Singh and Singh, 2005). Towards this, increase in number of productive tillers with the use of drip fertigation practice was evident in aerobic rice (Vanitha, 2008).
In spite of drought stress, reproductive stage is the most damaging to rice crop by the reduction of dry matter production and therefore, reduction of productive tillers, these genotypes having more panicles / plant indicating that most of their tillers bear panicles under drought conditions. This may be due to the increase in nitrogen content in their shoot (Abd Allah et al., 2010).
188.8.131.52. Panicle length
Predominant additive gene action was observed for panicle length (Rao et al., 1996). Lengthy panicles are generally associated with higher number of spikelets per panicle resulting in higher productivity. Therefore, hybrids with positive heterosis are desirable. They indicate the presence of desirable genes for the expression of longer panicles. The SCA variance due to lines x testers were significantly higher for days to 50 per cent flowering, panicle length, spikelet fertility, 100 grain weight, harvest index, SPAD values and relative water content in aerobic condition. (Chen et al., 1995; Malarvizhi et al., 2010; Tiwari et al., 2011).
Higher number of grains per panicle with KRH-2 was due to higher panicle length (23.50 cm) and higher grain weight per panicle and also the test weight (Basavaraja et al., 2010).
184.108.40.206. Filled grains per panicle
In cereals, grains are the most important sink for carbon and nitrogen after anthesis. In rice, available carbon assimilate for grain production is determined by carbon assimilation during the grain-filling period plus assimilate reserves stored in the straw (Cock and Yoshida, 1972). Growing conditions can also influence the percentage of filled grains. In lowland rice, for example, drought stress during late panicle development sharply decreases the percentage of filled grains (Fageria, 2007). Delayed senescence during grain filling under drought stress conditions can result in more non-structural carbohydrate remaining in the straw, leading to a lower harvest index. Poor grain filling in two-line hybrid rice was generally considered to be closely associated with its stronger stay-green or delayed senescence until ripening stage, in comparison with three-line hybrid rice or conventional rice varieties ( Zhu et al., 1997; Wang et al., 1998; Chen et al.1999; Chen, 2001; Gu and Tang, 2001).
A ‘stay-green characteristic’ of rice could potentially increase grain yield through prolonged photosynthesis during grain filling. For hybrid lowland rice, delaying leaf senescence increases the percentage of filled grains. In lowland rice, N top dressing at anthesis usually impairs grain filling, especially when the soil N supply is high (Yang et al., 2001). The contribution rate of 70 per cent at the grain-filling stage was smaller than that of other crops, such as lowland rice (81–98 per cent) or wheat (77–92 per cent) (Yang Xiaoguang et al., 2002)
Rice grains at the apical primary branches of the panicle are classified as ‘superior’, while those at the proximal secondary branches are classified as ‘inferior’. Fast synchronous grain filling usually results in high yields, while slow synchronous or asynchronous grain filling usually results in relatively low yields. Equation to simulate grain filling procedure of different crops (lowland rice, wheat, etc.), such as Richard’s equation and ‘the beta growth function’, can estimate grain filling rate and duration precisely. Hence, we suppose that the HD297 grain-filling pattern was either slow synchronous or asynchronous, resulting in poor grain filling and low grain yield of this variety (Yin et al., 2003).
Reducing the irrigation (and hence increasing drought stress) decreases the percentage of filled grains in aerobic rice (Bouman et al., 2007).
Fertile grain is an important contributory factor to grain yield. Grain filling is the final growth stage in rice when fertilized
ovaries develop into caryopsis and grain quality is mainly
formed during this period (Wiangsamut and Mendoza, 2008). Production of grains and their filling percentage was significantly influenced by the irrigation regimes and fertilizer levels with drip system of irrigation in aerobic rice (Vanitha, 2008). Top dressing at anthesis increases grain filling of large panicle hybrid rice through delayed senescence of leaves and roots (Li et al., 2008).
Drought stress at booting and flowering stages reduced number of filled grains/panicle and induced sterility (%), whereas, these genotypes have high number of filled grains/ panicle and low sterility (%). This may be due to higher sugar in their stems (Abd Allah et al., 2010).
Significant and positive effects desirable for filled grains per panicle were recorded for three females APMS6A, IR58025A and PMS3A and 12 males (Pradeep Kumar and Reddy, 2011)
The percentage of filled grains depends on the grain filling rate and grain filling duration of superior and inferior grains, which may be fast synchronous, slow synchronous, or asynchronous (Fengtong Wei et al., 2011).
220.127.116.11. Spikelet fertility
When stress occurs near flowering, the main yield component affected is the spikelet fertility. The way that spikelet fertility is affected by drought at flowering is acute. So it gives clear information on genotypic response to stress than does yield.
Maurya and O’Toole (1986) found large genotypic variation in filled grain percentage during the dry season. Reduction in leaf water potential at anthesis caused poor panicle exertion (O’Toole and Namuco, 1983; Cruz and O’Toole, 1984 and Ekanayake et al., 1989) and this increased the percentage of sterile spikelets because of pollination abnormalities. Water stress affects many complex and phenologically increasing biochemical events between panicle initiation and grain filling. Studies of panicle water relations (O’Toole et al., 1984; Garrrity et al., 1986 and Ekanayake et al., 1989), abnormalities of gamete formation (Namuco and O’Toole, 1986) and panicle exertion (O’Toole and Namuco, 1983; Cruz and O’Toole, 1984) have provided insight into the causes of sterility induced by water stress and the resultant decrease in grain yield. Numerous serial events (viz., panicle initiation, gamete production, panicle growth and exertion, anthesis and fertilization) if perturbed by stress can result in spikelet sterility or embryo abortion.
Grain yield is closely related to grain number, because grain weight is relatively stable across environments. Grain yield is mostly limited by sink capacity, the ability of grain to accept assimilate (Fukai et al., 1991). The number of spikelets determines the grain number at the anthesis and pollination of spikelets, which produce grains (filled grain percentage). The number of spikelets is directly related to the rate of assimilation between panicle initiation and anthesis, regardless of whether the assimilate production is altered by water stress or shading (Boonjung, 1993).
Filled grain percentage on the other hand is related to assimilation around anthesis and is particularly susceptible to water stress (Cruz and O’Toole, 1984; Boonjung, 1993). Garrrity and O’Toole (1994) established a high correlation between spikelet sterility and grain yield. Retention of spikelet fertility appears important in maintaining high grain yield under flowering stage drought.
Chandra Babu et al. (2003) and Kumar and Kujur (2003) concluded that drought delayed flowering which in turn resulted in lower spikelet fertility and lower grain yield.
Recent research showed that drought tolerance and high spikelet fertility under reproductive stage stress in aerobic adapted germplasm are strongly associated with short duration and minimal delay under stress (Atlin et al., 2005).
The spikelets in the unexcerted portion of panicle remain sterile (Mackill et al. 1996). Rapid peduncle elongation occurs when the panicle attain its full length. Drought causes a reversible inhibition on peduncle elongation. Therefore under drought stress, the peduncle elongation is blocked and upon rewatering, the growth continues but the peduncle does not achieve its full length (Ji et al., 2005).
Traits like spikelet fertility, leaf drying and dry root weight greatly influenced the grain yield both directly and indirectly hence should be given priority during selection for enhancing grain yield under drought stress situation (Sheeba et al., 2010).
Number of spikelets per panicle is one of the most important yield components that improves yield. Expression of desirable genes for this trait decides the grain filling per cent in hybrids which is due to the restoration ability of the male parents (Malarvizhi et al., 2010).
The largest number of spikelets per panicle was observed in well water, while Suphan Buri 1 had higher spikelet per panicle than Pathum Thani 1 for the rice that suffered water stress at the vegetative growth stage (Sanusan et al., 2010).
18.104.22.168. Total dry matter production (TDMP)
Lilley and Fukai (1994) reported that water deficit reduced biomass production in rice varieties and the degree of reduction dependant upon the severity of moisture stress. Dry matter yield of rice genotypes were reduced by 11-37 per cent and 30-65 per cent under mild and severe moisture stress respectively (Yang et al., 1995). Boonjung and Fukai (1996) also reported similar results. Chauhan et al. (1996) reported that cultivars differed in their ability to produce dry matter under stress. The total biological yield was the highest in continuous irrigation and the lowest was observed in irrigation at eight-day interval after tillering initiation (Surek and Korkut, 1998). Aerobic treatment with flooding in the reproductive stage exhibited 14 per cent lower biomass production than continuously flooded treatment and the continuously aerobic produced 32 per cent lower biomass than continuously flooded treatment (Nieuwenhuis et al., 2002).
Rajeswari (1990) reported that N application increased total dry matter production in rice. As opined by Belder et al. (2005), the total dry matter production of aerobic rice could be improved by the N application as compared to the flooded rice.
22.214.171.124. Hundred grain weight
A mild water stress at vegetative stage resulted in linear decrease in 100 grain weight in rice and its influence on grain yield was not significant (Cruz et al., 1986). Water stress at booting and heading to flowering stages reduced number of productive tillers, grain number per plant and 100 grain weight (Liu et al., 1993). The reduction in grain yield in rice with water stress was mainly due to decrease in the number of filled grains per panicle and 1000 grain weight depending on severity of stress (Surek and Korkut, 1998).
When the crop was exposed to aerobic condition, the number of grains per panicle and the 1000 grain weight were significantly lowered and the percentage spikelet sterility was increased when compared to continuous flooding in rice (Nieuwenhuls et al., 2002). When the crop was exposed to aerobic condition, number of grains per panicle and 1000 grain weight were significantly lowered and the percentage of spikelet sterility was increased when compared to continuous flooding in rice (Nieuwenhuis et al., 2002; Gowri, 2005).
Conversely, Tao et al. (2006) observed a higher harvest index and 1000 grain weight in aerobic rice than that in lowland treatments. Spikelet weight in rice is generally expressed in terms of 1000 grain weight (test weight) in grams. Spikelet size is rigidly controlled by hull size and under most conditions, the 1000 grain weight of rice is very stable varietal character (Fageria, 2007). The test weight showed wide variations for irrigation as well as fertilizer level in aerobic rice given through drip system (Vanitha, 2008).
126.96.36.199. Harvest index
The Harvest index is an indicator of the efficiency of carbohydrate partitioning to the grains. In general, higher harvest index indicated efficient translocation of assimilates for grain production or economic yield. Harvest index is the major determinant of yield due to its direct effect and indirect contribution from N-uptake ability, CGR, LAI (flowering), sink weight and sink volume through harvest index. Harvest index had shown a consistent association with grain yield in rice (Reuben and Katuli, 1990). Genotypes having greater tolerance to water stress recorded higher number of grains and grain weight per panicle, grain yield per plant and harvest index (Saxena et al., 1996). The continuous flooded irrigation had the highest harvest index, where as, the irrigation at eight-day interval after tillering initiation had the lowest value (Surek and Beser, 2003).
Generally, dry matter had positive associations with grain yield and N is important for improving harvest index (Fageria and Baligar, 2005; Fageria et al., 2006). Harvest Index is the ratio of grain yield to total biological yield and the values for harvest index in cereals and legumes are normally less than one (Fageria and Baligar, 2005).
Higher harvest index meant that greater amount of carbohydrate was translocated to the grains (sink) (Wiangsamut and Mendoza, 2008). Increased value of harvest index was noticed with the biofertigation practice in aerobic rice (Vanitha, 2008).
Malarvizhi et al. (2010) reported that eight genotypes had desirable genes for expression of high harvest index which in turn increased the grain yield. The hybrids developed from these parental lines were found superior for most of the yield and physiological traits under aerobic condition.
188.8.131.52. Single plant yield
Soil water stress during the earlier growth phases (vegetative) affect the production of effective tillers resulting in the reduction of grain yield, while water stress during the later growth phases (reproductive) appeared to affect the reproductive physiology, by interferrring with pollination, fertilization and grain filling in the reduction of grain yield. Continued flooding is not essential for higher grain yield and practice of intermittent submergence at critical stages of crop gives yield comparable to continuous flooding (Waan, 1978). The biological yield of a cereal crop is the
total yield of plant and is an indication of the yield of the photosynthetic
capability of a crop (Yoshida, 1981; Garrity et al. 1986). Nevertheless, Osaki et al. (1995) opined that t
he biological yield is a function of crop growth duration and crop growth rate at successive growth stages.
Fussel et al. (1991) reported that the reduction in grain yield was more than 40 per cent when water stress was imposed during mid-flowering period. Lilley and Fukai (1994) observed a reduction in grain yield up to 70 per cent due to the occurrence of water deficit during reproductive stage. The grain yield of rice was reduced by 5-38 per cent under mild water stress while severe water stress reduced the grain yield by 25-67 per cent (Yang et al., 1995). Water shortage during flowering and grain filling stages reduced the yield drastically (Boonjung and Fukai, 1996).
Grain yield, relative grain yield and spikelet fertility were significantly and negatively correlated to the number of days beyond the beginning of stress period that cultivars flower (Garrity and O’Tooole, 1994). Lilley and Fukai (1994) observed grain yield reduction up to 70 per cent due to occurrence of water deficit during reproductive stage. Water shortage during flowering and grain filling reduces yield drastically (Boonjung and Fukai, 1996).
The effect of water stress on yield decrease of rice is very pronounced during certain period of growth called the moisture sensitive periods. The most sensitive periods to water deficit are flowering and head development (Surek and Korkut, 1998). In dry season, the rice variety Apo, the treatment with aerobic conditions in the reproductive stage and flooding in the vegetative and grain filling stages, recorded an yield reduction of 13 per cent compared to continuous flooding. Aerobic condition in the vegetative and reproductive stages recorded about 29 per cent yield reduction with that of continuously flooded treatment, but it is only 12 per cent in the case of IR 43. The flooding in reproductive stage did not result in an yield increase over keeping the soil continuously aerobic (Nieuwenhuis et al., 2002). Bouman et al., (2002) also recorded yield reduction under aerobic condition. Aerobic rice tended to yield
lesser than the flooded rice and yield reductions were dramatic when
water deficit occurred (Lafitte et al.
, 2002). According to Peng et al.
(2006), the yield difference between aerobic and flooded rice was
attributed more to difference in biomass production than to
Nevertheless, higher grain yield was registered in aerobic rice ecosystem especially with drip biofertigation practice than that in the conventional method of irrigation (Vanitha, 2008).
Stress-selected lines had an yield advantage of 25 to 34 per cent over random lines when evaluated at stress levels similar to those in which they were selected. Yield gains under very severe stress occurred only in a population derived from a highly tolerant parent. The lowland parents IR64 and IR72 had much lower yields under stress than Vandana and Apo (Venuprasad et al., 2007).
The hybrids involving IR 72875-94-3-3-2 viz., IR 68886A x IR 72875-94-3-3-2, IR 68888A x IR 72875-94-3-3-2 and COMS 14A x IR 72875-94-3-3-2 had higher grain yield under aerobic condition. Similarly PSBRC 80, the male parent best suited for aerobic condition had better performance for most of the traits like harvest index, high relative water content, total dry matter production, root dry weight and grain yield (Malarvizhi et al., 2010).
The type of rice cultivar did not significantly affect seed yield (g plant-1), number of spikelets (grains) per panicle and the 1000-seed weight (g), but seeding depths and water stress significantly affected grain yield (Sanusan et al., 2010).
2.5.2. Putative traits
Putative traits are traits that might be useful as selection criterion if it improves an intermediate process such as plant water uptake. These traits are hypothesized to be of value on the basis of our understanding of crop physiology and biochemistry. Several putative traits contributing to drought tolerance in rice have been suggested (Fukai and Cooper, 1995).
Measurement of physio-morphological traits permits the rapid identification of potentially tolerant plant materials and cross combinations. Higher levels of days to 70 per cent RWC and chlorophyll stability index observed in drought tolerant lines than in the susceptible ones (Michael Gomez and Rangasamy, 2002 and Anbumalarmathi, 2005).
184.108.40.206. Root characters
Root characteristics such as root thickness, depth of rooting, root length, root pulling force, root penetration ability, branching of root system, root to shoot ratio, root density and dry weight of roots below 30 cm are commonly considered to play an important role in water deficits (Fukai and Cooper, 1995 and Nguyen et al., 1997).
220.127.116.11.1. Root length, dry root weight and dry shoot weight
Cruz et al. (1986) observed that a mild water stress at vegetative phase decreased total root dry mass and total root length and density. All the japonica varieties of rice had high root dry weight per plant and this could have been due to increased root length and thickness (Sinha et al., 2000).
In aerobic rice systems, rice roots fully explore the soil profile and effectively absorb water at deeper layers (Lafitte and Bennett, 2002). Among the several factors contributing to enhance resistance to drought, roots were the main organ for plant water uptake. Maximum root length and root dry weight were good indicators ofdrought resistance in upland rice (Nguyen et al., 1997 and Mane et al., 2003). Pradhan et al. (2003) reported that in rice the root length and root number increased due to moisture stress.
Shoot dry weight increased significantly in the vegetative aswell
as reproductive growth stages. Nitrogen, P and K fertilization influenced shoot dry weight
as shown by Fageria et al., 2003 and Fageria and Baligar, 2005.
Most of the studies on roots under drought conditions reveal the possibility of enhancing water availability through adaptation of the better root system (Cuikehei et al., 2004). Singh et al. (2004 b) reported that putative traits like root length showed a strong relationship with grain yield as well as stress indicators (RWC) and phenology traits (R / S ratio and biomass) under water limited environments. Ravindra Kumar et al. (2004) reported that well developed root system helped in maintaining high plant water status along with a short delay in flowering which ultimately reflected in yield stability under drought conditions.
Breeding for a large root system contributes towards enhanced drought tolerance in rice crop. Maximum root length, root dry weight and root volume were recorded highly heritable. Higher expected genetic advance as per cent of mean was obtained for root volume, root dry weight, total root number, shoot dry weight and number of tillers. Hence, good response to selection for these characters could be anticipated (Adnan kanbar et al., 2004). Fageria and Baligar (2005) reported that root: shoot ratio was increased under moisture stress condition.
Fageria (2007) observed that maximum root length followed a significant quadratic response with the
advancement of plant age from 19 days after sowing. He further showed a linear
increase in root length from tillering initiation to flowering. Thereafter, root
length was more or less constant or reached a plateau.
Root biomass production could be ascribed to
adventitious root development at the phytomer level or more
simply to dynamic changes in the number of adventitious roots
and in individual root growth (Kato et al., 2007). The increase in shoot weight was mainly associated with the increase in leaf and culm weights during these growth stages and considered important because it was significantly associated
with grain yield (Fageria, 2007).
According to Richards (2006), for rice to adapt to
aerobic culture, the plants would have to change their rooting pattern
to that of the dryland crops. Thicker roots were more likely
to grow under unsaturated, harder and surface soil in
aerobic culture (Clark et al., 2008).
A deep root system is needed for acquiring water and nutrition from the relatively wet deep soil layer to obtain a stable yield under rainfed conditions (Naoki Matsuo and Toshihiro Mochizuki, 2009).
Significant variations in the root length of genotypes were recorded both under control and stress conditions. Seedling dry weight was reduced significantly at lower water potential (-0.75 MPa) as compared to control. Under stress, dry weight per seedling ranged from 0.47 (L3 x T3) to 2.40 mg (L6) with a mean of 1.31 mg. Genotypes such as Nootripathu, Norungan, PMK2, Norungan x PMK2 and Nootripathu x PMK2 had recorded high germination percentage, shoot, root length and seedling dry weight under reduced water potential indicating their drought tolerance behaviour (Vikas et al., 2009).
Abd Allah et al. (2009, 2010) reported that root system plays an important role under drought conditions. The nature and extent of root characteristics are considered to be major factors affecting plant response to water stress. The root development has long been recognized as an important factor in determining the adaptability of a given plant species to varying water conditions.
Ganapathy et al. (2010) reported that increased root thickness improves drought resistance as the roots are capable of increasing root length density and water uptake by producing more and larger root branches. Root size, morphology and root depth and length are important in maintaining high leaf water potential against evapotranspirational demand under water stress.
The difference in specific root length between
aerobic and flooded cultures was large for Akihikari and IRAT109
rice varieties (Kato et al., 2009). They further indicated that the adaptive response of
root branching might be a minor part of root system development in
aerobic culture, unlike in flood-based culture.
Extending the root
growth to the subsurface layer might thus alleviate the temporary
plant dehydration occurring between irrigation events in
aerobic culture. Root biomass was reduced in aerobic culture primarily on account of fewer adventitious roots (Kato et al., 2009).
Root traits such as thickness, depth and penetration ability help to avoid drought by increased water uptake from deeper soils. Greater hydraulic conductance, xylem thickness and osmotic adjustment are secondary root traits enable better extraction of available soil moisture (Chandra Babu, 2010).
18.104.22.168.2. Root – Shoot Ratio
Yield efficiency was noted for upland rice varieties having relatively high root shoot ratio under moisture stress conditions (O’Toole and Chang, 1979).
Morrison and Laignet (1983) observed a
greater allocation of carbon to root system thus resulting
in higher root volume, root proliferation and root activity. This could also be reasoned out for increase in R/S ratio under moisture stress situations as observed by Turner (1986).
Root characteristics in rice are genetically controlled but they are also strongly affected by soil conditions and crop management practices (Hasegawa et al., 1984).
The root to shoot ratio was high in drought resistant upland rice cultivars (Namuco et al; 1993; Simane et al., 1993).
Deep root system ensures greater extraction of water held deep in the soil profile and therefore the maintenance of high leaf water potential during drought period (Fukai and Cooper, 1995).
The R/S ratio
increased under water-limiting upland conditions
(Kondo et al., 2000; Singh et al.,
2000; Price et al., 2002). Azhiri-
Sigari et al. (2000) and
Banoc et al. (2000) observed a reduction in R/S ratio
under lowland conditions. However, this effect would differ if the
water stress occurred at the seedling stage or with very severity in the stress (Asch et al., 2004).
Higher root to shoot ratio under drought contributes significantly to flowering stage drought tolerance in rice (Kumar and Kujur, 2003; Yang et al., 2004). Root to shoot ratio was higher in Ciza 175(2.10), GZ 6296-12-1-2-1- 1(2.00), GZ 8450-19-6-5 3(2.20) and SIS R215 (2.00), while Augusto (0.70), Handao 11(0.67) and Qinai (0.77) had low values. Root to shoot ratio had highly significant genotypic correlation with flag leaf area, leaf angle, flag leaf dry weight and grain yield characters. The varieties with high root: shoot ratio was more drought tolerant (Abd Allah et al., 2010). Temporary decline in soil water
potential in the surface layer during the early growth stage was
detrimental to the R/S ratio in aerobic rice culture
(Kato et al., 2009).
22.214.171.124.3. Relative water content (RWC)
RWC is one of the measures which gives an idea of tissue water status.
The mechanism of controlling plant water status may involve water uptake or water conservation by the plant and also internal plant water conductance during drought and the plant water status can differ significantly among cultivars exposed to the same period of water exclusion (O’Toole and Moya, 1978).
The genotypes maintaining higher relative water content accumulated solutes and had higher photosynthesis (Krishnayya and Murthy, 1991) and higher recovery upon stress relief plants (Saxena et al., 1996). Relative water content (per cent) in leaf tissue decreased in all the rice genotypes under moisture deficit (Nadarajan and Kumaravelu, 1993). According to Lawlor (1995), a RWC of 100-90 per cent was related to stomatal closure
and decreased cell expansion and growth of organs and 90-80 per cent to
changes in composition of tissues and some alterations in the relative rates of photosynthesis
and respiration. Water deficit reduced water status of leaves in rice plants (Saxena et al., 1996). Progressive decline in RWC was reported by Silva et al. (2007) in sugarcane. Jamauex et al., 1997; Altinkut et al., 2001 and Colom and Vazzana, 2003 have shown that maintenance of a relatively high RWC during mild drought is an indicative of drought tolerance.
Further, with the RWC of below 80 per cent (around water potential
of -1.5 MPa), changes in metabolism become marked, with cessation of photosynthesis,
much increased respiration and accumulation of proline and abscissic acid (Cabuslay et al., 2002).
Thus, moisture stress had an inhibitory effect on RWC as a result of soil dryness (Sarker et al., 1999).
The maintenance of plant water status more than plant functions, controls crop performance under drought. Leaf water potential is closely related to leaf RWC, but it is confounded by osmotic adjustment, stronger correlations might be found between yield and RWC under water stress near flowering was necessary but not sufficient to ensure good yield (Lafitte, 2002). Mean leaf relative water content across the DH lines of rice declined to 68 per cent under stress (Babu et al., 2003). Maintenance of higher water status under drought plays a central role in stabilizing the various plant processes and yield (Kumar and Kujur, 2003). Singh et al. (2004 a) reported the positive relationship of RWC with shoot biomass and grain yield and indicated that RWC could be used as a reliable and simple screening technique for vegetative stress. Liu et al. (2004) reported the positive correlation between RWC and leaf water potential.
Vanitha (2008) indicated that drip fertigation at appropriate level of water and fertilizer maintained higher values of RWC in aerobic rice. The trait days to attain 70 per cent RWC positively correlated with root traits such as root length, dry root weight and root: shoot ratio suggesting that selection based on days to attain 70 per cent RWC is highly fruitful in developing drought tolerant genotypes as it will bring simultaneous improvement of these traits (Sheeba et al., 2010). Parents with high relative water content can be used as potential donors which would result in rice hybrids with tolerance to water deficit conditions (Malarvizhi et al., 2010). The plants grown well-watered had higher RWC (70 %) than plants that suffered water deficit (59.28 per cent). RWC was closely associated with the lengths of roots of rice seedlings (Sanusan et al., 2010).
Measures of relative water content (RWC) and water potential (WP) are indices of plant water status, which are useful in monitoring the development of stress in plants growing under drought conditions (Bimpong et al., 2011).
126.96.36.199.4. Chlorophyll stability index (CSI)
Higher levels of days to 70 per cent RWC, chlorophyll stability index and lower levels of leaf rolling, leaf drying and drought recovery rate observed in drought tolerant lines than in the susceptible ones (Michael Gomez and Rangasamy, 2002 ; Anbumalarmathi et al., 2005).
Wider variation was noticed between the control and drought stress plants for chlorophyll stability index. Under a normal growth condition, a slight variation for CSI was seen among the genotypes. Significant differences were noticed when the same genotypes were exposed to water deficit condition. Higher CSI indicated the level of polyunsaturated lipids stabilized chloroplast membrane and increased adaptive response to tolerance under water stress condition (Deivanai et al., 2010).
188.8.131.52.5. Proline content
Singh and Singh (1983) observed that proline accumulation under drought condition is a good indicator of drought tolerance capacity of plants. Proline is one of the important osmolytes which accumulates during moisture stress condition. It helps to maintain turgor and promotes continued growth in low water potential soils (Mullet and Whitsitt, 1996). High proline content is a good index for water deficit
resistance in genotypes. Under moisture stress condition,
the protein degraded and consequently the proline
Proline accumulation in plant cells exposed to salt or water stress is a widespread phenomenon (Lin and Kao, 1996). In rice, proline accumulated in response to water stress (Zhang et al., 1997), more in shoots than in roots (Pandey et al., 2004).
Proline is a non-protein amino acid that forms in most tissues subjected to water stress and together with sugar, it is readily metabolized upon recovery from drought (Singh et al., 2000). Rapid accumulation of free proline could be a typical response to drought or a high salt content in the soil (both leading to water stress); many plants accumulated higher amounts of proline, in some cases, several times the sum of all the other amino acids (Mansour and Al-Mutawa, 2000).
Upon desiccation, many plants accumulated non-toxic or ‘compatible’solutes such as proline, mannitol and glycine betaine (Chen, 2001). Increased proline accumulation was often observed with N supply in green bean plants (Sanchez et al., 2002). Among the common response in plants to abiotic stresses is the production of different types of organic
solutes (Serraj and Sinclair, 2002), which include small
molecules such as proline (Shao et al., 2006; Tan et al.,
2008; Szabados and Savouré, 2010).
Production and accumulation of proline by plant tissue during drought is an adaptive response. Proline acts as a compatible solute that adjusts the osmotic potential in the cytoplasm. It can be used as a metabolic marker in relation to stress (Caballero et al., 2005). Vendruscolo et al. (2007) stated that proline might confer drought stress tolerance to wheat plants by increasing the antioxidant system rather than as an osmotic adjustment.
Tatar and Gevrek (2008) reported that the stress enhanced proline content in leaves. It may possibly play an important role in the osmoregulation under moisture stress condition. Mostajeran and Rahimi-Eichi (2009) suggested that the production of proline is a common response of plants under drought conditions. High proline content is a good index for moisture resistance in genotypes. Under moisture stress condition the protein degrades and consequently the proline content increases (Roy et al., 2009). Both free proline and ABA showed generally an increase under water stress conditions and the varietal differences in the accumulation of these osmolytes were also reported (Abdellah Akhkha et al., 2011).
184.108.40.206.6. SPAD chlorophyll meter reading
Chlorophyll pigments play decisive role in plant productivity, as they are the only pigments responsible for photosynthesis. Water stress decreased chlorophyll content in rice leaves (Zhu and Huang, 1994; Sheela and Alexander, 1996). The chlorophyll content of drought tolerant upland varieties was higher than that of the transplanted paddy under water deficit conditions (Peng et al., 1996). Abdellah Akhkha et al. (2011) also observed similar decrease in chlorophyll content under moderate and severe water stress situations.
The chlorophyll meter (SPAD-501) provides a simple, quick and non destructive method for estimating leaf chlorophyll content (Watanabe et al., 1980). Jiang and Vergara (1986) reported that SPAD-501 was reliable in determining the relative chlorophyll content due to the stresses such as drought. They also reported that a significant positive correlation of 0.989 was obtained between the SPAD reading and chlorophyll content. Leaf chlorophyll-SPAD value was positively and significantly correlated to filled grain percentage and grain number panicle-1 (Maibangsa, 1998). The Minolta SPAD-502 chlorophyll meter (Spectrum Technologies, Inc., 1998) determines the leaf greenness which is the direct measure of leaf chlorophyll content and indirect method of determining the N content of the leaf. Hand held chlorophyll meters have been used successfully for field determination of leaf N concentration in several agronomic crops like Corn (Ahmad et al., 1999) and rice (Mahendar et al., 2001). These meters make quick and easy measurements of leaf greenness, which is positively correlated to leaf chlorophyll content (Fahrurrozi et al., 2001). Recent research in Walnut trees (Simorte et al., 2001) indicated a close correlation between leaf chlorophyll content measured with a chlorophyll meter and leaf N content in these plantation species. Rajkumar (2001) reported that the chlorophyll content was reduced due to drought. Total chlorophyll was found to decrease with the severity of stress and this decrease was strongly cultivar-dependent. The decrease in chlorophyll is associated with a reduction in the flux of nitrogen into the tissue, aswellas alteration in activity of enzyme systems such as nitrate reductase (Deivanai et al., 2010).
2.6. COMBINING ABILITY AND GENE ACTION
Combining ability is the ability of an inbred to transmit the desirable performance to its hybrid progeny. It helps in identification of parents with high general combining ability (gca) effects and gives an idea about the relative magnitude of additive and non-additive type of gene action in expression of trait.
The successful development of rice hybrids by utilizing the cytoplasmic genic male sterility system and fertility restoration system mainly depend on the availability of stable sterile lines and economically viable seed production technology. The success can be further hastened by choice of suitable elite parents with favorable alleles which on crossing would give heterotic hybrids and the concept of combining ability was put forward by Sprague and Tatum (1942).
Sprague and Tatum (1942) developed the concept of combining ability
by using single crosses in Maize. It was further elaborated by Henderson (1952),
Rojas and Sprague (1952), Griffings (1956), Carnaham et al. (1960) and Perraju and Sarma (1999). The average performance of a line in a cross combination is referred to as general combining ability and is mainly due to additivity of genes, while dominance and epistatic gene action results in specific combining ability of a cross which are the deviations of crosses from expected values based on average performance of parental lines. The general combining ability is used to designate the average performance of a line in hybrid combination and the mean performance of a line can be statistically expressed as a deviation from the mean of all F1s involving this line as a common parent. The specific combining ability is used to designate those crosses in which certain combinations do relatively better or worse than that would be expected on the basis of the average performance of the lines involved. Statistically, the deviation of the mean of a cross from the sum of the general combining abilities of two parents is the specific combining ability.
The ability of parents to combine well cannot be judged by phenotypic performance and adaptation qualities (Khattak, 1999). Therefore, the choice of parental material and breeding methodology become convoluted for improvement or development of new cultivars (Thirumeni and Subramanian, 2000). Combining
ability analysis provides guide line for the assessment of relative breeding potential
of the parents and help in choice of parents (Gnanasekaran et al., 2006) which
may be hybridized either to exploit hybrid vigor by accumulating unfixable gene
effects or to evolve cultivars by accumulating fixable gene effects (Nadarajan and Gunasekaran, 2005).
The concept of combining ability helps the breeder to determine the nature of gene action involved in the expression of quantitative traits of economic importance. The choice of suitable breeding method for the improvement of drought tolerant traits primarily depends on the relative importance of GCA and SCA variances. Proper choice of parents on the basis of their combining ability status for putative drought tolerant attributes as well as productive traits and selection in typical target environment will help in combining complex traits such as productivity and drought tolerance (Hanamaratti et al., 2004). A hybrid is commercially valuable only when it exhibits significantly high standard heterosis over the best locally adapted variety or hybrid. Apart from high vigor and yield, the hybrids can be a potential genetic source for better root system with higher efficiency to absorb moisture effectively for tolerating drought condition. Existence of heterosis for drought tolerant traits will be a boon to drought tolerance breeding since most of the hybrids developed so far lack tolerance to abiotic and biotic stresses. The estimation of additive and non-additive gene action through this technique may be useful in determining the possibility of commercial exploitation of heterosis and isolation of pure lines among the progenies of the good hybrids.
Among the different genetic analysis, Line x Tester model is an important
one to find out the combining ability of parents with rapidity and confidence.
This design was proposed by Kempthorne (1957) and is being widely used in all crops. Dhillon (1975) indicated that combining ability of parents gave useful information on the choice of parents in terms of expected performance of their hybrid progenies.
Latest research findings on the combining ability effects and nature of gene action governing different yield contributing and drought related traits are reviewed character wise below:
2.6.1. Gene action for different biometrical and physiological traits
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2.7. VARIABILITY STUDIES
2.7.1. PHENOTYPIC AND GENOTYPIC CO EFFICIENT OF VARIATION
The breeders need sound information on variability consisting of phenotypic and genotypic variance to obtain better results for selecting superior genotypes. Population variation is due to both genotypic and phenotypic. The observable variation within the population is called phenotypic variance. This phenotypic variance is an interaction of genotype and environment. Genotypic variance is independent of environmental effect and is an inherent character. Effective selection programme for any trait in crop breeding depends on variability which is expressed in the form of phenotypic and genotypic co efficient of variation (PCV and GCV).
Elayaraja et al. (2005) reported that the variability was observed in characters such as number of productive tillers, panicle length, number of grains per panicle, 100-grain weight and grain yield per plant in M2 generation.
Estimates of variability, heritability, genetic advance as percentage of mean and correlation coefficients were worked out in 22 semi-deep rice genotypes for yield and its attributing characters. High genotypic coefficient of variation was observed in panicles per plant followed by panicle weight, productive tilllers per plant and grain yield per plant (Bhandarkar et al., 2003).
Estimation of genetic variability was studied for 12 characters in which high coefficients of variation was observed for grain yield per plant followed by harvest index and biological yield per plant (Shukla Vivek et al., 2005).
All the characters tested showed significant variation among the varieties.
The highest genetic variability was obtained in flag leaf area and filled grains per panicle (Hasib et al., 2005).
A wider genetic variability was observed among the genotypes for most of the characters studied. The highest genotypic coefficient of variation was recorded for grain yield, percent unfilled grains per panicle, number of grains per panicle and number of filled grains per panicle (Ahmed Mustafa and Yassir Elsheik, 2007).
Muhammad Rashid et al. (2007) revealed that the analysis of variance indicated that the differences among the genotypes were highly significant for all the characters studied viz., days to 50 per cent flowering, 50 per cent heading day, plant height, productive tillers per plant, panicle length, 1000 seed weight, fertility and yield.
Analysis of variance revealed highly significant differences among treatments, parents, parents vs crosses and crosses for flag leaf area, plant height, panicle density, harvest index, biological yield per plant and yield per plant (Saleem et al., 2008).
Padmaja et al. (2008) reported that genetic variability, genotypic and phenotypic coefficients of variation, heritability and genetic advance for 11 characters in 150 genotypes including five check varieties of rice. The analysis of variance revealed that there were highly significant differences for all the characters except leaf width and 100 seed weight among the genotypes. The estimates of genotypic and phenotypic coefficients of variation (GCV and PCV) were high for all the characters except days to 50 per cent flowering and panicle length.
Kole et al. (2008) reported that genotypic and phenotypic coefficients of variation were high for flag leaf angle and panicle number; moderate for grain number per panicle, straw weight, harvest index and grain yield per plant; and low for days to flowering, plant height, panicle length, spikelet number, spikelet fertility (%) and test weight .
Sabesan et al. (2009) revealed that the PCV values were slightly greater
than GCV, revealing little influence of environment in character expression for
Chandra and Pradhan (2003) showed that the phenotypic coefficient of variation (PCV) was higher than genotypic coefficient of variation (GCV) for all the
12 characters studied indicating the influence of environment on the characters.
Grains per panicle had maximum GCV followed by plot yield, grain yield per plant, harvest index, panicle number, plant height and 1000-seed weight.
2.8. HERITABILITY AND GENETIC ADVANCE
Heritability refers to ‘the extent of transmission of variation for any trait to the progeny’. Estimate of heritability helps in discriminating the variance in a population into genotypic component and environment interaction component and explain the relative importance of environment effect and inheritance levels for the variation in population. Heritability in broad sense denotes the functioning of the genotype as a whole (Lush, 1940), while heritability in narrow sense refers to additive genetic variance. Genetic advance is a measure of the gain for the character that could be achieved by further selection. This, in general is expressed as per cent of mean. Heritability along with genetic advance estimates helps in programming the breeding programme to obtain best results of genetic gain for any economic trait.
Bhandarkar et al. (2003) reported that high heritability with high genetic advance as percentage of mean were observed for panicle weight. Heritability was also higher than 80 per cent for all parameters showing heritable variation among genotypes. Heritability for kernel per row, plant height and grain yield per plant was higher than the other characters (Muhammad Rafique et al., 2004).
High values of heritability coupled with high-expected genetic advance were observed for the characters grain yield/plant, harvest index and biological yield per plant. Hence, selection should be based on these traits (Shukla Vivek et al., 2005).
A high heritability associated with a moderate to high genetic advance as per cent of mean was observed for number of productive tillers, panicle length, number of grains per panicle, 100-grain weight and grain yield per plant in M2 generation (Elayaraja et al., 2005)
Girish et al. (2006) exhibited that high heritability coupled with high GA was observed for several plant traits viz., number of tillers, plant height, total number of spikelets per panicle, biomass per plant, straw weight, harvest index and grain yield per plant and hence offered good scope for selection.
Heritability and genetic advance were studied in three New Plant Type (NPT) based crosses of rice for 13 characters. High heritability estimates coupled with high genetic advance as per cent of mean was seen in all the crosses for days to 50 per cent flowering, days to maturity, plant height, panicle length, L: B ratio and 1000 grain weight, while high heritability with moderate genetic advance was seen in average grain length and grain breadth. Spikelets per panicle and filled grains per panicle had moderate heritability and high genetic advance. Productive tillers per plant, spikelet sterility and grain yield per plant showed low to moderate heritability coupled with low to moderate genetic advance as per cent of mean (Bharadwaj et al., 2007).
Estimates of broad sense heritability and expected genetic advance in response to selection in next generation were high for all the traits (Saleem et al., 2008).
Padmaja et al. (2008) reported that the heritability and genetic advance were high for all the characters except days to 50 per cent flowering and panicle length, which had moderate genetic advance along with high heritability indicating the involvement of additive type of gene action in controlling these characters.
High heritability accompanied by high to moderate genetic advance for
flag leaf angle, panicle number, grain number, straw weight and grain yield
indicated the predominance of additive gene action for the expression of these characters (Kole et al., 2008).
High values of heritability along with genetic advance were observed
for grain yield per plant, 100 grain weight, productive tillers per plant, grains per panicle, grain length, grain breadth, kernel length, panicle length and plant height (Sabesan et al., 2009).
Gupta et al. (2009) observed high heritability (0·58) and high genetic advance (0·53) for 100-seed weight, high heritability (0·93) and moderate genetic advance (0·37) for seed yield per plant and high heritability (0·60) and low genetic advance (0·13) for number of days to mid-flowering.
Abd Allah (2009) revealed that high heritability values were associated with high genetic advance for days to heading and plant height in all the crosses studied while there was no association with high genetic advance in the other characters.
Heterosis refers to the increased or decreased vigour of F1 hybrid over its parents. The term heterosis was coined by Shull (1948), implies the excellence of F1over its strictly homozygous parents. According to him, the term heterosis refers to the increased vigour, growth, yield or functions of hybrid over the parents those resulting from crossing genetically diverse individuals. Jones (1926) was the first to report increased vigour in culm number and grain yield of F1 hybrids over their parents in rice. Similar to irrigated lowland rice, heterosis is also important in aerobic rice. Apart from high vigour and yield, the hybrids can be a potential genetic source for better root system with higher efficiency to absorb moisture effectively for tolerating drought under aerobic condition. Existence of heterosis for drought tolerant traits will be a boon to aerobic rice breeding since most of the hybrids developed so far lack tolerance to abiotic and biotic stresses and also the yield penalty. Hence, the reports on different traits for the expression of heterosis in rice hybrids are reviewed hereunder:
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2.10. ASSOCIATION ANALYSIS
2.10.1. Correlation studies
Grain yield in rice is a complex quantitative trait influenced by several component traits. Knowledge on association among different yield components under aerobic condition is a prerequisite in developing appropriate breeding strategy that would bring about the simultaneous improvement of these traits. A positive association between traits warrants the simultaneous improvement of both the traits while restricting selection to any one of the associated traits. On the other hand, a negative relationship between two traits necessitates equal weight to be given on both the traits during selection process. Relative importance of drought tolerant attributes may be decided based on highly correlated trait with grain yield and with other major mechanisms of drought tolerance under aerobic condition. The review on correlation between yield and traits conferring drought tolerance in rice is given below.
220.127.116.11. Correlation of grain yield with component traits
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