S. Fukai

Development of drought-resistant cultivars using physiomorphological traits in rice

Abstract Drought is a major problem for rice grown under rainfed lowland and upland conditions, but progress in breeding to improve drought resistance has been slow. This paper describes patterns of water-stress development in rice fields, reviews genetic variation in physio-morphological traits for drought resistance in rice, and suggests how knowledge of stress physiology can contribute to plant breeding programmes that aim to increase yield in water-limiting environments. To provide a basis for integrating physiological research with plant-breeding objectives we define drought resistance in terms of relative yield of genotypes. Therefore, a drought-resistant genotype will be one which has a higher grain yield than others when all genotypes are exposed to the same level of water stress. A major reason for the slow progress in breeding for drought resistance in rice is the complexity of the drought environment, which often results in the lack of clear identification of the target environment(s). There is a need to identify the relative importance of the three common drought types; early-season drought which often causes delay in transplanting, mild intermittent stress which can have a severe cumulative effect, and late stress which affects particularly late-maturing genotypes. In addition, in rainfed lowland rice, flooded and non-flooded soil conditions may alternate during the growing season, and affect nutrient availability or cause toxicity. Several drought-resistance mechanisms, and putative traits which contribute to them, have been identified for rice; important among these being drought escape via appropriate phenology, root characteristics, specific dehydration avoidance and tolerance mechanisms, and drought recovery. Some of these mechanisms/traits have been shown to confer drought resistance and others show potential to do so in rice. The most important is the appropriate phenology which matches crop growth and development with the water environment. A deep root system, with high root length density at depth is useful in extracting water thoroughly in upland conditions, but does not appear to offer much scope for improving drought resistance in rainfed lowland rice where the development of a hard pan may prevent deep root penetration. Under water-limiting environments, genotypes which maintain the highest leaf water potential generally grow best, but it is not known if genotypic variation in leaf water potential is solely caused by root factors. Osmotic adjustment is promising, because it can potentially counteract the effects of a rapid decline in leaf water potential and there is large genetic variation for this trait. There is genotypic variation in expression of green leaf retention which appears to be a useful character for prolonged droughts, but it is affected by plant size which complicates its use as a selection criterion for drought resistance. There is a general lack of drought related research for rice in rainfed lowland conditions. This needs to be rectified, particularly considering their importance relative to upland conditions in Asian countries. We suggest that focussing physiological-genetic research efforts onto clearly defined, major target environments should provide a basis for increasing the relevance of stress physiology and the efficiency of breeding programmes for development of drought-resistant genotypes.

Processes determining intercrop productivity and yields of component crops

Abstract This review examines how intercrop productivity is determined, by analysing several key physiological processes which affect yields of component crops. Availability of environmental resources to each of the component crops is important in determining combined intercrop productivity, and hence the analysis is based on capture of environmental resources and efficiency of conversion of captured resources into growth of harvested organs of the component crops. It is emphasized that the competitive abilities of component crops, which determine their biomass production and often yields, vary greatly according to growth environment, and hence cultural manipulation can adjust the balance of their yields. Intercrops are most productive when their component crops differ greatly in growth duration so that their maximum requirements for growth resources occur at different times. For high intercrop productivity, plants of the early-maturing component should grow with little interference from the late-maturing crop. The latter may be affected somewhat by the associated crop, but a long time period for further growth after the harvest of the first crop should ensure good recovery and full use of available resources. Compared with a sole crop, the reduced size of non-harvested organs of the late-maturing crop can result in improved assimilate partitioning to the harvested organ during the later part of the growth period and consequently a higher harvest index. Because of the differences in growth rhythm between component crops, there tends to be little interaction between relative performance of component crops and growth environment and hence productivity of this type of intercrop is often insensitive to management interventions. In contrast, when growth durations of component crops are similar the crops compete more intensely for available resources. Their relative performances can then be greatly affected by small changes in growth environment. ‘Additive’ intercrops of this type may nevertheless be productive, particularly where growth resources are more completely captured than in corresponding sole crops. However, if non-replenished growth resources are utilized too rapidly, the less-competitive component may suffer greatly. ‘Replacement’ intercrops of this type are not so productive in high-yielding environments. When the growth environment is not favourable however, their total lower plant populations compared to additive intercrops may allow yields of replacement intercrops to be less depressed. Where similar-duration crops are grown in variable environments, replacement intercrops may therefore be preferred due to their greater yield stability. Where a dominant crop uses available resources excessively and inefficiently, agronomic manipulation in favour of the usually suppressed component seems most likely to improve the productivity of the whole intercrop. Intercrop productivity depends on the genetic constitution of component crops, growth environment (atmospheric and soil) and agronomic manipulations of microenvironment. The interaction of these factors should be optimized so that the limiting resource is utilized most effectively in the intercrop. An understanding of the sharing of resources among component crops will help identify more appropriate agronomic manipulations and cultivars for intercrops.

IMPROVING EFFICIENCY OF WATER USE FOR IRRIGATED RICE IN A SEMI - ARID TROPICAL ENVIRONMENT

Irrigation water accounts for almost 40% of total variable production costs for rice (Oryza sativa L.) cropping in the Burdekin River Irrigation Area, northern Australia. Increasing the efficiency of water use would improve the economic viability of growers and long-term environmental benefits would also be likely due to lower water tables and decreased salinisation in irrigation areas. The aim of these studies was to maximize grain yield by optimizing its functional components: water use, efficiency of water use for dry matter production (WUEdm) and harvest index (HI). The responses of dry matter and yield in rice (cv. Lemont) to five methods of irrigation were studied in a wet and dry season in the region. Applying a permanent flood at sowing, the 3-leaf stage (traditional) and prior to panicle initiation were compared with two unflooded methods: saturated soil culture (SSC) and intermittent irrigation at weekly intervals. Saturated soil culture consisted of growing rice on raised beds of height 0.2 m and width 1.2 m, with water maintained in the furrows (0.3 m wide) some 0.1 m below the bed surface. The results of these studies show that it is not necessary to flood rice to obtain high grain yield and quality. The trend was for yield to increase with water supply, but there was no significant difference in yield and quality between SSC and traditional flooded production, although SSC used about 32% less water in both seasons. Therefore the efficiency of water use for grain production (WUEg, g m−2 mm−1) was higher in SSC than in traditional flooded production in the wet season and a similar trend existed in the dry season. There were no differences between SSC and the traditional method of irrigation in any of the grain quality components measured, indicating that this water saving method did not lower grain quality. Weed growth was generally higher in unflooded treatments, although weed populations in SSC and traditionally flooded rice were equivalent in the dry season, suggesting that weeds can be controlled in SSC. Saturated soil culture provides a viable alternative to flooded rice production for growers in semi-arid tropical environments. Substantial reductions in variable costs of production are attainable by reducing water use without reducing yield and quality.

Leaf water potential and osmotic adjustment as physiological traits to improve drought tolerance in rice

A sample of recombinant inbred lines (RILs) was derived from a bi-parental cross between Lemont and BK88-BR6, which contrasted in maintenance of leaf water potential (LWP) and expression of osmotic adjustment (OA). Genotypic variation for LWP and OA, and their associations with yield determination under water deficit, was studied in a series of five field experiments. Genotypic variation in the maintenance of high LWP was consistent across water deficit experiments. In the determination of genotypic variation in the maintenance of LWP, rate of water deficit was not an important factor influencing ranking, but degree of water deficit, and phenological development stage were important, particularly around heading. Genotypic variation in expression of OA was also observed under water deficits during both vegetative and flowering stages but ranking was inconsistent across experiments. This was in part because of large experimental errors associated with its measurement, but also because the expression of OA was associated with extent of decline of LWP. The relationship between OA and LWP was demonstrated when data were combined across experiments for vegetative and flowering stages. Under water-limited conditions around flowering, grain yield reduction was mainly due to a increased spikelet sterility. Variation in OA was not related to grain yield nor yield components. There were however, negative phenotypic and genetic correlations between LWP and percentage spikelet sterility measured at flowering stage on panicles at the same development stage during a water deficit treatment. This suggests that traits contributing to the maintenance of high LWP minimized the effects of water deficit on spikelet sterility and consequently grain yield

Yield response of rice (Oryza sativa L.) genotypes to different types of drought under rainfed lowlands. Part 1. Grain yield and yield components

Responses of rice genotypes to drought stress may be different when characteristics of the drought stress environments differ. The performance of 128 genotypes was examined under irrigation and four different types of drought stress, to determine genotypic consistency in yield and factors determining yields under different drought stress conditions. The different drought conditions were mild drought during grain filling, short and severe drought at flowering, prolonged severe drought during the reproductive to grain filling, and prolonged mild drought during vegetative and grain filling. Genotypic grain yield under mild stress conditions was associated with yield under irrigated conditions, indicating the importance of potential yield in environments where the yield reduction was less than 50%. However, yields under irrigated conditions differed over time and locations. Under prolonged or severe drought conditions, flowering time was an important determinant of grain yield. Earlier flowering genotypes escaped the severe stress and had higher grain yields indicating large genotype by environment (G x E) interactions which have implications for plant breeding even for mild stress. It is suggested that variations in flowering time, potential yields and drought patterns need to be considered for development of drought-resistant cultivars using specific physiological traits

Screening for drought resistance in rainfed lowland rice

Abstract In this review, reasons for the slow progress in development of drought resistant cultivars for rainfed lowland rice are considered first. Recent advances in screening methods for development of drought resistant cultivars from mostly our research in Thailand in the 1990s, are then described for drought that develops early or late in the wet-season. There now appears to be a good prospect for developing drought resistant cultivars that produce higher yield than existing cultivars when drought develops late in the wet-season. Appropriate phenology to escape late-season drought and high potential yield under well-watered conditions are important characters for cultivars adapted to rainfed lowland conditions. In addition, ability to maintain higher leaf water potential when drought develops late in the season is another desirable character. Maintenance of leaf water potential just prior to flowering is associated with higher panicle water potential, reduced delay in flowering time, and reduced spikelet sterility, and hence contributes to higher yield. Genotypes that are adapted to areas of late-season drought should also have high harvest index, intermediate height and rather small total dry matter compared to existing traditional cultivars, under well-watered conditions. This combination of characters would ensure high potential yield under favourable conditions and also contribute to resistance against late-season drought. Screening against early-season drought that develops during the vegetative stage is more difficult, as the genotype’s ability to recover from the stress appears more important than drought tolerance during the stress period. Other than appropriate phenology, high potential yield and ability to maintain high leaf water potential, no specific physiological and morphological characters appear to contribute directly to higher yield under drought conditions in rainfed lowland rice in Thailand, where drought develops rather rapidly due to the prevailing coarse textured soils. It is thus appropriate to develop a breeding program that is primarily based on selection for grain yield. There are large genotype by environment interactions for yield in rainfed lowland rice and hence it is important that genotypes are selected for yield under appropriate target environments. Addition of a drought screening program that is conducted in the field in the wet-season to the overall breeding program would enhance the opportunity to select for drought resistance within the breeding materials and increase the chance of developing high yielding cultivars adapted to the drought-prone rainfed lowland environments.

Yield response of rice (Oryza sativa L.) genotypes to drought under rainfed lowland. 3. Plant factors contributing to drought resistance

A series of experiments were conducted in drought-prone northeast Thailand to examine the magnitude of yield responses of diverse genotypes to drought stress environments and to identify traits that may confer drought resistance to rainfed lowland rice. One hundred and twenty eight genotypes were grown under non-stress and four different types of drought stress conditions. Under severe drought conditions, the maintenance of PWP of genotypes played a significant role in determining final grain yield. Because of their smaller plant size (lower total dry matter at anthesis) genotypes that extracted less soil water during the early stages of the drought period, tended to maintain higher PWP and had a higher fertile panicle percentage, filled grain percentage and final grain yield than other genotypes. PWP was correlated with delay in flowering (r = -0.387) indicating that the latter could be used as a measure of water potential under stress. Genotypes with well-developed root systems extracted water too rapidly and experienced severe water stress at flowering. RPR which showed smaller coefficient of variation was more useful than root mass density in identifying genotypes with large root system. Under less severe and prolonged drought conditions, genotypes that could achieve higher plant dry matter at anthesis were desirable. They had less delay in flowering, higher grain yield and higher drought response index, indicating the importance of ability to grow during the prolonged stress period. Other shoot characters (osmotic potential, leaf temperature, leaf rolling, leaf death) had little effect on grain yield under different drought conditions. This was associated with a lack of genetic variation and difficulty in estimating trait values precisely. Under mild stress conditions (yield loss less than 50%), there was no significant relationship between the measured drought characters and grain yield. Under these mild drought conditions, yield is determined more by yield potential and phenotype than by drought resistant mechanisms per se

Rainfed lowland rice: physical environment and cultivar requirements

The physical environment of the rainfed lowland ecosystem is often characterised and grouped according to the surface hydrology of rice paddies and rice cultivars have been developed for each subecosystem. Rainfall is an important determinant of the yield of rainfed lowland rice, but other factors such as topography and soil fertility also affect grain yield and choice of cultivars. The growing environment and also rice yield vary greatly within small areas as well as across seasons. This causes great difficulty in determining the target population of environments for a rice breeding program. This paper reviews past work on characterising the variability in the physical environment, and rice production in the rainfed lowland ecosystem. It examines possible connections between this variability and slow progress in developing new cultivars that are widely adapted to the rainfed lowland rice ecosystem.

Effect of timing and severity of water deficit on four diverse rice cultivars II. Physiological responses to soil water deficit

Rice shows significant genotypic variation in physiological response to water deficit. This work examines whether this variation is due to genotypic differences in water extraction capability. Leaf elongation rate, leaf rolling, leaf death, and predawn leaf water potential were studied for four rice (Oryza sativa L.) cultivars during water deficit imposed either before panicle initiation (vegetative-stress), or after panicle initiation (reproductive-stress). The four rice cultivars chosen, CPIC8, Lemont, Rikuto-Norin 12 (RN), and Todoroki-Wase (TW), were known to have differing responses to water deficit when grown under upland conditions. Paper I of this series showed differences in rooting pattern and soil water extraction. The cultivar differences in water extraction resulted in differing rates of development of stress. Cultivar RN had poor water extraction and was most sensitive to water deficit, with most rapid decline in leaf elongation, most rapid leaf rolling, and greatest leaf death. TW also had poor water extraction but plants were small and this cultivar escaped severe stress, particularly in the vegetative phase. CPIC8 and Lemont extracted more soil water and were less sensitive to water deficit. After accounting for differences in water extraction ability, cultivar differences in sensitivity of physiological processes to water deficit were small. Leaf elongation rate was more sensitive to water deficit than leaf rolling while pre-dawn leaf water potential, measured after overnight recovery of water status, declined only after those processes almost ceased. Duration of physiological activity was determined to a large extent by the ability of the cultivars to extract water from deep soil layers. Greater extraction prolonged physiological activity and therefore crop growth. This characteristics could be particularly important in an environment where several short periods of water deficit occur.

RAINFED LOWLAND RICE BREEDING STRATEGIES FOR NORTHEAST THAILAND. I. GENOTYPIC VARIATION AND GENOTYPE ENVIRONMENT INTERACTIONS FOR GRAIN YIELD

The magnitude and nature of genotype-by-environment (G x E) interactions for grain yield, days-to-flower and plant height of rainfed lowland rice in Northeast Thailand were examined using random F-7 lines from seven crosses developed by the Thai breeding program. A total of 1116 lines and checks were evaluated in a multi-environment trial conducted across three years (1995-1997) and eight sites. The G x E interaction was partitioned into components attributed to genotype-by-site (G x S), genotype-by-year (G x Y) and genotype-by-site-by-year (G x S x Y) interactions. The G x S x Y interaction was the largest G x E interaction component of variance for all three traits. There was little G x S interaction for grain yield and days-to-flower. The G x S interaction was significant for plant height, but was the smallest component of variance for that trait. The G x Y interaction component of variance was significant for all three traits, but was small relative to the genotypic component for days-to-flower and plant height. For grain yield the G x Y interaction component was comparable in size to the genotypic component. Partitioning the genotypic and G x E interaction components of variance into among-cross and within-cross components indicated that there was significant variation both among and within the crosses for each trait. The relationships between the three traits differed among the crosses and the environments. A major factor contributing to the large G x S x Y interactions for grain yield was the genotypic variation for days-to-flower in combination with environmental variation for the timing and intensity of drought. Some of the interactions associated with timing of drought were repeatable across the environments sampled in the multi-environment trial, and to some extent the environments were characterised on the basis of whether there was pre-flowering, intermittent, or terminal drought. There was genotypic variation for grain yield after taking into consideration the influences of timing of drought in relation to plant development, Three of the seven crosses involved the Thai cultivars KDML105 and RD6 as parents. These crosses produced an array of progeny with lower yield than the Thai cultivars, suggesting it would be difficult to improve on the yield of these cultivars. In contrast three of the remaining four crosses, which did not have a Thai cultivar as a parent, produced progeny that had higher yield than KDML105 and RD6 indicating that yield of rainfed lowland rice could be improved above that of the these popular cultivars