Yield response of rice (Oryza sativa L.) genotypes to different types of drought under rainfed lowlands - Part 2. Selection of drought resistant genotypes

Drought frequently reduces grain yield of rainfed lowland rice. A series of experiments were conducted in drought-prone northeast Thailand to study the magnitude and consistency of yield responses of diverse, rainfed lowland rice genotypes to drought stress environments and to examine ways to identify genotypes that confer drought resistance. One hundred and twenty-eight genotypes were grown under non-stress and four different types of drought stress conditions. The relationship of genotypic variation in yield under drought conditions to genetic yield potential, flowering time and flowering delay, and to a drought response index (DRI) that removed the effect of potential yield and flowering time on yield under stress was examined. Drought stress that developed prior to flowering generally delayed the time of flowering of genotypes, and the delay in flowering was negatively associated with grain yield, fertile panicle percentage and filled grain percentage. Genotypes with a longer delay in flowering time had extracted more water during the early drought period, and as a consequence, had higher water deficits. They were consistently associated with a larger yield reduction under drought and in one experiment with a smaller DRI. Genotypes, however, responded differently to the different drought stress conditions and there was no consistency in the DRI estimates for the different genotypes across the drought stress experiments. The results indicate that with the use of irrigated-control and drought test environments, genotypes with drought resistance can be identified by using DRI or delay in flowering. However, selections will differ depending on the type of drought condition. The inconsistency of the estimates in DRI and flowering delay across different drought conditions reflects the nature of the large genotype-by-environment interactions observed for grain yield under various types of drought in rainfed lowland conditions. (C), 2002 Elsevier Science B.V. All rights reserved

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. (C) 2002 Elsevier Science B.V. All rights reserved

Yield response of rice (Oryza sativa L.) genotypes to different types of drought under rainfed lowlands - Part 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. (C) 2002 Elsevier Science B.V. All rights reserved

Rainfed lowland rice breeding strategies for northeast Thailand. I. Genotypic variation and genotype x 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. (C) 1999 Elsevier Science B.V. All rights reserved

Rainfed lowland rice breeding strategies for northeast Thailand. II. Comparison of interstation and intrastation selection

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

Relationships of leaf color chart and SPAD values to leaf N content of Jasmin rice in Northeast Thailand

Strong correlations among the leaf color chart (LCC), chlorophyll meter (SPAD) readings, and leaf N concentrations in modern rice varieties provide simple and economical methods for indirectly determining leaf N content. The LLC and SPAD have not been used widely with traditional varieties. This study aimed to determine whether the LCC and SPAD could be used to indicate the leaf N status of jasmine rice (KDML 105). Field experiments were conducted in June-November 2003 in farmers' fields in Kha Khom village, Ubon Ratchathani (UR), and in Chumpae Rice Experiment Station, Khon Kaen. Experimental treatments included four N levels (farmers’ practice, 0, 60, and 120 kg N per ha) in UR, and six N levels (0, 19, 38, 58, 75, and 113 kg N per ha) in Khon Kaen. For both sites, treatments were laid out in an RCBD with four replioations under well-watered conditions. LCC and SPAD measurements were taken on fully expanded leaves of 16 plants from maximum tillering to flowering stage. The same leaves were destructively taken to determine the leaf N content. Leaf N content was 1.7-3.5%, SPAD reading 26-43, and the LCC reading 1-4. Leaf N content was linearly related to SPAD (R2= 0.71-0.86, P is less than 0.01) and LCC readings (R2= 0.52-0.77, P is less than 0.01) and the slopes of regression lines differed among growth stages. For any given SPAD reading, KDML 105 had a lower leaf N content than modern varieties. SPAD and LCC readings estimated leaf N content of jasmine rice. The SPAD and LCC can be used for site-specific N management of jasmine rice in Northeast Thailand