J. W. M. Basnayake

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

Genotype differences in nutrient uptake and utilisation for grain yield production of rainfed lowland rice under fertilised and non-fertilised conditions

A large number of rainfed lowland rice genotypes were grown under two fertiliser conditions to identify whether a genotypes ability to extract more nutrients or to use absorbed nutrients more efficiently to produce grain yield was more important in determining genotypic variation in grain yield. From the yield responses of lines to fertiliser application, 16 contrasting lines were selected, and dry matter and nutrient (N, P and K) contents were determined for grain and straw separately for crops grown under non-fertilised and fertilised (60-13-16 N-P-K kg ha(-1)) conditions at three locations in Laos. There were significant effects of both genotype and genotype-by-fertiliser interaction for grain yield, which were closely associated with total N and P content at maturity. There was, however, also significant genotypic variation in nutrient-use efficiency (grain yield per unit nutrient absorbed), and this also contributed to the genotypic variation for grain yield. There was a large effect of genotype and a smaller effect of genotype-by-location interaction for nitrogen-use efficiency for grain yield. Nitrogen-use efficiency was negatively correlated with grain N concentration at two of the three locations. Both N- and P-use efficiency were consistent across fertiliser levels, and hence are likely to be used as selection criteria. The work also indicates that genotypes with high harvest index (HI) are likely to perform well in different fertility conditions

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

Genotypic variation of osmotic adjustment and desiccation tolerance in contrasting sorghum inbred lines

Abstract Variation in maximum osmotic adjustment and desiccation tolerance were determined before anthesis for 21 lines of Sorghum bicolor (L.) Moench, selected on the basis of their capacity for osmotic adjustment and for their putative drought resistance. A reproducible and controlled imposition of gradual water stress was achieved by withholding water from plants grown in a controlled environment with a constant evaporative demand. Even though the rate of imposition of stress was lower in the second of two experiments, the maximum level of osmotic adjustment expressed by the lines was fairly consistent across both. The highest osmotic adjustment was 1.71±0.06 MPa for TAM422 in Exp. 2, while the lowest was 0.78±0.09 MPa for QL27 in Exp. 1. The difference in maximum osmotic adjustment between the highest and the lowest lines was 0.75 and 0.87 MPa, respectively, for Exp. 1 and 2. There was also variation in desiccation tolerance among the 20 lines in Exp. 2; 58% to 68% for lethal relative water content and −3.1 to − 3.9 MPa for lethal leaf water potential. Mean values of lethal realtive water content and lethal leaf water potential were, respectively, 62% and −3.4 MPa. Maximum osmotic adjustment was inversely related to desiccation tolerance; lethal relative water content and lethal leaf water potential increased linearly as maximum osmotic adjustment increased. Thus lines with high osmotic adjustment died at a higher relative water content and lower leaf water potential, than those with low osmotic adjustment. Despite their reduced tolerance of desiccation, lines with high osmotic adjustment survived 10 days longer. In both experiments, a high level of repeatability of line mean discrimination was identified for osmotic adjustment (0.75±0.15 and 0.96±0.09, for Exps. 1 and 2, respectively). The repeatability on a line mean basis across experiments was also high (0.84±0.07), as was the genetic correlation between the line means in both experiments (0.86±0.08). The high levels of repeatability suggest that the screening procedure developed in this study in robust, and that it could be used to evaluate the inheritance of osmotic adjustment in breeding populations.

Cold tolerance in rice varieties at different growth stages

Low temperature is a common production constraint in rice cultivation in temperate zones and high-elevation environments, with the potential to affect growth and development from germination to grain filling. There is a wide range of genotype-based differences in cold tolerance among rice varieties, these differences often reflecting growth conditions in the place of origin, as well as breeding history. However, improving low temperature tolerance of varieties has been difficult, due to a lack of clarity of the genetic basis to low temperature tolerance for different growth stages of the rice plant. Seeds or plants of 17 rice varieties of different origins were exposed to low temperature during germination (15°C), seedling, booting, and flowering stages (18.5°C), to assess their cold tolerance at different growth stages. Low temperature at the germination stage reduced both the percentage and speed of germination. Varieties from China (B55, Banjiemang, and Lijianghegu) and Hungary (HSC55) were more tolerant of low temperature than other varieties. Most of the varieties showed moderate levels of low temperature tolerance during the seedling stage, the exceptions being some varieties from Australia (Pelde, YRL39, and YRM64) and Africa (WAB160 and WAB38), which were susceptible to low temperature at the seedling stage. Low temperature at booting and flowering stages reduced plant growth and caused a significant decline in spikelet fertility. Some varieties from China (B55, Bangjiemang, Lijiangheigu), Japan (Jyoudeki), the USA (M103, M104), and Australia (Quest) were tolerant or moderately tolerant, while the remaining varieties were susceptible or moderately susceptible to low temperature at booting and flowering stages. Three varieties from China (B55, Lijianghegu, Banjiemang) and one from Hungary (HSC55) showed consistent tolerance to low temperature at all growth stages. These varieties are potentially important gene donors for breeding and genetic studies. The cold tolerance of the 17 rice varieties assessed at different growth stages was correlated. Screening for cold tolerance during early growth stages can therefore potentially be an effective way for assessing cold tolerance in breeding programs.

Effects of Soil Clay Content on Water Balance and Productivity in Rainfed Lowland Rice Ecosystem in Northeast Thailand

Abstract Water availability is one of the determinants of productivity of rainfed lowland rice (Oryza sativa L.). Quantifying water losses from a paddy field, such as deep percolation and lateral seepage, assists estimation of water availability to the rice crop and development of appropriate water management in the lowlands. The main objective of this study was to evaluate paddy water availability and productivity across various soils in Northeast Thailand. The daily rate of downward water flow from standing water in the field (D) varied between 0 and 3 mm day-1 from clayey to sandy soils when the standing water was connected to groundwater table. However, when the standing water was separated from groundwater table, D increased up to 5 mm day-1 on soils with very low clay content in the topsoil. Daily net lateral water flow from the field (L) averaged over the season varied between 5 and 24 mm day-1 for the outflow and between 3 and 16 mm day-1 for the inflow. Both the inflow and outflow tended to be associated negatively with the soil clay content. The seasonal water loss through D plus L during the growing season in the lowlands was also negatively related to the soil clay content. The yield of a major rainfed lowland rice cultivar in Northeast Thailand (KDML105) varied from 2 to 4 t ha-1 across the region, and the water productivity (the ratio of grain yield to cumulative rainfall from transplanting/seedling establishment to maturity) ranged from 3 to 9 kg ha-1 mm-1. High clay soils could provide good standing water until late in the growing season, so the high production efficiency was measured on such soils.

Estimating Percolation and Lateral Water Flow on Sloping Land in Rainfed Lowland Rice Ecosystem

Abstract Quantifying water losses in paddy fields assists estimation of water availability in rainfed lowland rice ecosystem. Little information is available on water balance in different toposequence positions of sloped rainfed lowland. Therefore, the aim of this work was to quantify percolation and the lateral water flow with special reference to the toposequential variation. Data used for the analysis was collected in Laos and northeast Thailand. Percolation and water tables were measured on a daily basis using a steel cylindrical tube with a lid and perforated PVC tubes, respectively. Percolation rate was determined using linear regression analysis of cumulative percolation. Assuming that the total amount of evaporation and transpiration was equivalent to potential evapotranspiration, the lateral water flow was estimated using the water balance equation. Separate perched water and groundwater tables were observed in paddy fields on coarse-textured soils. The percolation rate varied between 0 and 3 mm/day across locations, and the maximum water loss by lateral movement was more than 20 mm/day. Our results are in agreement with the previously reported findings, and the methodology of estimating water balance components appears reasonably acceptable. With regard to the toposequential variation, the higher the position in the topoesquence, the greater potential for water loss because of higher percolation and lateral flow rates.

Influence of rate of development of water deficit on the expression of maximum osmotic adjustment and desiccation tolerance in three grain sorghum lines

Abstract An experiment was conducted to quantify the effect of rate of development of water deficit (Rψ) on the expression of maximum osmotic adjustment (OA) and desiccation tolerance (DT) in three grain sorghum lines, TAM422, Tx2813 and QL27. DT was determined as the lethal relative water content (RWCL) and lethal leaf water potential (ψL) of leaves, i.e. the value of these traits just prior to tissue death. Five rates of development of water deficit were generated by using different soil volumes in which the decrease in leaf water potential ranged from 0.093 MPa day−1 to 0.153 MPa day−1. The expression of maximum OA increased markedly with a reduced Rψ, for all three lines. In contrast, DT decreased only slightly with a reduced Rψ. The three lines differed for expression of maximum OA and DT. Both TAM422 and Tx2813 had higher maximum OA, higher RWCL and lower ψL than QL27. There was a significant line-by-Rψ interaction for the expression of OA and DT. The interaction was much smaller than the effect of lines for the expression of maximum OA. Therefore, the absolute level of OA for the three sorghum lines and the magnitude of the difference between them changed with the Rψ without affecting their ranking. As with maximum OA, the interaction for RWCL was smaller than the line effect. The RWCL of TAM422 and QL27 was stable across rates of development of water deficit. The results indicated that low Rψ increased the expression of OA and decreased ψL but had little effect on RWCL. While line-by-soil-volume interactions were observed, these were largely explained by the increase in magnitude of the variation for the traits as Rψ decreased and were not the result of changes in rank of the lines among soil volumes. Therefore, the current screening strategy is considered to be appropriate for examining genetic variation for OA and DT in grain sorghum. Opportunities exist for using smaller pots and soil volumes where large numbers of genotypes are examined.

Combining ability variation for osmotic adjustment among a selected range of grain sorghum (Sorghum bicolor L. Moench) lines

Osmotic adjustment has been widely proposed as a plant attribute that confers adaptation to water stress. Genetic variation for osmotic adjustment among ten inbred sorghum lines was investigated by diallel analysis. Both general combining ability (GCA) and specific combining ability (SCA) were found to be significant (P < 0.05). The differences in patterns of GCA and SCA among the parents strong evidence that genetic variation for osmotic adjustment among the ten parents was controlled by more than one gene. Four parents (TAM422, Tx2813, Ajebsido and 40019), which phenotypically expressed high osmotic adjustment, expressed different patterns of GCA and SCA. This suggests that the genetic control for high osmotic adjustment differed among the four parents. Two major patterns of combining ability inheritance were identified among the high osmotic adjustment parents, one of these based on positive GCA inheritance (TAM422 and 40019) and the other on negative GCA inheritance (Tx2813 and Ajebsido). However, SCA influenced the expression of these patterns of inheritance and their expression was modified in specific parental combinations.