Lowland rice is an important cereal crop that plays a key role in the food security and the economy of Thailand. Terminal water stress (TWS) in rainfed lowland areas poses threats to rice productivity due to stress occurrence at terminal crop stages and extreme sensitivity of rice to TWS. A two-year study was conducted to characterize the performance of yield and yield attributes of twelve Thai lowland rice genotypes under TWS, to identify stress-tolerant genotypes using stress response indices and to identify promising stress indices which are correlated with grain yield (GY) under well-watered (WW) and TWS conditions for their use as rapid identifiers in a rice crop breeding program for enhancing drought stress tolerance. Measurements were recorded under WW and TWS conditions. Highly significant variations were observed amongst assessed genotypes for their yield productivity responses. According to stress response indices, genotypes were categorized into stress-tolerant and stress susceptible genotypes. Genotypes Hom Pathum, Sang Yod, Dum Ja and Pathum Thani-1 were found highly stress tolerant and relatively high yielding; genotypes Look Pla and Lep Nok were stress tolerant, whereas genotypes Chor Lung, Hom Nang Kaew and Hom Chan were moderately tolerant genotypes. Hence, stress-tolerant genotypes could be potentially used for cultivation under rainfed and water-limited conditions, where TWS is predicted particularly in southern Thailand to stabilize rice productivity. Stress tolerance indices, including stress tolerance index (STI), geometric mean productivity (GMP), mean productivity index (MPRO) and harmonic mean index (MHAR), indicated strong and positive associations with GY under WW and TWS; thus, these indices could be used to indicate stress tolerance in rice crop breeding program aimed at a rapid screening of lowland rice genotypes for stress tolerance.

Crop models can provide rapid and cost effective means to deal with rice crop management. The objectives of this study included, exploring the ability of CSM-CERES-Rice in scheduling irrigation and to simulate the effect of drought stress on upland rice yield. Irrigation treatments 100, 70 and 50 % of field capacity (FC) were applied from 80 days after planting (DAP) at flowering stage until maturity and CSM-CERES-Rice was used to predict irrigation amount for each water regime for treatment duration. Results showed that, at 70 and 50 % FC, performance of an upland rice genotype, Dawk Payawm was decreased significantly as compared to 100 % FC. Normalized root mean square error (RMSEn) values less than 10 % for each treatment indicated a strong agreement between simulated and observed grain yield (GY) and biomass. d-index approaching to unity and RMSEn less than 10 % indicated a good agreement between simulated and observed soil moisture contents (SMC) for all irrigation treatments. Overall, it was concluded that drought stress had negative correlation with GY and CSM-CERES-Rice was able to predict irrigation amount for all treatments assuring that, model has potential for its use as a tool to schedule irrigation for experiments under water limited conditions.

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Estimating genetic variability and cluster analysis of grain yield and yield contributing traits need to require for rice breeders to choose the best breeding programs. Ten upland rice genotypes were conducted from farmers’ fields during the years of 2017 at three provinces of southern Thailand. Extreme broad sense heritability and genetic gain values for flag leaf length, leaf area index, harvest index, total dry weight and filled grains showed that assortment of these yield contributing traits would be effective. Cluster analysis categorized genotypes into three groups. In each group some genotypes such as Dawk Pa-yawm or Dawk Kha 50 (group I), Nahng Kian (group II) and Khao/ Trai (group III) showed that genotypes had different better traits. These studies revealed that high broad sense heritability traits and the best genotypes Nahng Kian and Khao/ Trai would be useful for improving new upland rice varieties in southern Thailand.

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