Background: Most of the rice-growing areas are exposed to the threat of drought stress due to the effects of climate change. In order to meet this challenge and support an increasing population, it is desirable to breed rice varieties with improved drought tolerance. Breeders' success in developing improved drought resistance lines depends entirely on a simple and accurate screening process. All available drought assessment methods are time consuming, labour oriented and indirect indicators of drought.
Result: We explain a method which efficiently evaluates the drought tolerance degree (DTD) of lowland indica rice varieties. DTD is defined as the average of the ratio of green leaf length to the total length of the top three leaves of each rice seedling after drought treatment, and therefore takes a value from 0 to 1. To test the potentiality of the DTD method, 118 doubled haploids with two parents showing different degrees of drought tolerance were assessed, from which the DTD value has been determined. Of the 118 doubled haploids identified by the drought tolerance assessment system under severe drought stress, DH102 showed the strongest drought resistance, with the highest DTD value, and DH22 was found to be the weakest drought resistance with the lowest DTD value. Further correlation analysis revealed a high association of DTD values with RWC, leaf tip drying scores, and leaf rolling scores. Based on these characteristics, the entire population under study was divided into two main groups. PCA analysis suggested three principal components with its first principal component possessing the DTD value, RWC, leaf rolling score, leaf tip drying score, and chlorophyll content index with maximum loading score.
Conclusion: This new method will help in assessing the drought tolerance of indica rice varieties. This study indicates the DTD method is simple, cost-effective, direct and relatively accurate for drought-tolerant screening of rice varieties.