Morpho-Physiological Characterization of Diverse Rice Lines for High- and Low-Temperature Tolerance.

Developing quantitative screening tools for low- and high temperature tolerance for crop cultivars will help breeders to select lines with desired traits to develop new cultivars with superior performance across the potential crop belts. An experiment was conducted in sunlit plant growth chambers to characterize early-season morphology and physiology of 64 Indica and Japonica lines and cultivars at low (22/14°C), optimum (30/22°C) and high (38/30°C) temperature conditions. The temperature treatments were imposed 10 days after seeding and several morpho-physiological parameters including root traits were measured at harvest, 21 d after planting. Significant temperature stress × genotype interactions were found for most of the parameters measured. A cumulative low temperature response index (CLTRI) and cumulative high temperature response index (CHTRI) were developed by summing the individual response indices of all cultivars. Based on CLTRI, 13, 19, 24, and 8 rice lines were identified as cold-sensitive, moderately cold-sensitive, moderately cold-tolerant and cold-tolerant, respectively. Similarly, based on CHTRI, 10, 31, 12, and 11 rice lines were classified as heat-sensitive, moderately heat-sensitive, moderately heat-tolerant and heat-tolerant among the 64 lines tested. 'The rice lines RU1504114 (a tropical Japonica) and IR65600-18-5-2-3 (an Indica) were the most cold- and heat-tolerant among the lines tested. According to principle component analysis, most of the Indica lines were identified as heat tolerant and japonica lines as cold tolerant. A poor correlation was observed between CHTRI and physiological traits (r² = 0.02) and this implies that shoot and root traits are key for selecting heat tolerance during early-season. Also, poor correlation between CHTRI and CLTRI indices (r² = 0.01) indicates that heat and cold tolerance mechanisms are different and have to be selected independently in developing tolerance to low and high temperatures. The identified rice genotypes will be a valuable resource for breeders in developing early season high- and low-temperature tolerant genotypes for a given environment.