Developing Screening Tools for Temperature Tolerance in Rice Cultivars during Early-Season.

Temperature is one of the major abiotic stress factors that affects plant growth and development at various stages of plants. In the US Midsouth, rice plats will be exposed to variable temperatures during the, depending on the planting date. We hypothesize that rice cultivars vary in their response to temperature, and developing a method to identify low and high temperature tolerance will help producers and breeders to select cultivars for management and breeding. Four rice cultivars, CL152, Bowman, Antonio, and Mermentau along with two hybrids XL 753 and CLXL 745 most commonly grown in the US Midsouth evaluated in this study for temperature tolerance. Five day/night temperature treatments, 20/12 (very low), 25/17 (low), 30/22 (optimum), 35/27 (high), and 40/32°C (very high), were imposed after the seedling establishment, 13 days after planting (DAS). Growth and developmental including several root traits and physiological parameters using the WinRHIZO root image analysis system were recorded from plants harvested at 40 DAS. Rice cultivars and hybrids exhibited substantial and significant variability in their response to low and high temperatures. Based on total low- and high-temperature response index methods, relative temperature response scores were derived. Total low temperature index values ranged from 13.53 to 20.05 whereas total high temperature responses index values ranged from 30.03 to 47.11.  Antonio, CL 152, and Mermentau were identified as sensitive to cold- and heat-sensitive and Bowman and CLXL 745 and XL 753 as moderately and highly cold/heat tolerant cultivars/hybrids, respectively, among the six cultivars tested. These results may be useful for breeders to develop new rice cultivars which could withstand low and high temperature conditions during seedling stages. However, further large scale studies are needed to evaluate all the cultivars or lines both in the controlled environments and field settings before recommending them to the producers and breeders.