Identification of Heat-Tolerant Rice Genotypes through Phenotypic Selection and Multi-Trait Characterization for Genetic Diversity

Rice (Oryza sativa L.) is highly vulnerable to elevated temperature, particularly during reproductive development, when impaired spikelet fertility and grain filling can substantially reduce yield. This study evaluated heat-stress responses among 49 rice genotypes, including released varieties, advanced breeding lines and tolerant and susceptible checks, during kharif 2021 at the Regional Agricultural Research Station, Maruteru. Genotypes were grown under ambient control conditions and in a polyhouse where heat stress was imposed from panicle initiation to maturity. During the stress period, mean maximum and minimum temperatures were 30.3 °C and 25.4 °C under ambient conditions and 35.3 °C and 27.9 °C inside the polyhouse, respectively. Phenological, morpho-physiological and yield-related traits were assessed to identify genotypes with stable performance under heat stress. Heat stress reduced mean chlorophyll a from 3.20 to 2.75 mg g⁻¹ FW, chlorophyll b from 1.29 to 0.76 mg g⁻¹ FW, cell membrane stability from 55.9% to 47.4%, spikelet fertility from 89.7% to 50.4% and grain yield from 29.4 to 15.9 g plant⁻¹. Considerable genotypic variation was observed across traits. N22 maintained the strongest overall performance under stress, with 81.4% membrane stability, 84.0% spikelet fertility and 21.5 g plant⁻¹ grain yield. Rasi, L 663, L 672, MTU 1239, CL 448 and CL 452 also showed comparatively stable responses, while Vandana, MTU 1166 and MTU 1001 were susceptible. The tolerant group comprised MTU 1223, MTU 1239, L 663, L 672, L 674, CL 448, CL 452, N22 and Rasi. The findings indicate that chlorophyll retention, membrane thermostability, spikelet fertility and grain-yield stability are useful selection criteria for identifying rice genotypes with improved tolerance to reproductive-stage heat stress.