Phenotyping and characterization of heat stress tolerance at reproductive stage in rice (Oryza sativa L.)

Rice crop is known to be sensitive to heat stress particularly at the flowering stage. Breeding approaches for improving heat tolerance in rice needs understanding of heat tolerance mechanisms and suitable heat tolerance donors. A study was planned for screening of rice genotypes and identification of novel heat tolerant donor(s) and physiologically characterize the component traits using contrasting set of genotypes in green house environment. Genotypes were categorized as tolerant and sensitive to heat stress using heat susceptibility index and cumulative stress response index for spikelet fertility, pollen viability, and grain yield. Among the set of genotypes screened, IET 22218 recorded high spikelet fertility (> 85%), pollen viability (> 95%) at high temperature (39-44 degrees C) with relative humidity (> 60-80%). This genotype recorded higher photosynthesis, canopy temperature depression, and accumulation of endogenous level of polyamines both under optimum and heat stress environments. Moreover, IET 22218 genotype recorded lower H2O2 accumulation, membrane damage and higher activity of antioxidant enzymes. Heat stress tolerance in IET 22218 was at par with heat tolerant checks, i.e., Nagina22 (N22) and Nerica L-44 (NL-44). Interestingly, IET 22218 also maintained lower chalkiness (< 34%) and higher head rice yield (> 85%) under heat stress. Based on above traits IET 22218 was selected as the novel donor for heat tolerance. The study concludes that induced polyamines and antioxidant enzymes activity in IET 22218 under stress were associated with lowering oxidative stress and maintained higher pollen viability and spikelet fertility under heat stress environment. However, more studies are recommended to understand the role of polyamines in heat stress tolerance specifically in rice.