Sheathed Panicle Phenotype (cv. Sathi) Maintains Normal Spikelet Fertility and Grain Filling under Prolonged Heat Stress in Rice

The failure of panicle exertion or sheathed panicle (SP) phenotype in rice (Oryza sativa L.) has been associated with spikelet sterility and grain yield loss under both stress and nonstress conditions. A unique SP phenotype (cv. Sathi) having the potential to maintain normal seed-set and grain weight (exposed to 38 degrees C for 20 d) was identified. Despite the higher spikelet tissue temperatures (38-40 degrees C) at anthesis, pollen count and germinated pollen grains on the stigma were greater in number than the threshold required for successful seed-set. Panicle length, total spikelet number, and grain thickness in SP phenotype were not affected significantly by heat stress. In addition, lower lipid peroxidation in spikelets and nonsignificant reduction in carbohydrate metabolism enzymes (invertases and starch synthase) were recorded during grain filling, even after 10 d of heat stress exposure. Minimal impact on carbohydrate metabolism enzymes would support normal sucrose unloading and starch synthesis in the grain and normal grain size under prolonged heat stress exposure. Our findings open novel avenues for genetic and molecular dissection for inducing greater heat stress resilience during anthesis and grain filling in sensitive, popular rice cultivars to sustain global food security under future uncertain environmental conditions.