Effect of Heat Stress on Reproductive Success and Grain Yield in Spring Wheat.

Heat stress is increasingly becoming a serious threat to wheat production in major wheat growing regions. Exposure of wheat genotypes to temperatures above the critical threshold during flowering and grain-filling can induce reproductive failure and grain yield reduction. In our study, we have exposed 28 diverse spring wheat genotypes to heat stress (34^oC/16^oC, day/night temperature) during flowering (for 10 days) and grain-filling (for 30 days) and the genetic variability in reproductive success (pollen viability), physiological (leaf level photosynthesis, and chlorophyll fluorescence), and yield related parameters (biomass, grain number and weight) were explored under controlled environment conditions. Pollen grains collected from the flowers just beginning to open, between 05:30 to 06:30 A.M. were grown on liquid in vitro pollen germination media. Averaged across wheat genotypes, a significant reduction in pollen germination (39.7%; P<0.001) was recorded from pollen exposed to heat stress compared to control (24°C /16°C; day/night). Heat stress during flowering induced significant reduction in seed number (15.4% and 23.0%), and grain weight (32.3 % and 34.6 %) on main spike and primary spikes, respectively compared with control. Impact of heat stress during grain-filling was more pronounced on grain weight (16% and 22%) than the grain number (2.7 % and 9.4 %) averaged across all genotypes on main and primary spikes, respectively. Reduced grain yield with heat stress exposure during grain-filling was not compensated by higher grain number. Despite a non-significant effect of heat stress on biomass, differences were larger and significant among genotypes for total grain weight and harvest index. Our results suggest that 10 days of heat stress during flowering induced greater damage to spring wheat than the 30 days of heat stress at grain-filling. Physiological mechanisms and check points restricting pollen germination on the stigma and their interaction with heat stress will be discussed.