Resilience of Pollen and Post-Flowering Response in Diverse Sorghum Genotypes Exposed to Heat Stress under Field Conditions

The predicted increase in global temperatures will increase the probability of exposing sorghum [Sorghum bicolor (L.) Moench] to heat stress during critical reproductive developmental stages, such as flowering and post-flowering periods. Greenhouse and field studies were conducted to quantify the impact of heat stress on pollen germination and other post-flowering physiological processes affecting grain yield. Pollen collected from 24 diverse sorghum genotypes grown under greenhouse conditions were tested for their tolerance to heat stress. Using the same set of genotypes, field-based heat tents were used to impose heat stress from booting stage to maturity. Pollen grains from field experiments were tested under three different types of heat stress combinations to identify genotypes with pollen having true heat tolerance. Heat stress induced a significant reduction in grain yield (16-73%), pollen germination (2-95%), photosynthesis (0.5-50%), and photochemical efficiency of photosystem II (1-8%) and increased thylakoid membrane damage (2-27%) compared with control conditions. Reduced grain yield with heat stress exposure was not compensated by grain weight increase. In vitro pollen germination revealed SC155 to possess true heat-tolerant pollen, even under severe stress exposure. Macia and BTx378 recorded higher relative grain yield and pollen germination, providing opportunities for mapping genomic regions responsible for heat tolerance using currently available biparental mapping populations in RTx430 and BTx623 backgrounds, respectively.