Response of Heat-Tolerant and Heat-Susceptible Common Bean Genotypes to Heat Acclimation and Heat Stress

Heat stress is one of the major limiting factors for common bean (Phaseolus vulgaris) production.  While management practices could reduce heat stress at a certain degree, breeding heat-tolerant varieties is one of the most important practices to reduce crop yield loss under heat stress.  Understanding heat tolerant mechanisms could help breeders improve crop varieties and may help growers better deal with heat conditions.  Because heat acclimation could significantly improve crop performance under heat stress conditions, we studied changes in leaf stomatal properties and ultrastructure in mesophyll cells during heat acclimation and heat stress using two heat-tolerant genotypes (85CT-249762 and Strike) and two heat-susceptible genotypes (BBL-247 and BBL-2240).  Common bean plants were grown under 25/20 C day/night temperature in growth chambers for one month and then heat-acclimated at 38 C for 12 hours.  The first trifolioate leaves were removed from plants, placed into sealed test tubes, and merged into 50 C water incubation for 5, 10, 20, 30, 40, and 60 minutes. The results showed that stomatal resistance in heat-tolerant genotypes was lower under normal growing temperature (25 C) and heat acclimation temperature (38 C).  Heat-tolerant genotypes maintained lower stomatal resistance for longer time under 38 C compared to heat-susceptible genotypes.  Starch grains in chloroplast became significantly less and smaller after heat acclimation in both genotypes, indicating that starch was hydrolyzed into soluble sugars under high temperatures.  Heat acclimation delayed ultrastructural destruction induced by heat stress at 50 C.  The delayed destruction is more significant in heat-tolerant genotypes than heat-susceptible genotypes.  In mesophyll cells, nucleolus was the most susceptible structure to heat stress. Thylakoid was more susceptible than chloroplast envelope in chloroplasts.  This study suggests that heat tolerant genotypes responded better to heat acclimation temperature and maintained structure intact for longer time under high temperature stress (50 C) compared to heat-susceptible genotypes.