Photosynthetic Responses to Heat Stress in Common Bean Genotypes Differing in Heat Acclimation Potential

Abstract

Photosynthesis is one of the most heat‐sensitive processes in plants. The common bean (Phaseolus vulgaris L.) is notoriously sensitive to heat. The objectives of this study were to determine the photosynthetic activities of six common bean genotypes under heat stress and whether these activities correspond with the genotypes' ranking in purported heat acclimation potential (HAP). The HAP is defined as the change in leaf heat tolerance, based on plasmalemma thermostability measured by electrolyte leakage, at the prefloral growth stage of plants after acclimation at 37 °C day/night for 24 h. Photosynthetic response to short‐term heat stress (5 min at 42 or 45 °C) was expressed by O2 evolution and chlorophyll fluorescence. Without heat acclimation, heat stress at 42 °C decreased O2 evolution in the six genotypes from 50 to more than 95%, compared with the controls, and heat stress at 45 °C almost totally inhibited O2 evolution in all genotypes. Heat stress had less effect on peak fluorescence level than on O2, evolution. Heat acclimation slightly reduced O2, evolution, compared with nonacclimated controls. In heat‐acclimated plants, heat stress at 42 °C had no effect on O2 evolution, whereas stress at 45 °C significantly reduced O2 evolution. Changes in levels of peak fluorescence under heat stress in heat‐acclimated plants showed patterns similar to changes in O2 evolution. Photosynthetic responses to heat stress did not totally correspond with the ranking of HAPs of the six genotypes obtained by conductivity test. Full recovery of O2 evolution from heat injury at 42 °C for 5 min occurred within 4 h in GNUI 59, whereas O2 evolution rates were still lower than the controls after 6 h in the other genotypes. The recovery of chlorophyll fluorescence was slower than that of O2 evolution.