The common bean (Phaseolus vulgaris L.) is a heat-sensitive plant species in which excessive abscission of reproductive organs occurs during hot weather. This results in yield reductions, and, in extreme heat stress, plants produce few or no pods. We evaluated 74 bean genotypes in terms of leaf heat tolerance (HT) and leaf heat acclimation potential (HAP), as expressed by heat killing time (HKT), the time in minutes needed to cause a 50% electrolyte leakage from leaf tissue heated at 50°C Leaf HT is defined as the leaf HKT of plants without prior conditioning at 37°C day/night temperature and leaf HAP as the change in leaf HT following exposure of the plant to 37°C day/night for 24-h. Among 74 bean genotypes examined leaf HT ranged from 5 to 30 min HKT, whereas leaf HAP ranged from 35 to 130 min HKT. Positive significant correlations were observed between leaf HAP and post-stress performance in photosynthetic activities, plant dry weight, pod set, pod weight and yield among bean genotypes. Correlations, however, were not significant between leaf HT and post-stress performance.
A relationship between heat resistance, consisting of the combination of HT and HAP, and heat injury is proposed. Interpretation of the differential amounts of heat injury among genotypes having different HAP, is discussed. We view leaf HT and leaf HAP as two distinguishable phenomena. We suggest that in breeding programs HAP may be the more important of the two, and should he evaluated as a selection criterion for improving crop performance in high temperature environments.