Cold tolerance in beans (Phaseolus spp.) as analyzed by their exotherms

Summary

Cold tolerance of common bean (Phaseolus vulgaris L.), runner bean (P. coccineus L.) and several bean lines was evaluated under artificial conditions. Seedlings were exposed to −6°C gradually, then to −12°C rapidly in the growth chamber with copper-constantan thermocouples attached to various parts of the plant. Seedling freezing curves with exotherms were then analyzed. Three basic parameters were measured: time required for exotherm to appear, temperature of the appearance of the exotherm and temperature rise caused by the exotherm. Exotherm appearance in beans was related to freezing injury and death of the seedlings. Exotherms of the common bean variety Bush Blue Lake 92 seedling recorded at the stem, primary leaf base, tip and petiole and true leaf appeared at the same time, however their shape was different. Exotherms of stem appeared at higher temperature than those of primary leaves. Their shapes always followed the same pattern. Beans with cold tolerance were found to produce exotherms later than susceptible ones. Exposing 3 weeks old bean seedlings to 12°C delayed the time of exotherm appearance in the cold susceptible cultivar Bush Blue Lake 92. Some plants survived 2 hours exposure to −6°C in the growth chamber.