Heat tolerance in food legumes as evaluated by cell membrane thermostability and chlorophyll fluorescence techniques

The genotypic variation for heat tolerance in chickpea, groundnut, pigeonpea, and soya bean was evaluated by testing membrane stability and photosystem (PS II) function in leaves at high temperatures. The legumes could be ranked from heat-tolerant to sensitive in the order: groundnut, soya bean, pigeonpea and chickpea. The damage to cell membranes (as reflected by an increased leakage of electrolytes) and PS Il (as reflected by a decrease in the ratio of variable to maximum fluorescence) was less, and recovery from heat stress was faster in groundnut than in other crops. Prior exposure of plants to 35 degrees C for 24 h led to a reduced leakage of electrolytes at high temperatures in all crops but the differences among legumes were consistent. Substantial genotypic variation for heat tolerance was found in all legumes. Membrane injury was negatively associated with specific leaf weight in groundnut (r=-0.69**) and soya bean (r=-0.56**) but not in the pulses. Electrolyte leakage and fluorescence ratio were negatively correlated in all legumes. The potential use of electrolyte leakage and fluorescence tests as screening procedures for breeding heat-tolerant legumes is discussed.