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 II (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 °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 .Abbreviations: RI -relative injury ; Fo -initial fluorescence ; Fm -maximum fluorescence ; Fv -variable fluorescence ; PS II -photosystem II ; PAR -photosynthetically active radiation
Among abiotic factors, high temperature is one of the major constraints to adaptation of groundnut (Arachis hypogaea L.) in tropical and subtropical areas. The aims of this study were (i) to evaluate three genotypes (ICG 1236, ICGS 44, and Chico) of groundnut for their heat acclimation potential (HAP), and (ii) to examine whether the growth, yield, and photosynthetic responses of these genotypes to temperature related to the HAP. We defined HAP as the change in leaf heat tolerance based on plasmalemma thermostability at 40 to 60°C measured by electrolyte leakage after acclimation at 35/30°C day/night temperature. Initially, plants were raised in a glasshouse maintained at 25/25°C day/night temperature. One half of the plants were shifted to another glasshouse maintained at 35/30°C after the appearance of the third leaf. Heat killing time (HKT), defined as the time required to cause 50% relative injury, indicated that the three genotypes acclimated to high temperature stress, with significant variations in HAP. All genotypes maintained greater vegetative growth and higher photosynthetic rates when grown under the higher temperature regime and genetic differences in photosynthetic rate were related to HKT. The higher temperature regime affected the reproductive growth adversely by increasing flower abortion and decreasing seed size, however. Differences in chlorophyll fluorescence and membrane thermostability between growth temperature were found only after incubating the leaf tissue at temperatures of 50°C or higher. Genetic differences in HAP were small and unrelated to growth differences.
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