Gas Exchange of Five Warm‐Season Grain Legumes and their Susceptibility to Heat Stress

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Abstract
Objective of this study was to compare the heat stress performance of four pulses from dry and hot areas (mungbeans, limabeans, and teparybeans and cowpeas) with that of soybeans. Two experiments were conducted in growth chambers, and data were pooled because results of both experiments were similar. Plants were raised up to flowering at 24/17 °C (day/night) and were then either exposed to these temperatures until maturity or stressed with 33/24 °C for 2 weeks starting at day 1 or 15 after onset of flowering (early vs. late stress). Before, during and after these stress intervals, gas exchange of representative upper leaves was examined; additionally, immediate effects of increasing leaf temperatures from 24 to 32 or 40 °C on chlorophyll fluorescence were assessed. Without heat stress rates of photosynthesis (Pn), and of transpiration (TR), stomatal and mesophyll conductance (gs, gm) and intrinsic transpiration efficiency (iTE) differed significantly among the five crops at each date. However, because of crop‐specific time‐courses ranking among unstressed crops was instable with time, so values were integrated or averaged over time. This procedure revealed high Pn potentials in mung‐ and teparybeans and high iTE values in limabeans compared to the other crops. Heat stress lowered Pn and gs considerably, but increased TR in all five crops. Relative lowering of Pn during heat stress displayed a crop‐specific pattern with limabeans being least susceptible to both early and late heat stress, while cowpeas were highly susceptible to early stress. Effects on Pn were mainly attributable to lowering of gs and only in part to gm. The latter was supported by very small changes (N, +16 %) pointed to an adaptive mechanism to avoid oxidative strains under heat. Leaf temperatures of 40 °C immediately provoked stronger changes in all fluorescence signals than 32 °C; substantial damages at 40 °C were indicated by effective quantum yield, photochemical quenching and ratio of fluorescence decrease in mungbeans and low ones in cowpeas and soybeans. Nevertheless, some adaptive responses of e‐rates and QN were observed in all crops and were most expressed in limabeans.

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