Improvement in photosynthesis, seed yield and protein content of common bean (Phaseolus vulgaris) by foliar application of 24-epibrassinolide under drought stress

Brassinosteroids (BRs), as a class of plant growth regulators, have been shown to affect different physiological traits of plants and counteract various stresses. We studied the possibility of increasing seed and protein yields of two genotypes of common bean (Phaseolus vulgaris L.) with the exogenous application of 24-epibrassinolide (EBL) in an experiment conducted during 2016 and 2017. Two levels of irrigation (optimal and drought stress) were applied to the main plots, and two genotypes of common bean (cv. Kusha and genotype COS16) and four concentrations of EBL (0, 2, 4 and 6 µm) were allocated to subplots in a factorial arrangement. At the flowering stage, drought stress was applied and bean plants were sprayed with EBL. The results indicate that drought stress reduced leaf area, yield components, seed yield and protein content. Moreover, substantial increase in intercellular CO2 concentration and decrease in transpiration rate, stomatal conductance and net photosynthetic rate were also recorded. However, exogenous application of EBL remarkably improved gas exchange attributes, leaf area, yield components, seed yield and protein content both under optimal irrigation and drought-stress conditions. Analysis of regression showed that, under both water-supply conditions, genotype COS16 would have highest seed yield when receiving 4.05 and 4.52 µm EBL, and cv. Kusha would have the highest seed yield by receiving 3.27 and 3.62 µm EBL. Therefore, EBL can be used as a plant growth regulator to enhance drought tolerance and minimise yield loss of common bean caused by water deficits.