The effect of salicylic acid in improving the physiological and agronomic characteristics of pinto bean under the end season drought stress

Introduction Drought stress is considered an important abiotic stress, one of the factors affecting the production of crops and agricultural sustainability around the world, especially in arid and semiarid regions. Pinto bean (spotted bean) (Phaseolus vulgaris L.) is one of the major pulse crops in Iran. It is widely cultivated in arid and semiarid regions. The protein content of the pinto bean varied from 21.4% to 23.6%. Drought is a major limiter of yield in pinto beans, decreasing food security for those who rely on it as an important source of protein. Protecting plants from adverse environmental conditions by using simple methods could be of great value under these conditions. Salicylic acid (SA) is one of the important growth regulators that modulate plant responses to environmental stresses including drought stress. Materials and methods A split plot experiment was conducted based on a randomized complete block design with three replications at the research farm in the East Azerbaijan province, Sarab, during 2019-2020. Factors experiment included irrigation at two levels (full irrigation as control, irrigation withholding in 50% of flowering stage) and foliar application of salicylic acid at three levels (foliar application with water as control, 100 and 200 mg.l-1). In this study, morphological traits, plant height, the number of sub-branches, the number of pods per plant, the weight of 100 grains, grain yield, and harvest index of pinto beans were investigated. Also, the measured traits included the chlorophyll index, malondialdehyde (MDA), hydrogen peroxide (H2O2), and Electrical Conductivity (EC). Results and discussion Drought stress reduces the value of all traits (chlorophyll index, plant height, the number of sub-branches, the number of pods per plant, the weight of 100 grains, grain yield and, harvest index) except MDA, EC and H2O2. However, salicylic acid foliar application was able to mitigate the adverse effects of water stress. Drought stress decreased chlorophyll index (about 14.2%), plant height (about 8.2%), number of sub-branches (about 22.9), the number of pods per plant (about 10.5%), the weight of 100 grains (about 8.9), grain yield (about 14.1) and harvest index (3.1%) and increase in the content of the MDA, EC and H2O2 (about 61, 2.1, and 92.2 present, respectively) as compared to the control treatment. Chlorophyll index, plant height, the number of sub-branches, the number of pods per plant, the weight of 100 grains, grain yield and harvest index in pinto bean were increased with the application of different concentrations of salicylic acid. Foliar spraying with 200 mg.l-1 salicylic acid caused a significant increase in the chlorophyll index, number of sub-branches, the number of pods per plant, (about 17.9, 33.02 and 18.4 present, respectively) and decreasing the MDA (about 49.33%) as compared to the control treatment. Under full irrigation and drought conditions, foliar spraying with 200 mg.l-1 salicylic acid caused a significant increase in the plant height (15.92 and 9.8%), the weight of 100 grains (17.8 and 12.8%), grain yield (27.3 and 20.4%) and harvest index (13.5 and 10.7%) and decrease in the content of the EC (1.7 and 2.3%) and H2O2 (28.9 and 81.9%), respectively. Conclusion In this study, by exacerbation of water stress, all agronomic traits were reduced. Foliar spray of salicylic acid improved the pinto bean plant tolerance to drought stress by regulating several physiological responses. Salicylic acid treatments with the increase of content of the MDA, EC and H2O2, mitigated the negative effects of drought stress, thereby increasing tolerance and growth of the pinto bean plant. The results also showed that the concentration of 200 mg.l-1 salicylic acid improved plant growth and drought tolerance in pinto bean plant more effectively than the concentration of 100 mg.l-1 salicylic acid.