The effect of 24-Epibrassinolide foliar application on the enzymatic and non-enzymatic defense system and yield of pinto beans (Phaseolus vulgaris L. cv. Pinto) under Low irrigation stress

Beans (Phaseolus vulgaris L.) are among the most valuable and strategic legumes globally and in Iran, primarily due to their high protein content and nutritional benefits. Growing global demand for protein-rich foods highlights the importance of enhancing bean production. However, water scarcity, exacerbated by Iran's location along the global desert belt and receiving less than one-third of the global average rainfall, poses significant challenges to sustainable agriculture and food security.  Brassinosteroids, a class of plant growth regulators, have emerged as a promising solution to mitigate environmental stresses like drought while improving crop growth and yield. This study aims to evaluate the effects of foliar application of 24-epibrassinolide hormone on various physiological traits, enzymatic and non-enzymatic defense mechanisms, and gain yield of pinto beans under low irrigation stress.

A field experiment was conducted during the 2022 growing season on a farm in eastern Shiraz, using a split-plot design within a randomized complete block design with three replications. The irrigation regime was applied at three levels: full irrigation (100% of the moisture requirement) and low irrigation stress (80% and 60% of the moisture requirement), as the main factor. Four concentrations of 24-epibrassinolide, including control (distilled water), 0.05, 0.1, and 0.2 mg/L were applied as the subplot factor. The experiment aimed to assess physiological traits such as chlorophyll index, relative water content, antioxidant enzyme activity (catalase, peroxidase, and superoxide dismutase), grain yield under different irrigation and hormone treatments.

The results revealed that both low irrigation stress and 24-epibrassinolide application significantly influenced all measured traits. Foliar application of 0.2 mg/L 24-epibrassinolide increased the chlorophyll index and leaf relative water content by 7.42% and 11.77%, respectively, compared to the control. Significant interactions between irrigation regimes and hormone concentrations were observed. At 60% and 80% moisture levels, the 0.2 mg/L hormone treatment increased antioxidant enzyme activities as follows (compared to the control): catalase (172% (60%) and 131% (80%)), peroxidase (105% (60%) and 115% (80%)), and superoxide dismutase (0.35% (60%) and 0.45% (80%). Similarly, seed yield improved significantly with 24-epibrassinolide application. Under 100%, 80%, and 60% irrigation levels, 0.2 mg/L application resulted in yield increases of 72.38%, 56.02%, and 44.35%, respectively, compared to the control.

The application of 24-epibrassinolide at a concentration of 0.2 mg/L effectively enhances the antioxidant defense system, reduces reactive oxygen species, prevents cell wall damage, and minimizes macromolecule degradation in pinto beans. These effects contribute to improved grain yield, under both full and low irrigation conditions. This approach offers a promising solution for mitigating water stress and promoting sustainable bean production in arid and semi-arid regions.