Physiological Responses of Vigna radiata to Zinc Stress Alleviated by 28-Homobrassinolide Priming

Zinc (Zn) toxicity is a major environmental stressor that hampers plant growth, physiology, and productivity. This study evaluated the efficacy of seed priming with 28-homobrassinolide (28-HBL), a potent brassinosteroid, in mitigating Zn-induced stress in Vigna radiata (L.) R.Wilczek under natural field conditions. Seeds were primed with varying concentrations (1 μM, 1 nM, and 1 pM) of 28-HBL and grown under 150 mM and 300 mM ZnSO4 stress. Zn toxicity significantly reduced shoot and root length, fresh and dry biomass, photosynthetic pigments, membrane stability, and yield attributes. However, 28-HBL priming particularly at 1 nM and 1 pM markedly improved morphological, physiological, and reproductive parameters. Notable improvements were observed in chlorophyll content, leaf area, proline accumulation, and membrane stability index, along with reductions in MDA and Zn accumulation in leaves. Reproductive success, including flowering time, pod length, and seed yield, was also restored. The findings suggest that seed priming with 28-HBL enhances Zn stress tolerance in V. radiata through modulation of growth, antioxidant defense, and physiological resilience. This study highlights the potential of 28-HBL as a sustainable biostimulant to improve legume productivity under heavy metal stress, warranting further exploration at molecular and field levels.