Alleviation of Oxidative Stress and Enhancement of Gas Exchange in Phaseolus vulgaris by Proline and ZnO Nanoparticles under Agricultural Drainage Water Irrigation

The increasing scarcity of freshwater resources has led many countries, including Egypt, to rely on saline agricultural drainage water (ADW) for irrigation. However, the elevated salt content of agricultural drainage water imposes salinity stress, disrupting water relations, ionic balance and photosynthetic processes, ultimately triggering excessive production of reactive oxygen species and oxidative damage in plants. Common bean (Phaseolus vulgaris L.) is particularly sensitive to salinity, therefore, this study investigated the potential of foliar-applied proline (1 g/L) and zinc oxide nanoparticles (ZnO-NPs, 20 µg/L), individually and in combination, to alleviate salinity-induced oxidative stress in plants irrigated with agricultural drainage water compared with freshwater irrigation during the 2026 growing season. Irrigation with ADW significantly increased the accumulation of H2O2, $${\text{O}}_{2}^{{\bullet - }}$$ and thiobarbituric acid-reactive substances (TBARS), while reducing membrane stability index and gas exchange parameters, including stomatal conductance, transpiration rate, and net photosynthetic rate. In contrast, application of proline and ZnO-NPs mitigated these detrimental effects by lowering reactive oxygen species and TBARS levels, enhancing membrane stability and promoting the accumulation of total phenolics and endogenous proline under saline conditions. Furthermore, these treatments enhanced antioxidant defense capacity through increased activities of superoxide dismutase, catalase, peroxidase, ascorbate peroxidase and glutathione reductase, accompanied by improvements in gas exchange characteristics. The co-application of proline and ZnO-NPs consistently produced the greatest protective effects. These findings demonstrate that integrating proline with ZnO-NPs is a promising strategy to enhance salinity tolerance, improve water use efficiency and reinforce the physiological and biochemical resilience of common bean under agricultural drainage water irrigation.