Protective role of Salicylic acid and sodium nitroprusside foliar application against copper stress in okra (Abelmoschus esculentus)

Copper (Cu) stress, an abiotic stressor, can severely damage plant cells. At elevated concentrations, copper becomes a toxic element within plants, triggering the generation of oxidative molecules and disrupting enzymatic activities. Salicylic acid (SA) is a plant growth regulator, while sodium nitroprusside (SNP) is a nitric oxide-releasing compound. Both play critical functions in modulating plant metabolism, growth, development, and mechanisms. They also influence gene expression and signaling pathways, with context-dependent effects that may be beneficial or detrimental. The application of these compounds at appropriate doses significantly contributes to alleviating abiotic stresses in various plant species, particularly by counteracting the toxic effects. In this study, the individual and simultaneous effects of SA (500 µM) and SNP (150 µM), alongside varying concentrations of copper sulfate (600 and 1200 µM), were evaluated on the physiological and morphological responses of the Clemson variety of okra (Abelmoschus esculentus) in a hydroponic culture system. The experiments were conducted using a completely randomized design with three replicates per treatment. High copper concentrations reduced growth parameters and essential nutrient levels while increasing malondialdehyde (MDA), hydrogen peroxide (H₂O₂), cell death, protein content, proline, soluble carbohydrates, enzyme activities, and copper content in the shoots and roots of okra plants compared to the control. In contrast, foliar application of SA and SNP improved the uptake of essential elements, increasing Mg by up to 41%, Fe by 51%, and Zn by 50%. It also enhanced antioxidant enzyme activities (67-92%) and significantly reduced copper concentration (58%), MDA content (15%), H₂O₂ levels (26%), and cell death (49%) in the shoots and roots of okra plants. Therefore, based on the results, the individual and combined application of SA and SNP significantly mitigated the adverse effects of copper sulfate stress, improving tolerance of okra plants. According to the findings of this study, SA demonstrated a considerably greater effect compared to SNP in enhancing the resistance of okra plants to copper-induced stress.