Exogenously Applied Vanillic Acid Confers Drought Resilience in Cabbage (Brassica oleracea var. capitata L.) by Modulating Morpho-Physiological and Biochemical Responses

Drought is a major factor limiting vegetation growth, ultimately affecting food production. A study was conducted to assess the impact of exogenously used vanillic acid (VA) on cabbage (Brassica oleracea var. capitata L.) plants under water-deficit conditions. The plants were subjected to two drought stress (DS) treatments: a control group (100% field capacity, FC) and DS (60% FC). After 30 days of maintaining 60% FC, the plants were foliar-applied with VA at various concentrations (0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 mM). The data showed that DS led to a decline in growth attributes, relative water content (RWC), and photosynthetic pigments. Additionally, DS significantly increased leaf free proline, relative membrane permeability (RMP), glycine betaine (GB), ascorbic acid (AsA), total phenolics, and the activities of the antioxidant enzymes catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). However, foliar application of VA resulted in significant improvements in plant morphology and in physiological and biochemical traits. VA-treated plants showed enhanced growth and chlorophyll content. However, VA lowers the RMP, reflecting VA’s role in stabilizing the cell membranes. Furthermore, the VA application increased the levels of proline, GB, AsA, and total phenolics, as well as the activities of CAT, SOD, and POD enzymes. Among the different VA concentrations, 2.0 and 4.0 mM were found to be the most effective in enhancing oxidative stress protection and osmotic regulation, thereby improving stress tolerance, maintaining cellular functions, and enhancing growth and development in cabbage plants.