Foliar application of salicylic acid and kaolin for enhancing drought tolerance, antioxidant activity and photosynthetic performance in Nagami kumquat (Fortunella margarita)

Abstract Nagami kumquat ( Fortunella margarita ) is sensitive to high summer temperatures and limited water, which reduce growth and fruit quality. To enhance drought tolerance, a greenhouse factorial experiment was conducted in Chaboksar, Iran, using kaolin (0, 3, 5%) and salicylic acid (0.0, 0.1, 1.0 mM) under three irrigation levels (100%, 75%, 50% field capacity). One-year-old grafted kumquat plants were foliar-treated, and physiological, biochemical, and leaf nutrient traits were measured after one month. While three-way interactions were not significant, two-way interactions significantly affected several biochemical traits including proline, chlorophyll a, carotenoids, flavonoids, malondialdehyde, superoxide dismutase (SOD), and ascorbate peroxidase (APX). Kaolin application improved potassium (K), manganese (Mn), CO₂ assimilation rate (AR), stomatal conductance (SC), and relative water content (RWC). Salicylic acid enhanced 1,1-diphenyl-2-picrylhydrazyl (DPPH), nitrogen (N), K, and Mn across irrigation levels. The highest chlorophyll a (Chl a) was observed with 1.0 mM salicylic acid without kaolin, while 5% kaolin increased carotenoids. Salicylic acid increased SOD under all kaolin levels; however, the magnitude of this increase was smaller under 5% kaolin than under 0% kaolin. Salicylic acid also promoted proline, whereas kaolin reduced it. Salicylic acid at 1.0 mM enhanced Chl a, Chl b, total chlorophyll, and carotenoid concentrations, and increased total phenols, and flavonoids. Multivariate analyses demonstrated clear separation between well-watered and water-stressed plants and distinguished salicylic acid-treated plants as having the strongest positive association with photosynthetic and antioxidant traits. APX activity increased under water deficit, particularly depending on salicylic acid and kaolin interactions. The first two components of principal component analysis (PCA) explained 64% of variation, showing correlations between RWC, SC, and AR. Biplot of PCA highlighted six responsive combinations, suggesting that extreme levels of irrigation, kaolin, and salicylic acid had the most pronounced effects, which were confirmed by cluster analysis. Finally, foliar application of 1.0 mM salicylic acid without kaolin was most effective in mitigating the harmful impacts of water stress by increasing photosynthetic efficiency, antioxidant capacity, and osmotic adjustment, while kaolin alone had limited benefits. These findings suggest that salicylic acid represented a practical approach to improving drought resilience under water-limited conditions. Future research can investigate long-term applications and the combined effects of salicylic acid and kaolin under field conditions.