Foliar Application of Kaolin Enhances Physiological and Biochemical Resilience of Guar (Cyamopsis tetragonoloba L.) Under Salt Stress

Abstract

Salt stress is a major abiotic constraint limiting crop productivity in arid and semi-arid regions. Guar (Cyamopsis tetragonoloba L.), an important industrial and forage crop, is highly sensitive to salinity, which adversely affects its physiological and biochemical functions. The present study was conducted to evaluate the effectiveness of foliar-applied kaolin in mitigating salt stress in guar plants. A pot experiment was performed using clay loam soil selected based on its physicochemical properties. Salinity stress was imposed by adding a calculated amount of NaCl to the soil, while kaolin was applied foliarly at concentrations of 2% and 4%. Physiological attributes including photosynthetic rate, stomatal conductance, substomatal CO₂ concentration, transpiration rate, and chlorophyll content were measured. Osmolyte accumulation (proline and soluble sugars) and antioxidant enzyme activities (SOD, CAT, and POD) were also assessed. The results indicated that salinity significantly reduced plant physiological performance; however, foliar application of kaolin markedly improved photosynthetic rate (1.83-fold), stomatal conductance (24%), substomatal CO₂ concentration (11%), and chlorophyll content, while reducing transpiration rate by 54% compared to untreated plants under saline conditions. Kaolin treatment enhanced osmolyte accumulation, with proline and soluble sugars increasing by (2.56-fold) and (1.37-fold), respectively, contributing to improved osmotic adjustment. Additionally, antioxidant enzyme activities were significantly elevated in kaolin-treated plants, facilitating reactive oxygen species (ROS) detoxification and reducing oxidative damage under salt stress. These findings suggest that foliar application of kaolin is an effective and sustainable strategy to improve guar growth, physiological efficiency, and biochemical resilience under saline environments.