Abstract
High temperatures profoundly limit the growth and productivity of leafy vegetables. Investigating heatresponsive morpho-physiological and biochemical mechanisms is an important tool for the thermo-tolerance of crops. This research work is about the response of varying levels of exogenous Ca2+ supplementation (0, 2, 4, and 6 mmol L-1) on growth & development, gaseous exchange, antioxidant enzyme activities, and osmolytes adjustments in spinach plants grown under heat related stress. A rise in temperatures from 25 to 35 degrees C decreased leaf area, number of leaves, chlorophyll contents, and leaf fresh biomass of spinach by 34.20%, 26.88%, 21.73%, and 24.68%, respectively. Meanwhile, Ca2+ treated plants showed higher antioxidant enzyme activities, soluble sugar levels, and lower membrane leakage induced by heat stress. Leaf osmotic potential values of seedlings treated with the Ca2+ solutions were measured to be lower than the control. Under heat stress, the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) significantly increased in spinach leaves. Exogenous Ca2+ supplementation enhanced heat tolerance in spinach by the maintenance of physio-biochemical, and a decrease in membrane lipid peroxidation and presence of phytoconstituents. Thus, an appreciable level of thermo-protection was induced by 4 mmol L-1 Ca2+ under heat stress conditions, indicating that the Ca2+ level required for growth under heat stress exceeds that required level for normal growth, regulation of enzymatic antioxidants, and osmolytes adjustments under normal conditions.