EXOGENOUS CALCIUM ALTERS ACTIVITIES OF ANTIOXIDANT ENZYMES IN TRIFOLIUM REPENS L. LEAVES UNDER PEG-INDUCED WATER DEFICIT

This study was designed to examine whether external calcium (Ca²⁺) would improve the tolerance of Trifolium repens L to polyethylene glycol (PEG)-induced water deficit, and to determine the physiological mechanisms of Ca²⁺ effect on plant tolerance to water deficit. T. repens seedlings were subjected to PEG-induced water deficit alone or combined with 5 mM calcium chloride (CaCl₂) for 72 h. During PEG-induced water deficit period, leaf relative water content (RWC) decreased gradually, and chlorophyll content increased after 24 and 48 h of water deficit but decreased below the control level after 72 h. The Ca²⁺-treated plants had higher RWC and chlorophyll content than untreated plants. Smaller amounts of thiobarbituric acid reactive substances (TBARS) and hydrogen peroxide (H₂O₂) accumulated in Ca²⁺-treated plants than in untreated plants during the period of water deficit. The activity of superoxide dismutase (SOD) increased gradually during the experimental period, and external Ca²⁺ treatment further promoted SOD activity under water deficit. The activity of the catalase (CAT) was not influenced after 24 and 48 h of water deficit and insignificantly increased after 72 h, whereas the activity of ascorbate peroxidase (APOX) increased linearly and glutathione reductase (GR) activity slightly increased over the course of treatment. Seedlings treated with Ca²⁺ had higher CAT, GR, and APOX activities than untreated plants under water deficit. These results suggested that exogenous Ca²⁺ application enhanced T. repens tolerance to PEG-induced water deficit, and this enhancement was related to alleviation of lipid peroxidation and maintenance of antioxidant activities.