Using pharmacological and biochemical approaches,the role of protein phosphorylation and the interrelationship between water stress-enhanced kinase activity,antioxidant enzyme activity,hydrogen peroxide(H2O2) accumulation and endogenous abscisic acid in maize(Zea mays L.) leaves were investigated.Water-stress upregulated the activities of total protein phosphorylation and Ca2+-dependent protein kinase,and the upregulation was blocked in abscisic acid-deficient vp5 mutant.Furthermore,pretreatments with a nicotinamide adenine dinucleotide phosphate oxidase inhibitor and a scavenger of H2O2 significantly reduced the increased activities of total protein kinase and Ca2+-dependent protein kinase in maize leaves exposed to water stress.Pretreatments with different protein kinase inhibitors also reduced the water stress-induced H2O2 production and the water stress-enhanced activities of antioxidant enzymes such as superoxide dismutase,catalase,ascorbate peroxidase and glutathione reductase.The data suggest that protein phosphorylation and H2O2 generation are required for water stress-induced antioxidant defense in maize leaves and that crosstalk between protein phosphorylation and H2O2 generation may occur. Using pharmacological and biochemical approaches, the role of protein phosphorylation and the interrelationship between water stress-enhanced kinase activity, antioxidant enzyme activity, hydrogen peroxide (H2O2) accumulation and endogenous abscisic acid in maize (Zea mays L.) leaves were investigated. Water- stress upregulated the activities of total protein phosphorylation and Ca2 + -dependent protein kinase, and the upregulation was blocked in abscisic acid-deficient vp5 mutant.Furthermore, pretreatments with a nicotinamide adenine dinucleotide phosphate oxidase inhibitor and a scavenger of H2O2 significantly reduced the increased activities of total protein kinase and Ca2 + -dependent protein kinase in maize leaves exposed to water stress. Plements with different protein kinase inhibitors also reduced the water stress-induced H2O2 production and the water stress-enhanced activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase.Th e data suggest that protein phosphorylation and H2O2 generation are required for water stress-induced antioxidant defense in maize leaves and that crosstalk between protein phosphorylation and H2O2 generation may occur.