In maize (Zea mays L.) cells,the source of apoplast hydrogen peroxide (H₂O₂) induced by water stress (-0.7 MPa induced by polyethylene glycol) have been proposed,using different inhibitors for each enzymes,the histochemical and cytochemical observation and the analysis of the eyzeme activities.The results indicated that,the plasma membrane (PM) oxidase,cell wall peroxidase (POD) and the polyamine oxidases (PAO) take part in the H₂O₂ accumulation induced by water stress.Among them,NADPH oxidase could be the major soure of the apoplastic H₂O₂ production induced by water stress.The activities of PM NADPH oxidase,cell wall POD and apoplast PAO were all decreased significantly compared to that in the tugstate-pretreated maize plants,which indicated that ABA accumulation induced by water stress participated in the activation of the three enzymes.The results suggested that ABA accumulation induced by water stress can lead to significant increases in apoplast H₂O₂ accumulation through up-regulating the activities of the PM NADPH oxidases,cell wall POD and apoplast PAO.