Previous studies have shown that H₂O₂ acting as a second messenger functions in UV-B-regulated diverse processes in plants,but its origins remain elusive.The purpose of this paper is to investigate the enzymatic sources of H₂OO₂ mediating different doses of UV-B-induced stomatal closure in Arabidopsis thaliana leaves.With stomatal aperture bioassay and confocal laser-scanning microscopy,we observed that 0.5 W·m^-2 UV-B induced H₂O₂ production in guard cells and stomatal closure in wild type (WT),which were significantly inhibited by diphenylene iodonium chloride (DPI,an inhibitor of NADPH oxidase),but not by salicylhydroxamic acid (SHAM,an inhibitor of cell wall peroxidase).Meanwhile,this dose of UV-B could not induce H₂O₂ generation and stomatal closure in either single mutants AtrbohD and AtrbohF or double mutant AtrbohD/F.In contrast,0.65 W·m^-2 UV-B could induce H₂O₂ generation and stomatal closure in both WT and the three mutants,which were largely inhibited by SHAM,but not by DPI.These results demonstrate that different doses of UV-B activate distinct sources of H₂O₂ to induce stomatal closure in A.thaliana leaves.The low dose of UV-B activates NADPH oxidases AtrbohD- and AtrbohF-sources of H₂O₂ to induce stomatal closure,while the high dose of UV-B mainly activates cell wall peroxidase-sources of H₂O₂ to induce stomatal closure in A.thaliana.