Abstract: It is well known that the osmotic stress may induce oxygen free radicalgeneration and cause various degree injuries to some plant species. In thispaper the leaves of P. cathayana Rehd. were treated with different osmoticpressures by PEG-6000 to study the mechanism of leaf injury. The resultsshowed that the generation rate of superoxide free radical (O₂^-) increasedwith the increase in PEG concentration. The stress strength was in directproportion to the amount of O₂^- generation between -2.5 to -1.25 MPa. Thetrend of change in MDA contents was similar to that in O₂^- generation rate.In certain range of stress strength, the activity levels of SOD and CAT co-rresponded to the change in O₂^-. There was a positive correlation betweenincrease of cytoplasmic membrane permeability, leaf water potential decreaseand increment of MDA content. The conclusion obtained from this study maybe that osmotic stress damage of leave in P. cathayana is caused by membr-ane-lipid peroxidation firstly, meanwhile increase in MDA content, and cy-toplasina system destruction.

Key words: Osmotic stress, Superoxide free radical, Membranelipid peroxidation, Malondialdehyde (MDA), Superoxide dismutase (SOD), Populus cathayana Rehd