Abstract: Studies on the toxicity effect of monocrotophos on Platymonas sp.cells show that with increasing concentration of monocrotophos culture medium, the content of active oxygen in Platymonas sp.cells increased obviously, and correspondingly, that of malondialdehyde(MDA), peroxidation product of membrane lipid, and membrane permeability also increased.At the meanwhile, the activity of peroxidase(POD)and superoxide dismutase(SOD), two key enzymes for eliminating active oxygen, decreased gradually.These results indicate that under monocrotophos stress, the decline of PODand SODactivities broke the normal balance between production and eliminating of active oxygen, making active oxygen excessive and accumulated.The excessive active oxygen resulted in the aggravation of lipid peroxidation and the increase of membrane permeability, and therefore, the damage of Platymonas sp.appeared.

Key words: Platymonas sp, Active oxygen, Monocrotophos, Membrane lipid peroxidation, water availability, clonal integration, arid region, Populus euphratica, stable isotope, photosynthetic activity