Abstract: Nitric oxide (NO) is an active molecule involved in many biological pathways, but its effects on photosynthesis of tree leaves have not been established yet.This paper studied the effects of exogenous NO,sodium nitroprusside (SNP),on the water status,photosynthesis and scavenging enzyme activities in poplar leaves.Different levels of SNP treatments had remarkable effects on the water content of leaves,which increased with increasing SNP levels.When the SNP level exceeded 500 μmol·L^-1,differences in leaf water content were no longer significant between different SNP treatments.Exogenous NO increased the photosynthesis rate,photochemical efficiency of PSII Fv/Fm,and Fm/Fo and Fv/Fo ratios,and the effects decreased with increasing duration of water stress.The SOD and POD activities in poplar leaves were higher in 1 hour water stress treatment than in 3 h treatment. Treating with SNP could markedly increase POD activity,but SOD activity did not change much.POD and SOD activities increased initially,and then decreased with increasing SNP levels.The results indicated that exogenous NO delayed the accumulation of active oxygen by increasing POD and SOD activities,and thereby,alleviated the effects of water stress on photosynthetic organization of poplar leaves.

Key words: rotational tillage system, summer fallow period, winter wheat, soil water, yield, Southern Ningxia arid area