Abstract: The effects of exogenous NO on chlorophyll fluorescence parameters and photosynthesis rate in Malus hupehensis（Pamp.）Rehd. seedlings were studied in simulating drought condition of nutrient solution cultivation containing 20% polyethylene glycol 6000（PEG6000）and sodium nitroprusside（SNP）. The results showed that the Fo significantly increased，however，the Fv/Fm，qP，ETR，Yield,chlorophyll contents and photosynthesis rate all markedly decreased when the seedlings treated with PEG6000. Meanwhile，the qN increased within 3 days and then drastically decreased. On the other hand，the increase of Fo and the decrease of Fv/Fm，qP，ETR，Yield，chlorophyll contents and photosynthesis rate were all observed when the seedlings treated with 100700 μmol · L1 SNP. The results also indicated that exogenous nitric oxide with different treatments all had alleviated effects on Malus hupehensis（Pamp.）Rehd. seedlings and the alleviation function in the treatments was 300 μmol · L-1 SNP > 500 μmol · L-1 SNP > 100 μmol · L-1 SNP > 700 μmol · L-1 SNP. Moreover，there were remarkably alleviated effects with treatments from 300 μmol · L-1 SNP to 500 μmol · L-1 SNP，in which 300 μmol · L-1 SNP showed the best effect. 　　

Key words: Malus hupehensis（Pamp.）Rehd., water stress, nitric oxide, chlorophyll fluorescence parameters, photosynthesis rate