Abstract:The effects of sodium nitroprusside(SNP,an exogenous nitric oxide donor)on photosynthesis and ionic contents in tomato(Lycopersicon esculentum Mill.)seedlings were investigated under 100 mmol/L NaCl stress.The results showed that under NaCl stress,chlorophyll contents,net photosynthetic rate(Pn),stomatal conductance(Gs),chlom. phyll fluorescence parameters Fv/Fm and Fv/Fo were significantly increased,but intercellular CO2 concentration(Ci)was obviously decreased by exogenous nitric oxide.At the same time,contents of K+、Ca2+ and Mg2+ in dry weight of mot and leaves were markedly increased,but contents of Na+ were significantly decreased by exogenous nitric oxide.The above results indicated that exogenous nitric oxide donor(SNP)could alleviate the unfavorable effects of NaCl stress on photosynthesis;delay the inhibition of tomato seedlings by NaCl stress.