Abstract: A hydroponic experiment was conducted to study the effects of exogenous sodium nitroprusside (SNP), a NO donor, on the active oxygen metabolism and photosynthetic characteristics of tomato (Lycopersicon esculentum Mill.) seedlings under Cd stress. The results showed that under the stress, applying 100 μmol·L^-1 SNP promoted the activities of plant superoxide dismutase (SOD) and catalase (CAT) significantly, increased the leaf- and root calcium (Ca) and iron (Fe) contents and the leaf chlorophyll content, net photosynthetic rate (P_n), transpiration rate (T_r), and stomatal conductance (G_s), and decreased the contents of H₂O₂ and MDA and the concentration of intercellular CO₂ (C_i). The addition of hemoglobin, a NO scavenger, eliminated the effects of SNP, while applying 100 μmol·L^-1sodium nitrate or nitrite (the decomposition products of NO or its donor SNP) or 100 μmol·L^-1 sodium ferrocyanide (an analog of SNP) had no significant alleviation effects on Cd stress. This study suggested that exogenous NO could promote the scavenging of reactive oxygen, keep the mineral nutrition in balance, and alleviate the damage of Cd stress to the leaf photosynthetic apparatus, making the tomato seedlings preserve their photosynthetic efficiency.

Key words: nitric oxide, cadmium stress, tomato, active oxygen, photosynthesis, mineral element, degraded alpine meadow, Stellera chamaejasme, herbage, coverage, spectral feature parameter.