The study investigated the effect of gas transmitter H₂S (by adding H₂S donor NaHS) on the physiological and biochemical characteristics of hydroponic growth tomato seedlings under nitrate stress.The result showed that:(1)with the increasing nitrate stress treatment time,the shoot height,root length,fresh weight and dry weight,chlorophyll a and b contents,net photosynthetic rate,gas conductance and transpiration rate decreased significantly,while the intracellular CO₂ concentration,the MDA and H₂O₂ contents increased.The activities of superoxide dismutase,catalase,peroxidase,and ascorbate peroxidase and the antioxidant substances of ascorbate and GSH contents decreased after nitrate stress treatment.(2)When NaHS was added,the shoot height,root length,fresh weight and dry weight,chlorophyll a and b contents,net photosynthetic rate,gas conductance and transpiration rate increased significantly after NaHS added for 1,3,5 days,while the intracellular CO₂ concentration decreased,compared with the nitrate stress treatment alone.The MDA and H₂O₂ contents decreased,while the antioxidant enzyme activities of superoxide dismutase,catalase,peroxidase,and ascorbate peroxidase increased,besides,the antioxidant substances of ascorbate and GSH contents increased after NaHS was added.The activities of nitrate reductase,glutathione synthetase,and glutamate synthase increased after NaHS was added.NaHS treatment stimulated the increase of L-desulfhydrase activity,which in turn induced accumulation of H₂S content.These results indicate that the alleviation of the nitrate stress damage on tomato by H₂S might be fulfilled through increasing the antioxidant enzyme activities and the antioxidant substances contents.