Abstract: In order to study effects of different nitrogen sources on the responses of tomato plants to salt stress，the effects of 150 mmol ? L-1 NaCl on growth，cell ultrastructure，root vigor and photosynthesis of tomato seedlings were investigated by supplying with ammonium（3 mmol ? L-1 nitrogen），nitrate（3 mmol ? L-1 nitrogen）and ammonium nitrate（3 mmol ? L-1 nitrogen）in nutrient solution. The results showed that salt-stressed leaf cells were damaged under nitrate source，but were not obviously affected underammonium nitrate source，that leaf cell ultrastructure showed obvious damage under ammonium source，and salinity aggravate the damage. Under NaCl stress，the biomass and root vigor of plants were both significantly decreased fed with three nitrogen sources，with ammonium nitrate fed seedlings retaining the highest biomass and root vigor. It showed that salinity caused a significant decrease of Pn and Tr of all the plants fed with different nitrogen sources，and Pn and Tr of plants fed with ammonium nitrate was higher significantly than that fed with the other two nitrogen sources. The WUE and Ls of nitrate- and ammonium nitrate-fed plants were increased under salinity stress，however，those of ammonium-fed plants were decreased by salinity. To sum up，under salt stress，ammonium nitrate-fed tomato seedlings showed better leaf cell ultrastructure，higher root vigor and higher photosynthesis than nitrate-fed tomato seedlings，so that better salt tolerance was achieved. However，pure ammonium application produced greater damage or decline to cell ultrastructure，root vigor and photosynthesis，and salinity worsened these phenomena.

Key words: tomato, seedlings, NaCl stress, nitrogen form, cell ultrastructure, photosynthesis