Effects of exogenous Spd on contents of NO
−-N (A) and NH
+-N (B) in leaves of tomato seedlings under salinity-alkalinity mixed stress (nutrient solution with 75 mmol/L saline-alkaline solution; NaCl:Na
= 1:9:9:1). Data represent means of three replicates. Bars represent the mean ± SE (P < 0.05) of at least three independent experiments.