With the salinity-alkalinity-sensitive tomato cultivar ‘Zhongza No.9’ as the experimental material,the effects of foliar-spraying Spd (0.25 mmol·L^-1) on the growth paramaters,the net photosynthetic rate (P_n) and contents of chlorophyll biosynthesis precursors in tomato seedlings grown hydroponically under 75 mmol·L^-1 mixed saline-alkaline solution were studied.The results showed that:(1)Under saline-alkaline stress,the contents of chlorophyll biosynthesis precursors-protoporphyrin Ⅸ(Proto Ⅸ),Mg-Protoporphyrin Ⅸ(Mg-Proto Ⅸ),protochlorophyll (Pchl) were significantly decreased,while the contents of δ-aminolevulinic acid (ALA),porphobilinogen(PBG),uroorphyrinogen Ⅲ(Uro Ⅲ) were significantly accumulated in leaves of tomato seedlings.It was showed that stress restrained the transformation obstruction from Uro Ⅲ to Proto Ⅸ and lead to decrease in contents of Chl a,Chl b and total Chl,and reduced the P_n,leaf area,leaf relative water content,fresh and dry weight of shoot and root.(2)Under saline-alkaline stress,foliar-spraying Spd could effectively promote tomato seedlings growth and suppress the accumulation of ALA,PBG,URO Ⅲ and also increase contents of Proto Ⅸ,Mg-proto Ⅸ and Pchl,and enhance the P_n value.These results indicated that exogenous Spd could significantly relieve the degree of transformation obstruction from Uro Ⅲ to Proto Ⅸ,promote the tomato leaf chlorophyll synthesis and increase the chlorophyll content and net photosynthetic rate and also alleviate the stress damage under saline-alkaline stress.