Effects of Exogenous Spermidine on Photosynthesis and Activities of Antioxidant Enzymes in Tomato Seedlings under Ca(NO_3)_2 Stress

With tomato variety shanghai 'Hezuo 903' as material,we investigated the effects of exogenous spermidine(Spd,1mmol·L-1) on plant growth,chlorophyll fluorescence and photosynthetic characteristics,and active oxygen metabolism in tomato seedlings under 75 mmol·L-1 Ca(NO3)2 stress by hydroponic culture,and studied the mechanism of how exogenous spermidine ease the Ca(NO3)2 stress.The results showed that Ca(NO3)2 stress can significantly reduce plant growth.However,in the treatment of Spd for 9 d under Ca(NO3)2 stress,the seedlings height,stem diameter,the fresh and dry weight were significantly increased by 70.9%,15.8%,43.4%,41.4%,respectively.The chlorophyll a content,chlorophyll b content,Pn,Gs,Tr,rETR,qP were increased by 6.6%,18.0%,31.0%,4.9%,5.0%,respectively,while Ci,qN were decreased by 21.5% and 8.1%,respectively.The activities of SOD and CAT in leaves were significantly enhanced by 10.7% and 37.5%,respectively,while the accumulation of malondialdehyde(MDA) and O-·2 producing rate were significantly decreased by 34.5%,17.1%,respectively.These results showed that exogenous Spd can effectively remove reactive oxygen species by increasing the activities of antioxidant enzymes in tomato seedlings,maintain the stability of the photosynthetic apparatus,increase their photosynthetic rate,and thus ease the Ca(NO3)2 stress on tomato seedling damage.