Regulation Of Salicylic Acid On Photosynthesis And Nitrogen Metabolism Of Cucumber (Cucumis Sativus L.) Seedlings Under Water Stress

Salicylic acid is a phenolic derivative of phenylpropanoid metabolism, involved in cell signal transduction in plant. It has direct involvement in plant growth, thermogenesis, flower induction, uptake of ions and stress resistance. Cucumber (Zhongnong No.203) was applied as experimental material, 60% and 50% substrate relative water content treatments were used as light water stress and severe water stress respectively, this papper studied the effect of the rhizosphere SA (0.1mmol·L-1) pretreatment on the water use efficiency, chlorophyll fluorescence and nitrogen metabolism of cucumber seedlings under water stress, and analyzed the differential protein expressions of cucumber leaf after the rhizosphere treatment by exogenous salicylic acid. The main results were as follows:Under water stress, the relative water content of cucumber seedlings decreased significantly, the MDA content and electrolyte leakage in leaves increased markedly, Meanwhile, leaf water use efficiency, net photosynthetic rate (Pn), PSⅡmaximal photochemical efficiency (Fv/Fm), actual PSⅡefficiency (ΦPSⅡ), PSⅡpotential activitiy (Fv/Fo), PSⅡantenna conversion efficiency (Fv'/Fm'), and photosynthetic active fluorescence quenching (qP) declined, while non-phochemical fluorescence quenching coefficient (NPQ) increased. In addition, the total nitrogen content, soluble protein content, nitrate reductase (NR) and glutamine synthetase (GS) activity declined, while the inorganic nitrogen content, free amino acid and proline content increased.SA pretreatment increased significantly the relative water content, decreased markedly MDA content and electrolyte leakage, enhanced the water use efficiency, alleviated the decreases of Pn, Fv/Fm,ΦPSⅡ, Fv/Fo, Fv'/Fm'and qP, inhibited the increase of NPQ in cucumber seedlings under water stress. Compared with the light and severe stress, the leaves water use efficiency and Pn increased 7.24%, 6.51% and 29.36%, 16.89% respectively after the treatment of SA. SA pretreatment increased the total nitrogen content, soluble protein content and NR and GS activities, promoted the accumulation of the free amino acids and proline content in the early period of stress, decreased the accumulation of the inorganic nitrogen content and the free amino acids and proline content in the late period of stress, additionly, NR and GS activities increased 19.63%, 12.62% and 21.92%, 12.72% respectively after the treatment of SA. Furthermore, SA induced differential expression of 22 proteins, among which 8 specific proteins expressed, 7 proteins were significantly up-expressed, 5 proteins were significantly down-expressed, and 2 proteins silenced compared with the control. These differential proteins might be involved in cell growth, secondary metabolism and stress resistance of cucumber seedlings.The above results showed that, SA may increase the stability of cell membrane and PSⅡactivity in leaves, enhance nitrogen assimilation and ammonia assimilation ability of cucumber seedlings under water stress, increase the net photosynthetic rate (Pn) and water use efficiency, and improve the nitrogen metabolism, which were the key factors of alleviating the growth inhibition of cucumber seedlings under water stress.