Physiological Effects Of Salicylic Acid On Cucumber Leaves Under Water Stress

Salicylic acid (SA) is a simple hydroxybenzene in plant. As a endogenesis signal molecule, the effects of SA on many physiological progress in plant have been paid more and more attention. In recent years, the ability of SA to improve the endurance of plants in abiotic stress, such as salt, cold, hot and drought has been found. We tried to study the effects of exogenous SA pretreatment on the physiological functions and photosynthesis system of cucumber seedlings under water stress, and primarily evaluated the ability of exogenous SA to improve the endurance of cucumber seedlings in drought.The material for study is cucumber {Cucumis sativus L.): chunqiudafeng. After 24 hour pretreatment with SA by sprinkling to the leaves, cucumber seedlings were treated with the culture containing 10% (w/v) PEG-6000 for water stress from Od to 8d, every 2d as a standard.After pretreatment with SA for 24 hours, seedlings presented slight stressing state. Relative water content decreased slightly. Electrolyte leakage and the content of MDA and H2O2 increased. The activity of antioxidant enzyme POD and SOD increased, but the activity of CAT and APX decreased. In all the stress conditions, the physiological indexes of seedlings pretreated with SA are all better than control. Relative water content decreased, the decrease of MDA content and H2O2 content is slower than control. The activities of 4 kinds of antioxidant enzyme are all higher than control. Especially for the activity of POD and APX, they evidently increased in the initial stage. We can suggest that, as a signal molecule, SA could bind to a soluble SA-binding protein (SABP), and SABP has been confirmed as a catalase (CAT). Thebinding of SA to CAT could decrease the activity of CAT, and the content of H2O2 would increase. As a second messenger, excessive H2O2 induced the express of H2O2 eliminating enzymes like POD and APX, enhanced the efficiency of active oxygen eliminating system and the endurance of plant in stress.After the same pretreatment with SA for 24 hours, the content of Chla, Chlb, carotenoid and soluble proteins in leaves increased slightly. In all the stress conditions, the content of pigments in seedlings with SA pretreatment increased more slowly than control. SDS-PAGE of total proteins showed that the dissolution of soluble proteins is slower than control, especially the content of Rubisco in kalvin cycle. At the same time, Northern blotting for mRNA of rbcL. and rbcS, who code the big and small subpart, of Rubisco showed that the changes of mRNA were identical to the corresponding protein. These proved that proper concentration of pretreatment with exogenous SA can alleviate the injury of water stress on light capture pigments and CO2 fixation system. Proper concentration of SA can enhance the efficiency of active oxygen eliminating system, alleviate the injury of membrane and photosynthesis system, and improve the endurance of cucumber to stress.