Physiological And Biochemical Mechanism Of Exogenous Melatonin And Glutathione On Cucumber Seedlings Under Cadmium Stress

With the accelerated development of economy and industrialization,soil heavy metal pollution has become a major constraint on our agricultural development,and has also brought potential threats to the environment we live in.In soil heavy metal pollution,Cadmium(Cd)pollution was the most outstanding,Cd was listed as one of the five poisons(Cd,Hg,As,Cr,Pb).When plants were stressed by high concentrations of heavy metals,their own cellular mechanisms were damaged due to the decline of their antioxidant and detoxification abilities.Melatonin(N-acetyl-5-methoxytryptamine)as a free-radical scavenger in cells,has been shown to slow the damage caused by Cd stress in plants.Glutathione(GSH)is an important water-soluble antioxidant in plants.Studies have shown that GSH treatment can enhance plants’ resistance to salinity,high temperature and heavy metals to resist stress.The cucumber ‘Jin You 315’(Cucumis sativus L.)was used as experimental material to study the changes of antioxidant system,photosynthetic system,ascorbic acid and glutathione content in cucumber seedlings treated with melatonin and glutathione under Cd stress by hydroponic culture experiments.The physiological effects of melatonin and glutathione under Cd stress were also discussed.It provides a theoretical basis for the application of melatonin and glutathione in vegetable cultivation.The experimental results are as follows:(1)Exogenous melatonin and glutathione treatment improved the stem diameter,plant height,dry weight and total fresh quality of cucumber seedlings under Cd stress.The root activity was enhanced and the relative electrical conductivity of leaves was decreased.The soluble protein content in cucumber leaves was increased.The concentration and transport coefficient of Cd in roots,stems and leaves decreased significantly compared with that of Cd stress.The results showed that melatonin and glutathione treatment could increase the growth of cucumber under Cd stress.(2)The exogenous melatonin and glutathione processing to enhance the antioxidant capacity of cucumber leaf under Cd stress and increase the superoxide dismutase(SOD),peroxidase(POD)activity,increased As A content,reduced the malondialdehyde(MDA)content,melatonin and glutathione common treatment improves reducibility glutathione(GSH)content,GSH/GSSG ratio.The results showed that melatonin and glutathione could effectively inhibit the production of reactive oxygen species(ROS)under Cd stress,slow down the cell membrane damage,increase the antioxidant content of cucumber seedlings,improve the activity of related antioxidant enzymes,and enhance the resistance of cucumber seedlings to the stress of Cd.(3)Treatment with exogenous melatonin and glutathione increased the chlorophyll content and dry matter accumulation in cucumber leaves under the stress of Cd.Cd stress condition,the Pn have been rising after falling tendency,separate melatonin and glutathione on Pn no significant relief effect after processing,Gs,Ci and Tr were significantly higher than that of single Cd stress after fertilization of melatonin,glutathione processed Gs,Ci also were higher than Cd stress,but the Tr decreases,melatonin and glutathione jointly handle the Gs was slightly lower than controls,Ci and Tr was close to the reference value;melatonin and glutathione treatment under non-stress conditions had no significant effect on the gas exchange parameters of cucumber leaves compared with the control.(4)Exogenous melatonin and glutathione significantly improved the photosynthetic efficiency of cucumber seedling leaves under the stress of Cd.also ΦPSⅡ rose after the first drop trend changes,but still below the level control,ΦPSⅡ changes maintained at about 0.5 to 0.7,melatonin,glutathione and the common treatment improves the cucumber leaf ΦPSⅡ,and with the extension of time ΦPSⅡ returning to control levels,the change trend of Fv’/Fm’ and ΦPSⅡ line;Cd stress significantly increased NPQ,and melatonin and glutathione combined treatment significantly decreased NPQ.melatonin treatment increased q P value of cucumber leaves under Cd stress,while glutathione and combined treatment had little effect on q P.