The Effect Of Exogenous Selenium On The Physiology And Secondary Metabolites Of Tea(Camellia Sinensis)

Tea is an important cash crop in China and has become a popular beverage worldwide due to its unique flavor and health benefits.Selenium-rich tea not only serves as an important source of supplemental selenium,but also possesses rich secondary metabolites of tea itself,making it a hot research topic as the demand for selenium-rich tea continues to increase.However,the effect of exogenous selenium application on tea quality and resistance to low temperature stress has been little studied.In this paper,we investigated the effects of selenium spraying on the physiological characteristics of photosynthesis,antioxidant enzyme activity,malondialdehyde synthase and the determination of tea polyphenols,alkaloids,amino acids and soluble sugars,which are the main secondary metabolites in tea leaves under low temperature stress,to assess the effects of selenium application on the quality of tea leaves under low temperature stress.The experimental results were as follows.Different concentrations(0.5,1,2,4 and 6 mg/L)of exogenous selenium treatment groups increased the relative water content of tea leaves under low temperature stress by 14.88%,36.97%,46.73%,26.36% and 8.06%,respectively,compared with the no-selenium control group,with the best exogenous selenium concentration of 2.0 mg/L.The content of MDA in the selenium application treatment group was significantly lower than that in the no-selenium application control group,decreasing by 19.28%,36.35%,22.1%,51.33%,and 19.14%,respectively.The exogenous selenium application improved the scorched buds and purple-brown leaves of tea plants under low temperature,and the photosynthetic rate(Fv/Fm)was also improved compared with the group without selenium treatment.The optimum selenium application concentration of 2.0 mg/L was selected under a comprehensive comparison.The 2.0 mg/L exogenous selenium treatment increased the net photosynthetic rate of leaves under low temperature stress by 10.63% compared with the non-selenium group,alleviating the effects of low temperature stress.2.0 mg/L exogenous selenium reduced malondialdehyde and hydrogen peroxide by 31.59% and23.94%,respectively,stabilizing plant photosynthesis and membrane system and improving the resistance of tea plants to low temperature stress.The plant photosynthesis and membrane system were stabilized and the resistance to low temperature stress was improved.The exogenous selenium treatment also reduced the proline content by 34.64 % and increased the activities of superoxide dismutase,peroxidase,ascorbate peroxidase and catalase by 30.41%,34.44%,44.83%,and41.99%,respectively,in the selenium-applied group compared with the non-selenium-applied group under low temperature stress,which alleviated the oxidative damage caused by low temperature stress in tea trees.In addition,metabolomic changes in tea tree metabolites under low temperature stress revealed that exogenous administration of selenium mainly increased the content of compounds such as assamicain C,gallocatechin 3-o-gallate,robinetinidol-4α-olisorhapontin,and decreased epigallocatechin epigallocatechin-(4 β8)-epicatechin 3-O-gallate and its oxidation products theaflavin-3-gallate,theaflavin digallate,and other polyphenol oxidation products.Meanwhile,the levels of L-phenylalanine,L-lysine and L-glutamic acid were significantly down-regulated in tea plants under low temperature stress.In addition,the upstream substances synthesized by L-lysine,namely citric acid,L-aspartic acid and diamino-pimelic acid,were up-regulated.The application of exogenous selenium significantly improved this situation and increased the synthesis of L-phenylalanine,L-lysine,L-glutamic acid and L-arginine in tea plants.It was verified by further experiments that exogenous selenium treatment could regulate the changes of major secondary metabolites in tea under low temperature stress,including reducing the oxidation products of polyphenols,increasing the content of major polyphenols,with EGCG increasing by 15.11% and EGC increasing by 23.73%,increasing the accumulation of sugars,with the content of soluble sugars increasing by 36.03%,promoting the synthesis of various amino acids in tea,with the free amino acid content increased by 16.3%,thus improving tea quality.