Effects Of Melatonin On Physiological Metabolism Of Tea Plants Under Temperature Stress

China is the origin of the tea plant and owns the world’s largest tea garden.At present,with the global consistent warming,the frequent appearing of extreme weather such as“cold spell in later spring”and high temperature and drought,it has made a very significant impact on tea production.Therefore,it is of great significance to study the cultivation and control measures to improve the ability of tea trees to resist extreme temperature stress.Melatonin,also known as pineal voxel,was first discovered in plants in 1995.In addition to regulating plant growth,Melatonin also exerts multiple physiological functions in plants,which includes playing an important role in plant resistant reaction to adversity stress such as temperature stress.In this paper,we use the variety of tee tree"Longjing 43"as experimental materials,and the melatonin content of different tissues of tea tree was determined by High Performance Liquid Chromatography-Mass Spectrometry（HPLC-MS）.After spraying melatonin on the surface of the tea seedlings,the seedlings were treated with low temperature and high temperature sequentially,and then the effects of melatonin on the photosynthetic system and antioxidant system of tea plants under low temperature conditions as well as the effect of melatonin on quality and metabolic physiology of tea trees under high temperature conditions were studied with the methods of gas exchange measurement,photosynthetic fluorescence measurement,antioxidant enzyme activity analysis and gene expression analysis.The conclusions are as follows:1.The difference about content of melatonin in every organ of tea tree is significant,and the distribution character of the content is:fruit>stem>root>leaf>flower.It’s in the fruit the content of melatonin is the most abundant,up to 0.57 ng·g^-1,but it is only 0.026 ng·g^-1 in flowers.The expression levels of melatonin synthetase genes in different organs of tea plants are also different.We screened two candidate genes of serotonin N-acetyltransferase（SNAT）and three candidate genes of N-acetylserotonin methyltransferase（ASMT）in tea plants,and a candidate gene respective for each of tryptophan decarboxylase（TDC）,tryptamine 5-hydroxylase（T5H）,and catechol-O-methyltransferase（COMT）.Candidate genes for SNAT and ASMT in tea plants were highly expressed in tea leaves,while the remaining synthetase candidate genes were expressed most in roots.2.Melatonin can maintain the normal operation of the photosynthetic system and antioxidant system inside the tea trees under low temperature conditions,and improve the resistance of tea trees to low temperature.After the tea tree is treated by melatonin,the maximum photochemical efficiency（Fv/Fm）of tea tree was increased under low temperature conditions,and the net photosynthetic rate（Pn）of tea tree also increased significantly.Melatonin treatment has also significantly increased the activity and the expression degree of the related genes of some antioxidant enzymes inside the tree,like ascorbate peroxidase（APX）,superoxide dismutase（SOD）,catalase（CAT）and glutathione reductase（GR）under low temperature conditions.The total content of glutathione（GSH+GSSG）and the ratio of GSH/GSSG in tea tree have also both increased under melatonin treatment,while the contents of hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）have decreased significantly.It can prove that melatonin has the functions to increase the efficiency of antioxidant system of tea trees under low temperature stress and reduce the content of reactive oxygen species（ROS）.3.Melatonin will exert a significant effect on the quality components of tea under high temperature conditions.The melatonin treatment can increase the content of theanine,caffeine and tea polyphenols under high temperature conditions.According to the result of analysis on components of catechin,we can find that melatonin treatment can increase the content of catechin components such as EGCG,GC and EGC in the tea plant.Correspondingly,melatonin can increase the expression of genes related to theanine synthesis including CsGOGAT and CsTS1;the expression level of CsANR,CsF3H,CsDFR,CsCHS,CsCHI and CsLAR that related to catechin synthesis is also improved;The expression levels of CsTIDH and CsTCS1 were also increased too.