Effects Of Tea Polyphenols On Photosynthetic Characteristics And Antioxidant Responses In Wheat Seedling Leaves Under Salt Stress

Soil salinization is a global resource and ecological issue,and it has become one of the major factors limiting agricultural production in China.Salt stress affects almost all important physiological processes of plants.Photosynthesis is the basis of plant growth and development.It provides the necessary materials and energy for the growth and development of plants.Therefore,the change of photosynthesis will affect the growth,yield and quality of crops.Salt stress led to the accumulation of reactive oxygen species(RDS)in plants,which can lead to the degradation of proteins,DNA,carbohydrates and other biological molecules,thereby interfering with the normal physiological activities of cells.In this experiment,"Longchun 30" wheat was used to study the effects of tea polyphenols(TP)on the growth and physiological and biochemical characteristics of wheat seedlings under salt stress,in order to further explore the physiological and biochemical characteristics of TP under salt stress.The main results are as follows:1.150 mmol·L-1 NaCl treatment inhibited the growth of wheat seedling stems,reduced the relative water content,chlorophyll a,chlorophyll b and total chlorophyll contents and increased the chlorophyll a/b value of the leaves.Two concentrations of TP alleviated the inhibition of salt stress on the growth of wheat stems,and rose the chlorophyll a,chlorophyll b and total chlorophyll contents in wheat leaves under salt stress.2.Salt stress treatments alone reduced actual photochemical efficiency of PSII(Y(?))and electron transport rate(ETR),but increased non photochemical quenching coefficient(NPQ).The addition of TP increased Y(?),but lowered NPQ in leaves of wheat seedlings under salt treatment.In addition,high concentration of TP also rose qP and ETR.3.NaCl treatment alone induced the decrease of net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs)and intercellular carbon dioxide concentration(Ci)in wheat seedlings.The addition of 25 mg·L-1 TP resulted in a further decrease of Pn and Tr and increased Gs in wheat leaves exposed to NaCl treatment;the addition of 100 mg·L-1 TP led to a further decrease of Pn.4.NaCl treatment increased superoxide anion(O2·-),hydrogen peroxide(H2O2)and hydroxyl radical(OH)contents.and the content of O2·-reached the maximum at 2 days.100 mg·L-1 TP + NaCl reduced O2·-content on the 4th and 6th days.The addition of TP lowered H2O2 in wheat leaves exposed to NaCl treatment.Except that 100 mg·L-1 TP slightly increased OH content at the 4th day under salt treatment,TP treatment reduced the accumulation of ·OH.5.The activities of ascorbate peroxidase(APX)and glutathione reductase(GR)were increased in wheat leaves exposed to NaCl treatment under different treatment time.Salt stress merely increased superoxide dismutase(SOD)activity at 6 days,while catalase(CAT)and Peroxidase(POD)activities were inhibited.Different concentrations of TP enhanced POD activity,but inhibited APX and GR activities in leaves of wheat seedlings under salt treatment;high concentration of TP inhibited CAT activity.6.Treatment with salt alone increased cell wall bound POD(Cw-POD)activity at 2 d and 4 d.Diamine oxidase(DAO)and polyamine oxidase(PAO)activities were increased under salt treatment at 4 d and 6 d.The addition of two concentrations of TP inhibited Cw-POD activity on salt stress at 4 d and 6 d.The addition of low concentration of TP inhibited DAO activity under NaCl treatment,while high concentration of TP inhibited this enzyme activity only at 2 days.The addition of two concentrations of TP inhibited PAO activity under NaCl treatment.In conclusion,our results suggested that salt stress decreased chlorophyll content.Actual photochemical efficiency of PSII,electron transfer efficiency and the ability of PSID to accept electrons were weakened,while the proportion of light energy consumed in the form of heat dissipation was increased.Stomatal limitation also affected the photosynthetic rate of the wheat leaves.Therefore,the decrease of the photosynthetic capacity eventually affected the growth of wheat seedlings.In addition,salt stress induced the activities of Cw-POD,DAO and PAO may participate in the accumulation of H2O2,while the increase of SOD,APX and GR activities alleviated the accumulation of ROS.Low concentration of TP treatment inhibited the length of wheat stems,while the effect of high concentration is opposite to that.This may be related to changes in photosynthetic properties,ROS and the enzyme activity.Adding different concentrations of TP increased chlorophyll content,Actual photochemical efficiency of PSII,the acceptability of PSII to electrons and electron transfer efficiency in leaves of wheat seedlings under salt stress;thence reduced the proportion of excess light energy released through heat dissipation.The photosynthetic ability of wheat seedlings was enhanced,and the inhibitory effect of salt stress on the growth of wheat was alleviated.Moreover,the addition of TP alleviated the accumulation of ROS under salt stress,and inhibited the activities of CAT,APX,GR,Cw-POD,DAO and PAO,but increased POD activity.However,the addition of TP further reduced Pn in leaves of wheat seedlings under salt stress,which requires further study.