Alleviation Of Exogenous Melatonin On Grape Leaves Under O₃ Stress

In recent years,the concentration of Ozone（O₃）in the atmosphere has been increasing,and O₃ has become the main pollutant under the clear sky in summer.O₃ has the strong oxidability,o₃ entrance the leaves through the stomata,close the stomata,damage the leaf surface and reduce the photosynthesis,and ultimately results in the reduction of plant growth accompanied by yield reduction.Melatonin play an important role in plant resistance for abiotic stresses.‘Cabernet Sauvignon’grapes were used as materials to study the effect of melatonin under O₃ stress.The results are as follows:（1）Compared with O₃stress,melatonin treatment significantly increased chlorophyll content,net photosynthesis rate（P_n）,stomatal conductance（G_s）and transpiration rate（T_r）;the photosynthetic carbon assimilation was improved,which was manifested by the increase of Rubisco activity and electron flux for photosynthetic carbon reduction（Je（PCR））by 10.44%and 39.56%;the photorespiration was improved,which was manifested by the increase of photorespiration rate and electron flux for photorespiratory carbon oxidation（Je（PCO））by 48.65%and 71.33%.Compared with O₃stress,melatonin treatment also significantly improved starch content and activity of soluble acid invertase by 140.25%and 87.50%,reduced the content of reducing sugar,sucrose,total sugar,sucrose synthase（synthesis direction）（SS-Ⅱ）and sucrose phosphate synthase（SPS）by 4.50%,17.63%,10.64%,47.79%and 9.44%.（2）The inhibition of PSI and PSⅡ on the donor side and acceptor side after ozone stress was alleviated by melatonin treatment,which was manifested by the increase maximal photochemical efficiency of photosystemⅡ（Fv/F_m）and actual PSⅡ efficiency（Y（Ⅱ））by 21.78%and 100.03%,reduced the quantum yield of non-regulated energy dissipation of PSⅡ（Y（NO））and the quantum yield of regulated energy dissipation of PSⅡ（Y（NPQ））by 13.17%and 5.42%;increased net photosynthetic rate（P_m）and actual PSI efficiency（Y（I））by 37.92%and 8.44%,reduced the acceptor side limitation of PSI（Y（NA））by 9.82%.Compared with O₃stress,melatonin treatment reducedβ/α-1 and non-photochemical quenching（NPQ）by 30.97%and 29.45%,reduced the imbalance between photosystem I（PSI）and photosystem Ⅱ（PSⅡ）.D1protein,xanthophyll cycle and cyclic electron flow pathways after stress involved in relief O₃ stress,adding of D1 protein inhibitor Lin,xanthophyll cycle inhibitor DTT and cyclic electron flow inhibitor MV significantly reduced F_v/F_m and F_v’/F_m’after ozone stress,increased NPQ.Melatonin treatment alleviated ozone stress by increasing D1 protein,lutein cycle and cyclic electron transfer,and alleviated the F_v/F_m and F_v’/F_m’decrease caused by Lin,DTT and MV treatment after ozone stress.（3）Melatonin treatment significantly reduced the production of reactive oxygen species in grape leaves under ozone stress,increased the content of antioxidant enzymes and antioxidants in leaves.Compared with O₃ stress,melatonin treatment significantly reduced the H₂O₂ content and O₂^-.production rate by 13.85%and 52.27%,increased the activity of SOD,POD,CAT,APX and GR,significantly increased the content of ASA,GSH,GSSG and GSH+GSSG by 28.06%,9.63%,11.92%and 24.10%,significantly reduced GSH/GSSG by 24.10%.Compared with control,the difference between GSSG and GSH/GSSG was no significant.（4）Compared with O₃stress,melatonin treatment reduced ethylene release rate and gene expression related to ethylene synthesis pathway of grape leaves.Both melatonin treatment and ethylene inhibitor can alleviate ozone stress,and melatonin has a significant effect.Ethephon increased ozone stress.Which was manifested by melatonin and 1-MCP reduced the H₂O₂content and O₂^-.production rate after O₃ stress,increased Chlorophyll content and Fv/F_m.Compared with O₃ stress,ethrel treatment increased the leaf blade damage,the H₂O₂ content and O₂^-.production rate,reduced Chlorophyll content and Fv/F_m,but ethre and ethylene inhibitor 1-MCP had little effects on melatonin levels.