Physiological Effects Of Exogenous Melatonin In Alleviating Drought Stress In Garden Chrysanthemum "Xuanqiu Ninghong"

Drought stress affects plant growth and development,photosynthesis,oxidative stress,water dissipation,osmoregulation,hormone metabolism and signal transduction to varying degrees,requiring a variety of adaptation and mitigation strategies to cope with drought stress.And melatonin(MT),as a highly efficient and low-toxic growth regulator,is considered to be associated with a variety of adversity stresses.As an important worldwide horticultural crop,drought stress in chrysanthemum limits its ornamental quality and landscape applications.In the present study,we used PEG 6000 to simulate drought stress on the seedlings of chrysanthemum "Xuanqiu ninghong" as the test material,and analyzed the effects of exogenous MT on the growth phenotype,physiology and endogenous hormones of chrysanthemum seedlings under drought stress to reveal the physiological regulation mechanism of exogenous MT on chrysanthemum seedlings under drought stress.The main findings were as follows:1.PEG stress had a certain inhibitory effect on the growth of chrysanthemum seedlings in all cases.Among them,the seedlings treated with 25%PEG showed permanent death,while the inhibitory effect of 20%PEG treated chrysanthemum seedlings was significant and did not seriously affect their growth,so 20%PEG was determined as the subsequent simulated drought stress concentration for this study.Meanwhile,exogenous addition of melatonin at different concentrations could alleviate the growth inhibition of chrysanthemum seedlings by drought stress,but the best alleviation effect was achieved by spraying 100 μmol·L-1 MT.2.Under drought stress,chlorophyll content(Chla,Chlb and total Chl content),relative leaf water content(RWC),biomass(aboveground biomass,belowground biomass and total biomass)and root vigor of chrysanthemum seedlings were significantly reduced by 40.38%,29.63%,36.26%,21.22%,38.33%,17.20%,32.17%and 38.74%,while exogenous spraying of melatonin increased Ch1(67.74%,42.11%,56.90%),RWC(26.94%),biomass accumulation(62.15%,20.78%and 47.43%),and root vigor(63.24%)of seedlings under drought stress;reactive oxygen content increased and cell membrane integrity decreased under drought stress,and The exogenous suitable concentration of MT increased the antioxidant system under drought stress,increased reactive oxygen scavenging substances in chrysanthemum leaves,decreased hydrogen peroxide(H2O2),relative electrical conductivity(REC)and malondialdehyde(MDA)and pro line(Pro)contents(38.24%,18.46%,37.96%,20.37%),and soluble protein(SP)and soluble sugar(SS)contents increased by 1.10 and 1.44 times,respectively.It eventually alleviated the inhibitory effect of drought stress on the growth of chrysanthemum seedlings.3.Exogenous spraying of MT improved the photosynthetic performance of chrysanthemum seedling leaves under drought stress.Drought stress reduced the net photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr)and water use efficiency(WUE)of chrysanthemum seedling leaves by 38.46%,26.52%,18.07%and 9.50%,respectively;the maximum light energy conversion efficiency(Fv/Fm),photochemical burst coefficient(qP),potential photochemical efficiency(Fv/Fo)of PSII,reduced apparent electron transfer rate(ETR),and maximum fluorescence yield(Fm)were significantly decreased by 20.97%,32.00%,18.14%,27.09%,and 14.95%,and photosynthesis was inhibited.Under MT+PEG treatment,the values of all fluorescence parameters showed significant increases except for the non-photochemical burst coefficient(NPQ).The above results indicated that exogenous MT could alleviate the effects of drought stress on photosynthetic mechanisms and thus promote seedling growth.4.Drought stress decreased the contents of growth hormone(IAA),gibberellin(GA),cytokinin(CTK),salicylic acid(SA),and endogenous melatonin(MT),and significantly and significantly increased the contents of abscisic acid(ABA),ethylene(ETH),and jasmonic acid(JA)in leaves of chrysanthemum seedlings compared with CK treatment.In contrast,exogenous spray melatonin treatment increased IAA,GA,CTK,SA,and MT contents and decreased ABA,ETH,and JA contents in leaves of chrysanthemum seedlings under drought stress,thus enhancing the ability of chrysanthemum seedlings to resist drought stress.