The Response Mechanism On Flooding Stress Of Eight Landscape Eco-tree Species Seedlings In The Yangtze River Delta

This study simulates the seasonal occurrence of floods in the Yangtze River Basin,using one-year-old cuttings of 8 species of Taxodium distichum,Quercus nuttallii,and Q.palustris,Malus hupehensis,M.baccata,Nyssa aquatica and Liquidambar formosana as materials.Taking normal water management as control,the dynamic changes of growth,morphology and physiological indexes under flooding treatment were measured.Meanwhile,high-sensitivity and simple and easy-to-measure evaluation indicators were screened,and an early evaluation system for evaluation of waterlogging tolerance of tree species were established.The aims of this study were to study the adaptation mechanism of different types of ornamental tree species under flooding stress and to explore the effects of high temperature and flooding stress on plants in the summer flood season of the Yangtze River Basin,which can provide a theoretical basis for the screening of water-resistant ornamental trees in the Yangtze River Basin,and enrich the biodiversity and landscape diversity of wetlands.The main conclusions are as follows:(1)The survival rate and growth of tree species were significantly affected by flooding.Survival rate ranged from large to small of eight landscape eco-tree species was:T.distichum=M.hupehensis=N.aquatica(100%)>Q.nuttallii>T.'zhongshansha'>L.formosana>Q.palustris>M.baccata(<50%).Under flooding stress,the growth of tree species were not significantly inhibited by flooding(ratio of treatment and control>1,P<0.05),except M.baccata.According to the indexes of survival rate and growth,the ability of waterlogging tolerance of eight landscape eco-tree species were evaluated by fuzzy membership function value method.The tolerance of eight landscape eco-tree species from strong to weak was that T.'zhongshansha'>N.aquatica>T.distichum>M.hupehensis>Q.palustris>Q.nuttallii>L.formosana>M.baccata,and M.baccata was not tolerant to flooding.(2)Plants with stable leaf color had strong tolerance,whereas plants with instable leaf color does not reflect its non-tolerance.During the whole flooding period,the difference of leaf color between T.'zhongshansha'and N.aquatica treatment was smaller than that of CK(?E_{T.'zhongshansha'}<22;?E_N.aquatica=23?28),the results of pigment ratio(Chl/(Anth+Car))were higher than that of CK.However,the difference between treatment and CK of other six species increased with the extension of flooding stress time.Moreover,the leaf color of T.distichum and L.formosana changed to reddish yellow(hue angle h°<90)in the late stage of flooding,and the pigment ratio was less than 1.(3)There were significant differences in physiological responses of different tree species to waterlogging stress.The four tree species,such as the T.distichum,the T.‘zhongshansha',the N.aquatica and M.hupehensis,adapt to the flooding stress mainly through two ways:morphological avoidance and physiological adaptation.Other tree species mainly used physiological adaptation.During the flooding period,the T.distichum,T.‘zhongshansha'and N.aquatica resisted flooding stress through by upregulating IAA and ZR content,which can promote the formation of adventitious roots and alleviates root hypoxia stress in plant morphology.In physiological metabolism,it mainly regulated photosynthetic capacity(P_n,T_r,G_s,and C_i)by up-regulating ABA and JA content,ensuring light energy utilization rate and energy transfer efficiency(Electron transfer probability?_o,quantum yield of electron transfer?E_o and quantum yield of the terminal electron acceptor were significantly higher than that of CK in the early stage of flooding).Meanwhile,these trees increased its Alcohol dehydrogenase activity(ADH)and reduced the accumulation of toxic such as acetaldehyde to ensure the normal operation of life activities under flooding period.The M.hupehensis mainly up-regulated GA₃ content(?GA₃:0?12.85 ng·g^-1·FW)to promote stem elongation and increased the antioxidase activity(?SOD:0?958.53 u·g^-1·FW;?POD:0?10.45 U·min^-1·g^-1·FW)and osmotic adjustment ability(soluble sugar content)to improve its water-resistant ability.The Q.nuttallii and Q.palustris mainly used physiological metabolic pathways,such as increasing ADH activity;The L.formosana mainly used improving physiological metabolism levels such as photosynthetic energy transfer efficiency(The PI_total value of the comprehensive performance parameters in the early stage of flooding was significantly higher than that of CK;the light compensation point(L_CP)in the late stage of flooding was significantly lower than that of CK.).(4)The comprehensive evaluation system of tolerance based on physiological indicators can better evaluate the waterlogging tolerance ability of trees.According to the morphological and physiological indicators,correlation analysis,principal component analysis and membership function method were used to comprehensively evaluate the tolerance of eight landscape eco-tree species.The result showed that the waterlogging tolerance of eight landscape eco-tree species from strong to weak:T.distichum>T.‘zhongshansha'>N.aquatica>M.hupehensis>L.formosana>Q.nuttallii>Q.palustris>M.baccata.There was a very significant correlation(R=0.86,P=0.007)between this evaluation results and the results based on the evaluation of growth indicators.(5)Six photosynthesis and fluorescence indexes(R_d,L_sp,ABS/CS_m,RC/CS_m,TR_o/CS_mand DI_o/CS_m)with significant correlation(P<0.01)with growth and survival rate were screened according to the correlation of growth,morphology and physiological indexes.It can be used as an early evaluation index for evaluation of waterlogging tolerance ability of trees,which provided a reference for the early rapid screening of water-resistant ornamental trees in the Yangtze River Basin.(6)The high temperature and high humidity in summer mainly affected the photosynthesis and fluorescence of eight landscape eco-tree species.The results of correlation analysis among temperature,humidity factor and main functional indicators in waterlogging tolerance evaluation indicated that:The highest temperature was significantly positively correlated with the hue angle h°,and was significantly negatively correlated with the dark respiration rate(R_d)and the light compensation point(L_cp)(P<0.01).Interestly,there were extremely significant between the R_dand the seedling height.Inaddition,The mean temperature and relative humidity were significantly correlated with stomatal conductance and fluorescence parameters RC/CS_m,TRo/CS_m,PI_total and soluble sugar content(P<0.05).