Physiological And Ecological Characteristics Of Four Ecological Restoration Tree Species In Wet And Dry Seasons,Jinsha River Dry-hot Valley Of Southwest China

Dry-hot valley in Southwest China with a fragile ecology,Jinsha River dry-hot valley is characterized by hot and dry climate and serious vegetation degradation,making it difficult to rehabilitate former ecosystem structure and functioning.In order to more fully and truly understand the growth adaptabilities of the main ecological restoration tree species in this restored area,the study focuses on four typical such tree species,namely Leucaena leucocephala,Dodonaea viscosa,Trema tomentosa and Jatropha curcas in this region.Water physiological characteristics,anatomical characteristics,autioxidant enzyme activities,osmotic substances,photosynthetic physiological characteristics and chlorophyll fluorescence characteristics of the above tree species were measured in dry(November)and wet(July)seasons,respectively.Their changes of physiological and ecological characteristics in each season were analyzed.In addition,the drought resistance and adaptation strategies of different tree species to the dry-hot valley were discussed.Grey correlation analysis method was adopted to comprehensively evaluate the drought resistance and adaptation capabilities of different species.Here is what we found:(1)Water physiology.Compared with the wet season,Relative water content(RWC)of Dodonaea viscosa,Trema tomentosa and Jatropha curcas decreased to different extent in dry season and the leaf water deficit deepened,while Predawn leaf water potential(?pl)and Midday leaf water potential(?ml)of the three tree species correspondingly reduced to improve the water absorption capacity of soil.Leucaena leucocephala could still maintain a relatively high ?pl,?ml and RWC in dry season,indicating that its leaf water physiology was less affected by drought.There were two kinds of stomatal regulation strategies for the four tree species: In dry season,Dodonaea viscosa and Jatropha curcas reduced water loss by declining the Stomatal conductance(Gs)and lowering Transpiration rate(Tr),while Leucaena leucocephala and Trema tomentosa increased the Gs and Tr to enhance gas exchange.Compared with the wet season,Water use efficiency(WUE)of Leucaena leucocephala and Jatropha curcas increased significantly in dry season,while that of Trema tomentosa decreased.Leucaena leucocephala and Dodonaea viscosa adapted themselves to drought by increasing their consumption,so Leaf mass per area(LMA)decreased significantly in dry season compared with the wet season,while as Trema tomentosa and Jatropha curcas needed to increase dry matters to withstand drought,their LMA increased in dry season.WCDI(Weighted comprehensive drought resistance index)of each tree species in water physiology was ranked as Jatropha curcas> Leucaena leucocephala >Dodonaea viscosa>Trema tomentosa.(2)Morphological anatomy.There were three morphological and anatomical adaptation strategies for the four tree species in dry and wet seasons as follows: First,Leucaena leucocephala and Trema tomentosa maintained a relatively high Palisade tissue thickness/Spongy tissue thickness(P/S)and Cell tense ration(CTR)during both dry and wet seasons.In addition,it increased the upper epidermal thickness(UET),Lower epidermal thickness(LET),Palisade tissue thickness(PTT),Spongy tissue thickness(STT)and Leaf thickness(LT)to improve their water storage capacity and reduce water loss of the leaves.Second,Jatropha curcas maintained a relatively high LT in both wet and dry season.In order to reduce water loss of leaves,it changed the composition ratio of mesophyll tissue and increased PTT,P/S and CTR in dry season.As a result,the LT decreased slightly.Third,compared with the wet season,the UET,LET,PTT,STT,LT,PTT,P/S and CTR of Dodonaea viscosa declined markedly in dry season while SR increased significantly.WCDI of each tree species in morphological anatomy was ranked as Trema tomentosa > Leucaena leucocephala > Jatropha curcas > Dodonaea viscosa.(3)Biochemical characteristics.In dry season,Trema tomentosa increased its Superoxide dismutas activity(SOD)and Peroxidase activity(POD)to clear excess Reactive oxygen species(ROS)in leaves to prevent damages to the plasma membrane.Jatropha curcas increased its Proline(Pro)content to maintain the water balance of leaf cells.WCDI of each tree species in leaf biochemical characteristics was ranked as Jatropha curcas > Trema tomentosa > Leucaena leucocephala > Dodonaea viscosa.(4)Photosynthetic physiology.In dry season,Initial carboxylation efficiency(CE),Maximum carboxylation rate(Vcmax),Maximum electron transport rate(Jmax),Triose phosphates utilization rate(TPU),Jmax/Vcmax,Actual photochemical efficiency of PSII(?PSII),Photochemical fluorescence quenching(q P),Electron transport rate(ETR),Photosynthetic capacity(Amax)and Photorespiration rate(Rp)of all the four tree species decreased to different extent,which indicated that they adopted a passive consumption adaptation strategy with decreased photosynthetic enzyme activity and quantities of leaves,carboxylation efficiency and potential photosynthesis capacity,as well as the deeply damaged photosynthetic organs.?PSII and ETR,two indicators of chlorophyll fluorescence parameters,well indicated the photosynthetic capacity of the four ecologically restored tree species in the region.WCDI of each tree species in leaf photosynthetic physiology was ranked as Leucaena leucocephala > Trema tomentosa > Dodonaea viscosa > Jatropha curcas.In summary,the four ecological restoration tree species showed different adaptation strategies in dry-hot valley,and the WCDI in different physiological and ecological aspects were also quite different.Combining with 43 indicators in leaf water physiology,morphological anatomy,biochemical characteristics and photosynthetic physiology,a comprehensive evaluation towards the drought resistance adaptability of four ecological restoration tree species was made.WCDI of each tree species was ranked as Leucaena leucocephala > Trema tomentosa > Jatropha curcas > Dodonaea viscosa.