Impacts Of Drought Stress Under The Growth And Biochemical And Physiological Characteristics Of Albizia Odoratissima Seedlings

Albizia odoratissima is a tall deciduous tree of the genus Albizia in the family Leguminosae,which is a valuable local tree for soil and water conservation in China.In recent years,natural forests of Albizia odoratissima have been seriously damaged by human activities,and their distribution area is shrinking day by day,and the endangered status has become more and more serious.However,no studies on the effects of drought stress on the growth and physiological and biochemical properties of Albizia odoratissima are available at home or abroad.Therefore,in this study,three typical climate zones Leye in Guangxi,Xinping in Yunnan and Pingxiang in Guangxi（GM,XP and PX）of Albizia odoratissima annual seedlings were used as experimental materials.The growth,leaf structure,physiological and biochemical characteristics of Albizia odoratissima seedlings from different provenances under natural drought stress were compared and analyzed by indoor pot cultivation method.At the same time,the drought resistance of Albizia odoratissima was evaluated comprehensively to explore the drought resistance mechanism.And the good drought resistance provenance was preliminarily screened,which provided scientific basis for the breeding and large-scale planting of Albizia odoratissima.（1）Under natural drought stress,the soil water content of Albizia odoratissima seedlings showed a significant decrease（P<0.05）,which was lower than that of the control group,and the decreasing range was as follows:GM<XP<PX.On the 5th day of natural drought,a small number of GM and XP seedlings showed the phenomenon of terminal bud collapse,among which,the branches and leaves of PX seedlings drooped,the leaves were yellowing,and compound leaves fell off.In 10 d,GM and XP seedlings terminal buds were wilting and drooping,and a few leaves were yellowing or wilting,while PX seedlings was already partially dead,dying after 10 days（soil moisture content of 1.87%）.GM and XP seedlings died after 20 days（soil water content of 3.52%and 5.43%）,respectively.The phenotype score size was GM>XP>PX.（2）Under natural drought stress,Compared with CK,PX seedling stem diameter and height decreased the most in the three provenances.The fresh weight of total biomass,aboveground part biomass（Stem,branch and leaf）and belowground biomass（root）showed a decreasing trend.GM and XP seedlings showed a decrease in root to crown ratio,while PX seedlings showed the opposite trend.Range of variation of height and stem diameter,total biomass,aboveground biomass and belowground biomass of PX seedlings was greater than that GM and XP seedlings in terms of fresh weight and root-shoot ratio.（3）Natural drought stress also resulted in significant changes in leaf structure characteristics of three species of Albizia odoratissima seedlings:the thickness of leaf,spongy tissue,epidermis and palisade tissue became thinner with the increase of drought days,which were all less than that of the CK,and the thickness of the size was GM>XP>PX.The thickness of sponge tissue thickness,Pal/Ps thickness,main vein and xylem thickness of GM and XP seedlings decreased,while PX seedlings（except main vein）increased.The thickness of main veins and xylem of GM seedlings was lower than that of the other two provenances.（4）Under natural drought stress,total chlorophyll content,chlorophyll Pn,Tr,and Gs of seedlings of three species of Albizia odoratissima showed different degrees of decrease（P<0.05）,and the magnitude of decrease was PX>XP>GM.WUE of GM seedlings increased first and then decreased,while WUE of XP and PX seedlings decreased.Ci of GM and XP seedlings increased,while PX seedlings decreased first and then increased.Fo increased,while Fv,Fm/F_O,Fv/Fm and Fv/Fo decreased,and the PSII structure or function of Albizia odoratissimas eedlings was damaged to a certain extent,resulting in the reduction of the capture efficiency of primary light.（5）Natural drought stress also resulted the contents of proline,soluble protein and soluble sugar three Albizia odoratissima species sources were significantly increased（P<0.05）,and the increasing rangeand and content were GM>XP>PX.MDA,POD and CAT activities showed an overall increasing trend to different degrees and SOD decreased.On the whole,SOD activity of GM seedlings showed the least decline,POD and CAT activity showed the rose the most,SOD,MDA and POD showed the last peak and the highest content,XP seedlings were in the middle,PX seedlings were the last.The leaf and relative conductivity increased and the relative water content decreased significantly（P<0.05）,with the magnitude of the decrease being PX>XP>GM.Variability in endogenous hormone content was also observed:all showed different degrees of decrease,and all were significant（P<0.05）overall compared to the control group,and GM seedlings showed the greatest decrease in the content of zeatin nucleoside ZR,indoleacetic acid IAA and gibberellin GA with the smallest content.（6）Principal component analysis and membership function method were used to comprehensively evaluate the drought resistance of Albizia odoratissima seedlings from three origins.The results showed that GM（0.625）>XP（0.543）>PX（0.359）.Based on the results of principal component analysis and stepstep regression method,a mathematical model for evaluating drought resistance of Albizia was established:Y=-535.387+0.068X₁₃-8.443X₁₇+75.896X₃-6.217X₂₁(X₁₃,POD activity;X₁₇,Pro content;X₃,total chlorophyll content;X₂₁,Fv/Fo;R²=0.993,sing=0.000<0.05).