Growth And Physiological Responses Of Rhododendron Delavayi Seedlings To Soil Drought And Acid-base Stres

Soil provides plants with the necessary water and nutrients for their growth and development.With global warming,population growth,and economic development,human exploitation and utilization of natural resources have led to severe degradation of land resources.Drought and acid-base stress have become important ecological issues that limit the development and utilization of plant resources and the sustainable development of human society.The Baili Rhododendron Reserve in Guizhou is an important rhododendron germplasm resource bank in China and even the world.In recent years,with the intensification of climate change,scenic area construction,and human interference,the soil environment of the scenic area has been threatened from various aspects.Rhododendron delavayi,as the main constructive species of the Hundred Mile Rhododendron,has a shallow root system and is sensitive to changes in soil environment.In some areas,unknown causes of death occur,seriously threatening the ornamental and ecological functions of the scenic area.It is urgent to investigate the response of the growth of Rhododendron delavayi to soil p H and drought stress gradient,in order to lay the foundation for the protection and utilization of important germplasm resources of Rhododendron delavayi.Therefore,in this study,two-year seedlings of Rhododendron delavayi were used as test materials,and 80% of Field capacity was used as control(CK).The soil moisture content of other treatments was 60%(FC60),40%(FC40),30%(FC30),20%(FC20),and 10%(FC10)of Field capacity,respectively.There were six soil moisture gradients in total;Six acid-base gradients with soil p H=3,4,5,6,7,and 8 were used for drought and acid-base stress experiments.The main research results of measuring the growth indicators,physiological indicators,and photosynthetic indicators of plants during stress are as follows:1 The effect of drought stress on Rhododendron delavayi.(1)Both higher and lower soil moisture content are not conducive to the growth of relative plant height,bud length,new leaf number,total leaf number,and the formation of plant water retention capacity of Rhododendron delavayi.Except for the relative moisture content,the rest reached the highest level under FC60 treatment;The increasing drought significantly suppressed the relative ground diameter,single leaf area,leaf length,leaf width,and leaf thickness;The drought has accelerated the withering of old leaves.In addition,excessive drought can also inhibit the growth of roots and aboveground parts,and Rhododendron delavayi can resist a certain range of drought stress by increasing the root to shoot ratio.(2)As the degree of drought increases,soluble proteins and soluble sugars in the early and middle stages of stress first decrease and then increase;The soluble sugar gradually increases in the later stage;Proline first increases and then decreases.The osmotic regulatory effect of soluble proteins in the early stage of stress is stronger than that in the middle and late stages;In the later stage,soluble sugars play a dominant role in osmotic regulation;Under mild to moderate drought,the permeability of proline in the late stage of stress is stronger than that in the early stage,while under severe drought,the permeability in the early stage is stronger than that in the later stage.Overweight stress at earlier stage inhibited the activity of Malondialdehyde(MDA)synthase.The activities of Superoxide dismutase(SOD),Catalase(CAT)and peroxidase(POD)increased first and then decreased.Both higher and lower soil moisture levels continue to promote an increase in SOD activity;CAT activity showed an increasing trend with the prolongation of stress duration;POD activity only gradually increased with the prolongation of stress duration under moderate drought.(3)Severe drought inhibited the synthesis of chlorophyll a and total chlorophyll,and Chlorophyll b was continuously inhibited in the early and late stages;Drought stress increased the concentration of Lutein.Under mild drought treatment,both chlorophyll a and total chlorophyll gradually increased with the prolongation of stress duration;Chlorophyll b and Lutein in the late stress stage were lower than those in the early stress stage.The net photosynthetic rate(Pn),transpiration rate(Tr),and mid-term stomatal conductance(Gs)all increase and then decrease with the increase of drought severity;During the early and late stages of stress,Gs decreased continuously with the increase of drought severity;The intercellular CO2concentration(Ci)first decreases and then increases.Pn and Gs have their maximum values in the middle stage of stress,while Ci shows an increasing trend with the prolongation of stress duration in environments with high and low soil moisture content(CK);Tr gradually decreases with the prolongation of stress duration in lower soil moisture content.2 The effect of acid-base stress on Rhododendron delavayi.(1)The relative plant height,relative ground diameter,bud length,number of new leaves,leaf length,leaf width,leaf thickness,single leaf area,total leaf number,relative leaf water content,root biomass,and aboveground biomass all increase and then decrease with the increase of soil p H,and all indicators reach their highest water level at p H 4 and 5;The lowest levels were all at p H 8 treatment.As the p H increases,the number of fallen leaves first decreases and then increases,with a minimum value at p H 4;The root to shoot ratio fluctuates.(2)As p H increases,soluble protein,soluble sugar,and proline overall decrease first and then increase in the later stages of stress.As the duration of stress prolonged,the soluble protein stress in the later stage was generally higher than that in the earlier stage;Soluble sugars gradually increase;Proline gradually increased with only p H 4treatment.MDA,SOD,CAT,and POD activity decreased first and then increased during the later stages of stress.As the duration of stress prolonged,MDA first decreased and then increased;The SOD activity of the treatment with p H 6 was lower in the late stage of stress than in the early stage of stress;The POD activity of the treatment with p H 3-7 was lower in the later stage of stress than in the earlier stage,while the treatment with p H 8 gradually increased with the prolongation of stress duration.(3)Chlorophyll a,chlorophyll b,total chlorophyll,and lutein all increased first and then decreased with the increase of p H during the middle and late stages of stress;At the early stage of stress,Lutein gradually increased with the increase of p H.With the prolongation of stress duration,the photosynthetic pigments under p H 5 and 6treatments showed an increasing trend;Chlorophyll b increased gradually at p H 6,7and 8;At p H 5 and 6,Lutein in the late stage of stress was greater than that in the early and middle stages.With the increase of p H,Pn,Gs,Tr and Ci at different stress stages first increased and then decreased;During the early and late stages of stress,Ci decreased first and then increased.The overall performance of Pn,Gs,Ci,and Tr under different p H treatments reached their highest levels in the middle stage of stress;The highest levels of Pn,Gs,and Tr in the middle stage of stress were higher under p H 4 treatment than other treatments.