Effects Of Drought And Low Light Stress On The Growth Of Fine Roots And Physiological Characteristics Of Seedlings Of Three

Fine roots(root diameter ≤ 2 mm)are an important part of the material and energy cycle in forest ecosystems.They have a huge absorption surface area and are the main organ for plants to absorb water and nutrients.Fine roots are very sensitive to environmental changes,and there are great differences in biomass,morphological structure and physiological and biochemical of fine roots in different habitats.At the same time,the risk of drought in subtropical forests will increase sharply in this century,seriously hindering the growth and development of forest trees.In the process of forest succession,light intensity is the main factor affecting the structure and function of forest.But so far,the responses of fine roots to drought and low light stress are mostly concentrated in the tropics and temperate regions,and the research objects are mostly herbs.This limits the comprehensive understanding of the response of fine roots of woody plants to external environmental stresses in subtropical regions.To this end,this study took Juniperus formosana Hayata,Quercus variabilis Bl.and Phoebe zhennan S.Lee as typical subtropical afforestation tree species.,According to the randomized block experimental design,including two factors of light and moisture,the light intensity is divided into 3 levels,namely normal light(L1),50% light(L2),and 25% light(L3);the water gradient is divided into 3 levels.The levels are normal water content(75-85% field capacity)(W1),moderate drought(45-55% field capacity)(W2),severe drought(15-25% field capacity)(W3),carried out potted experiments in greenhouses,and adopted the regular harvesting method.Biomass,morphological structure,physiological indicators and nutrient characteristics of fine roots under different treatment conditions in spring(Early April),summer(Early July)and autumn(Early October),and compared the response differences of fine roots of three afforestation tree species to changes in water and light,in order to further explore the adaptation mechanism of fine roots of J.formosana,Q.variabilis and P.zhennan to water and light and the physiological mechanism under adversity.The results show:1.Under drought conditions,the fine root biomass of the three tree species was inhibited,but the R/S was higher than the normal water content.It shows that seedlings can adapt to drought stress conditions by reducing biomass and allocating relatively more biomass to the underground part to enhance the absorption of water and nutrients.Under low light conditions,the biomass of fine roots of J.formosana and P.zhennan,decrease while the biomass of Q.variabilis increased.The R/S of J.formosana and Q.variabilis was negatively correlated with the degree of shading,while that of P.zhennan was positively correlated with the degree of shading.Under the dual stress of drought and low light,shading treatment could alleviate the water deficit of J.formosana and P.zhennan under drought stress,and the biomass and R/S of fine roots increased significantly compared with that under single drought stress.2.Under soil water stress,the RD of J.formosana and Q.variabilis increased,SRL and SRA decreased,and RTD increased.Under drought stress,P.zhennan will adapt to the adversity environment by reducing RD,increasing SRL and SRA,and increasing RTD.Under low light stress,RD of J.formosana increased,SRL and SRA increased,RD of Q.variabilis and P.zhennan increased,and SRL and SRA decreased.Low light stress reduced J.formosana and P.zhennan,and increased the RTD of Q.variabilis,J.formosana RD,SRL,SRA,Q.variabilis and P.zhennan RD were all affected by interaction.Light shading would alleviate the decreasing trend of RD under drought conditions,but heavy shading would exacerbate the effect of drought on the morphological structure of plant fine roots.It showed that the fine roots of the three tree species had obvious morphological plasticity under different water and light gradients.3.Under mild drought stress,the content of MDA and osmotic adjustment substances of the three tree species were significantly increased,and the SOD of the fine roots of the three tree species was higher in normal water.Under low light stress,the MDA content of the three tree species decreased,and the SOD showed a trend of first increase and then decrease.Shading significantly increased the SP and Pro content of fine roots,but significantly decreased the SS content.The MDA,SOD and SP of J.formosana SP and P.zhennan were affected by the interaction between drought and weak light.Appropriate shading under drought conditions can significantly increase the content of osmotic regulators in fine roots,thereby enhancing the ability of plants to adapt to adverse conditions.4.Under drought stress,the carbon and N contents of fine roots of J.formosana and P.zhennan were first decreased and then increased with the degree of stress,while the carbon and N contents of Q.variabilis were negatively correlated.The total P of fine roots of J.formosana increased first and then decreased with the increase of drought stress,and the total P of fine roots of Q.variabilis and P.zhennan was negatively correlated with the degree of drought.The fine root C:N of each tree species was the largest in the normal treatment,and drought would increase the fine root C:P and N:P.Shading can promote the accumulation of carbon and N content in the fine roots of plants,and also increase the P content in the fine roots of Q.variabilis and P.zhennan,but it will reduce the P content in the fine roots of J.formosana.Shading significantly reduced C:N of the three tree species and was most significant in mild drought.J.formosana C:P was negatively correlated with the degree of low light stress,P.zhennan was positively correlated,and Q.variabilis showed a trend of first increasing and then decreasing.Different shading conditions can promote the increase of N:P in fine roots,and the most obvious in mild drought.J.formosana N,Q.variabilis N,P,C:N,C:P,N:P,and P.zhennan P were all affected by the interaction between drought and weak light.It can be seen that the effects of drought and low light on plant survival strategies changed the biomass,morphological structure,physiological indicators and nutrient characteristics of fine roots,thus affecting the environmental adaptability of plants.Different plants have different response mechanisms to different degrees of environmental stress,and the interaction between drought and weak light also has different effects on fine root parameters of different plants.Therefore,further understanding of the change characteristics of fine roots under drought and low light conditions is of great significance to reveal the response mechanism of fine root structure and function to different habitats and adaptation strategies to the environment.