Mechanical Responses Of Leaf And Root Morphological And Physiological Traits Of Cotinus Coggygria Seedlings To Drought And Barren Soil

Cotinus coggygria is one of the first picks for building landscape recreational forests and water and soil conservation forests in mountainous regions of North China.Beijing is the northern boundary of C.coggygria natural distribution and highly sensitive to environmental changes.In the rocky mountainous areas of Beijing,the current state of soil erosion is critical and the site becomes arid and barren nowadays,thus the growth of C.coggygria trees is greatly weakened by drought and barren soil in Beijing mountainous areas,and the weak trees have greater chances of suffering from tree disease and insect pest attacks.It is a difficult technical problem that should pay close attention to in the forestry production and practice of this region how to improve and maintain C.coggygria forest health and quality.Clarifying the mechanism of trees’adaptation and response to the surrounding environment is an important basis for scientific construction and management of forests.And rules of the morphological and physiological responses of trees to water-deficient and nutrition-deficient soil are the core contents of evaluating the adaptability of plants.Therefore,it is of great theoretical significance and practical value for scientific management and cultivation of C.coggygria forests and individual trees to explore the morphological and physiological response strategies of C.coggygria in arid and barren environments.In this paper,annual C.coggygria seedlings from different locations in North China were selected as test materials.A field experiment design with 2 factors（soil water content and location）was adopted to analyze the plastic changes and trade-offs of leaf and root functional traits of C.coggygria seedlings under drought stress,and to study the response of leaf gas exchange characteristics of C.coggygria seedlings to soil water deficiency.Through the pot experiment in the greenhouse with 3 factors（soil water content,soil barren degree and location）,the anatomical structure,morphological structure and plasma membrane H⁺-ATPase activity of the absorptive fine roots of the first two orders were investigated to reveal the resource absorption characteristics of C.coggygria seedlings in arid and barren soil,and the partition of biomass,non-structural carbon and non-structural reduced nitrogen in leaves and fine roots of the first two orders and nitrogen and water use efficiency were studied to illustrate the resource allocation and utilization pattern,relationships between carbon and nitrogen metabolism and relationships between water use and nitrogen use of C.coggygria seedlings in water and nutrient restricted soil.The results of this study provide reference for stress-resistant location selection and afforestation in difficult sites of C.coggygria.The main findings are as follows:（1）C.coggygria seedlings from Jiang County in Shanxi Province performed well during the cultivation trial in Beijing.Under severe drought stress,the root functional traits and functional traits representing root-leaf relative relation and their plasticity indexes of seedlings from this location were the highest among the studied locations.The fine root length to leaf area ratio,fine root length ratio,total root length to leaf area ratio,total root length ratio,specific fine root length and specific total root length were higher（P<0.001）than those under well-watered condition by 508.33%,389.60%,349.00%,335.42%,221.24%and 174.70%respectively,and their plasticity indexes were 0.450,0.428,0.494,0.483,0.313and 0.287 respectively.All in all,in severe drought environment,seedlings from Jiang County in Shanxi Province actively allocate more photosynthetic products to roots and fine roots,thus improving soil resource absorption efficiency effectively.（2）Plastic changes in leaf and root functional traits and trade-offs between leaf and root resource allocation are significant ecological strategies for C.coggygria seedlings to respond to drought stress.Under drought stress,the change level of seedling plasticity indexes of leaf and root functional traits was:functional traits representing root-leaf relative relation>root functional traits>leaf functional traits,and the plasticity index of fine root length to leaf area ratio was 0.339–the highest among all traits.There were significant negative linear relationships between specific leaf area and total root length to leaf area ratio（r=-0.426,P<0.001）,between specific leaf area and fine root length to leaf area ratio（r=-0.325,P=0.004）,between specific leaf area and total root length ratio（r=-0.307,P=0.007）,between specific leaf area and fine root length ratio（r=-0.302,P=0.008）,between specific leaf area and specific fine root length（r=-0.268,P=0.020）,and between specific leaf area and specific total root length（r=-0.264,P=0.022）.（3）Non-stomatal limitation is a crucial factor constraining the photosynthetic productivity of C.coggygria seedlings under severe drought environment.With the decrease of soil moisture content,the photosynthetic capacity of C.coggygria seedlings decreased significantly,and mesophyll diffusion limitation gradually replaced stomatal limitation as the dominant factor limiting seedling photosynthetic capacity.Overall,on the average,the light-saturated maximum net photosynthetic rate,the CO₂-saturated maximum photosynthetic rate,the maximum net photosynthetic rate limited by Rubisco activity and quantity,and the maximum net photosynthetic rate limited by Ru BP regeneration of the seedlings under severe drought stress were 22.74%,38.21%,43.63%,and 48.24%lower than those under water sufficient environment respectively,while the values of CO₂ stomatal limitation and mesophyll diffusion limitation were 4.61%and 24.57%higher than those under normal water treatment,respectively.The leaf CO₂conductance,nitrogen allocation in photosynthetic system and water status of C.coggygria seedlings were tightly correlated to the leaf CO₂ assimilation potential.A higher mesophyll conductance,a higher proportion of nitrogen allocated to photosynthetic carboxylation components and bioenergetic components,and a higher leaf dry matter content,leaf relative water content and leaf succulence were more favorable for C.coggygria seedlings to exert their potential maximum CO₂ assimilation capacity under drought stress.（4）The growth and development of fine roots of the first two orders determine C.coggygria seedlings’acclimation to drought and barren situation.In arid and barren environments,the adaptive changes of the anatomical structure,morphological structure and plasma membrane H⁺-ATPase activity of fine roots of the first two orders jointly influence the resource absorption capacity of C.coggygria seedlings.In general,the diameter of fine roots of the first two orders decreased,which accelerated fine root uptake rate,and the cortex ratio of fine roots of the first two orders decreased while the vascular tissue to root diameter ratio of fine roots of the first two orders increased,which was beneficial to strengthen the absorption and transportation capacity of the fine roots.Fine roots of the first two orders of the seedlings compensated for the loss of narrow root absorption range caused by stress by developing a higher specific root length and specific root area.The increase of plasma membrane H⁺-ATPase activity in fine roots of the first two orders was helpful to improve the seedling capacity of active absorption of soil resources.When the soil became drought and barren simultaneously,the diameter and the cortex ratio of fine roots of the first two orders went down by 12.04%～32.94%and 4.72%～25.26%respectively,while the vascular tissue to root diameter ratio rose by 8.61%～37.39%.Under the treatment of severe drought stress in sand-soil mixed matrix,the specific root length and specific root area of fine roots of the second order grew by 95.28%（P=0.011）and 47.36%（P=0.006）respectively.Under the treatment of severe drought stress in sand matrix,the plasma membrane H⁺-ATPase activity of fine roots of the first order increased by 17.82%（P=0.010）.（5）It is an important mechanism for C.coggygria seedlings during their adaption to drought and barren conditions to co-allocate carbon and nitrogen resource and balance the resource use efficiency.In arid and barren environments,although the biomass accumulation of fine roots of the first two orders and leaves were inhibited by 26.07%～69.27%,C.coggygria seedlings still could improve stress tolerance by increasing the biomass investment proportion of fine roots of the first two orders by 24.54%～79.12%.Seedlings adopted an active energy storage strategy by increasing the starch storage of fine roots of the first two orders and leaves to defense themselves against adverse environments,and the starch concentration of fine roots of the first two orders and leaves went up by 20.18%～54.78%under the treatments of drought stress in sand matrix.Seedlings increased more non-structural reduced nitrogen input to leaves than fine roots of the first two orders,thus promoting the photosynthesis and carbohydrate transfer,stimulating the morphogenesis of the sink organs,and contributing to the acquisition of soil resources.Seedling nitrogen use efficiency reduced but water use efficiency increased in drought and barren environments.There was relatively more starch produced by nitrogen per unit mass of fine roots of the first two orders and leaves,leading to the enhancement of seedling self-protection ability.The nitrogen use efficiency of fine roots of the first two orders was positively linked to the water use efficiency,while the water use efficiency was increased at the expense of leaf nitrogen use.