Coupling Response Mechanism Of Leaf Structure And Leaf Physiology Of Two Poplar Varieties To Drought Stress

Against a backdrop of increasing global drought,the research on the physiological and ecological mechanism of drought-resistant species and varieties in the semi-arid region of the loess plateau is the basis for improving the management level of plantation and forest productivity.Populus are widely used for vegetation construction and restoration and artificial timber forest in the semi-arid area of loess plateau,due to the advantages of fast-growing and strong adaptability.Its abundant genetic variation and plasticity to the environment bring about a variety of physiological,structural and growth response strategies to drought among different varieties.However,there are still a lot of controversy about the correlation and interaction among plant physiological,structural and growth characteristics under drought stress,the relationship between plant carbon,nitrogen and water resources remains to be systematically clarified,the drought resistance mechanism of different poplar varieties under different degrees of drought stress still needs to be further studied.In this study,Populus × popularis 35-44(P.QZ.)and Populus × euramericana cv ‘BYu'(P.BY.)were used as experimental materials,the pot experiment was conducted to simulate soil drought stress,the photosynthetic characteristics,water utilization and transport,the regulation mode of leaf morphological structure and the strategies for the utilization of leaf nutrients of two poplar varieties were compared,the influence of leaf physiological and leaf structural strategies on growth and drought resistance were analyzed,the drought resistance approaches and main mechanisms of two poplars were comprehensively evaluated,the key indexes affecting the drought resistance of poplar were screened out,the relationship between carbon,nitrogen and water resources was clarified,which is significant for the study of the drought resistance physiology of poplar in the semi-arid region of the loess plateau,and could provide scientific basis for breeding and cloning poplar varieties with strong drought resistance,furthermore has practical significance to improve the economic and ecological benefits of poplar plantation.The main results are as follows:1.Compared with P.BY.,P.QZ.had stronger photosynthetic performance,water utilization and transport ability however sensitivity to drought;had higher leaf nutrient content with lower use efficiency;had smaller leaf vein density and bigger stomatal density,which could maximize the photosynthetic carbon sequestration capacity and reduce the carbon investment cost for vein construction;finally increased productivity by prioritizing the allocation of photosynthetic products to growth.Compared with P.QZ.,P.BY had high transpiration and water consumption with low water use capacity;had the advantage of photosynthetic drought-tolerant,had lower leaf nutrient content while drought resistant of leaf nutrient use efficiency,had bigger leaf vein density and smaller stomatal density couldmaximize water transport and minimize water loss;finally had strong ability to acquire resources and adapt to adversity by prioritizing the allocation of photosynthetic products to storage.2.Under drought stress,the relationship was closer between leaf area and leaf vein density of P.BY.than that of P.QZ.,the match of leaf vein density and leaf nitrogen utilization was more active of P.BY.,the configuration of leaf vein density and stomatal density were more efficient of P.BY.,which were the drought-resistant phenotype and mechanism of leaf area adjustment,leaf vein construction,stomatal regulation,leaf nutrient utilization and photosynthetic carbon sequestration,water use,hydraulic transport,water balance and other morphological structure,physiological characteristics to obtain maximum carbon fixation for P.BY..3.There were significant difference about the drought-resistant approach and advantage between P.QZ.and P.BY..P.BY.with stronger drought ability had more active regulation of leaf morphological structure and leaf nitrogen utilization,and had drought tolerant advantages of photosynthesis under severe drought stress,which were beneficial to the coupling of photosynthesis and water transport and the effective accumulation of plant biomass;with weaker drought ability had better photosynthetic and water physiological performance under normal watering but more sensitive under moderate and severe drought stress,and had slower regulation of leaf morphological structure and lower efficiency of leaf nutrition utilization,which were to the disadvantage of maintaining the plant biomass accumulation.4.The most relevant mechanism to poplar drought resistance was leaf nutrition utilization;secondly was plant biomass accumulation with shoot biomass was the key drought-resistant index;next was leaf morphological structure with stomatal density,leaf vein density and leaf area as the representative drought-resistant index,then was leaf photosynthetic and water physiological mechanism with photosynthetic rate was the key index,and the relationship between root hydraulic conductance and drought resistance than leaf hydraulic conductance and drought resistance.5.The balancing and constraints,coupling and trade-off relationship among carbon – nitrogen – water resources of two poplars performed with a constraint relationship of leaf carbon-nitrogen resource between photosynthetic production and utilization strategies of leaf nutrients,a coupling relationship of leaf carbon-water resource between WUE and carbon yield,a restricting relationship between PNUE and WUE.In conclusion,the coupling response mechanism of leaf structure and leaf physiology for P.BY.under drought stress,presented a coupling relationship between carbon and nitrogen,a restrict relationship between carbon and water,and a balanced relationship between nitrogen and water,and the drought-resistant advantage of the active regulation in leaf structure,could provide a starting point for the molecular study of drought resistance.