Relationship Between Leaf Vein Network Traits And Leaf Morphology,Photosynthetic Physiological Function Of Heteromorphic Leaves Of Populus Euphratica

Vein is an important transport channel in leaves,which is closely related to the photosynthetic rate,morphogenesis and water transport of leaves.Plant leaves realize water balance and maximum utilization of light energy by adjusting the structural changes and combinations of vein characters and stomatal characteristics in the process of photosynthesis.Taking Populus euphratica at different developmental stages in the same habitat as the research object,this paper studied the differences and correlations of vein network traits,leaf morphology,anatomical structure and photosynthetic physiological function of heterologous leaves of P.euphratica from macro and micro perspectives by using statistical methods,and revealed the mechanism of regulating vein traits on morphological differentiation of heterologous leaves of P.euphratica.This study provided new insights into the evolutionary mechanism of P.euphratica adaptation and ecological strategies for desert arid environment.The results are as follows:（1）During the expansion period of leaf,the morphology and anatomical characters of the four heteromorphic leaves showed an increasing trend,and the vein density at all levels of the bar leaves and lanceolate leaves decreased gradually.Except for the first grade vein,vein density at all levels,vein spacing and midvein diameter of the ovate leaves and serrated broad-ovate leaves increased gradually.（2）With the increase of DBH,the heteromorphic leaves of P.euphratica showed a stronger xerophytic structure,and their drought resistance was as follows:D-type>C-type>B-type>A-type individuals,the leaf length,leaf shape index,spongy tissue thickness and stomatal density of P.euphratica individuals（types A,B,C and D）at different developmental stages decreased gradually,while the leaf width,leaf thickness,leaf area and specific leaf weight,palisade tissue thickness,number of vessels,catheter area and stomatal area gradually increased.Except for the veins of grade 1,the vein density at all levels showed an increasing trend.The significant differences in leaf morphology of heteromorphic leaves at different developmental stages were mainly caused by leaf length,width and thickness on the macro level,and by total vein density,vein spacing and fence tissue thickness on the micro level.（3）Leaf size was represented by leaf circumference and leaf area,and the standardized spindle analysis showed that except for the density of fine veins,the vein density at all levels of linear leaves and lanceolate leaves showed a significant negative correlation with leaf size,while the vein density at all levels of ovate and serrated broad-ovate leaves showed a significant positive correlation with leaf size（the first grade vein was opposite）.There was no correlation between the density of veinlets and leaf size.Heteromorphic leaves distributed in different canopy showed a tradeoff relationship between vein density and leaf size with the change of light conditions.Strip and lanceolate leaves growing in the lower canopy invested more biomass in leaf area in order to absorb more light to increase light energy capture,and the rate of increase of leaf area was greater than that of vein density.The growth rate of 2°vein density,3°vein density,main vein density and total vein density was higher than that of leaf area.In order to improve the utilization rate of water and nutrients,the conservative strategy of thickening leaves and increasing vein density was mainly adopted.（4）The net photosynthetic rate,transpiration rate,stomatal conductivity,intercellular CO₂concentration,water use efficiency and photosynthetic isotropy of the upper crown leaves were significantly higher than those of the lower crown leaves.The leaf area of the serrated broad-oval leaves was large and thick,and the vein density was developed.The anatomical structure of the serrated broad-oval leaves showed stronger xeromorphic structure,which was more suitable for high temperature and strong light environment,followed by the oval leaves.Linear leaves and lanceolate leaves in the lower part of the canopy are better suited to low light.The thickness of palisade tissue,the number of ducts,the area of ducts,the ratio of gate to sea,and the area of stomata increased,while the thickness of spongy tissue and stomatal density decreased gradually,which was conducive to improving the access and maximum utilization of limited resources.（5）The vein density of P.euphratica heterophylla was closely related to leaf morphology,anatomical structure and photosynthetic physiological function.The higher the vein density,the higher the water use efficiency;The higher stomatal density,the smaller stomatal area,the weaker photosynthetic capacity and the worse drought resistance.Through the change of leaf shape,the leaf structure and stomatal characteristics of P.euphratica were changed,and the heteromorphic leaves in different positions of the crown were separated to adjust the balance of water and nutrient supply and demand and adapt to the arid desert environment...