Structure And Hydraulic Conductivity Of The Xylem In Branches Affects The Morphology And Photosynthesis Diversity Of Leaves In Populus Euphratica Oliv.

Populus euphratica Oliv.is a deciduous tree belonging to the genus Populus in the family Salicaceae,which sustains the desert ecosystem and ecological balance.Indeed,P.euphratica is the naturally distributed arbor species in the desert and semi-desert regions of northwestern China.P.euphratica has the characteristics of heterophyll.In order to explore the physiological mechanisms that lead to the morphology and photosynthesis diversity of leaves in P.euphratica,the morphological and anatomical structures of upper(mainly dentate oval leaves)and lower(mainly lanceolate leaves)branches and their leaves were compared by means of paraffin section and optical microscope.Microcomputed tomography(Micro CT)was preliminarily established the technical indexes to scan the fresh branches and leaves of P.euphratica,and the differences in the water-conducting structures of the upper and lower branches were analyzed.The water conductivity(Ks)and the degree of natural embolization(PLC)were measured by HPFM.PMS Model 600 was used to measure the water potential and P-V curves of the branches with leaves.The physiological indices of Pn and Tr in leaves were measured using Li-6400,and the correlation of structural characteristics and water permeability of the two branches on leaf shape and photosynthesis were analyzed,so as to reveal the physiological mechanism of leaf shape change of P.euphratica.The major results were as follows:(1).The morphological features and anatomical differences of upper and lower branches and leaves were analyzed,and the results showed that the upper leaf tended to be xerophytic,with longer petiole,wider leaf width,smaller leaf area and more succulent leaf.upper branch has thicker cuticle and developed vascular bundle sheath structure,the vessel diameter in the upper branches were greater than the lower,and the vessel density in the lower branches was greater than the upper.There are many mucous cells in the upper branch cortex,phloem and pith,which have strong water-retaining ability.Both upper and lower petioles had a large number of parenchyma cells,but the xylem of the upper petiole had a larger diameter and the lower petiole had a smaller diameter.The pith of the upper petiole contains a large amount of parenchyma,and clusters of crystals can be observed in the cortex and phloem.Crystal clusters reduce the petiole osmotic potential,which is more conducive to the upper petiole's water transfer from branches to veins.The upper leaf has more developed palisade tissue and vein tissue than the lower leaf.Therefore,from the perspective of morphological and anatomical structure of branches and leaves,upper branches and leaves are more adaptive to arid environment than lower.(2).Micro CT system was established to scan the fresh branches and leaves of P.euphratica.In this dissertation,technical indicators for Micro CT scanning of branches and leaves of P.euphratica were preliminarily established.After the scanning was completed,CT-analyzer software was used to optimize the initial 2D images,which could be used to construct 3D images of samples according to the research needs.By adjusting the image pixel size and exposure time,the smaller structure of the sample can be observed.(3).Micro CT microstructures and scanning electron microscopy images of fresh and dehydrated branches of P.euphratica showed that Micro CT scanning of fresh branches could maintain the authenticity of microstructure to the greatest extent.The differences in xylem vessel structure of branches with different leaf shapes can be analyzed more accurately,which lays an experimental foundation for the subsequent acquisition of xylem structure characteristic parameters by Micro CT technology,and at the same time avoids a large amount of time for making slices.(4).Micro CT was used to analyze the xylem structure of the fresh branches,and the results showed that there were significant differences between upper and lower xylem of P.euphratica.The upper branch has a larger vessel diameter and a larger hydraulic diameter,indicating that the upper branch has a more developed water-conducting structure,which can more effectively transport the water absorbed by the roots to the upper canopy.And the lower branches has larger vessel density,this is because the vessel diameter and vessel wall thickness is small,so the unit area of the vessel is more,it also reflects to some extent the conductivity and the water retention of the lower branches are weaker.Lower branches located in the lower canopy,from the roots of the water transportation distance is short,it is adapted to its good water status.(5).Through the analysis of the water characteristics of upper and lower branches with leaves,the results showed that the upper branches had strong ability of hydraulic conductivity and water storage,make its can maintain a higher water potential,at the same time,the upper leaves with strong osmotic regulation ability,enhances its ability to absorb water from the branches and maintain turgor pressure in the drought stress.Therefore,the synergistic effect of the upper branches and leaves ensures the water supply of leaves under drought stress,which reflects their good drought resistance.(6).The differences of photosynthetic efficiency between upper and lower leaves were found by Pn,Tr,LUE and WUE measurements.The photosynthetic physiological indexes of upper leaves were higher than those of lower leaves.The morphology and anatomical structure of upper leaves make it easier to absorb water from branches,improve its water use efficiency(WUE),and maintain a high photosynthetic efficiency.This is also the reason why lower leaves are eventually replaced by upper leaves.(7).Pearson correlation analysis was carried out between the xylem structure and water conduction characteristics of upper and lower branches with leaf photosynthesis.The results showed that the water use efficiency(WUE)of upper leaves was positively correlated with the diameter of xylem vessel and hydraulic diameter of the corresponding branches(P < 0.01),and negatively correlated with the vessel wall thickness(P < 0.01).It means that the xylem structure and water conduction characteristics of upperbranches had a great effect on photosynthetic efficiency of upper leaves.The morphology and function of upper leaves were adapted to its poor water condition.The WUE of lower leaves was significantly positively correlated with the diameter of xylem vessel and hydraulic diameter of its corresponding branches(P < 0.05),but negatively correlated with the thickness of vessel wall,but not significant.It means that the effects of xylem structure and water conduction characteristics of lower branches on photosynthetic efficiency of lower leaves were limited.Therefore,the morphology and function of lower leaves were adapted to the better water condition in its habitat.It was suggested that the physiological mechanismthe of the morphological change of the leaves was affected by the water status of the branches and leaves in different canopy in P.euphratica.The leaf morphology changed from lanceolate to dentate oval due to the different degree of drought stress in different canopy,and the change tended to drought.This is a set of physiological and ecological behaviors of P.euphratica which is most suitable for its own growth and development in the process of long-term adaptation to drought stress so that it can not only survive in desert environment but also grow under better water conditions.