Characteristics And Mechanism Of Water Regulation Of Populus Tomentosa In Different Canopy Layers Under Various Soil Water Conditions

Water is the main limiting factor for tree growth.It is of great significance to understand the characteristics and mechanisms of tree water regulation to understand its drought resistance mechanism and survival strategy.In addition,it can also provide reference for the formulation and optimization of precise and reasonable irrigation strategies for plantations.In order to understand the water transport characteristics and drought resistance of different canopies,and to clarify the characteristics and mechanisms of water regulation of Populus tomentosa at different canopy layers under different soil water conditions,this study took the triploid Populus tomentosa plantation as the research object,comparing and analyzing the water transport,water storage and hydraulic safety characteristics of Populus tomentosa at different canopy layers under sufficient water supply(full drip irrigation,DIFI)and rain-fed(no irrigation,CK)by measuring the branch-petiole-leaf hydraulic conductance,water potential at the turgor loss point,leaf saturation osmotic potential,leaf water capacity and water content of branch and leaf,etc.The main results are as follows:(1)In the dry season,the upper and middle canopy layers under the DIFI treatment are more isohydric,while the lower canopy layer is more anisohydric;the lower canopy layer and upper canopy layer are more isohydric,while the middle canopy layer is more anisohydric under the CK treatment;the upper and lower canopy layer under the CK treatment are more isohydric than that of the DIFI treatment.In the rainy season,the isohydry of DIFI and CK treatments increase in order from the upper layer to the lower layer.The upper and middle canopy layers under CK treatment are more isohydric than that of the DIFI treatment.(2)There is no significant difference in branch hydraulic conductance between two soil water treatments(P>0.05),and no significant difference among different canopy layers under the same soil water treatment(P>0.05);The petiole hydraulic conductance in the upper and lower canopy layers under CK treatment is significantly greater than that of DIFI(P<0.05).The leaf hydraulic conductance of different canopy layers is not affected by changes in soil water conditions(only at 6:20 in the morning,the leaf hydraulic conductance in the upper canopy layer under the DIFI treatment is significantly greater than that of under the CK treatment(P<0.05)).During most of the day,the leaf hydraulic conductance among different canopy layers under DIFI and CK treatments is basically same,only the leaf hydraulic conductance in the upper and middle canopy layers under DIFI treatment is significantly greater than that of the lower layers at 12:00 am.(P<0.05).(3)The midday stomatal conductance under the DIFI and CK treatments has been on an upward trend from mid-April to early August,and the midday stomatal conductance gradually decrease from mid-August to the end of the growing season.The daily dynamic changes of stomatal conductance basically take on three forms,namely double peak,single peak,and stomata conductance gradually decreasing from early morning.The stomata at night is not completely closed,and the stomata conductance at night also tends to rise.The stomatal conductance of CK treatment peaked before DIFI treatment,and both treatments peaked before VPD(saturated water vapor pressure deficit).(4)?TLP and??100 of leaf in the lower canopy layer under the DIFI and CK treatments are smaller(more negative);The ?TLP and??100 of leaf in every canopy layer under CK treatment are relatively bigger than those under the DIFI treatment,and there is a significant difference in the middle canopy layers between two soil water treatments(P<0.05).(5)The leaf water capacity in the lower canopy layer under DIFI treatment is relatively large,and the leaf water capacity in the upper canopy layer under the CK treatment is relatively large.The leaf water capacity in every canopy layer under DIFI treatment is greater than that under CK treatment,and there is significant difference in the lower canopy layers between two soil water treatments(P<0.05)(6)The leaf absolute water content under the two soil water treatments show a trend of decreasing and then increasing from the dry season to the rainy season,and the absolute water content of the upper leaves is relatively lower compared with the middle and lower canopy layers The leaf relative water content does not change much in April and May,but it shows an obvious decrease in July.There is no significant difference in the leaf absolute water content and relative water content between the two soil water treatments(P>0.05).The average values of absolute water content and relative water content of the branch are smaller than the average values of absolute water content and relative water content of leaf,respectively.There is no significant difference in the branch absolute water content and the relative water content between the two soil water treatments(P<0.05)In summary,the leaf water regulation strategy will change under the conditions of changing soil water.Whether under the full drip irrigation or the rain-fed condition,the hydraulic conductance of branch-petiole-leaf does not fluctuate as the water transport pathway increases,maintaining the similar hydraulic conductance in different canopy layers.The leaves enhance the drought resistance by enhancing the water conducting capacity of the petiole and reducing the water potential at the turgor loss point.The leaves in every canopy layer of Populus tomentosa will reduce the stomatal conductance in time when the water potential is high,and adopt a more conservative water regulation strategy to avoid the threat of xylem embolism.