The Effects Of Vessel And Stomata Traits On Plant Water Transport Capacity

The ability of plants to resist drought stress is critical for survival under changing climate conditions.The direct negative effect of drought stress on plant growth is the failure of hydraulic transport system.The long-distance transport of water in plants is mainly carried out by the vascular system consisting of tracheary elements in xylem tissue of roots,stems and leaves.The water in transport is under metastable state,i.e.unstable status.Continuous water column will be impacted by cavitation and embolism,the processes by which tension keeps increasing and exceeds the threshold as a result of evaporation or insoluble from ice,when facing drought stress,frost stress,or plant diseases and insect pests.Embolism is an important factor affecting the hydraulic function of plants.Many studies have found that the xylem hydraulic structure of plants is closely related to its embolism resistance.A study focuses on the relationship of hydraulic traits of plants and transport capacity will help understand the mechanisms of drought resistance,provide a strong basis for selecting the dominant tree species in reforestation,in order to alleviate the ecological pressure in arid and semi-arid regions of China.In this article,we intend to study the relations between length and diameter in vessels through comparing two common approaches to measure vessel length in Acer,Populus,Vitis and Quercus,and selected the suitable method – air injection method to help choose appropriate tree species with long vessels for the research on transport capacity of “tracheid bridge”.The effect of “tracheid bridge” between solitary vessels on water transport efficiency and safety was studied by combining the anatomical characteristics of vessels and surrounding cells in the xylem of selected trees and the measurement of hydraulic conductivity.The stomata in the leaves,as a safe valve in the water cycle,is very sensitive to the decline of conductivity in plants.Additional experiments were done on 16 tree species with huge differences in phylogenetics to study the response of stomatal conductance and leaf conductivity to leaf potential in the process of dehydration,and the influence of low leaf hydraulic conductivity on the decrease in stomatal conductance was studied by using the model analysis.The main results are as follows:1.The air injection method yield a larger mean vessel length than the rubber injection method.The reason for the consistently larger values of vessel length was because the air injection method measured air flow rates in cut open vessels.The Hagen-Poiseuille law predicted that the air flow rate should depend on the product of number of cut open vessels times the fourth power of vessel diameter.An argument was advanced that the air injection method was more appropriate because it measured the length of the vessels that contributed most to hydraulic flow.2.Results showed that the wider vessels were also longer vessels through rubber injection method by which the values of vessel diameter were divided into vessel diameter size classes and the number of vessels in each bin size were used to assess vessel length.Also,mean vessel diameter of rubber-injected vessels increased with distance from the injection surface,which showed that vessel diameter kept unchanged over the length of individual vessel.3.The “tracheid bridge” contributed less than 2.2 % of the total conductance of the vessels in selected species but in theory it could offer very efficient transport connector-pathways between adjacent vessels that may or may not make direct vessel-to-vessel contact via pit fields.However,in some species the “tracheid bridge” may be the dominate pathway for water flow between vessels whereas in other species the “tracheid bridge” may be sub-dominate or virtually nil,due to its different function in hydraulic conductivity.4.We found strong variation in stomatal closure in response to leaf potential in the process of leaf dehydration among samples,and important and influential complexity of the coordination of stomatal closure with leaf hydraulic decline.Our model analysis showed that leaf conductivity decline can cause important declines in stomatal conductance,but their decline thresholds are independent.The stomatal closure would have a strong cost in photosynthetic carbon gain at mild water potentials but would protect the stem xylem from high tensions.