Hydraulic Vulnerability Segmentation And Its Correlation With Growth In Poplar

Hydraulic vulnerability segmentation is an important drought resistance mechanism of plants,which plays an important role in maintaining hydraulic safety of plants.So far,most of the verification of vulnerability segmentation hypothesis has focused on the interspecific level,and there is a lack of test at the intraspecific level.Three 4-year-old poplar hybrid clones（K,Z,M）[Populus alba‘I-101’×（P.alba×P.glandulosa‘84K’）]with different growth rates were selected as research materials,and three or six individual plants were selected from each clone.Aboveground biomass（AGB）and the midday leaf water potential(Ψ_middy)were measured,and the vulnerability curves of roots,branches and leaves were constructed(P₅₀).The hydraulic safety margin（HSM）of leaves and branches were calculated,and the anatomical traits of xylem vessels were measured,including the vessel diameter（D_V）,hydraulic weighted vessel diameter（D_H）,vessel density（V_D）,vessel lumen ratio（F_L）and（t/b）².The study on the differences of vulnerability segmentation among poplar individuals with different growth rates and the relationship between hydraulic strategy based on vulnerability segmentation and growth can not only provide experimental evidence for the test of vulnerability segmentation hypothesis,but also provide theoretical basis and practical guidance for the evaluation of drought tolerance and growth strategy of poplar under the background of frequent drought.The main conclusions are as follows:（1）In aboveground biomass（AGB）,three clones showed significant differences:K>Z>M.This trend was consistent with the growth trend in the first three years of the progeny of cuttings mother trees and 8-years old hybrid poplars in the nursery at Weihe experimental station,indicating that the AGB differences of K,Z and M could be maintained stably.（2）In different organs of same clone,K,Z and M all showed that the P₅₀ of branches was lower than that of leaves and root segments,indicating that branches had the highest embolism resistance.In the same organ of different clones,the P₅₀ of M was lower than K and Z significantly,indicating that the embolism resistance of branches of M was highest.While there were no significant difference in leaves and root segments among the three clones.（3）In different organs of same clone,the vessel diameter gradually decreased from root to leaf,and the vessel density gradually increased from root to branch.The difference of embolism vulnerability of different organs can be reflected by the D_V,（t/b）² and other vessel hydraulic traits.In the same organ of different clones,D_V of leaves,D_V,D_H,（t/b）² of branches,V_D of roots of M were all showed significant difference with K and Z.The difference in hydraulic traits of xylem conduits of branches may be the main reason for the different vulnerability segmentation degree of K,Z and M.（4）This study supported the vulnerability segmentation hypothesis.Compared with K and Z,which grow faster,the M with the slowest growth rate had a lower osmotic regulation capacity,a lower hydraulic safety margin,a higher resistance to branch embolization and a higher vulnerability segmentation degree.These phenomena indicated that M may have a higher priority to play the role of“hydraulic safety valve”of leaves to protect stems from hydraulic failure under drought stress,but this may be achieved at the expense of growth rate,indicating that the increase of vulnerability segmentation may be detrimental to plant growth.