Abstract:

Objective: Hydraulic vulnerability segmentation is one of important drought resistance mechanisms of plants. The study on the differences of vulnerability segmentation among hybrid poplar individuals with different growth rates and the relationship between vulnerability segmentation and growth can not only provide experimental evidence for testing vulnerability segmentation hypothesis, but also provide theoretical basis and practical guidance for evaluating drought tolerance and growth strategy of the poplar under the background of frequent drought. Method: Three clones (K, Z, M) of 4-year-old offsprings of hybrid poplar [I-101 (Populus alba) × 84K (P. alba × P. glandulosa)] with different growth rates were selected as the research materials, and 3–6 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. Result: 1) The clone K had the highest AGB, followed by clone Z, and both of them were significantly higher than clone M. 2) The P₅₀ of branches of M was significantly lower than that of K and Z, while the P₅₀ of leaves and root segments had no significant difference among the three clones. Among different organs, the P₅₀ of branches was lower than that of leaves and roots. 3) In terms of vessel anatomical traits, the D_H, D_V and b of branches of M were significantly lower than those of K and Z, and the (t/b)² was significantly higher than that of K and Z. The D_V of leaves of M was similar to Z, but significantly lower than that of K. The V_D of root segment of M was significantly higher than that of K and Z, and other indexes showed no significant difference. Among different organs, theD_V gradually decreased along the hydraulic path from root to leaf, and the maximum mean diameter of vessels in the root segments was 5.12 times that of the leaves, while the V_D gradually increased from root to branch, and the V_D in the root segments was only 1/4 that of the branches. 4) Embolism vulnerability segmentation existed in all three clones, and the segmentation degree of M was the largest, slightly higher than K, and twice as high as Z. The HSM of leaves of clone Z was wider, while that of K and M was narrower or even negative. The HSM of branches of the three clones was similar. Conclusion: All the three clones with different growth rates exhibit vulnerability segmentation, and clone M with the slowest growth rate has the largest degree of segmentation. The difference in vulnerability of different organs can be reflected by D_V, (t/b)² and other vessel anatomical traits. The difference in hydraulic structure of xylem conduit of branches may be the main reason for the different degree of vulnerability segmentation of the three clones. Compared with K and Z, M with the slowest growth rate has the higher embolism resistance and the higher degree of vulnerability segmentation. The two phenomena may work together to protect stems from hydraulic failure, but this may be achieved at the expense of growth rates, suggesting that the increased vulnerability segmentation may be detrimental to plant growth.

Key words: poplar, vulnerability segmentation, xylem hydraulic taits, growth rate