Department of Integrative Biology, University of California, Berkeley, CA 94720;
Fynbos Node, South African Environmental Observation Network, Newlands 7735, Cape Town, South Africa.
Proc Natl Acad Sci U S A. 2021 Mar 9;118(10). doi: 10.1073/pnas.2008987118.
Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks ( spp.). Optical visualization showed that 50% of embolism occurs at water potentials below -2.7 MPa in all 19 species, and -6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere's most ecologically and economically important plants.
定量了解木质部对脱水的物理耐受极限对于理解植物耐旱性至关重要，但在许多长导管被子植物中却缺乏这种知识。我们检验了这样一个假设，即木质部持续输水能力与下游组织功能之间的基本联系应该会选择避免长导管树木发生栓塞的木质部，我们通过量化北美西部橡树（ spp.）的木质部承受空气进入的能力来检验这一假设。光学可视化显示，在所有 19 个物种中，有 50%的栓塞发生在水势低于-2.7 MPa 的情况下，而在最具抗性的物种中，水势则低至-6.6 MPa。通过将木质部对栓塞的脆弱性进化图谱映射到北美西部橡树的化石年代系统发育树上，我们发现不同分支（组）之间存在很大差异，而每个分支内的密切相关物种在抵御空气进入的能力方面差异很小。木质部物理耐受性的系统发育保守性，以及物种分布与降雨梯度之间的显著相关性，表明密切相关的物种占据相似的气候小生境，并且物种的地理范围可能会根据其物理耐受性沿着干旱梯度发生变化。这些趋势，加上木质部栓塞抵抗力与其他与水力相关特征之间的进化联系，为来自不同生境的橡树提供了抵御栓塞的广泛安全边际。血管系统对干旱的进化响应支持栓塞避免假说，并揭示了量化植物抵御木质部栓塞能力对于理解北半球一些生态和经济上最重要的植物的功能和生物地理学的重要性。
