Zhang Shu-Bin, Zhang Jiao-Lin, Cao Kun-Fang
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences Mengla, China.
Plant Ecophysiology and Evolution Group, Guangxi Key Laboratory of Forest Ecology and Conservation, College of Forestry, Guangxi University Nanning, China.
Front Plant Sci. 2017 Jan 18;7:2075. doi: 10.3389/fpls.2016.02075. eCollection 2016.
Vulnerability segmentation, the condition under which plant leaves are more vulnerable to drought-induced cavitation than stems, may act as a "safety valve" to protect stems from hydraulic failure. Evergreen, winter-deciduous, and drought-deciduous tree species co-occur in tropical savannas, but there have been no direct studies on the role of vulnerability segmentation and stomatal regulation in maintaining hydraulic safety in trees with these three leaf phenologies. To this end, we selected three Anacardiaceae tree species co-occurring in a Chinese savanna, evergreen , drought-deciduous , and winter-deciduous , to study inter-species differentiation in leaf and stem hydraulic safety. We found that the two deciduous species had significantly higher sapwood-specific hydraulic conductivity and leaf-specific hydraulic conductance than the evergreen species. Moreover, two deciduous species were more vulnerable to stem cavitation than the evergreen species, although both drought-deciduous species and evergreen species had drought-resistance leaves. The evergreen species maintained a wide hydraulic safety margin (HSM) in stems and leaves; which was achieved by embolism resistance of both stems and leaves and isohydric stomatal control. Both deciduous species had limited HSMs in stems and leaves, being isohydric in the winter-deciduous species and anisohydric in drought-deciduous species. The difference in water potential at 50% loss of hydraulic conductivity between the leaves and the terminal stems (P50) was positive in and , whereas, exhibited a lack of vulnerability segmentation. In addition, differences in hydraulic architecture were found to be closely related to other structural traits, i.e., leaf mass per area, wood density, and sapwood anatomy. Overall, the winter-deciduous species exhibits a drought-avoidance strategy that maintains the hydraulic safety of the more carbon-costly stems by sacrificing cheaper and more vulnerable leaves, while the evergreen species exhibits a hydraulic strategy of drought tolerance with strong stomatal regulation. In contrast, the drought-deciduous species lacks vulnerability segmentation and sheds leaves at the expense of top shoots during peak drought. This study demonstrates that even sympatric tree species that differ in leaf phenology can exhibit divergent adaptive hydraulic safety strategies.
脆弱性分割，即植物叶片比茎更易受到干旱诱导的空化作用影响的状况，可能充当一种“安全阀”来保护茎免受水力故障影响。常绿、冬季落叶和干旱落叶树种在热带稀树草原中共存，但尚未有关于脆弱性分割和气孔调节在维持具有这三种叶物候的树木水力安全中作用的直接研究。为此，我们在中国稀树草原中选择了三种共生的漆树科树种，常绿、干旱落叶和冬季落叶，来研究叶和茎水力安全的种间差异。我们发现，两种落叶树种的边材比导率和叶比导率显著高于常绿树种。此外，两种落叶树种比常绿树种更易受到茎干空化影响，尽管干旱落叶树种和常绿树种都具有抗旱叶片。常绿树种在茎和叶中维持着较宽的水力安全边际（HSM）；这是通过茎和叶的抗栓塞能力以及等渗气孔控制实现的。两种落叶树种在茎和叶中的HSM都有限，冬季落叶树种为等渗，干旱落叶树种为非等渗。叶和顶枝在导水率损失50%时的水势差（P50）在[具体树种1]和[具体树种2]中为正值，而[具体树种3]则缺乏脆弱性分割。此外，发现水力结构的差异与其他结构特征密切相关，即单位面积叶质量、木材密度和边材解剖结构。总体而言，冬季落叶树种表现出一种避旱策略，即通过牺牲更廉价且更脆弱的叶片来维持成本更高的茎的水力安全，而常绿树种表现出一种具有强大气孔调节能力的耐旱水力策略。相比之下，干旱落叶树种缺乏脆弱性分割，在干旱高峰期以牺牲顶梢为代价落叶。这项研究表明，即使是叶物候不同的同域树种也能表现出不同的适应性水力安全策略。
