Hultine K R, Koepke D F, Pockman W T, Fravolini A, Sperry J S, Williams D G
Department of Biology, University of Utah, Salt Lake City, UT 84112, USA.
Tree Physiol. 2006 Mar;26(3):313-23. doi: 10.1093/treephys/26.3.313.
We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.
我们在美国亚利桑那州东南部一个沙壤土场地和一个壤质粘土场地，研究了绒毛牧豆树（Prosopis velutina Woot.）水分吸收的水力限制因素。我们预测，在干旱期间，沙壤土上的树木木质部和土壤水势的负值较小，对木质部空化的抗性较低，并且在比壤质粘土上的树木更高的土壤水势下达到临界蒸腾速率（E(crit)，即无水力故障时的最大稳态蒸腾速率）。然而，在夏季干旱高峰期测得的黎明前叶片木质部水势最小值，在沙壤土场地（-3.5±0.1 MPa；所有误差均为95%置信限）显著低于壤质粘土场地（-2.9±0.1 MPa）。中午木质部水势最小值在沙壤土场地（-4.5±0.1 MPa）也低于壤质粘土场地（-4.0±0.1 MPa）。尽管叶片水势存在差异，但两个场地树木的根或茎木质部栓塞、平均空化压力或Psi(95)（导致95%空化的木质部水势）均无显著差异。用现场数据参数化的土壤-植物水力模型预测，由于根际导度受限，沙壤土上的临界蒸腾速率在比壤质粘土上高得多的土壤体积水势（Psi(s)）下接近零。该模型预测，沙壤土场地的蒸腾作用受临界蒸腾速率限制，并且在生长季节的大部分时间内与土壤体积水势紧密相关，这表明沙壤土场地的季节性蒸腾通量与年内降水脉冲密切相关。相反，该模型预测，壤质粘土上的树木在整个生长季节的蒸腾作用都低于临界蒸腾速率，这表明质地细腻土壤上的通量与年际降水变化密切相关。关于木质部和根际限制对不同土壤质地梯度下叶片水分供应的综合重要性的信息，有助于深入了解控制植物水平衡和更大尺度水文过程的机制。
