Department of Civil and Environmental Engineering, Duke University, Box 90287, Durham, NC, 27708-0287, USA.
Nicholas School of the Environment, Duke University, Box 90328, Durham, NC, 27708, USA.
New Phytol. 2013 Apr;198(1):169-178. doi: 10.1111/nph.12126. Epub 2013 Jan 29.
Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as E(max)). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near E(max). To address this question, we calculated E(max) across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits. The predicted E(max) compared well with measured peak transpiration across plant sizes and growth conditions (R = 0.86, P < 0.001) and was relatively conserved among plant types (for a given plant size), while increasing across climates following the atmospheric evaporative demand. The fact that E(max) was roughly conserved across plant types and scales with the product of xylem saturated conductivity and water potential at 50% cavitation was used here to explain the safety-efficiency trade-off in plant xylem. Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics.
土壤和植物水力学通过为水分运输设置物理限制，从而限制叶片对碳的吸收，从而限制生态系统生产力。虽然更负的木质部水势为水分运输提供了更大的驱动力，但它们也会导致限制水力传导率的空化。在中间水势下，驱动力和空化之间会出现最佳平衡，从而定义了木质部能够维持的最大蒸腾速率（表示为 E(max)）。这种最大值的存在提出了一个问题，即植物是否通过气孔调节蒸腾作用，以使植物接近 E(max)进行功能。为了解决这个问题，我们使用水力模型和植物水力特性的全球数据库，计算了不同植物功能类型和气候条件下的 E(max)。预测的 E(max)与不同植物大小和生长条件下的实测蒸腾峰值（R = 0.86，P < 0.001）高度吻合，并且在给定的植物大小范围内，在植物类型之间相对保守，而随着大气蒸发需求的增加，在气候之间呈增加趋势。E(max)在植物类型和尺度上大致保持不变，并且与木质部饱和传导率和 50%空化时水势的乘积成正比，这可以用来解释植物木质部中的安全-效率权衡。尽管木质部部分空化，但气孔导度允许达到最大蒸腾速率，这表明气孔调节和木质部水力特性之间存在协调。
