Knowledge about physiological stress thresholds provides crucial information about plant performance and survival under drought. In this study, we report on the triphasic nature of the relationship between plant water potential () at predawn and midday and describe a method that predicts at stomatal closure and turgor loss exclusively from this water potential curve (WP curve). The method is based on a piecewise linear regression model that was developed to predict the boundaries (termed ₁ and ₂) separating the three phases of the curve and corresponding slope values. The method was tested for three economically important woody species. For all species, midday was much more negative than predawn during phase I (mild drought), reductions in midday were minor while predawn continued to decline during phase II (moderate drought), and midday and predawn reached similar values during phase III (severe drought). Corresponding measurement of leaf gas exchange indicated that boundary ₁ between phases I and II coincided with at stomatal closure. Data from pressure-volume curves demonstrated that boundary ₂ between phases II and III predicted at leaf turgor loss. The WP curve method described here is an advanced application of the Scholander-type pressure chamber to categorize plant dehydration under drought into three distinct phases and to predict thresholds of stomatal closure and turgor loss.

有关生理胁迫阈值的知识提供了有关植物在干旱条件下的表现和生存的重要信息。在这项研究中，我们报告了黎明前和中午植物水势 () 之间关系的三相性质，并描述了一种仅根据该水势曲线（WP 曲线）预测气孔关闭和膨压损失的方法。该方法基于分段线性回归模型，该模型是为了预测分隔曲线三相和相应斜率值的边界（称为₁和_{2 ）而开发的。}该方法针对三种重要的经济木本物种进行了测试。对于所有物种来说，在第一阶段（轻度干旱）期间，正午比黎明前的负值要大得多，在第二阶段（中度干旱）期间，正午的减少幅度较小，而黎明前继续减少，在第三阶段（严重干旱）期间，正午和黎明前达到相似的值）。叶气体交换的相应测量表明，第一阶段和第二阶段之间的边界₁与气孔关闭时一致。来自压力-体积曲线的数据表明， II 期和 III 期之间的边界_{2预测了叶片膨压损失。}这里描述的 WP 曲线方法是 Scholander 型压力室的高级应用，用于将干旱下的植物脱水分为三个不同的阶段，并预测气孔关闭和膨压损失的阈值。