Predicting stomatal closure and turgor loss in woody plants using predawn and midday water potential.

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 (ψ) at predawn and midday and describe a method that predicts ψ at stomatal closure and turgor loss exclusively from this water potential (WP) curve. The method is based on a piecewise linear regression model that was developed to predict the boundaries (termed O1 and O2) 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 ψ was much more negative than predawn ψ during phase I (mild drought), reductions in midday ψ were minor while predawn ψ continued to decline during phase II (moderate drought), and midday and predawn ψ reached similar values during phase-III (severe drought). Corresponding measurement of leaf gas exchange indicated that boundary O1 between phase I and II coincided with ψ at stomatal closure. Data from pressure-volume curves demonstrated that boundary O2 between phase II and III predicted ψ 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 ψ-thresholds of stomatal closure and turgor loss.