The importance of cuticular permeance in assessing plant water use strategies

Plant water use strategies can be characterized across a spectrum of drought responses, from tight stomatal control (isohydric) to distinctly less stomatal control (anisohydric). A popular classification method of plant water use strategies utilizes the regression slope of predawn and midday leaf water potentials, σ, to reflect the coupling of soil water availability (predawn leaf water potential) and stomatal dynamics (daily decline in leaf water potential). This type of classification is important in predicting ecosystem drought response and resiliency. However, it fails to explain the relative stomatal responses to drought of two important temperate forest species (Acer sacharrum and Quercus alba). We propose that cuticular permeance influences plant water use strategies and that cuticular permeance variation provides a mechanistic explanation of how σ can be a successful indicator of isohydricity in one location but fail in others. We used empirical and modeling evidence to show that plants with more permeable cuticles are more often classified as anisohydric; the σ values of those species were very well correlated with measured cuticular permeance. Furthermore, we found that midday leaf water potential in species with more permeable cuticles would continue to decrease as soils become drier, but not in those with impermeable cuticles. We devised a diagnostic parameter, Γ, to identify circumstances where the impact of cuticular conductance could cause species misclassification. The results suggest that cuticular conductance needs to be considered to better understand plant water use strategies and to accurately predict forest responses to drought under future climate scenarios.