Aerodynamic control of stomatal optimality: Exploring thermal cooling returns under varying leaf-to-air coupling

Recent advances in stomatal optimization theory propose that under extreme heat, plants may prioritize thermal regulation over instantaneous carbon gain, leading to a decoupling of stomatal conductance (gs) from photosynthesis (A). In this framework, transpiration (E) is maintained to facilitate evaporative cooling even as A declines. However, the thermal benefit of transpiration is physically constrained by the leaf boundary layer and its sensitivity to wind speed, a factor often overlooked in standard leaf gas exchange measurements.

We explore how leaf cooling capacity is modulated by the boundary layer conductance in irrigated grapevines exposed to a temperature gradient (20°C to 40°C). Using a commercial portable gas exchange system with adjustable fan speeds, we investigate a range of aerodynamic coupling conditions. Parallel measurements with filter-paper replicas were used to independently quantify the boundary-layer conductance, and to establish a reference temperature (Tref) across fan speeds. These measurements allow us to estimate the thermal return on investment (ROI) of transpiration, defined as the reduction in leaf temperature per unit water loss (Tref-Tleaf)/E, and examine its relationship with boundary layer conductance.

By applying a unified stomatal model, we assess whether the model parameter g1, proportional to the marginal water cost of carbon gain (λ), remains constant across treatments. Finally, we propose the hypothesis that the efficacy of evaporative cooling is aerodynamically regulated, such that the thermal ROI is maximized under low-wind conditions where thick boundary layers enhance the relative contribution of latent heat. Conversely, we aim to demonstrate how high-wind conditions, typical of standard gas-exchange cuvettes, may decrease the thermal ROI by allowing convective heat exchange to dominate. We discuss how these mechanisms might mask the adaptive significance of "wasteful" water-use strategies in decoupled canopy environments.