Shade and Drought Shape Stomatal Kinetics in Beech Saplings

Stomata and photosynthesis respond at different rates to rapid changes in light availability, resulting in different dynamics of water and carbon fluxes. Here, we investigated how growth conditions, drought, and potassium availability affect the dynamics of stomatal conductance and photosynthesis in response to rapid changes of irradiance in beech saplings (Fagus sylvatica L.). Two greenhouse experiments were conducted, one comparing saplings grown under sunlit conditions to those grown in shaded conditions, and the other testing the effects of edaphic drought and potassium supplementation on saplings grown on potassium-poor forest soils. When irradiance was changed abruptly, stomatal conductance (g_s) changed from an initial steady-state value to a final steady state; this change has two characteristic time constants: a lag time and a response time. Environmental conditions strongly shaped stomatal dynamics. Shade-grown leaves had reduced changes of steady-state g_s, but more reactive stomata, characterized by shorter lag and response times. This allowed relatively greater CO₂ uptake during transient light periods than if they had the response and lag times of sunlit-grown leaves. Droughted saplings with sufficient potassium also showed reduced g_s amplitude and shorter lag and response times enhancing conservation of water compared to well-watered plants. However, potassium-deficient saplings showed no significant changes in stomatal dynamics regardless of soil water status. Stomata closed about 20% faster than they reopened across all treatments. The differences in stomatal dynamics were not associated with any changes in stomatal size or density, indicating that physiological rather than morphological traits drive the observed plasticity of stomatal responsiveness in European beech.