Drought-induced water losses after stomatal closure across vascular plants

Drought and heat events can impose high evapotranspiration demands, pushing plants to close their stomata to prevent excessive water loss. Yet, plant leaves are not perfectly hermetic and water losses continue through leaky stomata and the cuticle. Post stomatal closure residual water losses, known as minimum conductance (g_min), are relevant since they indicate the water depletion rates under severe stress. We present the first standardized dataset of g_min values for 101 species spanning high phylogenetic and ecological diversities, from ferns to flowering plants. Our sampling also included different growth forms and life cycles from annual herbs to longevous trees. We show that minimum water vapor conductance is highly variable across species, and g_min shows a weak phylogenetic signal across vascular plants. Residual water lossesdiverged across growth forms and phenologies with greater water losses in annual herbaceous as compared to woody plants. Moreover, deciduous species showed higher water lossrates as compared to evergreen species, highlighting the integration of g_min in leaf economics, where long-lived leaves show higher capabilities to retain water under stress. We used stomatal measurements to model the other side of the conductance spectrum and estimated the maximum (g_{th max}) leaf conductance capabilities. In the sampled vascular plants g_min was dissociated with g_{th max}, revealing the lack of a clear tradeoff between maximum potential conductance efficiency and water retention. Unlike g_min, stomata-driven g_{th max} has a high phylogenetic signal indicating that related species have similar maximal capacities of water conductance. Leaf conductance rates are negatively correlated with climate variables such as mean annual temperature and precipitation seasonality, revealing economies in water expenses in more seasonal, and hotter environments. As major drought events are coupled with significant heat stress, we further explored the relationship of g_min and photosynthetic thermotolerance (T_crit, T₅₀) in the diverse genus Quercus, to investigate potential interactions of thermal and drought sensitivities. Altogether, this presentation will provide recent advances on our understanding of the evolutionary physiology of water loss dynamics under heat- and drought-stress which will be cardinal to predict the fate of vegetation under global climatic changes.