Highly Wettable Leaves in Humid Uplands: Balancing Droplet Lifetime and Contact Area to Enhance Foliar Water Uptake

Water flux on leaf surfaces is associated with leaf wettability, which exerts both positive and negative effects on plant functions. Among these functions, foliar water uptake (FWU) is considered to provide a water subsidy that improves plant-water relations. However, whether leaf wettability affects FWU remains unclear. Herein, we determined the wettability and microstructures, including stomata, trichomes, and epicuticular waxes, of the leaf surfaces of 25 species in a subtropical mountain forest and analyzed their effects on FWU. In addition to measuring FWU based on water submergence experiments, we quantified foliar droplet uptake (FDU) by comparing the observed and calculated droplet lifetimes (derived from evaporation models) based on droplet deposition experiments. Moreover, we also analyzed the relationships between leaf surface traits and FWU/FDU. The frequency distribution of the droplet contact angles on both sides of the sampled leaves peaked within the 70°-90° range. All leaves were able to absorb water during submergence, whereas some of the leaf surfaces did not absorb droplets. The estimated FDU reached a maximum of 24.2% of the droplet volume. The droplet contact angle correlated negatively with FDU but exhibited no correlation with FWU. Intermediate contact angles may imply a trade-off between droplet lifetimes and contact areas. Droplets with relatively longer lifetimes and substantial contact areas are conducive to optimising FDU, positively contributing to alleviating short-term water deficits in plants.