Leaf surfaces constitute a major interface between the plant and its surrounding environment. Not only do they limit water loss during drought stress, but they may play a critical role in drought recovery of some species by facilitating absorption directly into the leaf. Despite its potential importance for understanding plant-level hydraulics, a general framework for quantifying foliar water uptake and a direct link to vascular hydraulics does not exist. To gain insight into the mechanisms underlying leaf water absorption from fog/rain, we analyzed the surface rehydration kinetics of leaves of four different species (Quercus lobata, Prunus dulcis, Ginkgo biloba and Olea europaea) experiencing moderate water stress levels (-2.0 to -1.5 MPa). By monitoring the temporal change in leaf mass and water potential during fog treatment, we estimated the instantaneous resistance to foliar absorption using an Ohm’s law analogy. Moreover, using this technique we explore potential links between leaf surface and vascular hydraulic properties.
Results/Conclusions
All the species examined absorbed water through the leaf surface, yet two hydraulically distinct groups emerged. P. dulcis and Q. lobata exhibited two to eight times faster recovery of leaf water potential and four to ten times higher surface water flux than G. biloba and O. europaea. Interestingly, higher surface flux in P. dulcis and Q. lobata leaves appeared to be limited more strongly by the water potential gradient (i.e., the driving force), suggesting a more direct link between the leaf interior and exterior in these species. In G. biloba and O. europaea higher surface hydraulic resistance (not driving force) seemed to limit leaf rehydration. In all species, rehydration via the leaf petiole showed a biphasic response, indicating two ‘compartments’ of water linked to the xylem via both low and high resistance pathways. Surface resistance was correlated with the relative volume of the slow-phase rehydration compartment. This coordinated high resistance compartmentalization at the petiole and leaf surface potentially allows these species to partially disconnect tissues to minimize water loss under drought conditions. In conclusion, we present a general framework for quantifying foliar water uptake and use the technique to identify novel links between leaf surface and vascular hydraulics.