Tracing the fate of water following foliar uptake in beech (Fagus sylvatica L.) using positron imaging.

BACKGROUND: Foliar water uptake (FWU) and its role in hydraulic redistribution are critical yet understudied mechanisms, particularly in temperate tree species of Europe. METHODS: This study investigates FWU in beech (Fagus sylvatica L.), with a focus on its contribution to the tree's water balance beyond leaf level. By using a combination of different imaging techniques such as silver nitrate tracing, positron emission tomography (PET) and autoradiography, we identified foliar water uptake from the point of entry to its subsequent transport. KEY RESULTS: The ionic tracer, silver nitrate (AgNO3), precipitated mainly at trichome bases and extended into subepidermal tissues, enabling the identification of water entry points. However, its inability to reach deeper vascular structures limited the ability to draw conclusions about further water transport and redistribution. Therefore, PET imaging and autoradiography were used and successfully visualized reverse sap flow of radiotracer-labelled water from treated leaves to connected branches, driven by a significant water potential gradient Δψ of 1.4 ± 0.9 MPa. Compartmental modelling quantified a net exchange rate eX-P of 0.15 ± 0.07 min-1 between xylem and surrounding parenchyma and a front velocity vFWU of 3.31 ± 0.56 mm min-1 under the imposed Δψ. CONCLUSIONS: These findings demonstrate that FWU may actively contribute to replenishing branch water pools, emphasizing its role as a critical hydraulic mechanism. This research underscores the utility of integrating PET imaging with complementary methods to better understand FWU dynamics and its implications for plant water budgets under changing climatic conditions.