Excessive mistletoe ( Viscum album) proliferation is considered dangerous for the survival of Scots pine ( Pinus sylvestris) stands, as mistletoe increases their sensitivity to drought stress. In order to better understand this sensitivity, the aim of this study was to explore in depth the hydraulic and gas exchange performance of V. album in relation to its host P. sylvestris during summer drought. We selected one of the Scots pine southernmost habitats that features oro-Mediterranean climatic conditions. Here we measured hydraulic traits, xylem embolism, water potential, gas exchange, plant conductance and branch transpiration in non-infected and infected pine branches and mistletoe. We concluded that 1) both species have similar xylem specific hydraulic conductivity, leaf specific conductivity, vulnerability to drought-induced cavitation and plant conductance, but the higher transpiration of V. album resulted in more negative shoot water potentials that entailed a higher risk of xylem cavitation; 2) infected pine branches adjusted stem conductivity to the supported leaf area, that could explain the lack of differences in leaf specific conductivity, gas exchange, water potential and branch conductance with non-infected pine branches; and 3) despite the pine hydraulic adjustment, V. album caused a water uncoupling, exacerbated with soil water deficit, in infected pine branches above an infection threshold.

As the single link between leaves and the rest of the plant, petioles must develop conductive tissues according to the water influx and sugar outflow of the leaf lamina. A scaling relationship between leaf area and anatomical traits of xylem and phloem is expected to improve the efficiency of these tissues. However, the different constraints compromising the functionality of both tissues (e.g., risk of cavitation) must not be disregarded. Additionally, plants present two main leaf habits (deciduous and evergreen) that may have different strategies to produce and package their petiole conduits to cope with environmental restrictions. In this study, we explore, in a diverse group of 33 oak species, the relationships between petiole anatomical traits, leaf area, stomatal conductance and photosynthesis rate. Results showed allometric scaling between anatomical structure of xylem and phloem with leaf area. We also found how photosynthesis and stomatal conductance at leaf-level are correlated with anatomical traits in the petiole. Nonetheless, the main novelty is how oaks present a different strategy depending on the leaf habit. Deciduous species tend to increase their diameters to achieve a greater leaf-specific conductivity. By contrast, evergreen oaks develop larger xylem conductive areas for a given leaf area than deciduous ones. This trade-off between safety-efficiency in petioles has never been attributed to the leaf habit of the species.