Viscum album shares hydraulic traits but causes a water uncoupling despite the adjustments of its host Pinus sylvestris.

Abstract

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.