<i>Viscum album</i> shares hydraulic traits but causes a water uncoupling despite the adjustments of its host <i>Pinus sylvestris</i>

Excessive mistletoe proliferation is considered dangerous for the survival of the host stands, as mistletoe increases their sensitivity to drought stress. To better understand this sensitivity, we aimed to explore in depth the hydraulic and gas exchange performance of Viscum album L. relative to its host, Pinus sylvestris L., during summer drought, by integrating a more comprehensive and detailed dataset. We measured hydraulic traits, xylem embolism, water potential, gas exchange, plant conductance and branch transpiration in non-infected pine branches, infected pine branches and in the mistletoe itself. We concluded that (i) although the two species exhibited similar xylem- and leaf-specific hydraulic conductivity, vulnerability to drought-induced embolism and plant conductance, V. album displayed higher transpiration rates, resulting in more negative stem water potentials, which indicate a reduced hydraulic safety margin and, consequently, a potentially greater risk of xylem dysfunction; (ii) the higher stomatal conductance of V. album may enhance its ability to uptake CO2, compensating for its lower mesophyll conductance and biochemical rates; (iii) infected pine branches adjusted stem conductivity to the supported leaf area, which could explain the lack of differences in leaf-specific conductivity, gas exchange, water potential and branch conductance with non-infected pine branches; and (iv) despite the pine hydraulic adjustment, V. album caused a water uncoupling effect, i.e., a lack of coordination between pine xylem conductivity and branch transpiration, in infected pine branches where mistletoe leaf area exceeds ~46% of the total leaf area of the branch; under soil water deficit, this value dropped to around 11%. These findings highlight that mistletoe-induced hydraulic uncoupling compromises the host's water balance, especially under soil drought, potentially accelerating tree decline in dry environments.