Mistletoe achieves aerial parasitism via high leaf hydraulic efficiency but this poses a risk of hydraulic failure under drought stress

The survival of mistletoe and its host under frequent drought stress has become a major focus of many studies but few studies have addressed the leaf hydraulic relations between mistletoes and their hosts that may provide new insights into their adaptation. Here, leaf water potential (ψ) at predawn and midday (ψpd, ψmid), leaf hydraulic conductance (Kleaf), stomatal conductance (gs), transpiration rate (E), net assimilation (An), pressure-volume curve traits, vein structure and anatomy were tested among mistletoe Loranthus tanakae and two of its hosts Quercus mongolica and Pyrus ussuriensis. We found that compared with the two host species, the mistletoe L. tanakae exhibited more negative ψpd, ψmid, higher Kleaf, less negative ψ at the induction of 50% loss of Kleaf (Kleaf P50), a less negative turgor loss point and a narrower leaf hydraulic safety margin (ψmid -Kleaf P50). Furthermore, the mistletoe also exhibited higher gs, E and An, and lower intrinsic water-use efficiency, a rapid decrease in the Kleaf to gs ratio in response to decreasing ψleaf, along with higher vein density and midrib xylem conduit area than its hosts. Our results suggest that the mistletoe L. tanakae exhibits profligate traits with a high-water consumption to sustain aerial parasitic life, but more hydraulic vulnerability to drought. Therefore, their populations may face an extinction threat under increasing drought and heat stress with future climate change.