As climate change brings more frequent and severe droughts, forests worldwide are experiencing higher mortality rates. Although biological interactions, such as parasitism, influence how plants cope with water stress, the underlying mechanisms remain poorly understood.

In a new study published in New Phytologist on July 9, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences found that, although parasitic mistletoes are highly tolerant of drought-induced blockages in their water-conducting tissues, they are critically constrained by the hydraulic limits of their host trees. Since the two are tightly linked, an increase in a host's vulnerability to drought directly translates to a greater risk for its mistletoes. This can amplify water stress and make entire forest communities more fragile.

Mistletoes are obligate stem hemiparasites: they lack true roots and depend entirely on their hosts for water and nutrients, though they perform photosynthesis themselves. To extract water from their hosts, they must maintain lower (more negative) leaf water potentials and sustain high transpiration rates.

To investigate how mistletoes cope with drought, the researchers examined 41 mistletoe-host species pairs from tropical China. They combined analyses of xylem vulnerability curves, vessel and pit anatomy, water-use traits, and phylogenetically informed statistical models to compare the hydraulic strategies of the parasites and their hosts.

The researchers discovered that mistletoes are more resistant to xylem embolisms (blockages caused by air bubbles) than their host trees are. However, this greater safety comes at the cost of lower hydraulic efficiency, a trade-off consistent with their need to operate under constant high tension.

Interestingly, while embolism resistance in host trees was strongly linked to anatomical traits such as vessel size and pit membrane properties, these same relationships were weaker in mistletoes and only became apparent when species-level differences were taken into account.

The researchers then found that the P50 of host trees (i.e., the water potential at which 50% of hydraulic conductivity is lost) directly affected mistletoe P50. Mistletoe anatomical traits played only a secondary role.

"Our results indicate that mistletoes operate within a shared mistletoe-host hydraulic continuum," said ZHANG Jiaolin of XTBG. "Rather than independently optimizing their hydraulic systems, mistletoes are constrained by the hydraulic vulnerability of their hosts. This means that when hosts are under stress, mistletoes are inevitably pushed closer to their own breaking points."

This study provides the first broad-scale evidence that host hydraulics constrain parasite hydraulic safety across a wide range of species pairs.

"Explicitly accounting for mistletoe-host hydraulic coupling will be essential for improving assessments of forest drought vulnerability, predicting drought-induced tree mortality, and forecasting the future distribution and ecological impacts of parasitic plants in forest ecosystems under climate change," said ZHANG Yunbing.

A kind of mistletoes. (Image by HUANG Xianyan)