Mistletoe is an important component of the ecosystem, and has a significant impact on ecosystem processes and their response to climate change. Mistletoes and their host plants are facing increasing challenges in drought, making comparative studies of their hydraulic traits particularly important.

In a study published in New Phytologist, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and collaborators investigated the water transport characteristics of mistletoes and their host plants under different drought environments, and presented the first comprehensive regional dataset, encompassing 12 xylem traits related to hydraulic efficiency, mechanical strength, and safety.

Focusing on xylem traits related to plant hydraulic functions, researchers tried to discern the adaptive strategies of mistletoes and their hosts along a gradient of aridity index (proxy of diverse water and temperature conditions). They compared the xylem features of 119 mistletoe-host species pairs (340 individual pairs) across four ecologically distinct sites in Yunnan Province, China. 

Researchers found that mistletoes have lower water use efficiency and hydraulic efficiency (smaller vessel diameter and theoretical hydraulic conductivity), but higher safety (lower vulnerability index and higher vessel wall reinforcement, vessel clustering index, and wood density).

Specifically, they revealed that mistletoes have smaller vessel diameters and lower theoretical conductivity, but have a lower vulnerability index, higher vessel wall reinforcement coefficient, higher vessel group index, and higher wood density. The higher embolism resistance of mistletoes was consistent with their high xylem tension (low water potential), but contradicted by their high transpiration and low water use efficiency.

These findings showed a trade-off between efficiency and safety in the water transport process of mistletoes. This trade-off reflects the unique adaptation strategy of mistletoes. Xylem traits of mistletoes are significantly affected by host water supply efficiency and environmental changes, suggesting that mistletoes may achieve high transpiration by maintaining a high stomatal conductance at low water potential.

“Our study reveals the unique hydraulic adaptation strategies adopted by mistletoe due to its parasitic lifestyle. It provides a new perspective for understanding the hydraulic adaptation mechanisms of parasitic plants,” said ZHANG Jiaolin, one corresponding author of this study.

A kind of mistletoe plants (Image by MO Haibo)