Even in juvenile stages, mistletoe can adjust its physiology according to its host

As parasites of economically valuable trees, mistletoes are generally considered undesirable. However, they play an important role in their ecosystem, providing a food source for the birds that are their pollinators and seed dispersers. The mistletoe Psittacanthus schiedeanus parasitizes both deciduous and evergreen trees, requiring it to adapt physiologically to the differing availability of its hosts’ resources. What is not yet known is how this affects the functioning of both the mistletoe and its host during the juvenile stages of the parasite’s life cycle.

In a new study just out in the American Journal of Botany, lead author Eliezer Cocoletzi and colleagues tracked the physiological changes of both host and parasite for mistletoe growing on both deciduous (Liquidambar styraciflua) and evergreen (Quercus germana) trees in a cloud forest in eastern Mexico. The study took place as a nursery experiment run over the course of a year using the fast-growing mistletoe P. schiedeanus, which reaches sexual maturity in only one year.

The authors found that the mistletoe’s photosynthetic capacity, carbon dioxide uptake, and nutrient content improved during its early life stages regardless of which host it parasitized. It also appears that the mistletoe can adjust its photosynthetic activity during development in response to the availability of host resources. Psittacanthus schiedeanus had a higher metabolic rate on the deciduous host, which had higher leaf nitrogen content, versus the evergreen.

In terms of the trees’ physiological responses to infestation, the two species reacted differently: the deciduous trees showed a decrease in nitrogen content and carbon dioxide assimilation, while the evergreens suffered decreased photosynthesis and a lowering of nitrogen, phosphorous, and carbon content. In both cases, the mistletoe had higher transpiration rates and lower carbon assimilation and water-use efficiency than the host on which it grew.

“With over 1500 xylem-tapping mistletoe species worldwide, more studies are required to determine the relationship between the ecophysiological capacity of the mistletoes and their host species, including the generalist mistletoe species that can parasitize multiple deciduous and evergreen tree host species,” write the authors.