Mistletoe ecophysiology: host–parasite interactions1

Mistletoes are highly specialized perennial flowering plants adapted to parasitic life on aerial parts of their hosts. In our discussion on the physiological interactions between parasite and host, we focus on water relations, mineral nutrition, and the effect of host vigour. When host photosynthesis is greatest, the xylem water potential of the host is most negative. To maintain a flux gradient and avoid stomatal closure and wilting, the mistletoe must tolerate a more negative water potential than the host. Succulent leaves enhance water storage and allow mistletoes to rehydrate before their hosts rehydrate. Mistletoe infections may disrupt the host stomatal control system, causing early and oscillating closure of host stomata, thereby diminishing host photosynthetic gain. Mistletoes lack the active uptake of minerals of a typical plant root system and rely upon the haustorium to connect with the host for the essentially one-way flow of photosynthates and nutrients from host to parasite. Modest growth rates, tolerance, succulence, and rapid leaf turnover are some means by which mistletoes avoid mineral deficiency or excess. We propose high concentrations of some mobile elements in the mistletoe by comparison with the host result not from active uptake, but from the inevitable accumulation by a parasite that utilizes host phloem sap. The relationship between host condition and mistletoe performance varies by situation and over time. In some cases, the host can outgrow the mistletoe, but favorable host status can also accelerate mistletoe growth. A better understanding of the mistletoe–host interaction can be utilized in improved management of infested forest plantations for resource production as well as for conservation of biodiversity and endangered species.