Carbon, water and nutrient relations of two mistletoes and their hosts: A hypothesis*

The carbon, water and nutrient relations of the xylem parasites Loranthus europaeus and Viscum laxum and their respective hosts. Quercus robur and Pinus sylvestris , were followed throughout clear days in July in order to study water and nutrient interactions in a simple system in which the plant growth depends on the host for its water and nutrients. At similar quantum flux densities, temperatures and vapour pressure deficits, the mistletoes had higher rates of transpiration and lower leaf water potentials than their hosts, but similar rates of CO 2 assimilation. Based on measurements of the nutrient content of the xylem and on seasonal measurements of the biomass and the tissue nutrient content, the present study suggests that the high rates of transpiration may be necessary for the parasites to take up sufficient nitrogen from the xylem of the host for production of biomass (leaves, fruits and stems).