Mineral nutrition and water relations of hemiparasitic mistletoes: a question of partitioning. Experiments with Loranthus europaeus on Quercus petraea and Quercus robur.

Hemiparasitic mistletoes thrive on xylem sap diverted from the host through direct xylem connections. There is no phloem link and no exchange of photosynthates. Mineral nutrition is therefore closely coupled with water consumption.Mineral nutrient partitioning between the mistletoe Loranthus europaeus and its host Quercus petraea was studied by monitoring nutrient incorporation into 3-year-old twigs of both species over 1 year. The mistletoes accumulated large amounts of potassium in comparison to other elements. It is hypothesized that this phenomenon is a consequence of the absence of a phloem connection between host and hemiparasite. In the host potassium is cycled between leaves and sites of photosynthate utilization in the wake of photosynthate transport in the phloem. In the hemiparasite it cannot cycle beyond the host-parasite interface and potassium imported with the xylem sap is thus trapped in the hemiparasite phytomass.The control of water partitioning in a host-hemiparasite system is another important aspect. As host and hemiparasite receive water from a common supply the otherwise independent stomatal control systems of both species become linked when transpiration demand exceeds supply. By measuring diurnal curves of transpiration and leaf water status in both species, by use of a special cuff-freezing technique to study stomatal response when water supply is cut off, and by measurements of leaf water capacitance with a pressure-volume curve technique, the fundamental data for a descriptive model of water partitioning in a L. europaeus - Q. robur system were collected. This model may be used to gain deeper insight into stomatal control of plant water status.