Viscum album is hemiparasitic, i.e. it depends for water and mineral nutrition on its respective host but is able to produce carbohydrates by photosynthesis. It contains all pigments, chlorophyll a and b as well as carotinoids that are necessary for photosynthesis (Luther and Becker 1987). Through radiotracer experiments it has been shown that mistletoes have the same photosynthetic activity per unit leaf surface as the respective host plants. As there is no phloem connection between mistletoe and host, organic substances from the host are only transported via the xylem. This transport includes amino acids, cyclohexols (Richter and Popp 1992) and thiols (Renneberg et al. 1994). Mistletoe have a much higher transpiration rate than their respective hosts. Schulze et al. (1984) found that mistletoes on pine trees have a more than 3-fold transpiration rate, calculated for leaf surface, as compared to the pine tree. In a broad study on three continents, Ehleringer et al. (1985) investigated transpiration and mineral nutrition of different mistletoes. They conclude that nitrogen supply is the limiting factor and that mistletoe transpiration is higher on hosts with low nitrogen content than on hosts with higher nitrogen content in their xylem sap. All mistletoes including V. album usually have a higher mineral content than the respective host and especially the infected host branch (Lamont 1983).
All mistletoes affect their respective host trees in many ways (Hawksworth 1983): They adversely affect height and diameter growth, lower the vigor of the host, induce premature mortality, adversely affect the quality and quantity of wood produced, reduce fruiting of infected trees, and predispose trees to attack by other agents, such as insects or decay fungi.
All these aspects also apply to V. album. Cervera and Villaescusa (1977) estimate that fir trees infected by V. album heave a reduced secondary growth of about 40%.
Table 1 Secondary compounds from Viscum album (for lectins and vicotoxins, see Pfuller, this book).
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