Mistletoes - epiparasitism
Mistletoes commonly exhibit transpiration rates higher than their terrestrial hosts, and keep their stomates open at lower water potentials. So how do epiparasites behave in this respect? A study in Sri Lanka compared an obligate epiparasite, Viscum capitellatum growing on Dendrophthoe sp., with an incidental epiparasite, Scurrula parasitica growing on Dendrophthoe sp. and Taxillus cuneatus . It showed that in the obligate epiparasite, Viscum, transpiration rates were very low and water potential was similar to that of the host mistletoe. This obligate epiparasite appears to have physiological stress-avoidance adaptations for its lifestyle. In the incidental epiparasite, Scurrula, transpiration rates and water potential both exceeded those of the host mistletoe. When growing as an incidental epiparasite, Scurrula aggressively outcompetes its mistletoe host, possibly to the eventual detriment of both.
Data on uptake of potassium and calcium by Viscum suggested that obligate epiparasites are likely phloem parasites as well as xylem parasites, and the primary mistletoe may be little more than a conduit between the epiparasite and the ultimate tree host.