INTEGRATED NITROGEN, CARBON AND WATER RELATIONS OF A XYLEM-TAPPING MISTLETOE FOLLOWING NITROGEN-FERTILIZATION OF THE HOST

Xylem-tapping mistletoes transpire large volumes of water (E) while co nducting photosynthesis (A) at low rates, thus maintaining low instant aneous water-use efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitro gen dissolved at low concentration in host xylem water; however, low A /E also results in substantial heterotrophic carbon gain. In this stud y, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to de termine whether mistletoe A/E would approach that of the host if mistl etoes were supplied with abundant nitrogen. Fertilization significantl y increased foliar N concentrations (N), net assimilation rates, and A /E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the o ther hand, when instantaneous water-use efficiency and A/N were calcul ated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budget s was constructed to analyze the relative benefits of nitrogen- and ca rbon-parasitism. The model assumes constant E and includes feedbacks o f tissue nitrogen concentration on photosynthesis. These results, comb ined with our earlier observation that net assimilation rates of mistl etoes and their hosts are approximately matched (Marshall et al. 1994) , support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain . However, the low ratio of A/E is interpreted not as a means of acqui ring nitrogen, but as an inevitable consequence of an imbalance in C a nd N assimilation.