HETEROTROPHIC CARBON GAIN OF THE PARASITIC ANGIOSPERM TAPINANTHUS-OLEIFOLIUS

Tapinanthus oleifolius is a xylem-tapping mistletoe, parasitising a wi de range of host trees including acacias, which are potentially nitrog en fixing, and Euphorbia virosa, which acquires CO2 mainly by nocturna l fixation via phosphoenolpyruvate carboxylase. Heterotrophic carbon g ain by T. oleifolius was estimated by two different approaches. Firstl y, delta(13)C values of leaves were calculated from c(i)/c(a) values a nd compared with measured delta(13)C ratios of these leaves and corres ponding host xylem saps. Secondly, carbon intake from the host was det ermined by measuring carbon content of the host xylem sap and transpir ation of the mistletoe leaves. This taken together with the photosynth etic CO2 net uptake allowed the calculation of a carbon budget and hen ce the proportion of heterotrophically-derived carbon. There was good agreement between the methods of determining heterotrophic carbon gain . For young leaves of T. oleifolius, heterotrophic carbon gain was aro und 55% on E. virosa and Acacia nebrownii. In old leaves parasitising E. virosa, heterotrophic carbon accounted for more than 80% of the tot al carbon. Although nitrogen content of mistletoe and host leaves was highly correlated, we could not confirm earlier findings that the diff erence in delta(13)C values of parasite-host associations depends on t he nitrogen content of the host. The photosynthetic characteristics (c arboxylation efficiency, photosynthetic capacity) of the parasite were only little affected by the host species being parasitised.