Heterotrophic Carbon Gain of the Parasitic Angiosperm Tapinanthus oleifolius

Abstract

Tapinanthus oleifolius is a xylem-tapping mistletoe, parasitising a wide range of host trees including acacias, which are potentially nitrogen fixing, and Euphorbia virosa, which acquires CO₂ mainly by nocturnal fixation via phosphoenolpyruvate carboxylase. Heterotrophic carbon gain by T. oleifolius was estimated by two different approaches. Firstly, δ¹³C values of leaves were calculated from c_I/c_a values and compared with measured δ¹³C ratios of these leaves and corresponding host xylem saps. Secondly, carbon intake from the host was determined by measuring carbon content of the host xylem sap and transpiration of the mistletoe leaves. This taken together with the photosynthetic CO₂ net uptake allowed the calculation of a carbon budget and hence 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 around 55% on E. virosa and Acacia nebrownii. In old leaves parasitising E. virosa, heterotrophic carbon accounted for more than 80% of the total carbon. Although nitrogen content of mistletoe and host leaves was highly correlated, we could not confirm earlier findings that the difference in δ¹³C values of parasite-host associations depends on the nitrogen content of the host. The photosynthetic characteristics (carboxylation efficiency, photosynthetic capacity) of the parasite were only little affected by the host species being parasitised.