CARBON-ISOTOPE COMPOSITION, GAS-EXCHANGE AND HETEROTROPHY IN AUSTRALIAN MISTLETOES

1. Photosynthesis, transpiration and carbon isotope ratios of 11 mistl etoes and their hosts were measured in central and eastern Australia t o test hypotheses regarding (i) similarity of photosynthetic rate betw een host and parasite and (ii) significance of host xylem carbon as a contributor to parasite carbon gain. 2. Differences in photosynthetic rates of mistletoes and their hosts were not statistically significant , although most mistletoes had slightly lower photosynthetic rates and much higher transpiration rates than their hosts. 3. Uptake of carbon dissolved in the xylem sap of the host was estimated from the discrep ancy between observed and predicted carbon isotope ratios. Dividing xy lem carbon gain by measured transpiration yielded a mean +/-SE xylem c arbon concentration of 10+/-2.0 mM. Addition of xylem carbon gain to p hotosynthesis for the mistletoes further reduced differences between m istletoe and host carbon gain. 4. Heterotrophic carbon gain, derived f rom the host xylem sap, represented approximately 15% of the total car bon gain on average. Patterns are discussed in terms of possible selec tion for equivalent shoot growth rates between parasite and host.