Oxygen and carbon isotope composition of parasitic plants and their hosts in southwestern Australia

We measured leaf dry matter δ¹⁸O and δ¹³C in parasitic plants and their hosts growing in southwestern Australia. Parasite/host pairs included two mistletoe species, three species of holoparasites, and five species of root hemiparasites. Among these parasite functional types, significant variation was observed in parasite/host isotopic differences for both δ¹⁸O (P<0.0001, n=65) and δ¹³C (P<0.0001, n=64). Mistletoes were depleted in both ¹⁸O and ¹³C compared to their hosts; parasite/host differences were -4.0‰ for δ¹⁸O (P<0.0001) and -1.9‰ for δ¹³C (P<0.0001). The lower δ¹⁸O in mistletoe leaf dry matter compared to their hosts is consistent with the frequently observed high transpiration rates of these parasites. Root hemiparasites were also depleted in ¹⁸O and ¹³C compared to their hosts, but not to the same extent as mistletoes; parasite/host differences were -1.0‰ for δ ¹⁸O (P=0.04) and -1.2‰ for δ¹³C (P=0.0006). In contrast to mistletoes and root hemiparasites, holoparasites were enriched in both ¹⁸O and ¹³C compared to their hosts; parasite/host differences were +3.0‰ for δ¹⁸O (P<0.0001) and +1.5‰ for δ¹³C (P=0.02). The enrichment in ¹⁸O for holoparasite dry matter did not result from more enriched tissue water; holoparasite tissue water δ¹⁸O was less than host leaf water δ¹⁸O by a difference of -3.8‰ when sampled at midday (P=0.0003). Enrichment of holoparasites in ¹³C compared to their hosts is consistent with a generally observed pattern of enrichment in heterotrophic plant tissues. Results provide insights into the ecology of parasitic plants in southwestern Australia; additionally, they provide a context for the formulation of specific hypotheses aimed at elucidating mechanisms underlying isotopic variations among plants.