Comparative ecology and physiology of Australian and Brazilian mistletoe-host relationships

Mistletoes are parasitic angiosperms that attach to the host branch by a modified root (the haustorium) and acquire water and nutrients exclusively via the host xylem, at a very low carbon unit-cost. The association between mistletoes and their hosts is a fascinating coevolutionary system and has long been used as a model to investigate plant resource use strategies. In this thesis I tackled resource use questions using a many-species, “comparative ecology” approach, focusing on mistletoe-host species pairs from both Australia and Brazil, with the overarching aim of identifying robust generalities among species. I investigated several key aspects of their ecology and physiology, including photosynthetic trait adaptations to aridity, nutrient resorption, leaf functional traits, investment in anti-herbivore defences, and herbivory itself. I also took the opportunity to revisit old hypotheses in the literature, such as the “N-parasitism hypothesis” and the “mimicry hypothesis”. My findings reveal that mistletoes show strong responses to environmental conditions, exhibiting similar trait-shifts as their hosts in relation to site aridity. Nevertheless, mistletoes are profligate water users compared to their hosts and showed substantially less efficient use of water in photosynthesis, both at wet and at dry sites. Mistletoes seemingly achieve reliably high rates of water flow from the host xylem via maintaining higher osmolarity in their leaf tissues than in hosts; this appears to have a substantial cost that is reflected in leaf respiration rates. Little support was found for the N-parasitism hypothesis in a global context, and in three low-P sites, where the lack of N resorption suggests that N is not a limiting nutrient for mistletoes. Moreover, natural selection may have favoured P resorption processes in mistletoes occurring in P-impoverished habitats, suggesting that an alternative ‘P-parasitism hypothesis’ could better explain these results, at least for these systems. I provided evidence that the evolution of mimicry in Australian mistletoes could be associated with higher N availability in the hosts and with alkaloid-positive hosts, illustrating a case of Batesian mimicry. In a broader context, my results deliver significant new insights into the evolution of resource use in mistletoes, and address fundamental theories related to mistletoe ecology and physiology. They also provide general insights into adaptations of plants along environmental gradients and into the evolution of mimicry in Australian mistletoes.