Haustoria in action: case studies of nitrogen acquisition by woody xylem-tapping hemiparasites from their hosts.

This review discusses studies conducted by the author and his colleagues on mistletoes and root hemiparasites native to Western Australia. Morphological characteristics of haustoria are described and their anatomical features are discussed in relation to uptake, transfer, and metabolism of xylem-borne nitrogenous solutes derived from a host. Experimental approaches used include comparisons of xylem sap composition of parasite and host(s), solute pool analyses and enzymatic properties of haustoria, host xylem feeding of 15N-labelled solutes to follow the fate of label in haustoria and body of the parasite, and studies using species-specific nonprotein amino acids to validate successful uptake from hosts or occasional backflow of xylem-borne solutes to a host. Field studies on promiscuous root hemiparasites assess frequencies of exploitation of different hosts. 15N natural abundance assays of host and parasite dry matter demonstrate marked preference by Santalum acuminatum for N2-fixing as opposed to nonfixing hosts. The ability of Olax phyllanthi to continue to exploit deep-rooted hosts ranks of importance when xylem water potentials of other hosts go out of sucking range during periods of water stress. Comparisons of xylem sap composition of parasites feeding on different hosts indicate remarkable versatility by haustoria in uptake and utilization of the different major nitrogenous solutes received from these hosts. Solute pools in parasites partly reflect metabolic transformations accentuated by haustoria while also indicating direct throughput from xylem of a host. The review concludes by showing how empirically based modelling techniques can be used to estimate proportional gains of N by parasites from single hosts and repercussions on host growth which accompany such exploitation.