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. Morphologic al characteristics of haustoria are described and their anatomical features art: discussed in relation to uptake, transfer. and metabolism of xylem-bo rne nitrogenous solutes derived from a host. Experimental approaches used i nclude comparisons of xylem sap composition of parasite and host(s), solute pool analyses and enzymatic properties of haustoria, host xylem feeding of N-15-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-born e solutes to a host. Field studies on promiscuous root hemiparasites assess frequencies of exploitation of different hosts. N-15 natural abundance ass ays of host and parasite dry matter demonstrate marked preference by Santal um acuminatum for N-2-fixing as opposed to nonfixing hosts. The ability of Olax phyllanthi to continue to exploit deep-rooted hosts ranks of importanc e when xylem water potentials of other hosts go our 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 trans formations accentuated by haustoria while also indicating direct throughput from xylem of a host. The review concludes by showing how empirically base d modelling techniques can be used to estimate proportional gains of N by p arasites from single hosts and repercussions on host growth which accompany such exploitation.