Haustorial structure and functioning of the root hemiparastic tree Nuytsiafloribunda (Labill.) R.Br. and water relationships with its hosts

Observations on the origin and mature structure of the haustorium of the We stern Australian Christmas tree (Nuytsia floribunda) corroborate and extend the findings of earlier workers. We show that the previously described scl erenchymatous 'horn' or 'prong' formed within the haustorium acts as a sick le-like cutting device which transversely severs the host root and then bec omes lodged in haustorial collar tissue directly opposite to that where it originated. The cutting process is deduced to be rapid and the gland-like f luid filled structure in the haustorium is suggested to generate a hydrosta tic force driving the device through the host root. The haustorial parenchy ma cells at the tight junction between the endophytic part of the haustoriu m and the cut face of the host root develop balloon-like outgrowths which i ntrude into the lumina of severed xylem vessels of the host. Experiments fe eding 0.05% (w/v) basic fuchsin to freshly cut ends of host root segments d istal to terminally-attached mature haustoria demonstrate an apoplastic pat hway from host xylem elements fractured at the interface into haustorial pa renchyma, and thence through vascular tissue to the haustorium into the tra nspiring plant of Nuytsia. Application of labelled water (D2O) to uncut bas al roots of petted plants of Acacia acuminata parasitized by Nuytsia result s in labelling of leafy shoots of parasite and host, indicative of haustori al uptake of water by Nuytsia from host root xylem in the intact associatio n. Measurements of xylem water potentials of pot-cultured seedling Nuytsia associated with a range of hosts, or of mature trees of Nuytsia and partner woody hosts in the native habitat, demonstrate consistently more negative potentials in the parasite than host, suggesting that the parasite may regu larly obtain xylem water through its haustorial apparatus. (C) 2000 Annals of Botany Company.