Haustorial Structure and Functioning of the Root Hemiparastic Tree Nuytsia floribunda(Labill.) R.Br. and Water Relationships with its Hosts

Observations on the origin and mature structure of the haustorium of the Western Australian Christmas tree (Nuytsia floribunda) corroborate and extend the findings of earlier workers. We show that the previously described sclerenchymatous 'horn' or 'prong' formed within the haustorium acts as a sickle-like cutting device which transversely severs the host root and then becomes lodged in haustorial collar tissue directly opposite to that where it originated. The cutting process is deduced to be rapid and the gland-like fluid filled structure in the haustorium is suggested to generate a hydrostatic force driving the device through the host root. The haustorial parenchyma cells at the tight junction between the endophytic part of the haustorium and the cut face of the host root develop balloon-like outgrowths which intrude into the lumina of severed xylem vessels of the host. Experiments feeding 0.05% (w/v) basic fuchsin to freshly cut ends of host root segments distal to terminally-attached mature haustoria demonstrate an apoplastic pathway from host xylem elements fractured at the interface into haustorial parenchyma, and thence through vascular tissue to the haustorium into the transpiring plant of Nuytsia. Application of labelled water (D₂O) to uncut basal roots of potted plants of Acacia acuminata parasitized by Nuytsia results in labelling of leafy shoots of parasite and host, indicative of haustorial uptake of water by Nuytsia from host root xylem in the intact association. 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 regularly obtain xylem water through its haustorial apparatus