Hydraulic architecture and xylem structure of the dimorphic root systems of South-West Australian species of Proteaceae

Specific hydraulic conductivities (Ks) of trunks (stem), superficial lateral roots, and deeply penetrating sinker (tap) roots were examined in variously-aged tree or shrub species of Proteaceae (Banksia prionotes, Banksia ilicifolia and Adenanthos cygnorum) in native habitat on deep sand overlying water tables located at 1.8–3.8 m depth. Ks values (on the basis of organ transectional area) for sinker roots (range from 30 to 780 ×10−3 m2 MPa−1 s−1 were consistently greater than in associated laterals (2 to 50 × 10−3 m2 MPa−1 s−1, and much greater again than in trunks (0.5 to 9× 10−3 m2 MPa−1 s−1. Wood was essentially diffuse-porous and mean lengths of vessels in the trunk, laterals and sinker roots of B. prionotes were 4 cm, approximately 75 cm and greater than 1.5 m, respectively. A 5–20-fold increase in Ks from top to base of a sinker was associated with progressive increases in mean radii of conducting elements, proportional areas of stele occupied by xylem conduits and percentage of organ fresh weight displaceable by mild vacuum extraction of water from vessel lumina. Resulting from these changes, conductance (Kh) of a sinker was relatively constant along its length, or even increased with depth, despite a 10–15-fold downward attenuation in transectional area over the 1.3–1.9 m length of root studied. Correlation plots of mean vessel radius and Ks (lumen area basis) grouped data for laterals separately from that of sinkers and showed interspecific differences in Ks values for similar range classes of lumen radius. Increases in conduit flow with increasing radius were less than predicted from the Hagen-Poiseuille equation.