STRUCTURE, ENVIRONMENTAL EFFECTS ON THEIR FORMATION, AND FUNCTION OF PROTEOID ROOTS IN LEUCADENDRON LAUREOLUM (PROTEACEAE)

S ummary Proteoid roots are abundant on field and potted plants of Leucadendron laureolum (Lam.) Fourcade, accounting for up to 40% of the root mass (in 9‐month‐old plants in sand), and up to 400 g −1 of new season's roots (in 18‐month‐old plants in humus‐amended sand). There may be over 250 hairy rootlets per 10 mm of parent axis, giving a total surface area almost 15 times that of an equivalent mass of parent roots. Microsymbionts are absent, but chlorornycetin suppressed proteoid root formation, providing limited evidence of the need for a bacterial cofactor. Proteoid roots formed preferentially in humus‐amended sand but were elminated, with little effect on non‐proteoid roots, when organic fertilizer high in P was added. In the field, proteoid roots were confined to a 50 mm wide root mat just beneath the leaf litter around the parent plant. The greater soil volume explored directly by proteoid roots, despite lower soil P availability, may largely explain the greater rate of P uptake by plants in Clovelly sand than that m Hutton clay‐loam, although differential rates of solubilization and root respiration also appeared to be involved.