Aust. J. Bot., 1989, 37, 137-43 Chemical Characteristics of the Proteoid Root Mat of Banksia integrifolia L. P. F. Grierson and P. M. Attiwill School of Botany, University of Melbourne, Parkville, Vic. 3052. Abstract The proteoid roots of Banksia integrifolia are concentrated in the surface soil, forming a dense mat beneath the litter layer. Water extracts of the proteoid root mat contained a significantly greater amount of hydrogen ions, reductants and an unidentified chelating agent, than water extracts of soil beneath the root mat, of the litter layer and of soil from beyond the proteoid root zone. The results are discussed in relation to previously reported production of organic chelates by plants causing solubilisation of soil phosphates. It is suggested the proteoid roots of B. integrifolia chemically modify the soil environment thereby enhancing nutrient uptake. Introduction Purnell (1960) proposed the term 'proteoid root' to describe the dense proliferation of rootlets of limited growth arising from the lateral roots of many members of the Proteaceae. Since Purnell's studies, proteoid roots have been found in the Casuarinaceae, Fabaceae and Mimosaceae (Trinick 1977; Bowen 1981; Lamont 1982). These roots lack mycorrhizal or other microsymbiont associations (Purnell 1960; Powell 1975; Trinick 1977) although some microbial stimulus for their production has been reported (Lamont and McComb 1974; Malajczuk and Bowen 1974). Plants with proteoid roots are notable for their ability to grow on soil low in nutrients and in particular, low in available phosphorus. The earliest work on proteoid root production, using sand and water culture methods, indicated a greater production of proteoid roots at low levels of phosphorus (Groves 1964; Beadle 1968; Grundon 1972; Groves and Keraitis 1976). In the field proteoid roots tend to be concentrated in the top 10 cm of soil (Lamont 1982) and may form a dense, continuous mat at the soil-litter interface (Jeffrey 1967; Lamont 1973; Green 1976). It has been suggested that proteoid roots enhance plant uptake of phosphorus and other nutrients by providing an increased surface area for absorption of ions (Jeffrey 1967; Lamont 1980). On the other hand, the compact nature of the roots results in depletion zones, thus negating increased uptake due to greater surface area (Gardner et al. 1982). In addition to considerations of mass flow and diffusion, we must also con- sider the proposition that root-induced modifications of the rhizosphere are of crucial importance for the mineral nutrition of plants. The effect of substances exuded by roots would be far greater in dense aggregations than about a single root. That is, with increasing density of root hairs and lateral roots typical of the proteoid root system, the chances of root exudates mobilising sparingly soluble inorganic compounds is increased (Gardner et al. 1983). Gardner et al. (1983) have shown that the proteoid roots of Lupinus albus L. produce large amounts of reductants, protons and organic acids compared to the bulk soil. They 0067-1924/89/020137$03.00