Exudation of carboxylates in Australian Proteaceae: chemical composition

Roots of a wide range of plant species exude carboxylates, such as citrate, into the rhizosphere. In the present study, seedlings of a range of Australian Banksia, Hakea and Dryandra species (Proteaceae) were assayed for their exudation of carboxylates. All of these species (Hakea prostrata, Hakea undulata, Hakea petiolaris, Hakea baxteri, Banksia grandis, Banksia prionotes, Banksia occidentalis and Dryandra sessilis) form cluster roots when grown in nutrient solution with a low phosphate concentration. Exudation of carboxylates was studied for cluster roots and non-cluster roots separately, and for the entire root system. Cluster roots of these Proteaceae exuded malate, malonate, lactate, acetate, maleate, citrate, fumarate, cis- and trans-aconitate. The relative contributions of each of these carboxylates differed between species. Malate, malonate, lactate, citrate and trans-aconitate, however, were invariably present in large proportions of total carboxylate exudation. Non-cluster roots of H. prostrata exuded a spectrum of carboxylates (mainly malonate, lactate and citrate), which differed somewhat from the exudation pattern of cluster roots (mainly malate, malonate, lactate and citrate). The rate of exudation for cluster roots of the seven species was approximately 1.6 nmol g -1 FM s -1 , which is considerably higher than that reported for a variety of crop and native species that do or do not form cluster roots. Contrary to what occurs in the cluster roots of Lupinus albus, which release carboxylates accompanied by protons so that the rhizosphere is acidified, the present Proteaceae exude the carboxylates as anions without concomitant proton release. The role of carboxylates in the mobilization of phosphate and other nutrients from soil is discussed.

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