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Root Exudation of Organic Acids: Importance to Nutrient Availability and the Calcifuge and Calcicole Behaviour of Plants
Vol. 80, No. 3 (Dec., 1997), pp. 459-466
Stable URL: http://www.jstor.org/stable/3546618
Page Count: 8
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Many vascular plant species are unable to colonize limestone soils and the floristic composition of adjacent limestone and acid silicate soils differs greatly. Low-molecular organic acids (LOAs) in root exudates may greatly affect plant availability of nutrients and it is hypothesized that contrasting exudation of LOAs is a major mechanism controlling the calcicole and calcifuge behaviour of plants. Rhizosphere soil solution from two calcicole and two calcifuge species, grown in a pH-intermediate soil, was expelled by high-speed centrifugation. The concentrations of LOAs in these solutions were determined by an application of ion chromatography using a supported liquid membrane enrichment technique. Concentrations of dicarboxylic (mainly oxalic) and tricarboxylic (mainly citric) acids were much higher in the soil solution of the calcicole species, whereas there was no difference in monocarboxylic (mainly lactic + acetic) acids between rhizosphere soil solutions of the two species categories. A consistent difference in the relative molar proportion of mono-, di- and tricarboxylic acids was also demonstrated among all species, indicating a species specific exudation of LOAs from plant roots. The solubilizing effect of acetic, oxalic and citric acid and their Na-salts on Fe, Mn and phosphate in two limestone soils and in the pH-intermediate soil was also tested. Citric acid and/or Na-citrate were powerful solubilizers of Fe and Mn and oxalic acid and/or Na-oxalate of phosphate, whereas acetic acid and/or Na-acetate was quite weak in this respect. The results from this study strongly support the view that high exudation rates of di- and tricarboxylic LOAs is a major mechanism controlling calcicole behaviour of plants.
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