DISTRIBUTION OF EXUDATES OF LUPIN ROOTS IN THE RHIZOSPHERE UNDER PHOSPHORUS DEFICIENT CONDITIONS

The distribution of secretory acid phosphatase and organic acids enhan ced by phosphorus deficiency in lupin rhizosphere was investigated usi ng a rhizobox system which separated the rhizosphere soil into 0.5 mm fractions. In the soil fraction closest to the root surface, the lupin exudates displayed an acid phosphatase activity of 0.73 u g(-1) dry s oil and citrate concentration of 85.2 mu mol g(-1) dry soil, respectiv ely. The increase of the acid phosphatase activity induced an apprecia ble depletion of organic P in the rhizosphere, indicating that lupin e fficiently utilized the organic P from soil through the enzyme activit y. The sterile treatments demonstrated that the acid phosphatase in th e rhizosphere was mainly derived from lupin root secretions. The secre tory organic acids enhanced considerably the solubility of the inorgan ic P in three types of soil and a sludge. However, the secretory acid phosphatase and organic acids from lupin roots were only detected in a considerable amount in 0-2.5 mm soil fractions from root surface.