Interspecific differences in the role of root exudates in phosphorus acquisition

Phosphorus (P) is highly fixed with aluminum (Al) and iron (Fe), and present as sparingly soluble forms in Andosols with low fertility. We tried to select crops which had superior P uptake ability in P-limiting soils, and examined their P uptake mechanisms. Field-grown sorghum took up large amounts of P in the low P soil. This ability depended on the development of a larger root system, and sorghum failed to survive when soil volume was limited. On the other hand, pigeonpea, groundnut, and rice were selected as crops that had higher P uptake ability in low P soils in a pot experiment compared with soybean, sorghum or maize. These selected crops seemed to have some mechanisms to take up P from Al- and Fe-bound forms. The ability was not related to their root development, microorganisms association, rhizosphere pH, P absorption characteristics (I-max, K-m, C-min), or Fe-III-reducing activity. In the examination of root exudate, only pigeonpea exudate could dissolve FePO4 and AlPO4. The solubilizing ability was attributed to the organic acids (malonic, citric, malic, piscidic and oxalic acids) which can make chelate rings with Al and Fe. The exudation of these active acids was not enhanced by plant P-deficiency, but may be controlled by plant growth stage. The application of artificial pigeonpea-root exudate, consisting of malonic and citric acid, increased the P uptake amounts of crops with low P uptake ability, such as soybean and maize in a low P Andosol. Therefore, it was demonstrated that organic acids exuded from roots play an important role for P uptake of pigeonpea from Al- and Fe-bound sparingly soluble forms in Andosols.