Plant responses to phosphorus-deficiency stress: the role of organic acids in P mobilization from iron oxide and P acquisition by sorghum

Abstract

Phosphate deficiency is frequently a limiting factor in crop production on acid soils due to the tendency of iron oxides to strongly bind phosphate through inner-sphere ligand adsorption. Two of the reported responses of some plants to P-deficiency stress are organic-acid exudation from the roots and increased phosphatase activity. Organic-acid-induced P release from iron-oxide surfaces was studied using synthetic iron oxides. The amounts of P released and Fe dissolved from ferrihydrite and goethite at pH 4.0, 5.5, and 7.0 by citric, malic, melodic, oxalic, succinct, and tartaric acids were measured. At low initial P-adsorption levels, ligand-induced dissolution of the oxide surface as the primary mechanism of P release from oxides was supported by the strong positive relationship between Fe dissolved and P released. At higher initial P-adsorption levels, ligand exchange played a greater role in P release. Ten cultivate of sorghum exhibiting a range of P efficiencies were obtained from EMBRAPA in Brazil and tested for organic-acid exudation and phosphatase activity under P-deficiency stress in hydroponic culture. HPLC analysis of root exudates revealed increases in oxalate and succinate under P- deficient conditions, with significant differences between cultivate. There was no evidence of citrate in the root exudates at either P level. Western blots using arabidopsis purple acid-phosphatase antibodies revealed varying increases in phosphatase activity under P deficiency among the different cultivate. It was concluded that organic-acid exudation might not be expected to significantly improve P uptake by plants grown on weathered soils dominated by well crystalline iron oxide under low P-fertility conditions. Under higher initial P-feritility conditions, organic-acid exudation might play a greater role in plant P acqusitions.