Phosphorus (P) Uptake Mechanisms of Crops Grown in Soils with Low P Status : II. Significance of Organic Acids in Root Exudates of Pigeonpea

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In our previous studies, pigeonpea (Cajanus cajan L.), groundnut (Arachis hypogaea L.), and rice (Oryza sativa L.) were found to have a higher ability to take up Fe- or Al-bound phosphorus (P) than soybean (Glycine max L.) and sorghum (Sorghum bicolor L.). Phosphorus absorption characteristics like I_<max> K_m, C_<min> and Fe^<III> reduction activity of roots, and root exudates in various crops were examined with a view to analyzing the mechanisms of P uptake. Phosphorus uptake ability was largely unrelated to variations in I_<max> K_m, C_<min> and Fe^<III> reduction activity of roots. Phosphorus-solubilizing activity in anionic fractions of root exudates was detected in pigeonpea but not in rice or groundnut. Malonic acid was the major component, followed by oxalic and piscidic acid. These organic acids were able to release P from FePO_4 and AlPO_4. The higher P uptake ability of pigeonpea in soils with low P fertility presumably depends on the secretion of such organic acids from roots.