Root cell-wall properties are proposed to contribute to phosphorus (P) mobilization by groundnut and pigeonpea

Groundnuts showed a superior ability to take up phosphorus (P) from two soils of extremely low fertility, where sorghum and soybean died of P deficiency. This ability could not be attributed to differences in root development, to P uptake parameters such as C_min, or to the excretion of root exudates capable of solubilizing iron- (Fe-P) and aluminum-bound P (Al-P), the sparingly soluble P forms in soils. A new P solubilizing mechanism (called `contact reaction') which occurs at the interface between root surface and soil particles, is therefore proposed. Isolated cell walls from groundnut roots solubilized more P from P-fixing minerals than those from sorghum and soybean roots. The P-solubilizing activity of groundnut root cell-walls might therefore be related to the superior growth of this crop under P-deficient conditions. The P-solubilizing active sites in groundnut root cell walls were located at the root surface and could act as chelating agent with Fe(III). This P-solubilizing active component in the cell walls could be extracted by NaOH, but not by HCl, and was identified as a small molecule through column chromatography with Sephadex LH-20. The P-solubilizing ability of pigeonpea root cell-walls was examined and found to be as high as that of groundnut. As pigeonpea plants excrete significant amount of root exudates with Fe-P solubilizing ability only after they flower, the P-solubilizing ability of root cell-walls may partially explain the high P efficiency of this species before it flowers.