Release of phosphate from calcium phosphates by rhizosphere products

Wheat plants were shown to be more efficient than maize or pea in phosphate uptake from ³²P]-labelled synthetic hydroxyapatite over a 24 h period. Each wheat plant produced on average 0.58 mg of water-soluble rhizoshpere products (a combination of root exudates and microbial products in the rhizosphere) during 12 days growth in sand. The rhizosphere products released phosphate from calcium phosphates.The dissolution of phosphate was associated with a negatively-charged fraction (obtained by ion-exchange chromatography) and with a u.v.-absorbing fraction (obtained by gel-filtration chromatography on Sephadex G-10) of the rhizosphere products. The release of phosphate from calcium phosphates by these active fractions or the crude rhizosphere products was associated with a drop in pH of the mineral suspensions compared with controls.