Mobilization of sparingly soluble inorganic phosphates by the external mycelium of an abuscular mycorrhizal fungus

Red clover (Trifolium pratense L.) and Glomus versiforme (Karsten) Berch growing in rhizoboxes were employed in two glasshouse experiments to study the mobilization of sparingly soluble phosphates by arbuscular mycorrhizal fungal (AMF) mycelium. In one experiment, four inorganic sources of phosphate, CaHPO(4). 2H(2)O (Ca(2)-P), Ca(8)H(2)(PO(4))(6). 5H(2)O (Ca(8)-P), Ca(10)(PO(4))(6).F(2) (Ca(10)-P) and AlPO(4). nH(2)O (Al-P), were chemically synthesized, labelled with (32)P in an atomic pile and applied to the hyphal compartments of the rhizoboxes. Shoot yield, (3)2P and total P uptake were measured in clover growing in the root compartments. A similar experiment was conducted simultaneously using the same phosphate sources unlabelled and clover mycorrhizal infection and soil pH were determined. Although AMF inoculation increased the P uptake and biomass of clover shoots, the contribution of AMF to shoot P uptake and biomass varied with phosphate source, and was greatest with Ca(2)-P and least with Ca(10)-P. (3)2P measurements indicated that external hyphae could mobilize Ca(2)-P, Ca(8)-P and Al-P, but not Ca(10)-P. This indicates that AMF not only mobilize the same types of phosphates that plants mobilize under stress conditions of low P, but give increased contact with phosphates in the soil compared with non-mycorrhizal root systems.