TRANSFER OF PHOSPHATE FROM FUNGUS TO PLANT IN VA MYCORRHIZAS - CALCULATION OF THE AREA OF SYMBIOTIC INTERFACE AND OF FLUXES OF P FROM 2 DIFFERENT FUNGI TO ALLIUM-PORRUM L

The aim of the work described in this paper was to calculate the fluxes of phosphate (P) across the intraradical interfaces between two vesicular-arbuscular mycorrhizal fungi [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe and Glomus sp. 'City Beach' (WUM 16)] and Allium porrum L. The inflows of P into roots via the fungi were calculated from inflows into mycorrhizal and non-mycorrhizal plants for four harvest periods up to 84 d. The areas of interface between intercellular hyphae and roots, and arbuscules and roots were calculated for the same harvest periods using image analysis of sections in which the fungus was stained with nitroblue tetrazolium. The invagination of the arbuscular interface was calculated using methods based on previously published data. The fungi colonized the roots to approximately the same extent (% infection), but G. mosseae produced more intercellular hyphae and arbuscules, and hence larger interfacial areas than Glomus sp. City Beach (WUM 16). Hyphal inflows were higher for Glomus sp. City Beach (WUM 16) than G. mosseae for all except the last harvest period, but because of the lower interfacial areas, fluxes via this fungus were higher (3-7-12.8 nmol m(-2) s(-1)) than for G. mosseae (0-8-3-2 nmol m(-2) s(-1)). These fluxes are in the same range as values for P influx into plant cells and considerably larger than efflux from the fungal hyphae of ecto and ericoid mycorrhizal fungi measured in cultured mycelia. We conclude that enhanced efflux from the fungus must be essential for symbiotic phosphorus uptake by plants via VA mycorrhizal fungi.