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 fluxe s of phosphate (P) across the intraradical interfaces between two vesi cular-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 fr om inflows into mycorrhizal and non-mycorrhizal plants for four harves t periods up to 84 d. The areas of interface between intercellular hyp hae and roots, and arbuscules and roots were calculated for the same h arvest periods using image analysis of sections in which the fungus wa s stained with nitroblue tetrazolium. The invagination of the arbuscul ar interface was calculated using methods based on previously publishe d 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 Be ach (WUM 16). Hyphal inflows were higher for Glomus sp. City Beach (WU M 16) than G. mosseae for all except the last harvest period, but beca use 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. W e conclude that enhanced efflux from the fungus must be essential for symbiotic phosphorus uptake by plants via VA mycorrhizal fungi.