Characterization of two arbuscular mycorrhizal fungi in symbiosis with Allium porrum: inflow and flux of phosphate across the symbiotic interface

Individual arbuscular mycorrhizal fungi can differ markedly in their abilit y to improve the phosphate nutrition and growth of host plants. In particul ar, Scutellospora calospora is relatively ineffective with some hosts and a growth depression is often observed. We have examined the abilities of S. calospora and Glomus sp. 'City Beach' strain WUM 16, grown in soils which p romote extensive mycorrhizal colonization, to transfer phosphate (P) to All ium porrum. Phosphate uptake from the low-P soils (P-0) was compared with u ptake from soils amended with extra P (P-1). In order to relate P transfer to physiological characteristics of the two fungi, inflow of P via fungus t o the plant was combined with the surface areas of intercellular hyphae and arbuscules (symbiotic interfaces) to calculate the amount of P transferred per unit area of interface (P fluxes). 'Hyphal inflows' and 'hyphal fluxes ' were also calculated on the assumption that P uptake by the plant was the same in mycorrhizal and equivalent nonmycorrhizal (NM) plants (the validit y of this assumption is discussed). With both soil P treatments, P was take n up by plants colonized by both mycorrhizal fungi to a greater extent than by the equivalent NM controls. Hyphal inflows to plants in P-0 soil that w ere colonized by Glomus sp. 'City Beach' were greatest from 14-21 d and dec reased at later harvests. With P-0 soil the inflow via Glomus sp. 'City Bea ch' peaked at a very high value at 21-28 d. Hyphal inflows into plants in P -0 soil that were colonized by S. calospora increased throughout the experi ment, and with P-1 soil they remained steady at high values. With both fung i, the surface areas of the symbiotic interfaces increased greatly after 14 d, and generally there was little effect of higher soil P. With P-0 soil, Glomus sp. 'City Beach' showed no significant differences in the ratio of s urface areas of the two interfaces over the course of the experiment. Scute llospora calospora tended to produce a lower percentage of interfacial area contributed by arbuscules. With the mycorrhizal plants growing in their re spective soils, there appeared no consistent differences between the two fu ngi with respect to fluxes of P across the interfaces. With P-0 soil, fluxe s via Glomus sp. 'City Beach' were initially higher than those via S. calos pora, but later they were higher with S. calospora. With P-1 soil the only difference was the 28-42-d period, when Glomus sp. 'City Beach' produced th e higher flux (reflecting the low surface area at the time). The results sh ow that relative inefficiency of S. calospora in its ability to transfer P, as reported by others, may result from different environmental conditions, use of different hosts, or even of different fungal isolates (strains).