Spatial differences in acquisition of soil phosphate between two arbuscular mycorrhizal fungi in symbiosis with Medicago truncatula

Responses of Medicago tuncatula to colonization by two arbuscular mycorrhiz al fungi, Scutellospora calospora isolate WUM 12(2) and Glomus caledonium i solate RIS 42, were compared in the light of previous findings that the for mer fungus can be ineffective as a beneficial microsymbiont with some host plants. The plants were grown individually in two-compartment systems in wh ich a lateral side arm containing soil labelled with P-33 was separated fro m the main soil compartment by a nylon mesh that prevented penetration by r oots but not fungal hyphae. Fungal inoculum was applied as a root-soil mixt ure in a band opposite the side arm. Nonmycorrhizal controls were set up si milarly, without inoculum. There were harvests at 28, 35, 42 and 49 d. Both sets of mycorrhizal plants grew better than nonmycorrhizal plants and init ially had higher concentrations of P in shoots and roots. Plants grown with S. calospora grew better than plants grown with G. caledonium, and this wa s associated with somewhat greater fungal colonization in terms of intrarad ical hyphae and numbers of arbuscules. Scutellospora calospora formed dense r hyphae at root surfaces than G. caledonium. By 28 d there were extensive hyphae of both fungi in the side arms, and after 35 d S. calospora produced denser hyphae there than G. caledonium. Nevertheless, there was very littl e transfer of P-33 via S. calospora to the plant at 28 d, and thereafter it s transfer increased at a rate only c. 33% of that via G. caledonium. The r esults showed that plants colonized by S. calospora preferentially obtained P from sites in the main soil chamber relatively close to the roots, compa red with plants colonized by G. caledonium. Hence formation of a highly ben eficial arbuscular mycorrhizal symbiosis does not necessarily depend on dev elopment of hyphae at a distance from the roots or on large-scale transloca tion of P from distant sites. The results are discussed in relation to prev ious studies with compartmented systems that have involved the same fungi. Possible causes of the variable effects of S. calospora in symbiosis with d ifferent host plants are briefly assessed. Differences in spatial abilities of individual arbuscular mycorrhizal fungi to acquire P might have strong ecological implications for plant growth in soils low in P.