Plant growth responses to vesicular-arbuscular mycorrhiza. XVI. Effectiveness of different endophytes at different levels of soil phosphate

Several vesicular-arbuscular mycorrhizal endophytes were inoculated separately on to onion plants in soils of low P status amended with five rates of Ca(H2PO4)2. H2O. Mycorrhizal infection was usually decreased by increasing additions of P, but the rate of root colonization and the infection plateau varied with different endophytes. Plant growth responses to mycorrhiza were large at low and medium P with Glomus mosseae and Glomus epigaeum and at medium P with Glomus macrocarpum and Gigaspora margarita. Glomus caledonium and Glomus sp. 'E3' were generally effective at low, medium and high P. Glomus clarum was ineffective at low P. The correlation between infection and growth response was higher for some endophytes than others. It was concluded that analysis of the VA symbiosis with any single endophyte cannot always be used as a basis for generalization about VA mycorrhizal fungi, and that the selection of field inoculants should take special account of endophyte behaviour in soils given extra phosphate.