PLANT GROWTH RESPONSES TO VESICULAR‐ARBUSCULAR MYCORRHIZA

S ummary The growth of mycorrhizal and non‐mycorrhizal plants was compared in several soils receiving increasing amounts of Ca(H 2 PO 4 ) 2 . In some soils mycorrhizal plants grew better at all levels of applied phosphate. In other soils, where high phosphorus concentrations built up rapidly in the plants, those with mycorrhiza grew worse when more than 0.2 g Ca(H 2 PO 4 ) 2 was added per kg of soil. Because their roots take up more phosphate, mycorrhizal plants may reach supra‐optimal phosphorus concentrations with smaller amounts of added phosphate. Mycorrhiza tended to die out when 1 g Ca(H 2 PO 4 ) 2 or more was given per kg soil. With added phosphate no arbuscules were formed, many attenuated hyphae ramified through the cortex and the host seemed to have lost control over fungal development. Eventually only certain, morphologically distinct cells in the sub‐epidermal layer were infected. With 6 g Ca(H 2 PO 4 ) 2 root infection disappeared completely, but some viable inoculum survived in the soil.