Comparative role of phosphate in soil or inside the host on the formation and effects of endomycorrhiza

Summary Non-mycorrhizal plants grown 5 weeks in a low-phosphate soil with different amounts of soluble P were transplanted to soil also with different levels of phosphate and inoculated with VA mycorrhiza. The intensity of mycorrhizal infection as affected by the interaction of differents levels of phosphate in soil and in the host was examined after a further 8 weeks. In the soil with no added phosphate mycorrhizal infection was not affected by the initial P content of the plants. When 0.8 or 1.5 g K₂PO₄ was given per kg soil both the external and the internal P negatively influenced the infection. In some conditions a P content that was supraoptimal for infection was not for plant growth. The critical P concentration in plants depends on the age of the host. An interaction between P and N as a factor contributing to phosphate toxicity is discussed.