THE EFFECT OF NITRATE AND PHOSPHATE ON THE VESICULAR‐ARBUSCULAR MYCORRHIZAL INFECTION OF LETTUCE

SUMMARY The application of increasing amounts of nitrate resulted in higher levels of vesicular‐arbuscular mycorrhizal infection in lettuce roots inoculated with Glomus mosseae. This effect was found at three phosphate levels. The application of nutrient solutions containing different concentrations of nitrate and phosphate resulted in different nitrogen and phosphorus levels in the roots. At high concentrations of phosphorus in the roots, infection was inhibited, but at low concentrations the infection level appeared to be related to the nitrogen content of the host tissue. Mycorrhizal infections in low‐nitrogen treatments were poorly developed with very few arbuscules and intercellular hyphae. Two methods of estimating infection (gridline intersect and yield of glucosamine) were compared.