Plant Growth Responses to Vesicular‐arbuscular Mycorrhiza

S ummary Onions, lucerne and barley were inoculated with one of two endophytes placed below the seed in a replicated field trial. After 13 weeks shoot growth was increased by 77% (onion), 79% (lucerne) and 33% (barley) with one inoculum and by four‐, six‐fold and 30 % respectively with the other. Lucerne and onion benefitted most from inoculation in the sub plots with most available P whereas barley responses were confined to those with less available P. The uninoculated plants reached approximately 45% infection from indigenous endophytes and the inoculated plants around 70%. Inoculation responses were not related to infection level. One introduced endophyte spored profusely and spread 22 cm from the point of inoculation, the other formed only few new spores; the indigenous fungi hardly sporulated at all. Spore production was little affected by host species or soil P level and was unrelated to percentage root infcetion. The rationale of field inoculation is discussed with particular reference to amounts of inoculum required.