Functioning of Vesicular–arbuscular Mycorrhizas Under Field Conditions

S ummary Although VA mycorrhizas are easily demonstrated as mutualistic associations in pot culture, on the basis of reciprocal exchange of carbon and (mainly) phosphorus, unequivocal field demonstrations are less numerous. This may be due to incompetence of widely occurring fungal types, to inter‐plant connections by hyphae, which dissipate the benefit, to grazing of external hyphae, or to problems associated with root longevity. The action of mycorrhizal roots in improving plant water relations and promoting phosphate uptake under field conditions is considered in the light of these constraints. It is suggested that much greater effort is required in the investigation of mycorrhizal associations in the field.