Revised for: Correspondence to: Plant and Soil

While agricultural research has traditionally focused on average environmental conditions, environmental variability, independent of the mean, can also have biological consequences. Using lettuce (Lactuca sativa) as a model system, we tested two hypotheses: (1) increased temporal variability in water supply impacts plant growth, yield, photosynthesis, water relations, and nutrition and (2) AM fungal associations benefit this agricultural crop, especially when plants experience temporal variability in water supply. The experiment used a randomized complete block design with two blocks and three variables (each with two levels): – mycorrhizal inoculation, high or low variability in watering intervals, and high or low total watering volume. Temporal variability in water supply, at a time scale similar to what is common in agricultural practices, had negative effects on lettuce production. Inoculation treatments were successful in doubling the extent of AMF infection in lettuce roots. There were no main effects of mycorrhizal inoculation on any measured variable, but augmented mycorrhizal associations interacted with variability in water supply to increase root/shoot ratios and decrease tissue concentrations of N and P. Successful application of AMF to sustainable agriculture probably requires a general theoretical framework for predicting when effects on plants will be beneficial versus neutral or even detrimental.