Augmentation of AM fungi fails to ameliorate the adverse effects of temporal resource variation on a lettuce crop

While agricultural research has traditionally focused on average environmen tal conditions, environmental variability, independent of the mean, can als o have biological consequences. Using lettuce (Lactuca sativa) as a model s ystem, we tested two hypotheses: (1) increased temporal variability in wate r supply impacts plant growth, yield, photosynthesis, water relations and n utrition and (2) arbuscular mycorrhizal AM fungal associations benefit this agricultural crop, especially when plants experience temporal variability in water supply. The experiment used a randomized complete block design wit h two blocks and three variables (each with two levels): +/- mycorrhizal in oculation, high or low variability in watering intervals, and high or low t otal 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 t he extent of AM fungal infection in lettuce roots. There were no main effec ts of mycorrhizal inoculation on any measured variable, but augmented mycor rhizal associations interacted with variability in water supply to increase root/shoot ratios and decrease tissue concentrations of N and P. Successfu l application of AM fungi to sustainable agriculture probably requires a ge neral theoretical framework for predicting when effects on plants will be b eneficial versus neutral or even detrimental.