Alfalfa (Medicago sativa L.) were exposed to O(3) concentrations varying between 118 x 10(-6) microg cm(-3) (0.06 ppm) and 157 x 10(-6) microg cm(-3) (0.08 ppm) for 6 h per day 5 days per week for several weeks. Typical plants were sacrificed weekly, and growth parameters were measured. O(3) reduced overall growth, relative growth rates and unit leaf rates in alfalfa before it was cut, indicating that O(3) had reduced photosynthesis. However, after the alfalfa was cut, these same parameters indicated that in some cases, O(3)-stressed plants had greater photosynthetic rates than controls during regrowth. O(3) also altered dry matter partitioning. Roots were most affected, followed by leaves and stems, respectively. In general, O(3) reduced photosynthate production and reduced the proportion of photosynthate partitioned to roots relative to leaves and stems. This could reduce starch reserves in alfalfa, and be detrimental to stand longevity. However, the post-cutting study indicated that at least some alfalfa cultivars may be able to acclimate to O(3)-stress, though plants did not fully recover from pre-cutting differences.