The concentration of ribulose-1,5-bisphosphate carboxylase was measured in the foliage of 'Moapa 69,' 'Ladak,' and 'Vernal' cultivars of alfalfa (Medicago sativa L.) after plants were exposed to 0.25 or 0.30 microliters per liter ozone for 2 hours. The quantity of ribulose-1,5-bisphosphate carboxylase decreased 80%, 68%, and 36% in leaves which did not exhibit ozone symptoms and to undetectable levels, 99% and 79% in symptomatic foliage for each cultivar, respectively.Ozone, a major component of photochemical smog, has been associated with reduction in photosynthesis ofmany plant species including alfalfa (9). There is evidence that 03 reduces CO2 fixation indirectly by inducing stomatal closure, thereby reducing CO2 uptake (9). There is also evidence that ozone may directly alter structural and functional elements of photosynthesis. Several researchers have described changes in the ultrastructure of the chloroplast of a number of plant species exposed to 03 (8). Crystalline structures have been observed in the stroma of chloroplasts of bean and cotton plants after an 03 exposure (8) and it has been suggested that these structures may be RuBPCase2 crystals (7). Nakamura and Saka (5) have reported that the activity of RuBPCase decreased in foliage of rice plants after an 03 exposure. Palazzolo (6) examined the effect of ozone on the profile of soluble alfalfa leaf proteins by SDS-PAGE; it appeared that foliage from 03-treated plants had lower levels of RuBPCase than did nontreated plants.In the present study, we extracted, partially purified, and quantified RuBPCase from three cultivars of alfalfa. The impact of ozone on the concentration of this enzyme was contrasted in foliage with and without visible injury. MATERIALS AND METHODSSeeds of Medicago sativa L. 'Moapa 69,' and 'Ladak' and M. sativa L. x M. falcata L. 'Vernal' were inoculated with Rhizobium meliloti (Agway, Inc., Syracuse, NY) and grown in a

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