Transgenic tobacco (Nicotiana tabacum cultivar W38) plants that overproduce petunia chloroplastic Cu/Zn superoxide dismutase were exposed to ozone dosages that injure control tobacco plants. Based on foliar injury ratings, there was no consistent protection provided to the transgenic plants. These Ozone damage is thought to result from the action of free radicals and other oxidants produced by the interaction of 03 and its degradation products with plant tissue constituents (9). Cellular antioxidant systems are a front-line defense against oxygen free radicals. The activity of SOD,2 one such enzymatic antioxidant, has been shown to increase in response to 03 (7,8,14). Whether this increase is part of a protective, defense response or is a secondary consequence of injury is subject to debate. Lee and Bennett (12) reported a substantial increase in SOD activity in bean plants treated with EDU, a compound capable of conferring tolerance on 03-susceptible plants (4). They also attributed the "natural" tolerance of young versus old leaves in the 03-susceptible BBL290 bean cultivar to a higher endogenous SOD content in young leaves. They suggested that SOD plays a major role in detoxifying 03-induced superoxide radicals which are at least in part responsible for ozone injury.We (200 ,g/ mL). Three plant types were tested: the control W38. 11, which had been transformed with the kanamycin resistance gene only, and independent transformants SOD 35/1.4 and SOD 13.22 (hereafter called SI and S2, respectively), which overproduced the petunia chloroplastic Cu/Zn-SOD (19). Vegetative plants aged 3 to 4 months with 9 to 11 nonsenescent leaves were used, uniform plants being chosen within each experiment. The newest leaf over 2 cm long was designated as leaf 1. Phenotypically there were no distinguishing features between the control and the two SOD transformants.Ozone fumigation tests were conducted with five plants of each genotype in a dynamic flow system contained within a greenhouse (13); an equal number were placed in a similar chamber minus 03. Ozone was produced by a generator (OREC model 0341-0) supplied with high grade oxygen. Ozone concentration in the chamber was measured by the neutral potassium iodide method (10). A dosage of 0.30 ppm 03 for 6 h was used because it was capable of eliciting a differential response from indicator tobacco cultivars Bel W3 and BelB, sensitive and tolerant, respectively. Forty-eight hours after ozonation, leaves 1 through 8 were rated for visible injury. The experiment was conducted three times. Injury data for leaves 5 through 8 were subjected to analysis of www.plantphysiol.org on May 8, 2018 -Published by Downloaded from