Six pairs of O3-sensitive and O3-tolerant cultivars, clones or populations of different plants (tobacco, plantain, clover, radish, poplar and loblolly pine) were taken through identical but short episodic exposures to O3 in a controlled-environment fumigation system. Emissions of ethene and concentrations of polyamines, total phenols and ascorbate as well as levels of reduced glutathione and ascorbate in fumigated and clean air controls were determined in these various cultivars, clones and populations. A large number of significant differences were detected between the various sensitive and tolerant pairs, but it is clear that different sequences of response involving these parameters occur in these various plant pairs to account for their individual O3 sensitivity or tolerance. Some O3-tolerant plants have increased polyamines, others total phenols and some appear to be able to form reduced ascorbate and glutathione more rapidly. In the case of ethene emissions, however, all O3-sensitive selections produce more ethene when fumigated while O3-tolerant ones either reduce their rates of emissions below those of clean air-grown controls or at least keep them at the same level.