Editor: Patrick S. HerendeenPremise of research. Reactive oxygen species (ROS) accumulate in plant tissues when the plant is exposed to stress, causing damage to cell structure and metabolism. Plants have a variety of mechanisms to prevent such damage, including a number of enzymes that scavenge ROS. The impact of specific enzymes, however, is as yet poorly understood.Methodology. The impact on plant tolerance to cold stress of enzymes involved in scavenging ROS was explored by comparing transplastomic tobacco lines overexpressing genes encoding glutathione reductase (GR), dehydroascorbate reductase (DHAR), glutathione-S-transferase (GST), and manganese superoxide dismutase, singly or in pairwise combinations, with nontransformed (NT) tobacco.Pivotal results. Chilling to 10ЊC resulted in less photoinhibition in lines overexpressing GR alone or in combination with GST, compared to NT tobacco, but more photoinhibition in lines overexpressing DHAR alone or in combination with GR. These differences were not, however, reflected in other indicators of stress. When chilled at 4ЊC, all lines showed similar damage to NT, except under relatively high photosynthetically active radiation, in which case photoinhibition was reduced in all transplastomic lines compared to NT. Growth over several weeks in a greenhouse was inhibited in the transplastomic lines when the duration included transient cool air temperatures. When optimal air temperature was maintained, growth over a similar duration was equal in transplastomics and NT tobacco. Conclusions.These experiments suggest a subtle interaction of enhanced ROS scavenging and tolerance of chilling stress.