Transgenic plants that over-express superoxide dismutase (SOD) have been developed in several laboratories. Although there are exceptions, these plants have been shown to have increased protection from oxidative stress caused directly by treatment with paraquat or indirectly through stress exposure. We have begun to test whether this approach is applicable for increasing stress tolerance in cotton. We used a chimeric gene developed by Dr. C. Bowler that contains coding sequences for the mature Mn SOD subunit from Nicotiana plumbaginifolia fused with a chloroplast transit peptide sequence from an Arabidopsis thaliana RUBISCO gene. This coding sequence is expressed under the control of a CaMV 35S promoter. The chimeric Chl-MN SOD gene construct was transferred to cotton plants via an Agrobacterium-mediated transformation procedure. Regenerated transgenic cotton plants were analyzed for the expression of novel SOD isoforms using a non-denaturing polyacrylamide gel negative staining technique. A unique SOD isoform was identified in extracts of several transgenic SOD cotton plants that correlated with tobacco Mn SOD. Suspension cultures derived from these plants were analyzed for growth in media that contained elevated levels of NaCl or polyethylene glycol (PEG). The increase in dry weight in PEG containing cultures was approximately two fold higher for cultures from transgenic SOD plants than for control plants. However, growth in NaCl containing cultures was similar for transgenic SOD cells and for control cells. Further tests of the oxidative stress resistance of transgenic cotton plants that express chloroplastic Mn SOD are currently under way. These include characterization of their paraquat resistance, analysis of photosynthesis during and after exposure to high light intensity and low temperature and drought tolerance.