Transgenic tobacco plants that express a chi- The inhibition of photosynthesis that can occur when excess excitation energy reaches the reaction center is commonly referred to as photoinhibition. High light intensity, especially at extreme temperatures or water deficit, can cause increased electron flow to 02, resulting in greater production ofO2 and H202. Although oxygen radicals appear to be involved in photoinhibition (9)(10)(11), the role of SOD in limiting the oxidative damage associated with photoinhibition has not been directly demonstrated (12, 13).To investigate the possible protective functions of SOD in plant chloroplasts, we have developed transgenic tobacco plants that overexpress chloroplast-localized Cu/Zn SOD. These plants were analyzed for photosynthetic rate when exposed to light and temperature conditions that inhibit photosynthesis and for their ability to recover photosynthetic capacity after stress. Our results indicate that these transgenic plants have improved photosynthetic function at chilling temperatures and moderate light intensity, and they recover more effectively from severe stress than control plants. These changes correlate with increased resistance to oxidative damage caused by the herbicide methyl viologen (MV).