Rising CO 2 concentrations and their effects on plant productivity present challenging issues. Effects on the photosynthesis/photorespiration balance and changes in primary metabolism are known, caused by the competitive interaction of CO 2 and O 2 at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase. However, impacts on stress resistance are less clear. Reactive oxygen species are key players in biotic and abiotic stress responses, but there is no consensus on whether elevated CO 2 constitutes a stress. Although high CO 2 increases yield in C 3 plants, it can also increase cellular oxidation and activate phytohormone defense pathways. Reduction-oxidation processes play key roles in acclimation to high CO 2 , with specific enzymes acting in compartment-specific signaling. Traditionally, acclimation to high CO 2 has been considered in terms of altered carbon gain, but emerging evidence suggests that CO 2 is a signal as well as a substrate. Some CO 2 effects on defense are likely mediated independently of primary metabolism. Nonetheless, primary photosynthetic metabolism is highly integrated with defense and stress signaling pathways, meaning that plants will be able to acclimate to the changing environment over the coming decades.