Comparatively little is known about mechanisms of potential changes in plant stress tolerance undere[CO2]. Defence metabolites such as antioxidants are, in part, directly linked to primary carbohydratemechanism and so potentially impacted by e[CO2]. It is unknown whether photoprotective and antiox-idative defence systems, key to plant stress tolerance, will be affected, and if so, whether the responsewill be strengthened or weakened by e[CO2]. Better understanding of underlying principles is particularly important because it is virtually impossible to test all possible stress factor combinations with e[CO2] inrealistic field settings.