ABSTRACT
Reactive oxygen intermediates (ROIs), such as hydrogen peroxide and superoxide, are inevitable byproducts of metabolism in all aerobic organisms. Failure to reduce (scavenge) ROIs results in oxidative damage to cellular macromolecules and membranes either directly or via the products of lipid peroxidation. Thus, oxidative stress occurs when the production of ROIs exceeds the capacity of the antioxidative systems to remove them. For plants the danger of oxidative stress is particularly acute because as well as carrying out the reduction of oxygen in processes such as respiration, they also engage in high rates of oxygen metabolism in the chloroplast during photosynthesis and in allied activities such as photorespiration and the Mehler reaction, all of which occurs in the vicinity of highly energised pigment beds.
