Chloroplast and mitochondrial mechanisms for protection against oxygen toxicity.

As a consequence of their oxygen rich environment, organelles of photosynthetic tissues are exposed to large fluxes of oxyradicals and reactive oxygen species. Superoxide, hydrogen peroxide, hydroxyl radical and singlet oxygen are all potential by-products of respiratory and photosynthetic systems. Strong reductants found in mitochondria and chloroplasts along with a steady flux of photosynthetically generated oxygen enhance the potential for oxyradical production. Unless neutralized by scavenger substrates or enzymes, these reactive intermediates pose a lethal threat. The presence of superoxide dismutases, catalases, various peroxidases and scavenger substrates are all means of defences available to protect organelles. A balance between oxyradical production and neutralization should exist. Perturbations in generation or in sequestration caused by environmental or nutritional factors might profoundly alter the steady state level of oxyintermediates.