Lewis structure of some of the<unk> . A: hydroxyl radical (HO•); B: hydroxide ion (HO–); C: triplet<unk>  (O22•); D: superoxide anion (O2•–); E: peroxide ion (O22–); F: hydrogen peroxide (H2O2); G: nitric oxide (NO•)<unk>  <unk>  are highly reactive chemicals formed from O2. Examples of<unk>  include peroxides, superoxide, hydroxyl radical, singlet<unk> , and alpha-oxygen. The reduction of molecular<unk>  (O2) produces superoxide (•), which is the precursor to most other<unk> : :O2 + e− → • Dismutation of superoxide produces hydrogen peroxide (H2O2): :2 H+ + • + • → H2O2 + O2 Hydrogen peroxide in turn may be partially reduced, thus forming hydroxide ions and hydroxyl radicals (•OH), or fully reduced to water: :H2O2 + e− → HO− + •OH :2 H+ + 2 e− + H2O2 → 2 H2O In a biological context,<unk>  are byproducts of the normal metabolism of<unk> .<unk>  have roles in cell signaling and homeostasis.<unk>  are intrinsic to cellular functioning, and are present at low and stationary levels in normal cells. In vegetables,<unk>  are involved in metabolic processes related to photoprotection and tolerance to various types of stress. However,<unk>  can cause irreversible damage to DNA as they oxidize and modify some cellular components and prevent them from performing their original functions. This suggests that<unk>  has a dual role; whether they will act as harmful, protective or signaling factors depends on the balance between<unk>  production and disposal at the right time and place. In other words,<unk>  toxicity can arise both from uncontrolled production and from the inefficient elimination of<unk>  by the antioxidant system. During times of environmental stress (e.g., UV or heat exposure),<unk>  levels can increase dramatically. This may result in significant damage to cell structures. Cumulatively, this is known as oxidative stress. The production of<unk>  is strongly influenced by stress factor responses in plants, these factors that increase<unk>  production include drought, salinity, chilling, defense of pathogens, nutrient deficiency, metal toxicity and UV-B radiation.<unk>  are also generated by exogenous sources such as ionizing radiation generating irreversible effects in the development of tissues in both animals and plants.
Reactive oxygen species
Reactive aldehyde species
Hydroxyl radical
Hydrogen peroxide
Cytokine
Neurotrophic factors
Nitric oxide
Lymphokine