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    STUDIA BIOLOGIA - Ediţia nr.2 din 2009  

  Rezumat:  During their life cycle, plants are continuously exposed to different types of environmental stress factors, such as extreme temperatures, drought, and high intensities of photosynthetically active radiation, air pollution and pathogens. As a direct result of stress exposure, plants’ metabolic functions are affected through the increased production of reactive oxygen species. These short-lived molecules inactivate enzymes and mediate damage of the cellular components. Depending on their nature, some of them are highly toxic and are rapidly detoxified by different cellular enzymatic and non-enzymatic mechanisms. In recent years, new roles of reactive oxygen species scavenging enzymes were providing the control and regulation of programmed cell death, hormonal signalling, stress responses, and development. Although reactive oxygen species perform a wide array of roles in plant cells, there is an evident controversy regarding their roles: they are necessary for the survival of plants, performing signal transduction and gene-activating roles, but they can also be lethal when they are accumulated. During optimal growth conditions there is equilibrium between the production and destruction of reactive oxygen species determining the fate of the plant. Under unfavourable growth conditions an imbalance occur between the production and destruction of reactive oxygen species that leads to oxidative stress. Plants possess a very efficient enzymatic and non- enzymatic scavenging system against reactive oxygen species. It is also important to mention, that the capacity and activity of the antioxidant defence system play a significant role in limiting oxidative damage by destroying reactive oxygen species that are produced in excess and damage metabolic processes of the plant. Here are described the most widely distributed mechanisms of reactive oxygen species generation and removal in plants.

Keywords: oxidative stress, reactive oxygen species, enzymatic defence system, non-enzymatic defence system
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