Article Information Reactive oxygen species (ROS) are a by-product of normal cell metabolism in plants; however, the balance between production and elimination is disturbed under stress conditions. Several reactive oxygen species are continuously produced in plants as secondary products of aerobic metabolism. Depending on the source of the ROS species, some of them are highly toxic. Plants cellular rapidly use various enzymatic and nonenzymatic mechanisms for detoxify ROS species. Enhanced

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... of ROS and absence of detoxify systems, can cause damage to biomolecules such as lipids, proteins and DNA and eventually cause to cell death. Despite their destructive activity, they are second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. This review paper describes the Variety of Reactive oxygen species, sources and roles of ROS in plants.