Figure 1.

Overview of reactive oxygen species in plants.A, main ROS in plants, separated whether they exist as nonradicals or free radicals. Color indicates their overall reactivity, pale orange = low reactivity, orange = intermediate reactivity, and red = highly reactive. Reduction potentials for oxygen species shown in parentheses are from the study by Imlay (183). B, factors involved in the generation of ROS in plants. C, summary of the relationship between the Mehler, Haber–Weiss, and Fenton reactions. The Mehler reaction oxidizes water and oxygen to O₂^•−, which is subsequently dismutated into H₂O₂ by superoxide dismutase (SOD). The Haber–Weiss and Fenton reactions oxidize H₂O₂ to highly reactive ^•OH and OH⁻ in plants (see text for more detailed description of these reactions). H₂O₂, hydrogen peroxide; O₂^•−, superoxide; ^•OH, hydroxyl radical; OH⁻, hydroxide; ROS, reactive oxygen species.