Reactive Oxygen Species Production and Detoxification During Leaf Senescence

The natural leaf senescence is the last stage of leaf development and is a highly genetically regulated process. Induced leaf senescence, instead, usually occurs in detached leaves or in plants placed in dark conditions under postharvest storage. In both cases, an oxidative burst is a typical feature of this phenomenon and includes a significant increment in reactive oxygen species (ROS), such as superoxide anion, hydrogen peroxide, hydroxyl radical, and singlet oxygen. Thus, ROS play a key role in leaf senescence process. Under normal (i.e., non-stressed) conditions, the levels of ROS are controlled and balanced by the antioxidant systems present in leaves. The leaf cells act in the defense response against the accumulation of ROS by enhancing the activity of some key enzymes such as catalase (CAT) and superoxide dismutase (SOD). In addition, the ascorbate–glutathione cycle, also known as Asada–Halliwell cycle, is the most important enzymatic antioxidant system acting in the leaf cells during senescence. The leaves are considered the main target of ROS production, and in particular, the photosynthetic apparatus and the related molecules act to support the light perception and photosynthesis activity (chlorophylls and carotenoids).