ABSTRACT
Oxygen (O2) is the most abundant molecule in the biological system and is often a source of free radicals as its partially reduced species are generated through normal metabolic processes. Most of the O2 consumed by nonphotosynthetic plant tissues is reduced to water by the terminal oxidase(s) of the respiratory electron transport chain in the mitochondria (Apel and Hirt, 2004). Reactive oxygen species (ROS) are the partially reduced forms of molecular oxygen that result from either the excitation of O2 to form singlet oxygen or the transfer of one, two, or three electrons to O2 to form, respectively, superoxide (O2
−), hydrogen peroxide (H2O2) or hydroxyl radical (OH·) (Hodges et al., 2004). Both O2
− and OH· are extremely reactive and can cause oxidative injury leading to cell death. The average life span of these ROS varies from nanoseconds (e.g., OH) to milliseconds (e.g., O2
−, H2O2). The OH· can also be generated by the interaction of O2
− and H2O2 in the presence of transition metal ions, so called “Haber-Weiss reaction.” Cells do not possess detoxication mechanism for OH· due to its very high reactivity, and rely on mechanisms preventing its formation. These mechanisms include the preceding elimination of O2
− and
11.1 Introduction .................................................................................................. 237 11.2 Mechanisms for ROS Scavenging ................................................................ 238
11.2.1 Enzymatic Antioxidants ................................................................... 239 11.2.2 Nonenzymatic Antioxidants ............................................................. 239
11.3 Postharvest Oxidative Stress......................................................................... 241 11.4 Factors Affecting Postharvest Oxidative Stress ........................................... 243
11.4.1 Genotype/Cultivar ............................................................................ 243 11.4.2 Harvest Maturity ...............................................................................244 11.4.3 Storage Temperature and Duration ...................................................246 11.4.4 Storage Atmospheres ........................................................................248 11.4.5 Postharvest Treatments ..................................................................... 252
11.4.5.1 1-Methylcyclopropene ........................................................ 252 11.4.5.2 Nitric Oxide ....................................................................... 253
11.5 Conclusions ...................................................................................................254 References .............................................................................................................. 255
H2O2 and/or sequestering metal ions that catalyze the Haber-Weiss reaction with metal-binding proteins such as ferritins or metallothioneins (Gechev et al., 2006). In addition to H2O2, O2
− can also react with nitric oxide radical (NO·) to form peroxynitrite (ONOO−), which can rapidly protonate to peroxynitrous acid (ONOOH), a powerful oxidizing agent. The reactions among various types of ROS can, therefore, generate intermediates or products that are capable of causing extreme levels of oxidative injury to the cell.
