Reactive Oxygen Species

Abstract

Molecular oxygen (O2) is essential for the survival of all aerobic organisms, because it acts as the final acceptor of mitochondrial electron chain. In this situation, O2 is reduced to H2O after the cytochrome-c oxidase reaction. Reactive oxygen species (ROS) are extremely reactive substances derived from the O2 that may be formed during the electron transfer reactions or through photon absorption by a sensitizer molecule and subsequent reaction either by energy or electron transfer reactions with O2. The ROS not only comprise free radicals such as superoxide (O2−•), hydroperoxyl (HO2•), hydroxyl (HO•), peroxyl (RO2•) and alkoxyl (RO•) radicals, but also non-radicals such as hydrogen peroxide (H2O2), singlet oxygen (1O2), and hypochlorous acid (HOCl). Radicals are unstable with variable reactivity. Figure 1 shows the typical ROS generation pathway. Several allelochemicals can generate an uncontrolled production and accumulation of ROS on target plants. This oxidative stress is often referred as part of harmful effects of these substances on target plants. Soil allelochemicals such as quinones or phenols may become semiquinone radicals that reduce the molecular oxygen to superoxide radical and then produce other ROS on soil matrix. It may be aided by catalytic metals such as aluminum or iron most abundant metals in soils. ROS and direct depolarization by some allelochemicals which cause cellular disruption and ultimately the cell death, can initiate the lipid peroxidation of plant root membranes. Soil allelochemicals also act as inhibitory agents of antioxidizing enzymes, such as superoxide dismutase and catalase, thus leaving the plant vulnerable to ROS mediated oxidative damage. ROS production by plant roots indicates the signal of plant defence mechanism upon microbial attack. ROS accumulates in plant cells in response to pathogen infections (causing damage to cells or finally the cell death). On the other hand, soil derived ROS e.g. H2O2, may contribute to the structural reinforcement of plant cell walls and coordinate the activation of defense genes and phytoalexin production. Many events cause ROS generation and allelochemicals production in soils, thus, soil microbiological, physicochemical and mineralogical studies often includes the ROS assessment, in soil matrix or plant roots. This chapter provides useful and sensitive tests to assess the ROS content in plant-soil system.