**2. ROS and antioxidant system**

Oxygen is an essential element for the life of aerobic organism but it may become toxic at higher concentrations. Oxygen molecule in its ground state is relatively unreactive; but its partial reduction gives rise to reactive oxygen species (ROS). ROS are highly reactive oxygen molecules consisting of free radicals. Free radicals are an atom or molecule having an unpaired electron which is extremely reactive, starting chain reactions that generate many more free radicals, that are capable of attacking the healthy cells, causing them to lose their structure and function [1–5]. Types of ROS include the hydroxyl radical, the superoxide anion radical, hydrogen peroxide, singlet oxygen, nitric oxide radical, hypochlorite radical, and various lipid peroxides [1–5] (**Table 1**). Reduction of oxygen leads to the formation of the superoxide radical (O2 •−), which is a molecule with an uncoupled electron and can react with other molecules to stabilize its energy. Hydrogen peroxide (H2 O2 ) result from the nonenzymatic reduction of O2 •− in the presence of H+ ions, or from the action of catalase on O2 •−. H2 O2 has a strong oxidizing capacity, and its life span is longer than that of superoxide. H2 O2 can also diffuse through membranes and therefore reach target molecules at some distance from its production site [1–5].


**Table 1.** Main reactive oxygen species (ROS) [1].


**Table 2.** Main plant antioxidants [2].

also proved to act as a positive signal in seed dormancy release [30–33]. The dual function of ROS in plants depends on the levels of antioxidant compounds, and enzyme activities release [34–38]. By this way, plants can eliminate potentially harmful ROS that is produced under stress conditions, or control ROS concentrations in order to regulate various signaling pathways [34–38]. This dual function of ROS is a very interesting subject in seed physiology. Even though there is a huge progress in this field, and the dual functions of ROS are quite well documented in the literature, it should also be regarded from a different point of view. The involvement of ROS in seed filling processes is not well documented, and the mobility of ROS in seeds has not yet been documented, thus, more data is needed on roles of ROS in seed germination and development physiology. Under light of the increasing progress made in the understanding of mechanisms driven by ROS, the role of ROS in seed biology may need to be revisited. To date, many distinct roles for ROS, apart from their toxic effects, have

Oxygen is an essential element for the life of aerobic organism but it may become toxic at higher concentrations. Oxygen molecule in its ground state is relatively unreactive; but its partial reduction gives rise to reactive oxygen species (ROS). ROS are highly reactive oxygen molecules consisting of free radicals. Free radicals are an atom or molecule having an unpaired electron which is extremely reactive, starting chain reactions that generate many more free radicals, that are capable of attacking the healthy cells, causing them to lose their structure and function [1–5]. Types of ROS include the hydroxyl radical, the superoxide anion radical, hydrogen peroxide, singlet oxygen, nitric oxide radical, hypochlorite radical, and various lipid peroxides [1–5] (**Table 1**). Reduction of oxygen leads to the formation of the

•−), which is a molecule with an uncoupled electron and can react with

O2

ions, or from the action of catalase on O2

O2

O2 or 1 Δg ) result from the nonenzy-

•−. H2 O2

> O2 can

been identified.

168 Advances in Seed Biology

superoxide radical (O2

matic reduction of O2

its production site [1–5].

Superoxide, O2

**2. ROS and antioxidant system**

other molecules to stabilize its energy. Hydrogen peroxide (H2

**Free radicals Nonradicals**

Hydroperoxyl, HO2 Ozone, O3 Peroxyl, ROO· Singlet oxygen, 1

**Table 1.** Main reactive oxygen species (ROS) [1].

•− in the presence of H+

•– Hydrogen peroxide, H2

Hydroxyl, ·OH Hypochlorous acid, HOCl Alkoxyl, RO· Peroxynitrite, ONOO<sup>−</sup>

has a strong oxidizing capacity, and its life span is longer than that of superoxide. H2

also diffuse through membranes and therefore reach target molecules at some distance from

The Haber-Weiss and Fenton reactions involve superoxide radicals and H2 O2. In the presence of iron or other transition metals, O2 •− and H2 O2 lead to the formation of the hydroxyl radical, OH•, the most aggressive form of ROS, including the radical derivatives of oxygen (O2 •−, OH•), and also the peroxyl, alkoxyl or hydroperoxyl radicals, which are named as free radicals. Free radicals contain one or more unpaired electrons, but they also include nonradical derivatives of oxygen such as H2 O2 and singlet oxygen [2, 5]. These free radicals are highly toxic and electrically charged molecules, i.e., they have an unpaired electron which causes them to seek out and capture electrons from other substances in order to neutralize themselves, all are capable of reacting with membrane lipids, nucleic acids, proteins and enzymes, and other small molecules, resulting in cellular damage, thus generate oxidative stress in plants [1–5].

Plants have developed a wide range of defense strategies to combat with these free radicals and deactivate their harmful effects known as antioxidants. The evolution of efficient antioxidant systems has enabled plant cells to overcome ROS toxicity and to use these reactive species as signal transducers [4, 5]. Antioxidants have diverse physiological roles in plants, acting as a scavenging and deactivating agent against oxidation, and converting the radicals to less reactive species, even at relatively small concentrations. The antioxidative system copes up with the harmful free radicals both by enzymatic (superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductase (GR), polyphenol oxidase (PPO), etc.), and by nonenzymatic (ascorbic acid (vitamin C); α-tocopherol, carotenes, flavonoids, polyphenolics, etc.) systems (**Table 2**). Under unfavorable conditions such as extreme oxidative stress, this antioxidant system scavenges the toxic radicals, and thus helps the plants to survive through such conditions [1–5].
