**3. Spontaneous relationship between abiotic stress and oxidative stress**

The reactive oxygen species (ROS) that arise from normal metabolic processes are kept under tight control by various antioxidant mechanisms. ROS are important signal molecules that regulate many physiological processes, including environmental stress responses. Under steady state conditions, the ROS molecules are scavenged by various antioxidative defense mechanisms (Foyer & Noctor, 2005). The equilibrium between the production and the scavenging of ROS may be perturbed by various biotic and abiotic stress factors such as salinity, UV radiation, drought, heavy metals, temperature extremes, nutrient deficiency, air pollution, herbicides and pathogen attacks. The ability to utilize oxygen has provided plants with the benefit of metabolizing fats, proteins and carbohydrates for energy; however, it does not come without cost.

Oxygen is a highly reactive atom that is capable of becoming part of the potentially damaging molecules commonly called "free radicals" which appear to be a major contributor to aging and damage the cell. Fortunately, free radical formation is controlled naturally by various beneficial compounds known as antioxidants that protect cellular membranes and organelles from the damaging effects of active species. Antioxidants are the first line of defense against free radical damage, and are critical for maintaining optimum health and well being of the plant cells. The need for antioxidant becomes even more critical

Fig. 5. Visual symptoms of some abiotic stress(including drought, dust accumulation and heavy metal pollution) on some desert plants in arid environment (Ibrahim & El-Gaely,

**3. Spontaneous relationship between abiotic stress and oxidative stress** 

The reactive oxygen species (ROS) that arise from normal metabolic processes are kept under tight control by various antioxidant mechanisms. ROS are important signal molecules that regulate many physiological processes, including environmental stress responses. Under steady state conditions, the ROS molecules are scavenged by various antioxidative defense mechanisms (Foyer & Noctor, 2005). The equilibrium between the production and the scavenging of ROS may be perturbed by various biotic and abiotic stress factors such as salinity, UV radiation, drought, heavy metals, temperature extremes, nutrient deficiency, air pollution, herbicides and pathogen attacks. The ability to utilize oxygen has provided plants with the benefit of metabolizing fats, proteins and carbohydrates for energy; however, it

Oxygen is a highly reactive atom that is capable of becoming part of the potentially damaging molecules commonly called "free radicals" which appear to be a major contributor to aging and damage the cell. Fortunately, free radical formation is controlled naturally by various beneficial compounds known as antioxidants that protect cellular membranes and organelles from the damaging effects of active species. Antioxidants are the first line of defense against free radical damage, and are critical for maintaining optimum health and well being of the plant cells. The need for antioxidant becomes even more critical

2011).

does not come without cost.

with increased exposure to free radicals. Each organelle has potential targets for oxidative stress as well as mechanisms for eliminating the noxious oxyradicals. Therefore, plants are equipped with complex antioxidant systems composed of low molecular weight antioxidants non enzymatic compounds, like lipid soluble and membrane-associated tocopherol; ascorbate and glutathione (Foyer 1993), (Foyer & Noctor, 2005) as well as protective antioxidant enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidases (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2). Other components of this system, monodehydroascorbate radical reductase, and glutathione reductase serve to maintain the antioxidants in their reduced functional state (Schwanz et al.,1996) Whether this is the case or not, the antioxidant defenses appear to provide crucial protection against oxidative damage in cellular membranes and organelles in plants grown under unfavorable conditions (Smirnoff 1993 and Kocsy et al.,2000).

Ibrahim & Sameera, 2011 showed that the activity of peroxidise (POD) and CAT of *Lepidium sativum* treated with lead mainly displayed biphasic responses due to increased Pb2+ level. SOD activity under elevated lead stress was steadily stimulated with increasing metal ions level in medium up to 600 ppm. The results showed that, under high metal stress, POD and CAT activities were inhibited, while SOD activity was stimulated, indicating that those enzymes are located at different cellular sites, which had different resistance to heavy metals. Thus, the deterioration of cellular system functions by high metal stress might result in inhibition of enzyme activity (Fig. 5)

Fig. 6. Antioxidant enzyme activities POD (A), SOD (B) and CAT(C) of *Lepidium sativum* leaves subjected to various concentrations of Pb2+. Each value represents the mean ±SE of five replicates. Significant differences (P<0.05) between treatments according to LSD test are shown by an asterisk (Ibrahim & Bafeel, 2011).

Instinctive Plant Tolerance Towards Abiotic Stresses in Arid Regions 227

It has been recognized that during lipid peroxidation (LPO), products are formed from polyunsaturated precursors that include small hydrocarbon fragments such as ketones, malondialdehyde (MDA), etc and compounds related to them ( Garg & Manchanda, 2009) . Some of these compounds react with thiobarbituric acid (TBA) to form colored products called thiobarbituric acid reactive substances (TBARS) (Heath & Packer, 1968). LPO, in both cellular and organelle membranes, takes place when above-threshold ROS levels are reached, thereby not only directly affecting normal cellular functioning, but also aggravating the oxidative stress through production of lipid-derived radicals ( Montillet et

Hydrogen peroxide (H2O2) plays a dual role in plants: at low concentrations, it acts as a signal molecule involved in acclimatory signaling triggering tolerance to various biotic and abiotic stresses and, at high concentrations, it leads to programmed cell death (PCD) (Quan et al., 2008). H2O2 has also been shown to act as a key regulator in a broad range of physiological processes, such as senescence (Peng et al., 2005), photorespiration and photosynthesis (Noctor & Foyer, 1998), stomatal movement (Bright et al., 2006), cell cycle

Also, H2O2 is starting to be accepted as a second messenger for signals generated by means of ROS because of its relatively long life and high permeability across membranes (Quan *et al*., 2008). In an interesting study the response of pre-treated citrus roots with H2O2 (10 mM for 8 h) or sodium nitroprusside (SNP; 100 mM for 48 h) was investigated to know the antioxidant defense responses in citrus leaves grown in the absence or presence of 150 mM NaCl for 16d (Tanoua *et al*., 2009). It was noted that H2O2 and SNP increased the activities of leaf antioxidant enzymes such as, superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) along with the induction of related-

**4. Examples of oxidative stress indices** 

Fig. 8. Production of lipid-derived radicals via lipid peroxidation

(Mittler et al., 2004) and growth and development (Foreman et al., 2003).

isoform(s) under non-NaCl-stress conditions.

**4.1 Lipid peroxidation**

**4.2 Hydrogen peroxide** 

al., 2005).

Oxygen free radicals or activated oxygen has been implicated in diverse environmental stresses in plants and animals and appears to be a common participation in most, if not all, degenerative conditions in eukaryotic cells. The peroxidation of lipid, the cross-linking and inactivation of proteins and mutations in DNA are typical consequences of free radicals, but because the reactions occur quickly and often are components of complex chain reactions, we usually can only detect their ″footprints″. Accumulation of ROS as a result of various environmental stresses is a major cause of loss of crop productivity worldwide (Mittler, 2002), ( Apel and Hirt, 2004) , (Khan & Singh, 2008), (Mahajan & Tuteja, 2005) , (Tuteja , 2007; 2010).

Fig. 7. Changes in the rate of superoxide production rate in roots of untreated and plants of *Lepidium sativum* subjected to various concentrations of Pb2+ for 10 days (Ibrahim & Bafeel, 2009).

Oxidative stress is a condition in which ROS or free radicals, are generated extra- or intracellular, which can exert their toxic effects to the cells. These species may affect cell membrane properties and cause oxidative damage to nucleic acids, lipids and proteins that may make them non functional. It is well documented that various abiotic stresses lead to the overproduction of ROS in plants which are highly reactive and toxic and ultimately results in oxidative stress. In an environment of molecular oxygen (O2), all living cells are confronted with the reactivity and toxicity of active and partially reduced forms of oxygen: singlet oxygen (1O2), superoxide anion (O2.-), hydroxyl radical (HO.), and hydrogen peroxide (H2O2), which can lead to the complete destruction of cells (Mittler et al., 2004).

These reactive oxygen species (ROS) can show acute production under conditions such as ultraviolet light, environmental stress, or anthropic action through xenobiotics such as herbicides. However, their production is also directly and constantly linked with fundamental metabolic activities in different cell compartments, especially peroxisomes, mitochondria, and chloroplasts. In plants, the links between ROS production and photosynthetic metabolism are particularly important (Rossel et al., 2002).
