**4. Examples of oxidative stress indices**

### **4.1 Lipid peroxidation**

226 Artificial Photosynthesis

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;

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,

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

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).

2010).

2009).

(Mittler et al., 2004).

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 al., 2005).

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

#### **4.2 Hydrogen peroxide**

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

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

Instinctive Plant Tolerance Towards Abiotic Stresses in Arid Regions 229

Thylakoids are considered to be one of the major sites of superoxide production because of the simultaneous presence in chloroplasts of a high oxygen level and an electron transport system. Most of the superoxide is produced by photosystem I via the univalent reduction of oxygen through the ferredoxin / ferredoxin NADP+ oxidoreductase system (Mehler reaction). The use of DCMU, the known inhibitor of photosynthetic electron transport, and the use of the new spin trap DEPMPO have demonstrated that photosystem II also

The modifications of the chloroplast in response to various environmental stresses have been widely studied in different laboratories and, thus the literature in the area is vast. The stress is sensed at the levels of pigment composition, structural organization, primary

Spatial and temporal complexity of photosynthesis makes photostasis prone to stress. The sequence of photosynthesis is known to cover a wide time-span and begins with photophysical and photochemical events, i.e. light absorption, excitation energy transfer and charge separation in the timescale of femtoseconds (10–15 s) to nanoseconds (10–9 s). This is followed by electron transport in the microseconds (10–6 s) to milliseconds (10–3 s) range, and finally by enzyme mediated reactions in the milliseconds to seconds range. Relatively slow reactions are rate-limiting and thus, incompatible with the fast reactions. Further, the fast primary photochemical reactions are relatively stress-resistant compared to temperaturedependent, slow, enzyme-mediated reactions associated with the electron transport system and carbon dioxide fixation in the Calvin–Benson cycle (Krause & Jahns, 2004). This results in the development of excitation pressure at the source. Since plants are photoautotrophs, light at any intensity in combination with other environmental stresses can bring a change in photostasis in terms of accumulation of excess unutilized quanta because of weakened sink demand induced by stress. In addition, high light always accumulates excess energy at the 'source'. NPQ of excess quanta at the source is one of the major processes for restoration of

Fig. 10. Electron transport system in the thylakoid membrane showing three possible sites of

contributes to superoxide production (Navari-Izzo et al.,1998).

the balance and maintenance of photostasis(Biswal et al., 2011).

activated oxygen production (Elstner, 1991; Bryan, 1996).

photochemistry and the CO2 fixation(Biswal et al., 2003; Biswal, 2005).

Fig. 9. Involvement of H2O2 and NO in cellular responses to various stresses and stimuli(Desikan et al., 2004).
