Nature of damage
Transfers its excitation energy to other biological molecules and damages photosynthetic machinery
Reduces quinones and transition metal complexes (Fe2+ and Cu2+)
Oxidise the thiol group of enzymes and inactivates them
Reacts with DNA, proteins, lipids, and other constituents of cell, in excess can lead to cell death
Catalases, superoxide dismutase, acsorbate peroxidase, guaiacol peroxidases, glutathione peroxidases
No enzymatic mechanism for elimination
Tocopherol, ß-carotene, plastoquinone
Glutathione, peroxiredoxins, ascorbate, glutathione, flavonoids
Ascorbic acid, glutathione, proline, flavonoids
References for half life and diffusion distance - Pitzschke et al. (2006), de Carvalho (2008), Gill and Tuteja (2010); for nature of damage: Gill and Tuteja (2010) and Gechev et al. (2006); and for scavengers/anti-oxidants - Gechev et al. (2006); Ahmad et al. (2010)
molecular oxygen instead of NADP+, generating a burst of superoxide ions at PSI by Mehler reaction [PSI-+O2=PSI+O2-] (Asada 2006) (Fig. 7.3). The main electron donors to oxygen in ETC for the production of O2 - are Fe-S cluster and ferrodoxin in PSI and plastoquinones in PSII (Mehler 1951; Dat et al. 2000; Edreva 2005). The presence of transition metals such as Fe in ferro-doxin and Fe-S clusters or quinoids in plastoqui-nones assist in easy electron transfer to O2 (Edreva 2005) . 2
Peroxisomes have oxidative type of metabolism and hence probably one of the major sites of ROS production. Moreover, peroxisomes harbour various ROS producing enzymes including glycolate oxidase, acyl CoA oxidase, uricase, and xanthine oxidase (Rodriguez-Serrano et al. 2009). Under high light or other abiotic stress conditions, under insufficient CO2 for carboxylation by RuBisCO, would result in oxygenation of RuBP by RuBisCO leading to a process called photorespiration in C3 plants. Though photorespiration acts as a alternative sink for excess load of light energy, it generates H2O2 with the help of glycolate oxidase (Noctor et al. 2002) (Fig. 7.3). The other H2O2 production metabolisms in peroxisomes are the fatty acid b oxidation, the enzymatic reaction of flavin oxidases and the disproportionation of O2-radicals (Palma et al. 2009). Apart from H2O2, O2- are also produced in peroxisomes at two sites one inside the matrix in the presence of xanthin oxidase and another at membranes, which is dependent on NADH (del Rio et al. 1989, 1992, 1998, 2002) . Xanthine and hypo-xanthine in the presence of enzyme xanthine oxidase gives rise to uric acid and O2- (Fridovich 1986; Radi et al. 1992; del Rio et al. 2002) (Fig. 7.3).Whereas, at the peroxisomal membranes a small ETC composing of flavoprotien NADH:ferricyanide reductase and cytochrome b is known to be involved in the production of O2- (Fang et al. 1987; Lopez-Huertas et al. 1995; del Rio et al. 2002).
Comparatively, mitochondria generates smaller amounts of ROS (Foyer and Noctor 2005). Complex I and complex III of METC are the sites of ROS production (Rhoads et al. 2006; Moller et al. 2007) (Fig. 7.3). In METC, complex I and complex III harbours electrons with sufficient energy to directly reduce oxygen which is very essential for aerobic respiration. Ubisemiquinone intermediates formed at complex I and III donates electrons to generate O2- (Raha and Robinson 2000) while under drought, increased mitochon-drial respiration could enhance ROS production by transferring an electron from cytochrome to O2 (Norman et al. 2004). Further, under severe drought, increased demand for mitochondrial ATP compensating for reduced chlorophyll ATP synthesis could also enhance ROS production (Atkin and Macherel 2009). Among the two mitochondrial electron transport pathways, from ubiquinone to oxygen, alternate oxidase activity could result in maintaining normal metabolite levels with reduced ROS production under stress (Farooq et al. 2009) .
Another source for the production of ROS is the detoxification reactions carried out by cyto-chrome P450 in cytoplasm and endoplasmic reticulum. In this detoxification reaction, O+- is formed (Bolwell and Wojtaszek 1997). ROS is also generated at the plasma membrane or extra-cellularly in the apoplast in response to abscisic acid (ABA) and drought (Hu et al. 2005, 2006). NADPH oxidase of plasma membrane is also considered as a source of ROS production (Kwak et al. 2003) . Moreover, NADPH oxidase has a multimeric flavocytochrome that forms an ETC which has the capacity to reduce oxygen to superoxide. Apart from this, pH dependent cell wall peroxidases, germin-like oxalate oxidase, and amino oxidases in the apoplast are sources of H2O2 (Bolwell and Wojtaszek 1997).
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