Investigating physiological and biochemical

2.2.4. Reactive oxygen species (ROS) 
ROS are produced in plants as a consequence of electron leakage onto O
2
from 
electron transport activity of mitochondria, chloroplast and cell membranes or from 
different metabolic processes occurring in various cellular organs (Dell Rio et al., 
2006; Blokhina and Fagerstedt, 2010; Heyno et al., 2011). Any type of environmental 
stress whether biotic or abiotic stimulates the production of ROS due to disturbance of 
cellular homeostasis (Mittler, 2002; Hu et al., 2008; Han et al., 2009; Mishra et al., 
2011; Srivastava and Dubey, 2011).
When generation of ROS surpasses the scavenging or defense mechanism, cell 

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bears the state of oxidative stress. Cells under oxidative stress bears lipid peroxidation, 
protein oxidation, DNA damage, inhibition of enzyme activities, initiation of 
programmed cell death due to ROS activity which ultimately leads to cell death 
(Verma and Dubey, 2003; Wang et al., 2003; Vinocur and Altman, 2005; Pitzschke et 
al., 2006; Mishra et al., 2011; Srivastava and Dubey, 2011). To quench and scavenge 
the ROS, plants have well defined and efficient system of non-enzymatic (tocopherols, 
carotenoids) and enzymatic (SOD, CAT, POD, APX) antioxidants (Kim et al., 2005; 
Nawaz et al., 2010; Ali et al., 2011).
Many evidences showed that high level of antioxidants cause more resistance to 
oxidative damage by ROS in plants (Bhutta, 2011; Nabati et al., 2011). For example, 
Cavalcanti et al. (2004, 2007) observed the active role of SOD, APX, CAT and GR in 
salt tolerance of maize and cowpea. However, Bose et al. (2014) in their recent 
review, argued that salt tolerant species possessing efficient mechanisms of Na
+
exclusion from the cytosol may not require a high level of antioxidant activity, 
because they do not allow excessive ROS production in the first instance. They 
suggested that H
2
O
2
signatures may operate in plant signaling networks, in addition to 
well-known cytosolic calciumsignatures. They also suggested that intrinsically higher 
SOD levels in halophytes are required for rapid induction of the H
2
O
2
signature and to 
trigger a cascade of genetic and physiological adaptive responses, while the role of 
other enzymatic antioxidants may be in decreasing the basal levels of H
2
O
2
, once the 
signaling has been processed.