Oxidative Stress in Plants

Any changes in the amount and intensity of abiotic factors (temperature, relative humidity, light intensity, and nutrients), which leads to a change in the normal form of plant physiology, is known as stress (Pandey et al., 2017). Biologically, stress is the result of an inconsistency that tends to prevent the normal systems from operating (Negrão, Schmöckel, and Tester, 2017). Due to any kind of stress, reactive oxygen species (ROS) are produced (Schieber and Chandel, 2014). These species have high energy or an extra electron charge due to electron excitation. More precisely, under stress conditions, homeostasis of intracellular oxidation-reduction is impaired, which results in the formation of oxidative stress and reactive oxygen species (Nita and Grzybowski, 2016). Oxidative stress is produced by increasing the number of reactive oxygen species or free radicals, which results in early aging, increased permeability, ions leakage from the cell membranes, and reduced photosynthesis in plants (Sharma et al., 2012). Free radicals result in cellular damage through lipid peroxidation (mainly cell membranes) and the blocking of natural antioxidants. By measuring the malondialdehyde (MDA), which is the result of lipid peroxidation, we can find the amount of stress in the plant cells (Grotto et al., 2009). Under oxidative stress conditions, by producing oxygen intermediates, which are relatively reduced or energy-intensive forms of atmospheric oxygen (O₂), the plant finds itself under stress conditions and activates a variety of defense systems such as antioxidants to protect against stress (Carvalho, 2008). The basis for measuring the amount of stress created in the plant, in many cases, is the measurement of the activity of the peroxidase enzymes, such as hydrogen peroxide (H₂O₂). In the case of the excessive accumulation of these activated oxygens, a variety of cell damage such as DNA damage, membrane lipid peroxidation, RNA damage, protein oxidation, and enzymatic inhibition, occurs in the cell (Saini et al., 2018). Free oxygen radicals or lipid peroxidation reactions in the plant membrane will selectively break up unsaturated fatty acids and accumulate hydrocarbons, aldehydes, and the like (Frankel, 1984). Therefore, in some studies, in order to determine the effect of environmental stresses on the membrane of plant cells, the amount of lipid peroxidation products, such as malondialdehyde (MDA) or hydrogen peroxide (H₂O₂), is measured and their results suggest the involvement of free oxygen radicals in response to stress (Birben et al., 2012).86