## 5 The Mode of Action/Plant Response Mechanisms to the Crude Oil-Induced Abiotic Stresses
### Oxidative Stress  
Oxidative stress is defined as a \"state in which oxidation exceeds the antioxidant systems in the body secondary to a loss of balance between them\" ([131], p. 271) and according to [PERSON] ([132], p. 176) \"its development is either the reason, or common event of many pathological states, including aging\". In line with [PERSON] [132], oxidative stress occurs in situations where the equilibrium between ROS generation and elimination is upset leading to their enhanced steady-state level. [PERSON] et al. [92] referred it to a shift in balance between oxidant/antioxidant which favours the oxidants as shown in Figure 10. This perturbation of the equilibrium between generation and scavenging of ROS may be caused by various biotic and abiotic stress factors which are known to reduce global crop production [133]. The ROS consists of free radicals, reactive molecules, as well as ions that are derived from O\({}_{2}\), which in plants, depending on their concentrations, can either be harmful or beneficial [115]. The most common ROS are singlet oxygen (\({}^{1}\)O\({}_{2}\)), superoxide anion (O\({}_{2}\)\({}^{\bullet-}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), and hydroxyl radical (OH\({}^{\bullet}\)) [115]. At elevated concentrations, ROS bring about damage to biomolecules while at low/moderate concentration, they function as second messengers in intracellular signaling cascades that mediate several responses in plant cells [115] such as stomatal closure [115; 134; 135], programmed cell death [115; 136; 137], gravitropism [115; 138], and acquisition of tolerance to both biotic and abiotic stresses [115]. [PERSON] [139] added that ROS levels that are too low (cytostatic level) or too high (cytotoxic level) have a negative effect on plant growth and development while ROS level within the right range (basal level) support plant health. As noted by [PERSON] et al. [115], in plants, ROS are generated in both unstressed and stressed cells at different locations in chloroplasts, mitochondria, plasma membranes, peroxisomes, apoplast, endoplasmic reticulum, and cell walls. These authors added that the ROS are usually produced as a result of the leakage of electrons onto O\({}_{2}\) from the electron transport activities of chloroplasts, mitochondria, and plasma membranes or as byproducts of various metabolic pathways in different cellular compartments. Generally, under ideal growth conditions, the generation of ROS in organelles is low [140] but are formed excessively during adverse/stressful conditions [140; 141], which can bring about damage to biomolecules like lipids, proteins, and deoxyribonucleic acid (DNA) [115]. These reactions according to [PERSON] et al. [115] can affect intrinsic membrane properties such as fluidity, ion transport, loss of enzyme activity, protein cross-linking, inhibition of protein synthesis, and DNA damage, which  
Figure 10: Relationship between oxidants (ROS) and antioxidants (AOX) leading to oxidative stress. eventually lead to cell death. [PERSON] and [PERSON] (2019) pointed out that in addition to the ability of ROS to damage cells, they can also initiate responses such as new gene expression. Plants' Responses to ROS and Oxidative Stress (Antioxidant Systems)  
In line with [PERSON] and [PERSON] (2019), the balance between generation of ROS and scavenging at the proper site as well as time, determines whether ROS will act as damaging, protective or signaling factors. Hence, as a result of the multifunctional roles of these ROS, cells must control their levels to prevent any oxidative injury but not eliminate them completely ([PERSON] and [PERSON], 2019). The scavenging or detoxification of excess ROS is carried out by the antioxidant system comprising the non-enzymatic antioxidants and enzymatic antioxidants ([PERSON] and [PERSON], 2019).