## 5 The Mode of Action/Plant Response Mechanisms to the Crude Oil-Induced Abiotic Stresses
### Anaerobic Stress  
Higher plants, being aerobes, need molecular oxygen from their environment for survival [82]. However, under certain environmental conditions, there is a possibility of shortage in the supply of O\({}_{2}\) to plant tissues [83]. When this happens, that is, when there is restricted aeration of part or all of the plant, \"the resulting tissue hypoxia or anoxia inevitably suppresses oxygen-dependent pathways especially the energy-generating system, disturbs functional relationships between organs such as roots and shoots, as well as suppresses both carbon assimilation and photosynthetic utilization\" [82].  
This shortage in the supply of oxygen has more direct effect on underground organs such as the roots and seeds; the shoot systems are then indirectly affected as a result of the negative impact of the stress on the root functions upon which the shoots depend [82]. The mitochondria have also been identified by [PERSON] et al. [84] as suffering from oxygen deficiency before other cell organelles are affected. Most plants tissues can, however, withstand anoxia (lack of oxygen) only for a short period of time before irreversible damage occurs [85].  
Plants' Responses to Anaerobic Stress  
Depletion of oxygen affects cell physiology; it alters gene expression, energy consumption, cellular metabolism, growth, and development [86]. Based on findings by the authors, [PERSON] et al. [87] indicated that lack of oxygen primarily results in a decrease in adenylate energy charge, cytoplasmic acidification, anaerobic fermentation, increase in cytosolic Ca\({}^{2+}\) concentration, alterations in the redox state, and a decline in the membrane barrier function. Low-oxygen (hypoxia) stress is also reported to induce significant changes in the transcriptome as well as a shift from aerobic to anaerobic respiration [88].  
1. Pasteur effect: [PERSON] et al. [89] identified the Pasteur effect as a common eukaryotic response to oxygen deficiency at the cellular level. According to [PERSON] et al. ([90], p. 721), this definition \"that the rate of fermentation rises when oxygen is excluded\" for the Pasteur effect by [PERSON] [91] has long been accepted in place of that involving \"the inhibition of glycolysis by respiration\". [PERSON] et al. [85] added that most anaerobic-intolerant plants exhibit a pronounced Pasteur effect.
2. ROS production and oxidative stress: Like many other stress conditions, hypoxia is associated with the excess generation of ROS [87]. Along these lines, there are two models which suggest (1) a decrease in ROS under oxygen deprivation [low NADPH-nicotine adenine dinucleotidephosphate [92]--oxidase activity); or (2) an increase in ROS due to inhibition of the mitochondrial electron transport chain.
3. Gene expression: As the synthesis of several proteins involved in glycolysis and fermentation processes is induced in plants under anaerobic conditions [93], [PERSON] and [PERSON] [94] noted that plants respond to low O\({}_{2}\)-stress condition via specific alterations in gene expression. Generally, anaerbiosis gives rise to the alteration of gene expression in plants which leads to the accumulation of anaerobic proteins (ANPs) [95] many of which are metabolic pathway enzymes [94].