## 5 The Mode of Action/Plant Response Mechanisms to the Crude Oil-Induced Abiotic Stresses
### Water (Drought) and Osmotic Stresses
#### 5.1.1 Plants' Responses to Water Stress  
[PERSON] and [PERSON] [68] indicated that plant adaptation to dry environments can be expressed at four different levels which are phenological or developmental, morphological, physiological, and metabolic while the latter (metabolic or biochemical adaptation) is the least understood. Whereas some responses of plants to water stress occur at the leaf level [69], it is the general response at the whole-plant level, involving carbon assimilation and the distribution of photoassimilates to different plant parts and reproductive ability that eventually determine the survival of these plants and persistence under environmental stress [69]. Nonetheless, carbon assimilation at this whole-plant level always decreases as a result of (1) reduction in CO\({}_{2}\) diffusion into the leaf; (2) diversion of carbon distribution to non-photosynthetic organs and defence molecules; or (3) biochemical changes in the leaf leading to the down-regulation of photosynthesis [69]. The various responses of plants to water stress are summarised in Table 2.  
Figure 7: Illustration of the mechanistic link between crude oil contamination/stress and the crude oil-induced abiotic stresses as it affects agrifood production. \begin{table}
\begin{tabular}{c c c c} \hline
**Type of Response** & **Impact Indicators** & **Parts of Plants/Processes Affected** & **References** \\ \hline \multirow{6}{*}{Crop growth and yield} & \multirow{3}{*}{Crop growth and yield} & Reduced growth parameters such as height, leaf number, leaf area index as well as fresh and dry weight. & [PERSON] and [PERSON] [68] \\  & & well as fresh and dry weight. & [PERSON] et al. [70] \\ \cline{3-4}  & \multirow{3}{*}{Water relations} & * \({}^{*}\) Shoot growth may be more inhibited than root growth. & [PERSON] [71] \\  & & & [PERSON] and [PERSON] [72] \\ \cline{3-4}  & & \begin{tabular}{c} \({}^{*}\) Effects plant water relations, stomatal closure, gas exchange, transpiration, and carbon assimilation (photosynthesis). \\ \end{tabular} & [PERSON] [63] \\ \cline{3-4}  & & * \({}^{*}\)Stomatal opening and closing is more strongly influenced. & [PERSON] et al. [67] \\ \cline{3-4}  & & \begin{tabular}{c} Reduced ability of plant roots to absorb water and nutrients which could be \\ as a result of a decrease in nutrient element demand. \\ \end{tabular} & [PERSON] and [PERSON] [68] \\ \cline{3-4}  & \multirow{3}{*}{Nutrient relations} & Reduced availability, uptake, translocation and metabolism of nutrients. & [PERSON] et al. [67] \\ \cline{3-4}  & & Increase in K, Na, Ca, Mg, and Cl; decrease in P and Fe. & [PERSON] et al. [74] \\ \cline{3-4}  & & Increase in N; decrease in P; no effect on K. & [PERSON] et al. [67] \\ \cline{3-4}  & & \
\begin{tabular}{c} Lowering water deficiency impact and linked to the maintenance of \\ stomatal conductance, photosynthesis, leaf water volume, and growth. \\ \end{tabular} \\ \cline{3-4}  & & * \({}^{*}\)Most often temporary as plants usually respond quickly to increase in the level of available water. & [PERSON] and [PERSON] [68] \\ \cline{3-4}  & & * \({}^{*}\)Solutes accumulate with water stress and contribute to osmotic adjustment & \\ \cline{3-4}  & & * \({}^{*}\)In non-halophytes including inorganic cations, organic acids, carbohydrates, and amino acids. \\ \hline \multirow{2}{*}{Photosynthesis} & Negative effect on photosynthesis of crops and possibly a cessation in the photosynthetic process. & [PERSON] and [PERSON] [68] \\ \cline{2-4}  & &
\begin{tabular}{c} Often enhanced allocation of dry matter to the roots increasing root growth \\ which can support greater water uptake. \\ \end{tabular} & [PERSON] et al. [67] \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of plants’ responses to water stress.