# The Effect of Antagonist Abiotic Stress on Bioactive Compounds from Basil (_Ocimum basilicum_)
## 4 Discussion  
Flooding and drought stress modifies a plethora of plant physiological functions ranging from cellular to organ. Plant roots are the first organs that sense the water limitation or excess, signaling stomatal closure and decreasing the carbon dioxide assimilation. In our experiment, the stomata conductance (\(g_{s}\)) for plants under flooding and drought falls over the days of stress. In drought stress, stomatal conductance to water vapor decreases faster than for flooding. In contrast, the assimilation rate is more affected by flooding than drought. It has been shown that under drought conditions, the plant growth process is slowed down [42], and due to stomata closure, there are limitations of the diffusion of CO\({}_{2}\) to chloroplasts [43]. Long-time exposure to drought often modifies phytohormal levels in plants, including abscisic acids, salicylic acid, jasmonate, and other metabolites [44]. Such behavior has been found for different C\({}_{3}\) plants in which drought mainly binds photosynthesis through stomatal closure [45]. The assimilation rate is decreasing as stomatal limitations dominate, irrespective of any metabolic impairment for mild drought stress. Furthermore, after re-hydration, the \(g_{s}\) did not recover at the same values, suggesting permanent damage of the photosynthetic apparatus. Usually, high humidity determines the stomata opening, but all photosynthetic parameters decrease under the long flooding period. For example, soil flooding _Lyopersion esculentum_ plants induce stomata closure within a few hours [46] while _Pisum sativum_ L. stomata close after 24 h from the start of flooding [46]. This stomatal closure under flooding may be due to the discharge of stress ethylene [47]. Basil plants exposed to flooding have reduced oxygen availability (hypoxia) in plant roots, being that it is difficult for gas diffusion to occur in the plant cells, which thus limits the gas exchange for photosynthesis and transpiration [48]. For plants under antagonist stresses, both photosynthesis parameters are decreasing despite which stress is the first one. For the flooding\(-\)drought plants, stomatal conductance to water vapor and assimilation rate decrease drastically at the end of the second treatment (day 29). Such behavior could be explained by the fact that flooding decreases contents of ribulose biphosphate and adenosine triphosphate, and in drought, the processes continue [45]. Moreover, for the plants under drought\(-\)flooding stress, such metabolic processes are less important. Plant species are triggering emissions of various characteristic stress volatiles in response to different abiotic stress (drought and flooding). The first compounds emitted from leaves under stress conditions are green leaf volatiles (GLVs). Those compounds include various five- and six-carbon alcohols, aldehydes, and ketones and are formed through the oxylipin pathway from C\({}_{18}\)-polyunsaturated fatty acids as \(\alpha\)-linolenic acid or linoleic  
Figure 6: Total concentration of phenols in _O. basilicum_ leaves under different stress conditions. acid [49; 50]. The emission of GLVs from basil leaves under drought and flooding stress starts from the first day and increases until a maximum, followed by a plateau. The same effect has been shown for the lipoxygenase activity during drought stress in _Rosmarinus officinalis_[51] and the leaves of _Q. robur_ trees exposed to drought stress [52]. Such an increase in GLV emissions could be due to the damage of the physical membrane as the enrichment of the lipoxygenase activity [53]. For the combined treatments, drought\(-\) flooding and flooding\(-\) drought, the emission of GLVs initially decreases, followed by an increase until a maximum. Such behavior has been shown for combined stress like drought and ozone [52] and corresponds to the effects of the lipoxygenase and hydroperoxide lyase systems which release those aldehydes and alcohols.