# The Effect of Antagonist Abiotic Stress on Bioactive Compounds from Basil (_Ocimum basilicum_)
## 4 Discussion
Because sweet basil is a very well-known spice worldwide due to its distinctively sweet odor and flavor, many investigations have been conducted to investigate the composition of terpen in _O. basilicum_ emission [14; 15; 16; 28; 29; 54; 55; 56]. The volatile emission from basil leaves is mainly alpha-pinene, camphene, sabinene, beta-pinene, 3-carene, D-limonene, gamma-terpinene, linalool, camphor, and alpha-terpineol. The total emission of monoterenes increases for plants under drought and flooding stress. The same pattern has been found for constitutively monoterene-emitting species under drought [57; 58; 59; 60] and flooding stress [61]. For plants under a second stress, the emission increase at the second maxim suggests overexpression of the non-mevalonate (MEP) pathway genes as terpen synthase [62]. Under drought or flooding stress, the volatile isoprenoid compounds can protect against abiotic stress when the stomata close. Furthermore, volatiles can protect against abiotic stress by stabilizing membranes and serving as antioxidants [63]. Some other compounds that can act as an elicitor to improve secondary metabolism _S. officinalis_ plants were found to be nano-TiO\({}_{2}\)[64]. Drought stress is one of the most severe limitations to photosynthesis, decreasing carotenoid pigments in the leaves. Lower chlorophyll content under drought stress may help plants reduce photooxidative damage, which occurs when photosynthesis is inhibited [65; 66]. The chlorophyll a and b decrease in the leaves of plants under flooding stress compared with the control. Even more, the reduction is more production for plants under flooding\(-\)drought stress. The decline in photosynthetic pigments could be due to slow synthesis, fast destruction of chlorophylls, or a nitrogen deficiency due to dilution and leaching. Drought stress also induces leaf necrosis due to the lack of anthocyanin pigments in the late stage of vegetation [67]. The effects of drought stress can be compensated by applying a small quantity of putrescine (20 mg L\({}^{-1}\)) in _Thymus vulgaris_ L. plants, which leads to a higher yield of essential oil (23.07%) [68; 69]. The phenylpropanol pathway in plants is disturbed by different abiotic factors such as cold, heat, drought, salinity, heavy metal toxicity, etc. [70]. Drought stress usually enhances phenolic acids and flavonoids [71]. Under drought conditions, the accumulation of glycine betaine was recognized to have positive functions in maintaining the integrity and stability of the membrane of cells [72]. Numerous studies have reported that the quality of products obtained from medicinal plants can be improved by applying drought stress [73; 74; 75]. In case of drought, flooding, and salt stress, due to stomata closure, the stress-induced synthesis and buildup of secondary metabolites, such as isoprenoids and phenolic compounds, is observed. This is an effect of oversupply of NADPH\({}^{+}\)H\({}^{+}\) from the Calvin cycle due to a decrease in CO\({}_{2}\) fixation [73; 74], compared to plants grown in the normal condition which assimilates carbon and energy from photosynthesis and utilizes a part of it for cell maintenance, while the other part is used for plant growth and the synthesis of defensive compounds [73]. In contrast, in our study, the total flavonoids and phenols decrease for plants under stress conditions. It has been shown that severe drought induced a decrease of total flavonoids and phenols in leaves of _Quercus ilex_[76]. Such decreasing could indicate a lower capacity to regulate reactive oxygen species and the oxidative load in _O. basilicum_, a relative stress-sensitive species [77].