## 6 Drought Stress and Secondary Metabolites  
Secondary metabolites are produced by plants in the attempt to respond to various environmental stresses [90; 91]. It is recognized that the biosynthesis of secondary metabolites is regulated by environmental factors, for instance temperature, light regime and nutrient availability [92]. Improved production of secondary metabolites is usually observed under water deficit conditions, which is caused by reduction in biomass formation and destination of assimilated CO\({}_{2}\) to C-based secondary metabolites to avoid sugar-promoted feedback of photosynthesis (Figure 3) [93].  
In _Hypericum brasiliense_, concentration of phenolic acids is considerably enhanced when grown in water deficit conditions [94]. In two native sub species of Iranian _Origanum vulgare_, i.e., subsp. _gracile_ and subsp. _Virens_, the content of sesquiterpene (E) \(\beta\)-caryophyllene strongly increased by water limitation [95]. Under mild and mild/severe drought, the content of oleanolic acid and betulin increased in _Betula platpyphylla_[96] and level of triterpenoid glycyrrhizin in Glycyrrhiziza glabra [97]. The lignin content was increased in Dermudagrass Tifton-85, which is a variety of _Cynodon dactylIon_ [PERSON], under drought conditions [98]. The flavonoids content was enhanced under stress conditions and high-water deficit conditions improved the medicinal properties of _Labisia pumila_[99]. _Phaseolus lunatus_ under water deficit condition had an elevated level of cyanogenic glucosides [92]. In Lamiaceae family, the content of essential oils declined in _Lavandula latifolia_ and _Salvia sclarea_, whereas, in _Mentha piperita_, _Salvia lavandulifolia_, _Thymus capitatus_ and _Thymus mastichina_, the essential oil amount was enhanced under drought conditions and the increase was attributable to a higher concentration oil glands due to decrease in leaf area [100]. The amount of phenolics and flavonoids increased in _Achillea_ species against drought stress [76]. The content of phenolic acids simultaneously improved, while the level of flavonoids declined in _Achillea pachycephala_[101].  
Figure 3: Enhanced synthesis of secondary metabolites under drought stress. Light energy captured by the photosynthetic machinery is considerably greater than the energy essential for the CO\({}_{2}\) fixation. Energy dissipation takes place by non-photochemical quenching and re-oxidation of NADPH + H\({}^{+}\), i.e., via xanthophyll cycle and C\({}_{2}\) cycle. Endogenous CO\({}_{2}\) level is low because of the escalated diffusion resistance caused by closing of stomata. Hence, a smaller amount of NADPH + H\({}^{+}\) is utilized in the C\({}_{3}\) cycle for the fixation and reduction of CO\({}_{2}\), and, ultimately, a greater amount of energy has to be dissipated. Protective activities such as non-photochemical quenching, C\({}_{2}\) cycle and xanthophyll cycle are boosted by feedback mechanisms; a number of e\({}^{-}\) is transported to O\({}_{2}\) (Mehler reaction). Generation of O\({}_{2}\)\({}^{-}\) ions further produce various ROS. Due to the stress-associated stimulation of SOD and APX, detoxification of the O\({}_{2}\)\({}^{-}\) ions occurs and therefore results in reduction of generation of ROS. Greater enhancement in the reduction potential, i.e., the ratio of NADPH + H\({}^{+}\) to NADP\({}^{+}\), elevates the plants secondary metabolites synthesis (modified from [PERSON] and [PERSON] [90]).