## 2 Characteristics and Biosynthesis  
Citral is an acyclic monoterenoid aldehyde consisting of two geometrical isomers, gerani (citral A, _trans_-citral, or the _E_-isomer) and neral (citral B, _cis_-citral, or the _Z_-isomer), in a 3:2 ratio. Citral has the chemical formula C\({}_{10}\)H\({}_{16}\)O, also known as 3,7-dimethylocta-2,6-dienal [21]. Figure 1 shows the structural formula of citral isomers and their main physical and chemical properties [22]. This monoterpene is found in different plants, citrus fruits, herbs, and grasses, being one of the most common essential oils (EOs) of _Cymbopogon_ species, mainly East Indian (_C. flexuosus_) and West Indian species (_C. citratus_), commonly known as lemongrass, with a citral content of 70-85% [23; 24]. Citral is also present in other plants' oils, including _Lindera citriodora_, _Cylypranthes parriculata_, Petitgrain, _Aloysia citrodota_, lemon ironbark, lemon balm, lime, lemon, orange, lemon tea-tree, among others [25]. Citral content in plant tissues can vary significantly depending on the species and environmental conditions (Table 1). The difference in the quantity can be attributed to the plant's genetics, climatic factors, seasonal variability, geographic zones, collection period, plant organs, and extraction techniques [26; 27]. Citral shows a strong lemon-like aroma and is widely used in cosmetics, fragrances, pharmaceutics, and as a flavor additive in the food industry [39]. Citral shows various biological activities such as antimicrobial [4], antioxidant [40], anticancer [41], anti-inflammatory [42], and others. A growing number of reports published the bioactivities of citral and citral-rich EOs, increasing medicinal and pharmacological significance. Citral is biosynthesized from geranyl diphosphate (GPP) molecules after various secondary transformations such as isomerization and acetylation, deacetylation, cyclization, and dehydrogenation (Figure 2) [43]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Essential Oil** & **Plant Material** & **Citral Content** & **Collection Site** & **Extraction Method** & **Ref.** \\ \hline _Cymbopogon citratus_ & Entire plant & 48.92\% & India & Steam distillation & [28] \\  & Entire plant & 62\% & Vietnam & \begin{tabular}{c} Steam distillation \\ under vacuum \\ \end{tabular} & [5] \\  & Aerial part & 74\% & - & \begin{tabular}{c} Solvent-free \\ microwave extraction \\ \end{tabular} & [29] \\  & Dried leaves & 80.93\% & Brazil & Hydrodistillation & [30] \\  & Fresh leaves & 72.6 & South Africa & Hydrodistillation & [31] \\ _Cymbopogon flexususus_ & Entire plant & 43.1\% & India & Hydrodistillation & [32] \\  & Entire plant & 74.98\% & India & Steam distillation & [28] \\ _Cymbopogon citrus_ & Dried leaves & 66.53\% and 60.78\% & Taiwan & \begin{tabular}{c} Solar energy extraction \\ and hydrodistillation \\ \end{tabular} & [33] \\ _Lippi alba_ & Dried leaves & 69\% & Brazil & Hydrodistillation & [34] \\  & & & \
\begin{tabular}{c} Hydrodistillation and \\ solid phase \\ microextraction \\ \end{tabular} & [35] \\ _Pectis brevipedunculata_ & Aerial parts & \(>\)91\% & Brazil &
\begin{tabular}{c} Alroextraction \\ \end{tabular} & [36] \\ _Pectis elongata_ & Dried aerial parts & 90\% & Brazil & Hydrodistillation & [36] \\ _Citrus limon var. pompia_ & Fresh leaves & 17.9\% & Italy & Steam distillation & [37] \\ _Lippi citrodota_ & Kunth & Dried leaves & 59.2\% & Iran & Hydrodistillation & [38] \\ \hline \end{tabular}
\end{table} 
Table 1: Citral content of different plant species and their extraction methods. Figure 1: Chemical structures, main sources, and citral’s physical and chemical properties. GPP is considered a universal monoterpene precursor of all monoterepenes and is synthesized by the fusion of two isoprene (C5) units, the isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP) [43]. IPP is the structural unit of all the isoprenoids in plants.