## 2 A Brief Description of Terpenes  
Conifers are the most abundant source of terpene compounds in nature [13]. In conifers, volatile compounds are contained in the oleoresin. This is a mixture of volatile monoterpenoids and sesquiterpenoids (turpentine), and non-volatile diterpenoids which represent the resin acids [14]. Monoterpenoids are low molecular weight compounds (136 Da for hydrocarbons and up to 200 Da for oxygenated derivatives) (Figure 1). Highly volatile, they are also odorants, naturally emitted by plants to attract pollinators or repel predators. They can be captured by processes such as hydrodistillation, to form extracts known as essential oils. Terpenes are often removed from essential oils as undesirable components, whereas synthetic oxy-functionalised derivatives find broad applications in flavours and fragrances industries. In addition to their perfuming power, these molecules respectively possess medicinal properties, which in synergy make essential oils renowned in aromotherapy. Volatile terpenoids also appear to be involved in protecting plants against heat and oxidative stress [14]. Their role as a communication molecule is thought to attract pollinating insects, thus aiding reproduction, but they also act as \"signal\" molecules to preserve surrounding trees in the event of an attack [15]. Despite the diversity of terpene \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Essence** & **Parameters** & **Monoterpene** &
\begin{tabular}{l} **Oxygenated** \\ **Monoterpene** \\ \end{tabular} & **Sesquiiterpene** & **Esters** & **References** \\ \hline \multirow{4}{*}{Balsam fir} & Concentration & 84.9\% & \multirow{4}{*}{\(\alpha\)-pinene} & \multirow{4}{*}{1.3\%} & \multirow{4}{*}{0.6\%} & \multirow{4}{*}{9.1\%} & [10, 11] \\  & major compound & \(\beta\)-pinene & & & & \\  & Limonene & \(\alpha\)-Tepeniol & \(\beta\)-carephyllene & Bornyl acetate & \\  & Concentration & 63.5\% & 2.6\% & & & \\  & \(\beta\)-pinene & \(\alpha\)-tepeniol & \(\beta\)-carephyllene & Bornyl acetate & [10, 11] \\  & major compound & \(\beta\)-STE & \(\alpha\)-tepeniol & \(\beta\)-carephyllene & Bornyl acetate & [10, 11] \\  & Concentration & 85.9\% & 2.7\% & & & \\ Jack pine & major compound & \(\alpha\)-pinene & \(\alpha\)-tepeniol & \(\alpha\)-tepeniol & & \\  & & Lamphene & 1,8-cinee & 1,8-cinee & Bornyl acetate & [10, 11] \\  & & Limonene 95\% & & & & \\ Citrus terpenes & major compounds & \(\alpha\)-pinene & 2.5\% & & & & \\  & & camphene 2\% & & & & \\  & & \(\beta\)-myrcene 0.5\% & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of balsam fir, black spruce, jack pine and citrus essential oils. compounds, they share a common biosynthetic pathway. This pathway comprises four stages [16]. Briefly, the first involves the formation of isopentenyl pyrophosphate (IPP), the C5 unit of biological isoprene. The main route for the synthesis of IPP and its allylic isomer, dimethylallyl pyrophosphates (DMAPP), is via the mevalonate pathway. A second synthesis pathway is also present, the methylerythitol phosphate (MEP/DOXP) pathway. In the second step, C5 units condense to generate three larger prenylpyrophosphates (GPP, FPP and GGPP). Subsequently, in the third step, these three pyrophosphates undergo a wide range of cyclisation and rearrangement to produce the parent carbon skeletons for each terpene class. The final stage encompasses various oxidation, reduction, conjugation and other transformations that convert the carbon skeletons into thousands of distinct terpene metabolites. Although several in vitro studies have shown that many monoterrenes and diterpenes have significant bactericidal effects, it is not always easy to achieve the same efficacy in vivo due to the multiple presence of molecules present in extracted oils.