## 4 Discussion  
The tree produces a series of defensive responses to infection, such as resistance-related enzymes, hormones, and ROS, and synthesizes and releases a large number of VOS, which play an important role as a chemical signal in the host plant's resistance to the disease. This may be one of the reasons for the differences in the variation in resistance substances in pine tree with different resistance levels [33; 34]. In this study, we found that resistant _P. thunbergii_ showed substantially fewer PWD-related symptoms and that the migration rate of PWNs in resistant _P. thunbergii_ was lower than that in susceptible _P. thunbergii_, indicating that resistant _P. thunbergii_ had a stronger barrier to PWN migration than the susceptible one. PWN populations were faster in susceptible _P. thunbergii_ than in resistant _P. thunbergii_. Studies by [PERSON] and [PERSON] showed that PWN-resistant _P. thunbergii_ and _P. densiflora_ inhibited the population size and migration of PWNs, revealing advantages over susceptible pine trees [35; 36]. Further results showed that cellulase activity in PWNs from resistant _P. thunbergii_ was higher than that from susceptible _P. thunbergii_. According to [PERSON] et al. [37], the pathogenicity of PWNs is positively correlated with their cellulase activity, while resistant pine trees develop resistance to PWNs; therefore, more cellulase is secreted by PWNs in response. It is important to examine the changes in the resistant material in differentially resistant _P. thunbergii_ inoculated with PWNs to help the underlying resistance mechanisms. Previous studies have shown that terpenoids are closely related to resistance in pine. Earlier studies on pine resistance to PWNs mainly focused on ROS-related metabolites in the tree, but terpen play an important role in plant chemosensory effects, signaling, the onset of defense mechanisms, and the inhibition of microbial growth [33]. Terpenoid concentrations may vary among different tissues of trees [38], and strong positive correlation was found between proportional quantities of several terpen of the needles and wood, with the highest total monoter concentrations in the needles and the wood. \begin{table}
\begin{tabular}{c c c c c} \hline \hline _P. thunbergii_ **with Different Levels of Resistance** & **Terpenoids** & \multicolumn{3}{c}{**Inoculation Time**} \\ \cline{3-5}  & cis-\(\gamma\)-Bisabolene & 1.16 & & & \\  & Linalool & 0.5 & 1.14 & & \\  & Alloarmondendrene & 1.73 & 1.46 & & \\  & Germacrene B; & 5.38 & 2.71 & & \\  & Bicyclogermacrene & 4.06 & 0.62 & 0.22 & \\  & Dehydrosabinene & 0.63 & 0.59 & 0.43 \\  & Pseudolimonen & 10.37 & 9.27 & 7.63 \\  & \(\alpha\)-Thujene & 10.82 & & 14.14 \\  & \(\alpha\)-Terpineol & 0.18 & 0.24 & \\  & epi-\(\beta\)-Caryophyll & & & 1.21 \\  & Myrtrenal & & & 0.12 \\  & Benzene, n-butyl- & & & 0.37 \\  & Caryophyllene oxide & & & 0.12 \\  & Sabinene & & & 15.97 \\ \hline  & (-)-\(\beta\)-Cadinene & 4.39 & 4.7 & & \\ Susceptible _P. thunbergii_ & \(\alpha\)-Muurolene & 2.5 & 0.27 & 0.58 \\  & Bornel & & 0.14 & 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specific and induced terpenoids and their contents (%) in _P. thunbergii_ with different levels of resistance in the early stages of PWN infestation. [PERSON] et al. [39] found that there are many differentially expressed genes (DEGs) in the terpenoid biosynthesis pathway in the transcriptome analysis of _P. mansoniia_ with different levels of resistance. The regulation of this pathway contributes to PWN resistance. However, there is little research on how terpenoids affect _P. thunbergii_ with different levels of resistance. According to [PERSON] et al., _GGPPS_ is an important target for terpenoid synthesis [40,41], and the PT gene is involved in diterpenoid production. Therefore, we related the genes _GGPPS_, _HMDH1_, and _PT_ to the types and contents of terpene VOS in _P. thunbergii_ with different levels of resistance after inoculation and compared their differences.