## 3 Results
### Screening and Analysis of DEGs  
To investigate the regulatory mechanism of terpenoid synthesis, we conducted transcriptome sequencing of the WFLs and SFLs. A total of 1971 DEGs were identified in the WFLs, with 1338 up- and 633 down-regulated, relative to the SLFs (Figure S3). These DEGs were enriched in KEGG pathways associated with the tea tree terpene synthesis pathway, including \"plant hormone signal transduction\", \"sesquiterpenoid and triterpen biosynthesis\", \"MAPK signaling pathway-plant\" and \"indole alkaloid biosynthesis\" (Figure 4A).  
Figure 3: Aroma analysis of fresh leaves (FLs). (**A**) Concentrations of total aroma compounds, monoterpenoids and sesquiterpenes in spring FLs (SFL) and winter FLs (WFL). (**B**) The contents of substances in various chemical categories in SFLs and WFLs. (**C**) The concentrations of important aroma components in SFLs and WFLs. Student’s \(t\)-test was used to identify significant differences (\({}^{*}p<0.05\); \({}^{**}p<0.01\); ns indicates no significant difference).  
Figure 4: Analysis of differentially expressed genes (DEGs) in spring (SFL) and winter fresh leaves (WFL). (**A**) Compared to SFL, DEGs of WFLs exhibited distinct enrichment within KEGG pathways. (**B**) The terpene biosynthesis pathway, with relative expression of related genes in WFLs vs. SFLs. Abbreviations: AACT, acetoacetyl-_CoA_ thiolase; HMGS, 3-hydroxy-3-methylglutaryl synthase; HMGR, 3-hydroxy-3-methylglutaryl-_CoA_ reductase; MK, mevalonate kinase; PMK, phosphomevalonate kinase; MPDC, mevalonate diphosphate decarboxylase; DXP, 1-deoxy-_D_-xylulose 5-phosphate; DXS, DXP synthase; DXR, 1-deoxy-_D_-xylulose 5-phosphate reductoisomerase; MCT, 2-_C_-methyl-D-erythriitol 4-phosphate cytidyltransferase; CMK, 4-(cytidine 5\({}^{\prime}\)-diphospho)-2-_C_-methyl-D_-erythriitol kinase; MDS, 2-_C_-methyl-_D_-erythriitol 2,4-cyclodiphosphate synthase; HDS, 4-hydroxy-3-methylbut-2-enyldiphosphate synthase; HDR, 1-hydroxy-2-methyl-2-(_E_)-butenyl-l-4-diphosphate reductase; IDI, isopentenyl pyrophosphate isomerase; FPP, famresyl pyrophosphate; FPS, FPP synthase; GGPP, geranylgeranyl pyrophosphate; GGPPS, GGPP synthase; LMS, limonene synthase; GPP, geranyl pyrophosphate; GPPS, GPP synthase; IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; TPS, terpene synthase; FAS, famesene synthase; AFS, \(\alpha\)-famesene synthase; NES, nerolidol synthase; LIS/NES,linalool/neridolol synthase; LIM, limonene synthase; OCS, ocineme synthase; GDS, geranylgeranyl diphosphate synthase. (**C**) Expression levels of _CsUGT91A1_, _CsUGT83A1_, _CsUGT91Q2_, _CsUGT85K11_, _CsUGT73B4_ and _CsUGT73C2_ in SFLs and WFLs. Student’s \(t\)-test was used to identify significant differences (\({}^{\star}\)_p_\(<\) 0.05; \({}^{\star}\)_p_\(<\) 0.01; ns indicates no significant difference).  
Because triterpenoids make a great contribution to Oolong tea's aroma, we focused on the DEGs in that pathway. In all, 47 genes related to terpenene synthesis were analyzed (Figure 4B). During winter, the majority of the genes in the MVA pathway were up-regulated, while MEP pathway genes were largely down-regulated. _CsIDI_, which facilitates the interconversion of IPP and DMAPP, was up-regulated during the winter season. Furthermore, genes involved in limonene synthesis, namely _CsLMS_ and _CsLIM_, and the ocimene synthase gene, _CsOCS_, exhibited up-regulation during winter. Four terpene synthesis genes (_CsTPS5_, _CsTPS6_, _CsTPS11_ and _CsTPS28_) also had increased expression in winter. However, some terpene synthase genes, such as farnesene synthase (_CsFAS_) and nerolidol synthase (_CsNES_, _CsLIS_/_NES_), were down-regulated during winter. Additionally, five UDP-glycostransferase genes (_CsUGT91Q2_, _CsUGT85K11_, _CsUGT3B4_, _CsUGT73C2_ and _CsUGT83A1_) exhibited elevated expression levels in winter relative to spring. On the other hand, _CsUGT91A1_ exhibited lower expression levels in winter compared to spring (Figure 4C).