## 3 Results
### Screening DEGs in Terpenoid Synthesis Pathway  
Based on KEGG enrichment and differential expression gene identification analysis, we observed a significant enrichment of genes related to terpenoid biosynthesis during the bud and decay stages, leading us to screen for DEGs for analysis (Figure 7). We speculate that the MVA pathway in the cytoplasm is the primary pathway for the synthesis of YJ terpenoid floral fragrances, specifically, a large number of enzyme genes highly expressed in YJ, including _acetyl-CoA C-acetyltransferase (AACT)_, _hydroxymethylglutaryl-CoA synthase (HMGS)_, _hydroxymethylglutaryl-CoA reductase (HMGR)_, _phosphorcatalane kinase  
Figure 6: KEGG enrichment pathway of terpenoid-related genes in two rhododendron species compared at the corresponding developmental stages (**A**) YJA vs. NWA; (**B**) YJB vs. NWB; (**C**) YJC vs. NWC; (**D**) YJD vs. NWD.  
(PMK), _nevalonate decarboxylase_ (_MVD_), _Isopentenyl diphosphate delta-isomerase_ (IDI), _geranyl diphosphate synthase_ (_GDPS_), _geranylgeranyl diphosphate synthase_ (_GGPS_), _farnesyl diphosphate synthase_ (_FDPS_), and _TPS_.  
Figure 7: Gene expression profiles of MEP and MVA synthesis pathways at different developmental stages (A, B, C, D) of two rhododendron varieties. MEP pathway, 2-methyl-D-erythnitol-4-phosphate pathway; MVA pathway, mevalonate pathway; acetyl-CoA, acetyl coenzyme A; HMG-CoA, 3-hydroxy-3-methyl glutaryl coenzyme A; MVA, mevalonic acid; MVAPP, mevalonate-5-phosphate; GA-3P, glyceraldehyde-3-phosphate; DOXP, 1-deoxy-D-xylulose-5-phosphate; CDP-ME, 4-diphosphocytidyl-2-C-methyl-D-erythnitol; CDP-ME, 4-phospho-2-C-methyl-D-erythnitol cytidine diphosphate; IPP, isopentenyl diphosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl diphosphate; FPP, farnesyl pyrophosphate; GGPP, geranylgeranyl pyrophosphate; _AACT_, _acetyl-CoA C-acetyltransferase_; HMGS, _hydroxymethylglutaryl-CoA synthase_; HMGR, _hydroxymethylglutaryl-CoA reductase_; PMK, phosphomevalonate kinase_; _MVD_, mevalonate decarboxylase; _DXS_, _1-deoxy-D-xylulose-5-phosphate synthase_; DXR, 1-deoxy-D-xylulose 5-phosphate reductase_; MCT, 2-C-methyl-D-erythnitol 4-phosphate cytidylyltransferase; CMK, 4-diphosphocytidyl-2-C-methyl-D-erythnitol kinase; IDI, isopentenyl diphosphate delta-isomerase; GDPS, geranyl diphosphate synthase_; GGPS, geranylgeranyl diphosphate synthase; FDPS, farnesyl diphosphate synthase; TPS, _terpene synthase_.  
However, the number of genes annotated on the MEP pathway is relatively small, including 1-deoxy-D-xylulose-5-phosphate synthase (DXS), 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR), 2-C-methyl-D-erythnitol 4-phosphate cytidylyltransferase (MCT), and 4-diphosphocytidyl-2-C-methyl-D-erythnitol kinase (CMK).  
Further analysis was conducted on candidate genes involved in the MVA pathway, visualizing the expression of these genes in petals across various developmental stages through a heatmap (Figure 8). The visualization reveals that 39 of these genes exhibit higher expression in YJ compared to other conditions. The results of the transcriptome analysis are consistent with the metabolome analysis, indicating a higher abundance of terpene synthesis genes in YJ than in NW. Candidate genes exhibit diverse expression patterns at different growth and development stages. Specifically, _TPS_ is highly expressed in YJ and lowly expressed during the decay stage. Conversely, _FDPS_, _HMGS_, _MVD_, and _AACT_ are predominantly expressed during the decay stage. Meanwhile, a significant number of genes, including _IDI_, _PMK_, _FDPS_, _HMGS_, _MVD_, and _AACT_, show lower expression levels in the bud and first bloom stages. In contrast, the four genes _GGPS_ and _HMGR_ are specifically expressed at higher levels in the first bloom stage.