## 1 Introduction  
Tea (_Camellia sinensis_ L) is the most popular non-alcoholic beverage, consumed worldwide for its charming flavor. Based on its processing methods, tea is categorized into three main types--green tea, black tea and oolong tea--each possessing distinct flavor characteristics. Oolong tea, in particular, is renowned for its elegant fruity and floral notes [1]. Some famous oolong teas include Tieguanyin from Anxi in Fujian Province, Wuyi rock tea from Wuyi Mountain in Fujian Province, Dongding oolong tea from Taiwan and Fenghuzo Dancong (FHDC) from Chaozhou in Guangdong Province.  
FHDC stands out for its unique aroma and diverse fragrance profiles, which are shaped by factors such as its cultivar, manufacturing processes and environmental conditions [2; 3; 4]. Harsh conditions like low humidity, intense light and dry heat in summer and autumn often degrade the quality of tea leaves [1; 5], while the mild temperatures and high humidity of spring enhance it. In the birthplace of FHDC (Chaozhou, China.), the moderate winter temperatures from 10 \({}^{\circ}\)C to 20 \({}^{\circ}\)C support tea plants' normal physiological activities. FHDC produced in this season exhibits a more intense and lasting aroma, potentially linked to the lower ambient temperature [6]. However, the mechanism behind the enhanced aroma of FHDC produced in winter remains largely unexplored.  
The aroma of tea mainly stems from volatile terpen, esters, carotenoids, phenyl-propane compounds and their derivatives [7; 8; 9]. Oolong tea is rich in terpene aroma compounds, with monoterenes and sesquiterpenes, such as linalool and its oxides,\(\alpha\)-phellandrene, geraniol and (\(E\))-nerolidol, being the primary contributors to its fragrance [10; 11].  
Terpenoid biosynthesis primarily occurs through the mevalonate (MVA) and methyleryth-thiritol phosphate (MEP) pathways. The MEP pathway, occurring in plastids, facilitates the synthesis of monoterpenes and diterpenes [12]. In contrast, the MVA pathway, located in the cytoplasm, is responsible for producing sesquiterpenes, sterols and triterpenes [13; 14]. These pathways function independently but exhibit metabolic crosstalk [15]. The MEP and MVA pathways have distinct sets of enzyme-catalyzed reactions, seven for MEP and six for MVA [16], both of which lead to the production of the precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) [16]. These precursors, under the influence of terpene synthases, undergo rearrangement and cyclization reactions to produce various terpenoid compounds [17; 18].  
Cold environments stimulate enhanced synthesis and emissions of large quantities of terpene compounds such as linalool and nerolidol, supporting the cold tolerance of tea plants [19]. Tea processing methods, especially those involving external pressure, further amplify the release of these aroma compounds [1]. Furthermore, the action of UDP-glycostransferases (UGTs) enables terpenoid aroma compounds to combine with sugar molecules, forming glycosides [20]. Although tasteless, these glycosides are hydrolyzed into aromatic volatile compounds by glycosidases [21].  
Since the fresh leaves of tea trees in spring are the main raw material used to process dry tea, in this study, we analyzed the aroma of fresh FHDC leaves in both the spring and winter seasons, incorporating transcriptome sequencing to unveil the underlying formation mechanisms of aroma development. Our findings may offer insights into the aromatic characteristics of winter-harvested Guangdong oolong tea.