Bioscience Evidence 2026, Vol.16, No.4, 235-248 http://bioscipublisher.com/index.php/be 239 changes in major taste-related compounds, the authors suggested maintaining the withering temperature below 25 ℃ and limiting the withering period to 15~18 h to promote amino acid accumulation while avoiding excessive quality loss caused by over-withering. 3.2 Light conditions and changes in tea quality Light intensity and day length are major factors regulating seasonal differences in tea quality. Stronger light and longer sunshine duration generally promote the accumulation of flavonoids and catechins, whereas shaded conditions favor the retention of amino acids and compounds associated with fresh taste. Seasonal metabolic studies have shown that theanine reaches its highest level in spring, while catechins peak during summer. This pattern of "fresh spring tea and bitter summer tea" is mainly explained by light-induced changes in photosynthesis, which regulate the expression of transcription factors and structural genes involved in secondary metabolism. When spring and autumn have similar temperatures, the compositional differences observed in white tea are more closely related to differences in light intensity than to temperature itself. Ultraviolet radiation, especially UV-B, plays an important role in regulating flavonoid metabolism in tea plants. Removing ambient UV-B reduces the accumulation of bitter flavonol glycosides such as kaempferol, myricetin, and quercetin derivatives, while increasing some non-esterified catechins. In contrast, supplemental UV-B promotes flavonol accumulation but decreases catechin content. This regulatory pathway is mainly controlled by CsHY5, which activates the expression of CsMYB12 and regulates flavonoid biosynthesis genes including CsFLS, CsLARa, and CsDFRa, thereby altering the metabolic balance between flavonols and catechins (Lin et al., 2021). Flavonol glycosides are more sensitive to changes in light intensity than catechins. Under different shading levels and light spectrum treatments, total flavonol glycosides decreased by as much as 79.6%, whereas total catechins declined by a maximum of 38.7%. At the same time, the expression of flavonoid biosynthesis genes such as PAL, CHS, and F3H was significantly reduced (Ye et al., 2021). These findings indicate that changes in light conditions are an important reason for the seasonal differences in tea chemical composition and sensory quality. 3.3 Rainfall and water supply Appropriate soil moisture is essential for bud growth and photosynthesis. However, prolonged rainfall, high air humidity, or excessive water availability can alter shoot growth rate, leaf development, and the accumulation of secondary metabolites, thereby affecting tea flavor and quality. Hazra et al. (2021) compared 22 tea cultivars across the spring, rainy, and autumn seasons. Tea harvested during the rainy season generally contained higher levels of total phenolics, flavonoids, proanthocyanidins, and antioxidant activity. However, individual catechin components showed substantial differences among cultivars, and no consistent seasonal trend was observed. Photosynthetic rate, transpiration rate, leaf temperature, and intercellular CO₂ concentration were all significantly correlated with phenolic compounds and antioxidant capacity, indicating that rainfall influences tea quality not only by changing soil water availability but also by modifying leaf physiological activity and secondary metabolism. Benti et al. (2023) monitored the yield, harvesting cycle, and fresh leaf quality of five tea clones over four seasons in southwestern Ethiopia. They found that monthly rainfall and temperature significantly affected shoot composition and the proportion of marketable leaves. Fresh leaf quality was highest in June, when premium shoots accounted for 78.21% of the harvest. After July, increased rainfall and lower temperatures resulted in a 5%~10% increase in soft-stem leaves, while the proportion of premium shoots declined to 68.83%. Long-term observations further showed that tea yield and harvesting frequency were highest when the daily maximum temperature remained between 23.03 ℃ and 26.35 ℃. When the maximum temperature exceeded 28.34 ℃ or the minimum temperature fell below 10.38 ℃, both fresh leaf quality and yield declined. 3.4 Tea growth stage and leaf maturity Seasonal differences are also reflected in the developmental stage of fresh tea leaves. Tea harvested in spring, summer, and autumn usually enters the processing chain at different levels of maturity. In addition to marked differences in chemical composition, morphological traits such as bud length, bud density, mature leaf biomass,
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