Bioscience Evidence 2026, Vol.16, No.4, 235-248 http://bioscipublisher.com/index.php/be 240 and the weight of 100 buds also vary greatly among harvest seasons. Therefore, seasonal effects are closely associated with changes in the structure and maturity of harvested raw materials. Studies on commercial CTC black tea have shown that as leaves develop from one bud with two leaves to one bud with three or four leaves, total polyphenols, total catechins, caffeine, theaflavins, water extract, and antioxidant activity all decrease, whereas crude fiber increases. Tea made from one bud with two leaves consistently achieves the best sensory quality. The interaction between leaf maturity and harvest season also explains why spring tea is preferred for producing premium teas. The quality of black tea and dark tea largely depends on phenolic substrates present in young shoots. Longer harvesting intervals increase the proportion of coarse and mature leaves beyond the one-bud-two-leaf stage, reducing both yield and quality (Aaqil et al., 2023). After winter dormancy, spring shoots enter the harvesting period with relatively high levels of amino acids and theanine but lower levels of polyphenols, making them more suitable for non-fermented or lightly fermented teas. In contrast, summer and autumn leaves accumulate more polyphenols and are therefore better suited for black tea and other products requiring stronger oxidation during processing. Even within the same cultivar, total polyphenols, free amino acids, theanine, and caffeine vary considerably among spring, summer, and autumn harvests, indicating that the effects of leaf maturity interact with cultivar characteristics and growing environment to determine seasonal tea quality. 4 Field Evidence from Zhejiang Tea-Producing Regions 4.1 Longjing tea-why is spring tea the best Longjing tea is the most representative premium green tea in Zhejiang Province and one of the best-known green teas in China. For many years, tea growers in Zhejiang have followed the traditional belief that “pre-Qingming tea is as valuable as gold” and that spring tea produces the highest quality. Even when the same tea garden is managed with identical cultivation practices and processing methods, Longjing tea made from spring leaves usually has a fresher taste, a stronger tender aroma, and a higher market value. In contrast, tea harvested in summer becomes noticeably more bitter and astringent, resulting in lower quality and reduced market prices. This raises an important question: why does spring produce the best Longjing tea? Jiang et al. (2021) addressed this question using materials from the National Tea Germplasm Repository in Hangzhou, maintained by the Tea Research Institute of the Chinese Academy of Agricultural Sciences. The study compared 90 samples from 10 tea germplasm accessions, including Longjing Changye (01), Longjing Yuanye (02), Tengcha (03), Shuigucha (04), Houyezhong (05), Enbiao (06), Lantian (07), Lanshan Kucha (08), Jianghongzhong (09), and Jiangku No. 2 (10). All materials were grown under the same cultivation management and harvested at the one-bud-two-leaf stage during spring (April 3), summer (July 8), and autumn (September 30). This experimental design minimized the effects of cultivar and field management, allowing seasonal environmental conditions to become the major source of quality variation. Most tea accessions showed the same seasonal pattern. Tea quality was highest in spring, declined in summer, and changed further in autumn. The Catechin Quality Index (CQI), which integrates catechin composition, was highest in spring for most materials and decreased in both summer and autumn, indicating that spring provides the most favorable conditions for producing premium green tea. Lower temperatures and milder sunlight during spring slow the growth of young shoots and promote the accumulation of theanine and other umami-related compounds while limiting the formation of bitter and astringent substances. As a result, spring tea has a fresh, mellow taste and a pleasant aftertaste that match the quality requirements of Longjing tea. During summer, higher temperatures and stronger sunlight accelerate vegetative growth, leading to increased catechin and flavonoid accumulation and a lower proportion of free amino acids. Consequently, summer-harvested leaves produce green tea with stronger bitterness, lower freshness, and reduced smoothness (Figure 2). 4.2 Anji white tea-why is the albino stage the best Although its name contains the term "white tea," Anji White Tea is actually processed as a green tea. Its unique characteristic is that the leaves temporarily become white under low spring temperatures, giving the tea distinctive quality characteristics that differ from other green teas. In the Anji tea-growing region of Zhejiang, growers often
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