Comparative analysis of aroma-active compounds in large-leaf yellow tea and rice crust was carried out via headspace solid phase micro-extraction (HS-SPME) combined with gas chromatography-olfactometry-mass spectrometry (GC-O-MS) to elucidate the material basis of the rice crust-like aroma of large-leaf yellow tea. The results revealed that 72 and 45 volatile compounds were identified in large-leaf yellow tea infusion and rice crust, respectively. In total, 15 aroma-active components were found in both large-leaf yellow tea and rice crust. Among them, five pyrazines responsible for a roasted aroma were common to both, which made a key contribution to the characteristic rice crust-like aroma of largeleaf yellow tea. 2-Ethyl-3,5-dimethylpyrazine, 2-ethyl-5-methylpyrazine and 2,6-diethylpyrazine were the key aroma-active components of both large-leaf yellow tea and rice crust. 3,5-Diethyl-2-methylpyrazine was key to the formation of the rice crust-like aroma of large-leaf yellow tea, but contributed little to the characteristic aroma of rice crust. 3-Ethyl-2,5-dimethylpyrazine was key to the formation of the characteristic aroma of rice crust, butt contributed little to the characteristic aroma of large-leaf yellow tea.

Flavonoids are critical secondary metabolites that determine the health benefits and flavor of tea, while chlorophylls are important contributors to the appearance of tea. However, transcription factors (TFs) that can integrate both chlorophyll biosynthesis and flavonoid accumulation in response to specific light signals are rarely identified. In this study, we report that the GOLDEN 2-LIKE TF pair, CsGLK1 and CsGLK2, orchestrate UV-B-induced responses in the chlorophyll biosynthesis and flavonoid accumulation of tea leaves. The absence of solar UV-B reduced the transcriptional expression of CsGLKs in the tea leaves and was highly correlated with a decrease in flavonoid levels (especially flavonol glycosides) and the expression of genes and TFs involved in chlorophyll biosynthesis and flavonoid accumulation. In vivo and in vitro molecular analyses showed that CsGLKs could be regulated by the UV-B signal mediator CsHY5, and could directly bind to the promoters of gene and TF involved in light-harvesting (CsLhcb), chlorophyll biosynthesis (CsCHLH, CsHEMA1, and CsPORA), and flavonoid accumulation (CsMYB12, CsFLSa, CsDFRa, and CsLARa), eventually leading to UV-B-induced responses in the chlorophylls and flavonoids of tea leaves. Furthermore, UV-B exposure increased the levels of total flavonoids, CsGLK1 protein, and expression of CsGLKs and target genes in the tea leaves. These results indicate that CsGLKs may modulate tea leaf characteristics by regulating chlorophyll biosynthesis and flavonoid accumulation in response to solar UV-B. As the first report on UV-B-induced changes in flavonoid and chlorophyll regulation mediated by CsGLKs, this study improves our understanding of the environmental regulations regarding tea quality and sheds new light on UV-B-induced flavonoid responses in higher plants.

Larger-leaf yellow tea (LYT) is a characteristic type of Chinese tea produced in Huoshan County, Anhui Province, which is made by mature leaves with stems. According to recent report, LYT showed competitive effects in anti-hyperglycemia in comparison to other teas such as green or black tea. However, the bioactive compounds of LYT are still undiscovered so far. For this purpose, 5 fractions of LYT were prepared by sequential extraction. The in vitro bioassay results indicated that the ethyl acetate fraction of LYT had the strongest inhibitory effects on α-glucosidase and α-amylase. Fluorescence-quenching analysis and protein-binding test revealed that the compounds of ethyl acetate fraction could inhibit α-glucosidase and α-amylase activities through binding to enzymes or other mechanisms. All chromatographic peaks of high-performance liquid chromatography (HPLC) of ethyl acetate fraction were separated and collected. The purified compounds were identified by liquid chromatography-mass spectrometry (LC-MS), and subsequently screened by calculating their inhibition ratio on α-glucosidase at the real concentration in LYT infusion. The results showed that (–)-epigallocatechin gallate, (–)-gallocatechin gallate, caffeine, N-ethyl-2-pyrrolidone-substituted flavan-3-ols were effective inhibitors for α-glucosidase.

Roasting is a common manufacture technology for processing various teas. It is not only used in decreasing the water content of finished tea, but also improving the flavor of teas. In the present study, the roasted and non-roasted teas were compared by liquid-chromatography mass spectrometry and sensory evaluation. The roasted tea tasted less bitter and astringent. The content of main galloylated and simple catechins, caffeine and theobromine in roasted were significantly lower than non-roasted teas. Targeted taste-compounds metabolomics revealed that (–)-epigallocatechin gallate, kaempferol-glucose-rhamnose-glucose and (–)-epicatechin gallate were main contributors tightly correlated to astringent intensity. Flavonol glycosides including kaempferol-glucose, quercetin-glucose, kaempferol-glucose-rhamnose-glucose, and quercetin-glucose-rhamnose-glucose in roasted teas were also significantly less than non-roasted teas. To study the chemical changes during roasting, tea with a strong astringency was roasted under 80, 100, 120, 140, and 160 ℃. With the increase of roasting temperature, the bitter and astringent intensity of tea was gradually decreased, but the main astringent compounds including (–)-epigallocatechin, (–)-epigallocatechin gallate and kaempferol/quercetin glycosides were irregularly varied with temperature. The Pearson correlation coefficient analysis suggested procyanidin B2, coumaroylquinic acids and gallotannins were tightly correlated to the astringent and bitter perceptions, while N-ethyl-2-pyrrolidone-substituted flavan-3-ols were negatively correlated.

Wuyi rock tea (WRT), a type of oolong tea, is famous for 'rock flavor' with characteristic taste and aroma. The qualitative and quantitative changes of the nonvolatile constituents and taste profiles were investigated during the WRT production process by analyzing of polyphenols, caffeine, amino acids, L-theanine and total sugar, coupled with sensory evaluation and determination of dose-over-threshold values. The contents of L-theanine, total sugar, amino acids and some catechins varied considerably except for GA, GCG and GC that increased significantly during the process, while the amount of caffeine was relatively stable. Astringent, bitter, sweet and umami taste properties were predicted. In accordance with the sensory evaluation, there was no umami-like taste in the finished tea that underwent full fire processing, which is essential for WRT to enhance the mellow, heavy taste, and weaken the bitter taste. EGCG and caffeine contributed the most to the astringent, bitter taste of WRT. This study provides a comprehensive profile of the changes in taste characteristics during the WRT manufacturing process.

Drought stress is one of the main factors limiting yield in tea plants. The plant cell's ability to preserve K⁺ homeostasis is an important strategy for coping with drought stress. Plasma membrane H⁺-ATPase in the mesophyll cell is important for maintaining membrane potential to regulate K⁺ transmembrane transport. However, no research to date has investigated the possible relationship between plasma membrane H⁺-ATPase and mesophyll K⁺ retention in tea plants under drought and subsequent rehydration conditions. In our experiment, drought stress inhibited plasma membrane H⁺-ATPase activities and induced net H⁺ influx, leading to membrane potential depolarization and inducing a massive K⁺ efflux in tea plant mesophyll cells. Subsequent rehydration increased plasma membrane H⁺-ATPase activity and induced net H⁺ efflux, leading to membrane potential hyperpolarization and thus lowering K⁺ loss. A first downregulated and then upregulated plasma membrane H⁺-ATPase protein expression level was also observed under drought and subsequent rehydration treatment, a finding in agreement with the change of measured plasma membrane H⁺-ATPase activities. Taken together, our results suggest that maintenance of mesophyll K⁺ in tea plants under drought and rehydration is associated with regulation of plasma membrane H⁺-ATPase activity.