Tea ( Camellia sinensis ) is considered to be acid tolerant and prefers ammonium nutrition, but the interaction between root zone acidity and N form is not properly understood. The present study was performed to characterize their interaction with respect to growth and mineral nutrition. Tea plants were hydroponically cultured with NH ₄⁺ , NO ₃⁻ and NH ₄⁺ +NO ₃⁻ , at pH 4·0, 5·0 and 6·0, which were maintained by pH stat systems. Plants supplied with NO ₃⁻ showed yellowish leaves resembling nitrogen deficiency and grew much slower than those receiving NH ₄⁺ or NH ₄⁺ +NO ₃⁻ irrespective of root-zone pH. Absorption of NH ₄⁺ was 2- to 3·4-fold faster than NO ₃⁻ when supplied separately, and 6- to 16-fold faster when supplied simultaneously. Nitrate-grown plants had significantly reduced glutamine synthetase activity, and lower concentrations of total N, free amino acids and glucose in the roots, but higher concentrations of cations and carboxylates (mainly oxalate) than those grown with NH ₄⁺ or NH ₄⁺ +NO ₃⁻ . Biomass production was largest at pH 5·0 regardless of N form, and was drastically reduced by a combination of high root-zone pH and NO ₃⁻ . Low root-zone pH reduced root growth only in NO ₃⁻ -fed plants. Absorption of N followed a similar pattern as root-zone pH changed, showing highest uptake rates at pH 5·0. The concentrations of total N, free amino acids, sugars and the activity of GS were generally not influenced by pH, whereas the concentrations of cations and carboxylates were generally increased with increasing root-zone pH. Tea plants are well-adapted to NH ₄⁺ -rich environments by exhibiting a high capacity for NH ₄⁺ assimilation in their roots, reflected in strongly increased key enzyme activities and improved carbohydrate status. The poor plant growth with NO ₃⁻ was largely associated with inefficient absorption of this N source. Decreased growth caused by inappropriate external pH corresponded well with the declining absorption of nitrogen.

As the predominant secondary metabolic pathway in tea plants, flavonoid biosynthesis increases with increasing temperature and illumination. However, the concentration of most flavonoids decreases greatly in light-sensitive tea leaves when they are exposed to light, which further improves tea quality. To reveal the metabolism and potential functions of flavonoids in tea leaves, a natural light-sensitive tea mutant (Huangjinya) cultivated under different light conditions was subjected to metabolomics analysis. The results showed that chlorotic tea leaves accumulated large amounts of flavonoids with ortho-dihydroxylated B-rings (e.g., catechin gallate, quercetin and its glycosides etc.), whereas total flavonoids (e.g., myricetrin glycoside, epigallocatechin gallate etc.) were considerably reduced, suggesting that the flavonoid components generated from different metabolic branches played different roles in tea leaves. Furthermore, the intracellular localization of flavonoids and the expression pattern of genes involved in secondary metabolic pathways indicate a potential photoprotective function of dihydroxylated flavonoids in light-sensitive tea leaves. Our results suggest that reactive oxygen species (ROS) scavenging and the antioxidation effects of flavonoids help chlorotic tea plants survive under high light stress, providing new evidence to clarify the functional roles of flavonoids, which accumulate to high levels in tea plants. Moreover, flavonoids with ortho-dihydroxylated B-rings played a greater role in photo-protection to improve the acclimatization of tea plants.

Nitrogen (N) plays an important role in the formation of tea quality-related compounds, like amino acids and flavor/aroma origin compounds. Lipids, which have been reported to be affected by N deficiency, are precursors to the generation of flavor/aroma origin compounds in tea plant. However, there is no literature about the lipid profiles of tea plant affected by N fertilization. Hence, we hypothesize that the biosynthesis of flavor-related compounds in tea was affected by N through its regulation of lipid metabolism. In this study, mature leaves and new shoots of tea plant grown under three N levels at the rates of 0, 285 and 474 kg/ha were applied for ultra-performance liquid chromatography-mass spectrometry (UPLC/MS) based lipidomic analysis. Totally, 178 lipid species were identified. The results showed that the composition of lipid compounds in mature leaves and new shoots varied dramatically, which was also affected by N levels. The higher content of the storage lipid TAG and higher carbon (C)/N ratio in mature leaves than that of new shoots in tea plants grown under low N level (0 kg/ha) suggested that tea plants could remobilize the C stored in TAG to maintain their C/N balance and help to improve the quality of tea. N fertilization resulted in a higher content of the compounds 36:6 MGDG and 36:6 DGDG. Since these compounds contain linolenic acid (18:3), a precursor to the formation of aroma origin compounds, we suggested their increase could contribute to the quality of tea. Taken together, the present work indicated that appropriate application of N fertilizer could balance the lipid metabolism and the formation of flavor/aroma origin compounds, which help to improve the quality of tea. Moreover, excess N fertilization might deteriorate the aroma quality of made tea due to increases of precursors leading to grassy odor.