Targeted Metabolomics Reveals Impact of N Application on Accumulation of Amino Acids, Flavonoids and Phytohormones in Tea Shoots under Soil Nutrition Deficiency Stress

Abstract

The abundant amino acids and flavonoids in tea crucially contribute to its particular flavor and many health benefits. The biosynthesis of these compounds is significantly affected by carbon and nitrogen metabolism, which is regulated by the nitrogen conditions in the soil. However, exactly how N-starved tea plants use N absorbed from the soil for the biosynthesis of amino acids, flavonoids, and phytohormones is still little known. Here, tea plants that were deficient in nitrogen owing to long-term non-fertilization were subjected to a higher N application (300 kg/ha) or lower N application rate (150 kg/ha) as well as organic or inorganic N. The levels of 30 amino acids, 26 flavonoids, and 15 phytohormone compounds were analyzed using ultra-high-performance liquid chromatography quadrupole mass spectrometry (UPLC-Q-MS/MS). It was found that a continuous lack of fertilization generated a minimal availability of soil N; as a result, the yield and the theanine and soluble sugar contents were greatly decreased, while the accumulation of seven flavonoid compounds (e.g., epigallocatechin, vitexin, and genistein) increased notably. The levels of theanine, glutamate, and aspartate significantly increased with the supply of N, whereas multiple amino acids, such as alanine, phenylalanine, valine, etc., decreased, indicating that the absorption of nitrogen is preferentially used for the biosynthesis of theanine and glutamate-derived amino acids by a N-starved tea plant. Meanwhile, the changes in the accumulation of flavonoids in tea shoots with various N supplies clarified that a lower N application rate has a negative influence while higher N has a positive effect on the synthesis of flavonoids in a N-starved tea plant. In addition, following N supply, the N-deficient tea plant accumulated ABA (Abscisic acid), SA (Salicylic acid), JA (Jasmonic acid), CKs (Cytokinins), and ACC (1-Aminocyclopropanecarboxylic acid), at 2.03, 1.14, 1.97, 1.34, and 1.26 times, respectively, as high as those in a tea plant with normal fertilization. Furthermore, we performed the correlation network analysis among amino acids, flavonoids, and phytohormones. Its result confirmed that glutamate, aspartate, and hydroxyproline showed a significantly positive correlation with 8, 11, and 8 flavonoid compounds, respectively. Cis-OPDA (cis-12-oxo-phytodienoic acid) was also significantly negatively correlated with eight flavonoid compounds (e.g., naringenin, myricetin, and quercetin). Collectively, our tests suggested that a lower N application promotes the biosynthesis of the theanine and amino acids involved in theanine synthesis, thus inhibiting the accumulation of other amino acids, while greater N application promotes flavonoids in a N-starved tea plant.