Mechanism of Response of Camellia sinensis Cultivar ‘Ziyan’ Shoots Growth and Quality Metabolism to Nitrogen Levels

The shoots of ‘Ziyan’ (Camellia sinensis (L.) O. Kuntze) are rich in anthocyanins, making their flavor unique and providing health benefits. The problem is that ‘Ziyan’ has weak growth and low yield of the shoots. Nitrogen levels affect the yield and quality of tea shoots by regulating their growth and the metabolism of major quality components, but the underlying mechanism remains unclear. In this study, one-year-old seedlings of purple cultivar ‘Ziyan’ (Camellia sinensis) were grown hydroponically under three nitrogen levels (low nitrogen, moderate nitrogen, high nitrogen). The contents of quality-related chemical components and key enzyme activities were determined, combined with transcriptome analysis, to investigate the effects of nitrogen level on growth and quality component metabolism of ‘Ziyan’ tea seedlings. Results showed that compared with medium nitrogen (control), low nitrogen significantly decreased yield and total free amino acids, while high nitrogen did not markedly increase either index. Meanwhile, both low and high nitrogen significantly elevated phenylalanine content by 132.46% and 47.37%, respectively. Although the responses of EGCG and catechin contents to low nitrogen and high nitrogen are completely opposite. However, both low nitrogen and high nitrogen significantly reduced the anthocyanin content in shoots (8.15%, 25.26%), inhibiting anthocyanin synthesis. Kyoto Encyclopedia of Genes and Genome (KEGG) enrichment analysis revealed that flavonoid, phenylpropanoid and sucrose-related pathways were relatively active under both high and low nitrogen supply. Additionally, transcriptome analysis identified C4H, LAR and FLS as key genes, and transcription factors (e.g., GRF, bHLH, MYB) and auxins were actively involved in the nitrogen stress response related to ‘Ziyan’ shoot growth and major quality component metabolism. The findings help clarify the adaptive mechanisms of ‘Ziyan’ shoots under nitrogen stress and provide a scientific basis for the fertilization management.