Objectives Nitrogen is an indispensable nutrient in the growth and development of tea plants, and the supply level of nitrogen has a significant effect on the balance of carbon and nitrogen metabolism in tea. This experiment explored the effects of different nitrogen fertilizer application rates on the regulation of carbon and nitrogen metabolism in tea leaves, and provided theoretical value and practical significance for the actual production of tea.

Methods Four potting experiments were set up with four nitrogen fertilizer application rates: no nitrogen N0 (0 g/kg), low nitrogen N1 (0.375 g/kg), medium nitrogen N2 (0.75 g/kg) and high nitrogen N3 (1.125 g/kg) were set up to study the changes of carbon and nitrogen metabolism related biochemical indexes in the leaves of Baiye1 tea tree under different nitrogen fertilizer application rates, and the correlation between nitrogen fertilizer application rate and carbon and nitrogen metabolism of tea plants was analyzed.

Results The application rate of nitrogen fertilizer significantly affected the activities of key enzymes in tea leaves, which profoundly changed the distribution direction of carbon and nitrogen metabolism flow. Under low nitrogen conditions (N1), phenylalanine ammonia lyase (PAL) and phosphoenolpyruvate carboxylase (PEPC) had the highest activities, driving carbon flow to significantly biased the phenylpropane metabolism pathway, promoting the synthesis of tea polyphenols, catechin components (EGC, EC) and free amino acids, and inhibiting the activities of key enzymes of nitrogen assimilation (GS, NR, GDH). Under medium nitrogen conditions (N2), the activities of glutamine synthase (GS), nitrate reductase (NR) and glutamate dehydrogenase (GDH) reached their peaks, indicating that the nitrogen assimilation efficiency was maximized. At this time, carbon resources were mainly used for nitrogen assimilation and chlorophyll synthesis, resulting in an increase in total nitrogen accumulation and a significant decrease in the total amount of tea polyphenols and catechins. Under high nitrogen conditions (N3), the activities of GS, NR and GDH decreased, and the nitrogen assimilation efficiency decreased, but the activity of glutamate synthase (GOGAT) continued to rise to the highest. At the same time, PAL activity continued to be suppressed, the content of tea polyphenols was minimized, and the total nitrogen accumulation reached its peak. Correlation analysis further confirmed that the application rate of nitrogen fertilizer was significantly negatively correlated with PAL, tea polyphenols, EGC and EC, and positively correlated with GOGAT, NR, GDH, total nitrogen, soluble sugar and caffeine. Principal component analysis showed that nitrogen fertilizer had the most significant effects on total N, total P, soluble sugar, EGC, tea polyphenols, free amino acids, PAL and PEPC activities.

Conclusions PAL, GS/GOGAT/GDH/NR are the core nodes that regulate the metabolism and nitrogen assimilation efficiency of carbon flow to phenolic defense, respectively, and their activities with nitrogen supply levels determine the allocation balance between growth, defense and stress response. Therefore, the application of nitrogen fertilizer in moderation can help maintain the dynamic balance of carbon and nitrogen metabolism in tea plants, which is of great significance for increasing nitrogen fertilizer use efficiency and improving tea yield and quality, and is an important research goal to achieve high quality and high yield of tea.