Objectives Nitrogen is an indispensable nutrient for tea plant growth and development, and its supply significantly influeces the balance of carbon and nitrogen metabolism in tea leaves. This study aimed to explor the effects of different nitrogen fertilizer application rates on carbon and nitrogen metabolism in tea plant, providing theoretical and practical insights for optimizing tea production.

Methods A pot experiment was 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). Changes of carbon and nitrogen metabolism related biochemical indexes in the leaves of ‘Baiye 1’ tea tree under different nitrogen fertilizer application rates were measured, and the correlation between nitrogen fertilizer application rate and carbon and nitrogen metabolism of tea plants was analyzed.

Results Nitrogen fertilizer application rates 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 toward the phenylpropane metabolism pathway and promoting the synthesis of tea polyphenols, catechin components epigallocatechin (EGC), epicatechin (EC) and free amino acids, and inhibiting the activities of key enzymes of nitrogen assimilation (glutamine synthase (GS), nitrate reductase (NR) and glutamate dehydrogenase (GDH)). Under medium nitrogen conditions (N2), the activities of GS, NR, 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 change with nitrogen supply levels and determine the allocation balance between growth, defense and stress response. Therefore, appropriate nitrogen application can help to maintain the dynamic balance of carbon and nitrogen metabolism in tea plants, which is important for increasing nitrogen fertilizer use efficiency and improving tea yield and quality.