Integrated transcriptomic and metabolomic analysis reveal the response of tea plants to nitrogen and magnesium

• Nitrogen (N) is key to tea plant growth, while magnesium (Mg) is key to tea quality. • Under high N, Mg enhanced key quality components while reduced phenol-ammonia ratio. • The N-Mg interaction affected flavone, flavonol and phenylpropanoid biosynthesis. • Mg fertilization synergizes with high N inputs to boost tea garden productivity. Nitrogen (N) is a critical nutrient for the growth of tea plants, while magnesium (Mg) plays a pivotal role in determining the tea quality. Here, a hydroponical study was conducted to investigate the effects of different N (1, 4.5 mmol l -1 ) and Mg (0.01, 0.7 mmol l -1 ) concentrations on both tea plant growth and quality. The results showed that increasing Mg concentration significantly increased tea plant biomass under high nitrogen conditions, which was not observed under low N conditions. Additionally, higher Mg concentration had a positive effect on the absorption of essential nutrients. Similar trends were observed in tea quality indicators; supplementation with Mg at high N levels increased the levels of free amino acids and caffeine while reducing the phenol-ammonia ratio. Further metabolomics and transcriptomic analysis revealed that the interaction between N and Mg primarily affected the biosynthesis pathways of flavone, flavonol and phenylpropanoid biosynthesis. Specifically, Mg supplementation led to increased levels of quercetin, isorhamnetin, kaempferol, rutin by the upregulation of 4CL, C12RT1 , along with the downregulation of HCT, F5H, CYP73A , and the contents of specific metabolites (5-hydroxyferulic acid, P-coumaric acid and coniferyl acetate). Our findings highlight the importance of Mg supplementation in tea plantations, particularly under high N conditions, as this can lead to a synergistic improvement in both growth and quality.