Influence of different nitrogen sources on carbon and nitrogen metabolism and gene expression in tea plants (Camellia sinensis L.)

Nitrogen plays an important role in plant growth and development, with different nitrogen forms also having an impact on carbon/nitrogen metabolism. Unlike most plants, tea plants prefer ammonium over nitrate. In this paper, we focused on how different nitrogen sources regulate the carbon/nitrogen metabolism in tea plants. Tea seedlings of 'Longjing 43' were cultivated hydroponically in four different solutions (zero-nitrogen, only NH4+, only NO3- and mixed nitrogen (NH4+: NO3- = 1:1). We analyzed characteristic components of tea plants and related genes in carbon and nitrogen metabolism. Tea polyphenols and catechins representing carbon pool, increased when NO3- was supplied as the nitrogen source, and similar findings were recorded in the zero-nitrogen treatment. The expression of most catechins biosynthesis-related genes was up regulated under NO3- and zero-N treatment, that was associated with tea polyphenols and catechins changes. Compared with NO3- as the nitrogen source, NH4+ and mixed nitrogen treatments had a positive effect on the accumulation of amino acids, especially theanine, glutamate and arginine, and these components contribute to the freshness flavor of tea. The expression of ammonium-assimilation genes was also up-regulated with NH4+ supply. Under mixed nitrogen treatment, the ratio of total polyphenols to free amino acids (PP/AA) was between sole NH4+ and NO3- supply. Therefore, compared with single nitrogen source, carbon and nitrogen metabolism of tea plant was more balanced under mixed nitrogen treatment. The results suggested that NO3- as the nitrogen source promoted the biosynthesis of catechins enriching the carbon pool, whereas NH4+ supply was more conducive to nitrogen metabolism, indicating that different nitrogen sources could affect the carbon and nitrogen balance.