Currently, the effects of the differences between day and night temperatures (DIFs) on tea plant are poorly understood. In order to investigate the influence of DIFs on the growth, photosynthesis, and metabolite accumulation of tea plants, the plants were cultivated under 5 °C (25/20 °C, light/dark), 10 °C (25/15 °C, light/dark), and 15 °C (25/10 °C, light/dark). The results showed that the growth rate of the new shoots decreased with an increase in the DIFs. There was a downward trend in the photosynthesis among the treatments, as evidenced by the lowest net photosynthetic rate and total chlorophyll at a DIF of 15 °C. In addition, the DIFs significantly affected the primary and secondary metabolites. In particular, the 10 °C DIF treatment contained the lowest levels of soluble sugars, tea polyphenols, and catechins but was abundant in caffeine and amino acids, along with high expression levels of theanine synthetase (TS3) and glutamate synthase (GOGAT). Furthermore, the transcriptome data revealed that the differentially expressed genes were enriched in valine, leucine, and isoleucine degradation, flavone/flavonol biosyntheses, flavonoid biosynthesis, etc. Therefore, we concluded that a DIF of 10 °C was suitable for the protected cultivation of tea plants in terms of the growth and the quality of a favorable flavor of tea, which provided a scientific basis for the protected cultivation of tea seedlings.

Nitrogen (N) and selenium (Se) are beneficial for tea growth and tea quality; however, it is unknown how the combined application of N and Se affects tea quality and N uptake and utilization in tea plants. In the present study, a hydroponic experiment with three N levels (0, 2 and 4.5 mmol/L) and three Se levels (0, 0.3 and 3 mg/L) was carried out with ‘Chuancha No.2’ as the material, and the contents of tea polyphenols, amino acids and caffeine as well as the expression levels of genes related to N uptake and utilization in tea plants were tracked. The findings reveal that the contents of tea polyphenols, AAs and caffeine in new shoots were the highest when supplied with 0.3 mg/L Se and 4.5 mmol/L N, while the contents of total N, AAs and tea polyphenols in mature leaves were the highest at the concentrations of 3 mg/L Se and 2 mmol/L N. Se supply (0.3 and 3 mg/L) induced an increase in amino acid and tea polyphenol contents in tea shoots under N deficiency conditions, whereas total N content, tea polyphenols and AAs in mature leaves and total N content in tea roots decreased significantly. When supplied with N (2 mmol/L and 4.5 mmol/L), the contents of tea polyphenols and caffeine in new shoots first increased and then decreased with the increase in Se concentration, while the total N content in mature leaves and roots increased. In leaves, CsAMT1.1, CsAMT1.2 and CsAMT3.1 had similar trends, and their expression levels were remarkably upregulated when supplied with 0.3 mg/L Se or 2 mmol/L N, respectively; the expression level of CsGS1.1 was significantly induced by N and Se, while CsGS1.2 and CsTS1 were mainly induced by N. In roots, CsAMT1.1 could play a major role in N uptake under the combined application of N and Se; CsGS1.2 expression was significantly induced compared to CsGS1.1 under the combined application of N and Se. This study explored the potential of the interaction of Se and N to promote tea quality and N uptake and utilization in tea plants.