Objective The experiment was conducted to determine the effects of Si foliar application on the growth and tea quality, and accumulation of Si in tea plant parts for the safe use of Si fertilizer.

Methods Pot and field experiments were used in this research, sodium silicate (Na₂SiO₃·9H₂O) was used as the silicon fertilizer. First, seven spraying concentrations of Si (0, 50, 150, 300, 500, 750, and 1500 mg/L) were set up in the indoor pot experiment, sprayed three times in total and once every 7 days. On the seventh day after the third spraying, the chlorophyll content (SPAD), the chlorophyll fluorescence characteristics, and the stomatal characteristics were measured. The biomass of shoot and root was weighed, the phytoalexin content of tea seedlings was measured, and the optimal Si spray concentration was proposed. The second indoor experiment was then conducted using the optimum Si spray concentration as treatment and distilled water as control. The Si concentration in the top 1, 3, and 5 leaves, in stem and root of tea seedlings were measured after the first, second, and third spray treatments, respectively. At the same time, field experiment was conducted in a tea garden located in Shandong Province, taking the same treatment as in the pot experiment. The tea yield and active component contents related to tea quality were investigated in spring, summer and autumn, respectively.

Results Si300 and Si500 exhibited higher leaf SPAD, qNP, and electron transport rate (ETR) values than other treatments, and contained significantly higher phytoalexin content than Si0. Si500 also elicited the highest bud density and 100-bud weight, thus it was the optimum spraying concentration. The concentration was used as Si treatment and distilled water as control in the following pot and field experiment. After the first spray treatment in the pot experiment, silicon content was found to be higher in leaves than in stems and roots, and the silicon content in different leaves was in order: the top third leaf > the top fifth leaf > the top first leaf. After the third spray treatments, the top fifth leaf had higher silicon content than the top first and third leaves; and root had the highest Si content in tea plants. In the field experiment, Si spray increased the 100-bud weight and budding density of tea plants significantly in all the three seasons. The content of free amino acids in spring tea, summer tea, and autumn tea increased by 17.49%, 94.48%, and 15.06%, respectively; soluble sugar content increased by 2.72%, 91.69%, and 3.33%, while caffeine decreased significantly by 7.07%, 32.91%, and 16.59%, respectively.

Conclusions Foliar spraying of 500 mg/L silicon is beneficial to the growth of tea seedlings. Spraying silicon fertilizer before picking is beneficial to improve the yield and quality of tea, especially summer tea. After spraying silicon fertilizer, silicon is mainly accumulated in the roots and mature leaves of tea.