Objectives We studied the effects of foliar selenium (Se) spray at different growing stages of tea trees on yield, quality and Se content of summer tea, to provide a technical reference for Se-enriched summer tea production.

Methods A field experiment was carried out in Shengzhou City, Zhejing Province, the green tea cultivar ‘Zhongcha 108’ was used as test material. Split-plot design was used for the experiment, the main plot was Se fertilizer: water control (A0), sodium selenate (A1), sodium selenite (A2) and yeast selenium (A3); while the sub-plot was two spraying periods: before budding stage (May 12, B1) and at one bud and one leaf stage (May 20, B2). All the selenium fertilizer was applied at a concentration of Se 50 mg/L, and the spraying amount of selenium fertilizer solution was 1.8 L/m². When the new twigs with one bud and two leaves accounted for 30% of the tea tree canopy, 30 cm×30 cm of tea canopy was randomly selected in each plot to investigate the total number of new shoots, the number and length of twigs with one young and two open leaves, and the 100-young leaf biomass. At the same time, the samples of one bud and two leaves twigs were taken for the determination of polyphenols, catechins, caffeine, free amino acids and anthocyanins, and Se content.

Results Compared with A0B1, A3B1 significantly reduced the sprouting value by 0.14 but did not affect the total number of new shoots; A2B1 treatment significantly reduced the length of new shoots and the 100-bud biomass by 1.04 cm and 1.94 g, respectively; A1B2 and A3B2 treatments significantly reduced the total number of new shoots by 17.66 and 22.33, respectively, but did not affect the sprouting values significantly. A1B2 treatment significantly reduced the length of new shoots and the 100-bud biomass by 0.88 cm and 1.70 g, respectively. The A1B1 and A2B1 treatments significantly increased total Se content in tea leavesby 1.15 mg/kg and 1.47 mg/kg, and significantly increased their organic Se content by 1.13 mg/kg and 1.38 mg/kg, respectively, while A3B1 treatment did not significantly increase their Se content. The total selenium content of tea leaves in A1B2, A2B2 and A3B2 treatments increased by 5.97 mg/kg, 7.88 mg/kg and 2.61 mg/kg, while the organic Se increased by 5.17 mg/kg, 7.51 mg/kg and 2.48 mg/kg, respectively. A1B1 treatment significantly reduced the content of caffeine, gallate and catechin of tea; A1B2 treatment significantly reduced the content of free amino acids content, but significantly increased the total amount of tea polyphenols and catechins, and the ester-type catechins such as gallic acid, epigallocatechin gallate and (-)-epigallocatechin gallate in the composition of catechins were significantly increased.

Conclusions Foliar application of selenium fertilizer before budding of summer tea increases the total and organic selenium content of tea leaves and improves the quality of tea leaves, especially the effect of sodium selenate is better.