Effects of potassium fertilizer application time on the coloration and quality of grape fruit and the expression of potassium-absorbing genes

  Objectives  A sufficient potassium (K) supply benefits the grape coloring and taste quality. Here, we studied the effect of potassium fertilizer application times on berry coloration and grape quality and evaluated the necessity of applying K at specific periods using gene expression profiles.

  Methods  Field experiments were conducted in Xuyi of Jiangsu Province and Tiantai and Jinhua of Zhejiang Province, using 5-year-old ‘Summer Black’ grape as test materials. The treatments were four K₂SO₄ application times, including 4 days (T1), 8 days (T2), 12 days (T3), and 16 days (T4) after fruit expansion, at an application rate of 120 g/plant K₂SO₄. No K fertilization was used as the control. Sample color was observed before each treatment, and the fruit coloring rate was counted every 4 days after the treatment, and 30 fruits were collected for the determination of fruit equatorial diamete and polar diameter, fruit weight, firmness, soluble solids (SS), and titratable acid (TA). One hour before K fertilization (9: 00 am), 4−5 pieces of mature leaves at nodes 8−9 were collected to determine the relative expression levels of VvKUP1 and VvKUP2.

  Results  In Xuyi, Jinhua, and Tiantai, T3, T3, and T2 exhibited the best coloring effect of 95.0%, 80.6%, and 83.4%, respectively; T3, T3 and T1 had higher fruit weight, equatorial diamete and polar diameter, respectively. In Xuyi, all K treatments increased fruit SS content, and T2, T3, and T4 treatments reduced fruit firmness. In Jinhua, all treatments reduced the TA content and firmness but increased the SS content of the grape. In Tiantai, all treatments reduced the fruit TA content and firmness, and T1, T2, and T3 increased the SS content. Before and after K fertilization, there was a paradigm shift in the expression of the VvKUP1 gene from a decreasing to an increasing trend, and the expression of the VvKUP2 gene followed the opposite direction, consistent with the optimum fertilization period.

  Conclusions  Potassium fertilizer can induce fruit coloring and improve fruit quality. Applying K₂SO₄ at 12 days after fruit expansion had a better influence on the grape ‘Summer Black’ in Xuyi and Jinhua, and application at 8 days after fruit expansion had a higher impact on the grape ‘Summer Black’ in Tiantai. The observed gene (VvKUP1 and VvKUP2) expression change verified the suitable K application time in the three tested cities.