Repeated Applications of a Mixture of Gibberellin A3 and 6-benzyladenine Increase the Feather Number on Apple Nursery Trees in the Current Year of Grafting

The Tall Spindle apple (Malus × domestica Borkh.) planting system achieves high early and sustained yields and excellent fruit quality. This system requires high-density planting (1,000–1,500 trees/acre) of highly branched (feathered) nursery trees, which increases the total cost of nursery trees purchased. To supply large quantities of highly feathered nursery trees at low prices, the nursery cycle should be shortened to one year. However, it is difficult to naturally obtain one-year-old nursery trees with sufficient feather number and length. Repeated spraying with 6-benzyladenine (BA) has been used as a conventional method to induce feather formation in apple nursery trees. We recently demonstrated that gibberellin A₃ (GA₃) treatment also increases the feather number on young apple trees. Here, we investigated the effects of GA₃ (0, 100, 500, and 1,000 μM) and BA (0, 75, 150, and 300 mg·L⁻¹) combinations on the production of well-feathered trees over a one-year cycle. Among the 16 GA₃ and BA combinations, repeated (7 or 9 times) spraying with 1,000 μM GA₃ + 300 mg·L⁻¹ BA solution (3 or 5 mL) was the most effective method for producing highly feathered nursery trees of ‘Fuji’/JM7 over a one-year cycle. In two trials, trees treated with 1,000 μM GA₃ + 300 mg·L⁻¹ BA had 0.9–1.0-fold greater terminal shoot lengths, 1.5–1.9-fold greater feather numbers (≥ 1 cm), 2.0-fold greater feather numbers (≥ 5–< 50 cm), 1.1–1.2-fold greater feather lengths, and 0.9-fold greater trunk diameters than those treated with 300 mg·L⁻¹ BA (conventional method). The feathers of trees treated with 1,000 μM GA₃ + 300 mg·L⁻¹ BA were predominantly 1–30 cm long (≥ 94.5%), and only few large feathers were observed. Thus, compared with the conventional method, treatment with 1,000 μM GA₃ + 300 mg·L⁻¹ BA facilitated the production of highly feathered nursery trees over a one-year cycle. It also reduced the time and labor cost required to obtain a large number of well-branched nursery trees needed for high-density plantings such as in the Tall Spindle system.