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

Abstract

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.

Introduction

The Tall Spindle system, an apple (Malus × domestica Borkh.) planting system developed in the late 1990s, achieves high early and sustained yields and excellent fruit quality (Robinson et al., 2006). Tall Spindle trees have a height of approximately 3 m and a diameter of approximately 0.9–1.2 m, which increases their density in orchards. The Tall Spindle system has the following seven characteristics: 1) high planting densities (~1,000–1,500 trees/acre), 2) dwarfing rootstocks (e.g., ‘M.9’ and ‘B.9’), 3) highly branched (feathered) nursery trees (10–15 feathers), 4) minimal pruning at planting, 5) tying down the large feathers below the horizontal after planting, 6) no permanent scaffold branches, and 7) limb renewal pruning to remove and renew branches (Robinson et al., 2006). In particular, highly feathered nursery trees are an essential component of this system because the greater the feather number during planting, the higher the yields in the early years (Reig et al., 2019; Robinson et al., 2006). However, natural branching is inadequate in many apple cultivars (Ferree and Rhodus, 1987; Lanar et al., 2020).

Various mechanical and chemical branching-inducing methods (pinching of the youngest leaves, notching, additive N fertilization, and treatment with plant hormones or their inhibitors) have been used to increase the feather number on nursery trees (Lanar et al., 2018, 2020). Among these branching-inducing methods, cytokinin 6-benzyladenine (BA) is a plant growth regulator that is commonly used to induce feather formation in apple plants (Kender and Carpenter, 1972; Wertheim and Estabrooks, 1994). In Japan, highly feathered apple nursery trees are usually produced using BA solution over a two-year cycle. In the first year, scions are grafted onto dwarfing rootstocks (e.g., ‘JM7’ and ‘M.9’); then, one-year-old trees are excised at a height of 60 cm above the ground in late winter of the first year. In the second year, all buds are removed, except for the terminal shoot. After the terminal shoot reaches a length of 20 cm above the heading cut, a 2–5 mL aliquot of BA solution per tree (300 mg·L⁻¹ for easy-to-branch cultivars [e.g., ‘Fuji’] or 600 mg·L⁻¹ for difficult-to-branch cultivars [e.g., ‘Shinano Gold’]) is sprayed 6–9 times onto the top 10–20 cm region of the newly developing terminal shoot at 10–15-day intervals. Thus, the conventional method requires two years of nursery management. The ideal tree for high-density planting should have at least 10 feathers with a length of ≥ 5–< 50 cm (Komatsu, 2018).

The total cost of nursery trees purchased has increased dramatically with an increase in tree density (Robinson, 2007). To reduce the costs and increase the production of highly feathered trees, the duration of their nursery cycle should be reduced from two years to one year. However, it is difficult to naturally obtain one-year-old nursery trees with sufficient feather number and length (Doric et al., 2014; Sadowski et al., 2007). We recently reported that 1 mM gibberellin A₃ (GA₃) treatment increases the feather number on young apple trees (Okada et al., 2020). This result indicated that adding GA₃ to the BA treatment may enhance feather induction over a one-year cycle. Here, we investigated the effects of GA₃ and BA combinations on the production of well-feathered trees over a one-year cycle using the most commercially important apple cultivar, ‘Fuji’, which covers ~50% of the growing area in Japan.

Materials and Methods

Plant materials and growth conditions

The apple scion cultivar, ‘Fuji’, and the apple dwarfing rootstock cultivar, ‘JM7’, were used in this study. ‘JM7’, which exhibits 93% higher vigor than M.9EMLA, is characterized by easy propagation via hardwood cuttings and resistance to the wooly apple aphid and apple chlorotic leaf spot virus (Soejima et al., 2010). ‘JM7’ rootstocks were planted in nurseries at a distance of 1.0 × 0.2 m at the Institute of Fruit Tree and Tea Science, National Agriculture and Food Research Organization, Iwate, Japan.

Plant growth regulators

The plant growth regulators GA₃ (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) and BA solution (3.0% 6-benzyladenine) (Kumiai Chemical Industry Co., Ltd., Tokyo, Japan) were used in this study. No wetting agents or surfactants were used.

GA₃ and BA treatments in 2018 (Trial 1)

On May 16, 2018, the scions of ‘Fuji’ were grafted onto ‘JM7’ rootstocks at a height of 20 cm above the ground. Feathers that emerged with a terminal shoot length of < 35 cm were removed. When the terminal shoot lengths of all trees exceeded 35 cm, GA₃ (0, 100, 500, and 1,000 μM) + BA (75, 150, and 300 mg·L⁻¹) aqueous solutions (5 mL per tree [Fig. S1a]) were sprayed once a week for seven weeks onto the top 10–20 cm region of newly developing terminal shoots (from July 31, 2018 to September 12, 2018). Control trees were not treated.

GA₃ and BA treatments in 2019 (Trial 2)

From May 15–17, 2019, ‘Fuji’ scions were grafted onto the rootstocks of ‘JM7’ at a height of 20 cm above the ground. Feathers that emerged with a terminal shoot length of < 35 cm were removed. When the terminal shoot lengths of all trees exceeded 35 cm, GA₃ (0, 100, 500, and 1,000 μM) + BA (0, 75, 150, and 300 mg·L⁻¹) aqueous solutions (5 mL per tree [Fig. S1a]) were sprayed once a week for nine weeks onto the top 10–20 cm region of newly developing terminal shoots (from July 10, 2019 to September 6, 2019). In Trial 2, the new condition of 0 mg·L⁻¹ BA was added to investigate the effects of GA alone on feather formation.

GA₃ and BA treatments in 2019 (Trial 3)

From May 16–17, 2019, ‘Fuji’ scions were grafted onto the rootstocks of ‘JM7’ at a height of 20 cm above the ground. No feathers emerged with a terminal shoot length of < 35 cm. When the terminal shoot lengths of all trees exceeded 35 cm, 1,000 μM GA₃ + 300 mg·L⁻¹ BA aqueous solution (5, 3, or 2 mL per tree [Fig. S1a–c]) was sprayed once a week for nine weeks onto the top 10–20 cm region of newly developing terminal shoots (from July 10, 2019 to September 6, 2019).

Data collection and statistical analysis

After growth ceased, the terminal shoot length, feather number and length, and trunk diameter at a height of 20 cm above the graft were measured. The feathers that emerged at a height of ≥ 35 cm of the terminal shoot and had a length of ≥ 1 cm were enumerated. The flower buds were enumerated by observing the flowers in May of the following year.

To mitigate the impact of variation among trees, the average feather length for each tree was used to calculate the mean ± SE for each treatment and to conduct statistical analysis. Two-way analysis of variance was used to compare the effects of GA₃ and BA treatments. The Tukey-Kramer test was used to perform multiple comparisons (P < 0.05). Statistical analyses were performed using Mac Toukei Kaiseki Ver. 3.0 (ESUMI Co., Ltd., Tokyo, Japan).

Results

Effects of GA₃ and BA treatments on ‘Fuji’/JM7 nursery trees in 2018 (Trial 1)

To determine the optimal combination of GA₃ and BA treatments to induce feather formation, the shoots of ‘Fuji’ grafted onto ‘JM7’ rootstocks were sprayed with 12 combinations of GA₃ + BA aqueous solutions for seven weeks (Table 1). We observed that the 100 and 500 μM GA₃ treatments significantly reduced the terminal shoot length compared with 0 μM GA₃ treatment. Further, 1,000 μM GA₃ treatment significantly increased the feather number (both ≥ 1 and ≥ 5–< 50 cm) compared with the 0 μM GA₃ treatment. Similarly, the 150 and 300 mg·L⁻¹ BA treatments significantly increased the feather number (both ≥ 1 and ≥ 5–< 50 cm) compared with the 75 mg·L⁻¹ BA treatment, with the 300 mg·L⁻¹ BA treatment exerting a greater effect than the 150 mg·L⁻¹ BA treatment. The GA₃ × BA interaction had a significant effect on feather length, but none of the treatments significantly differed from the conventional method (0 μM GA₃ + 300 mg·L⁻¹ BA). Flower buds were not observed under any treatment. These findings indicate that 1,000 μM GA₃ + 300 mg·L⁻¹ BA is the optimal combination for producing nursery trees with a high feather number. Trees treated with 1,000 μM GA₃ + 300 mg·L⁻¹ BA exhibited a 0.9-fold greater terminal shoot length, 1.9-fold greater feather number (≥ 1 cm), 2.0-fold greater feather number (≥ 5–< 50 cm), and 1.1-fold greater feather length than those treated with the conventional method (Table 1). The predominant feather lengths in trees treated with 1,000 μM GA₃ + 300 mg·L⁻¹ BA were ≥ 1–< 5 (46.0%) and ≥ 5–< 10 cm (38.9%), which are comparable to the distribution of feather lengths obtained using the conventional method (Table 2).

Table 1

Effects of seven treatments with gibberellin A₃ (GA₃) and 6-benzyladenine (BA) solutions on ‘Fuji’/JM7 apple nursery trees in 2018.

Table 2

Distribution of feather length of ‘Fuji’/JM7 apple nursery trees treated with gibberellin A₃ (GA₃) and 6-benzyladenine (BA) in 2018.

Effects of GA₃ and BA treatments on ‘Fuji’/JM7 nursery trees in 2019 (Trial 2)

To determine the optimal combination of GA₃ and BA treatments to induce feather formation, the shoots of ‘Fuji’ grafted onto ‘JM7’ rootstocks were sprayed with 16 combinations of GA₃ + BA aqueous solutions for nine weeks (Fig. 1). The GA₃ × BA interaction had a significant effect on terminal shoot length, feather number (both ≥ 1 and ≥ 5–< 50 cm), feather length, trunk diameter, and flower bud number (Table 3). Compared with the conventional method (0 μM GA₃ + 300 mg·L⁻¹ BA), two treatments (0 μM GA₃ + 150 mg·L⁻¹ BA and 100 μM GA₃ + 150 mg·L⁻¹ BA) resulted in a significant decrease in the terminal shoot length. Further, compared with the conventional method, two treatments (500 μM GA₃ + 300 mg·L⁻¹ BA and 1,000 μM GA₃ + 300 mg·L⁻¹ BA) resulted in a significant increase in the feather number (both ≥ 1 and ≥ 5–< 50 cm), whereas ten treatments (0, 100, 500, and 1,000 μM GA₃ + 0 mg·L⁻¹ BA; 0, 100, 500, and 1,000 μM GA₃ + 75 mg·L⁻¹ BA; 0 and 100 μM GA₃ + 150 mg·L⁻¹ BA) resulted in a significant decrease. Moreover, compared with the conventional method, one treatment (500 μM GA₃ + 0 mg·L⁻¹ BA) resulted in a significant decrease in the feather length. There were no significant differences in trunk diameter or flower bud number between the conventional method and other treatments. These results demonstrate that two treatments (500 μM GA₃ + 300 mg·L⁻¹ BA and 1,000 μM GA₃ + 300 mg·L⁻¹ BA) are suitable for producing nursery trees with a high feather number. Trees treated with suitable GA₃ + BA combinations had a 1.0-fold greater terminal shoot length, 1.4–1.5-fold greater feather number (≥ 1 cm), 1.8–2.0-fold greater feather number (≥ 5–< 50 cm), 1.0–1.2-fold greater feather length, and 0.8–0.9-fold greater trunk diameter than those treated with the conventional method (Table 3). The predominant feather lengths in trees treated with suitable combinations were ≥ 10–< 30 (56.4–59.0%) and ≥ 5–< 10 cm (20.2–23.2%), whereas those in trees treated with the conventional method were ≥ 10–< 30 (45.0%) and ≥ 1–< 5 cm (33.1%) (Table 4).

Fig. 1

Representative tree architectures of ‘Fuji’/JM7 apple nursery trees treated with gibberellin A₃ (GA₃) and 6-benzyladenine (BA) in 2019. These photographs were captured in April 2020. The height of the white tarpaulin is 1.71 m.

Table 3

Effects of nine treatments with gibberellin A₃ (GA₃) and 6-benzyladenine (BA) solutions on ‘Fuji’/JM7 apple nursery trees in 2019.

Table 4

Distribution of feather length of ‘Fuji’/JM7 apple nursery trees treated with gibberellin A₃ (GA₃) and 6-benzyladenine (BA) in 2019.

Effects of the amount of 1,000 μM GA₃ + 300 mg·L⁻¹ BA solution on ‘Fuji’/JM7 nursery trees in 2019 (Trial 3)

To investigate the effects of different amounts of 1,000 μM GA₃ + 300 mg·L⁻¹ BA solution (5, 3, or 2 mL) required to induce feather formation, the shoots of ‘Fuji’ grafted onto ‘JM7’ rootstocks were sprayed with 1,000 μM GA₃ + 300 mg·L⁻¹ BA aqueous solutions for nine weeks (Table 5). The amount of solution had no significant effects on the terminal shoot length, feather length, or flower bud number. However, compared with the 5 mL treatment, the 2 mL treatment resulted in a significant decrease in the feather number (both ≥ 1 and ≥ 5–< 50 cm), and the 3 mL treatment resulted in a significant increase in trunk diameter. The predominant feather lengths in trees with 5 mL treatment were ≥ 10–< 30 (41.5%) and ≥ 5–< 10 cm (33.6%), whereas those in trees with 2 and 3 mL treatments were ≥ 10–< 30 cm (41.6–46.8%) and ≥ 1–< 5 cm (31.4–40.4%) (Table 6).

Table 5

Effects of the amount of 1,000 μM gibberellin A₃ (GA₃) + 300 mg·L⁻¹ 6-benzyladenine (BA) solution on ‘Fuji’/JM7 apple nursery trees in 2019.

Table 6

Distribution of feather length of ‘Fuji’/JM7 apple nursery trees treated with 1,000 μM gibberellin A₃ (GA₃) + 300 mg·L⁻¹ 6-benzyladenine (BA) in 2019.

Discussion

Among the 16 combinations of GA₃ and BA, we showed that 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 ‘Fuji’/JM7 cultivar nursery trees over a one-year cycle (Tables 1, 3, 5; Fig. 1). In 2019, 500 μM GA₃ + 300 mg·L⁻¹ BA treatment also had the same effect as 1,000 μM GA₃ + 300 mg·L⁻¹ BA treatment on the production of highly feathered ‘Fuji’/JM7 cultivar nursery trees over a one-year cycle (Table 3). Overall, nursery tree growth was higher in 2019 than in 2018 (Tables 1 and 3). Because growth vigor is likely to be regulated, at least in part, by endogenous gibberellin (GA) levels (Looney et al., 1988), the high endogenous GA activity in nursery trees in 2019 may have compensated for the GA deficiency of 500 μM GA₃ + 300 mg·L⁻¹ BA. Thus, our findings indicate that treatment with the GA₃ and BA combination induces greater feather formation in apple nursery trees over a one-year cycle compared with treatment with BA alone.

In Japan, trees suitable for high-density planting should have at least 10 feathers with a length of ≥ 5–< 50 cm (Komatsu, 2018). The conventional method (0 μM GA₃ + 300 mg·L⁻¹ BA treatment) met this requirement in 2019 (Table 3), but not in 2018 (Table 1). However, the 1,000 μM GA₃ + 300 mg·L⁻¹ BA treatment met this requirement in both years. Thus, the 1,000 μM GA₃ + 300 mg·L⁻¹ BA treatment induces more stable branching than the conventional method, particularly under poor growth conditions.

The production of slender nursery trees is required for the Tall Spindle system because it requires less labor to bend larger feathers below the horizontal during planting. Furthermore, slim nursery trees facilitate transportation and storage. In both years, the predominant feather length (≥ 94.5%) in trees treated with 1,000 μM GA₃ + 300 mg·L⁻¹ BA was 1–30 cm, with very few large feathers (Tables 2 and 4). Despite the growth-promoting effect associated with GA, its addition did not result in longer feathers or negatively affect the production of slender nursery trees compared with the conventional treatment (300 mg·L⁻¹ BA alone).

We used ‘Fuji’ as a scion and ‘JM7’ as a rootstock because the former is the most popular cultivar in Japan and the latter can be easily propagated via hardwood cuttings. Thus, this combination is appropriate as a model for mass production of nursery trees with high-density planting. From a practical viewpoint, the effectiveness of feather formation induced by GA₃ + BA may be affected by scion cultivars and rootstocks. Future research should focus on the effect of GA₃ + BA on feather formation using different scion cultivars (e.g., the difficult-to-branch cultivar, ‘Shinano Gold’) and dwarfing rootstocks [e.g., ‘M.9’ and ‘JM1’ (Soejima et al., 2010)]. It is also critical to monitor the growth of these nursery trees in the future (e.g., the interval from planting to production, cumulative yield, and fruit quality).

Previous studies have reported that the effectiveness of GA + BA treatments on the feather number was slightly contradictory. Lanar et al. (2020) demonstrated that treatment with a mixture of 500 mg·L⁻¹ GA_4/7 + BA (twice at 10-day intervals) was less effective at increasing the feather number on two-year-old apple trees (three-year cycle) (‘Rubinola’ and ‘Topaz’/M.9) than that with 1,000 mg·L⁻¹ BA alone (twice at 10-day intervals). Lordan et al. (2017) reported that four applications of either 400 mg·L⁻¹ BA or GA_{4 + 7} + BA at 10–14-day intervals provided the best results for inducing branching on apple nursery trees; however, GA_{4 + 7} + BA treatment was slightly less effective than BA treatment. Miller and Eldridge (1986) found that a single spray with 500 mg·L⁻¹ BA or GA_{4 + 7} + BA was equally effective in increasing branching on one-year-old apple trees (two-year cycle) of ‘Criterion Golden Delicious’. Doric et al. (2014, 2015) revealed that treatment with 450 μL·L⁻¹ GA_{4 + 7} + BA (three times at 7-day intervals) increased the total feather number more effectively than that with 450 μL·L⁻¹ BA alone (three times at 7-day intervals) on one-year-old apple trees (two-year cycle) (‘Golden Delicious’, ‘Gala’, and ‘Jonagold’/M.9 T337). Our results are in line with those reported by Doric et al. (2014, 2015). These discrepancies may be due to the differences in the doses and frequency of application. In particular, spraying in previous studies (1–4 times) was less frequent than that in the current study. Kender and Carpenter (1972) demonstrated that foliar applied BA only provides a localized stimulus, with buds responding within 5 cm of the application point. As a constant BA supply must be provided to the buds to break apical dominance, currently growing shoots may require repeated applications to achieve satisfactory branching. Thus, we believe that it is important to reapply the plant growth regulators at regular intervals to produce well-feathered trees. Furthermore, the members of the GA family may influence the differences because GA₃, which was used in this study, is resistant to deactivation by GA 2-oxidase (Cheng et al., 2021; Kanno et al., 2016) and has more stable biological activity than GA₄.

Axillary bud outgrowth can be classified into four stages: bud formation, inhibition by apical dominance, bud activation, and growth leading to branching (Tan et al., 2019). Our findings revealed that treatment with 1,000 μM GA₃ alone increased the feather number (≥ 1 cm), but it was less effective than treatment with 300 mg·L⁻¹ BA alone (Table 3). Similarly, Tan et al. (2019) reported that treatment with 5 mM GA₃ significantly stimulated axillary bud outgrowth in apple plants; however, it was less effective than treatment with 5 mM BA. Previous studies have suggested that the GA + BA combination plays a sequential role in feather production in apple plants. BA breaks the dormancy of buds; however, the shoot stops growing immediately and forms a short spur. In the presence of GA, the shoot elongates and grows into a feather (Cody et al., 1985; Doric et al., 2014; Hrotko et al., 2000; Rossi et al., 2004; Williams and Billingsley, 1970). Similarly, Kender and Carpenter (1972) reported that the difference in response to BA between growing and nongrowing shoots in apple plants may be attributed to the high GA activity in the apices of growing shoots, which was used to stimulate feather elongation, whereas axillary buds of nongrowing shoots were induced to grow, but their elongation was inhibited by low GA activity. In contrast, recent studies on the perennial woody plant, Jatropha curcus, have suggested that GA₃ promotes shoot branching and is required for inducing axillary bud outgrowth via BA (Ni et al., 2015, 2017). Although further research is needed on the effects of GA on apple bud break, this study indicates the importance of GA as a positive regulator of feather formation in apple nursery trees.

Conclusions

The repeated application of 1,000 μM GA₃ + 300 mg·L⁻¹ BA is a more effective and consistent method for increasing the feather number on ‘Fuji’/JM7 apple nursery trees than the conventional application of 300 mg·L⁻¹ BA alone. The GA₃ + BA treatment significantly facilitated the production of highly feathered nursery trees over a one-year cycle, which may help reduce the time and labor costs required to obtain a large number of well-branched nursery trees needed for high-density plantings such as in the Tall Spindle system.

Acknowledgements

The authors would like to thank Hiroshi Iwanami and Masatoshi Nakajima from the University of Tokyo, Japan for helpful discussions. We thank Enago (www.enago.jp) for the English language review.

Literature Cited

•  Cheng,  J.,  J.  Ma,  X.  Zheng,  H.  Lv,  M.  Zhang,  B.  Tan,  X.  Ye,  W.  Wang,  L.  Zhang,  Z.  Li,  J.  Li and  J.  Feng. 2021. Functional analysis of the gibberellin 2-oxidase gene family in peach. Front. Plant Sci. 12: 619158. DOI: 10.3389/fpls.2021.619158.
•  Cody,  C. A.,  F. E.  Larsen and  R.  Fritts. 1985. Stimulation of lateral branch development in tree fruit nursery stock with GA_{4 + 7} + BA. HortScience 20: 758–759.
•  Doric,  M.,  Z.  Keserovic,  N.  Magazin,  B.  Milic and  M.  Miodragovic. 2014. Induction of feathers on one-year-old apple trees cultivar golden delicious using benzyladenine and gibberellins 4 + 7. Contemporary Agriculture 63: 13–21.
•  Doric,  M.,  N.  Magazin,  B.  Milic and  Z.  Keserovic. 2015. Enhancing feathering of one-year-old Gala and Jonagold apple trees through application of 6-benzylaminopurine and gibberellins. Bulgarian Journal of Agricultural Science 21: 631–637.
•  Ferree,  D. C. and  W. T.  Rhodus. 1987. Early performance and economic value of feathered apple trees on semi-standard rootstocks. J. Amer. Soc. Hort. Sci. 112: 906–909.
•  Hrotko,  K.,  L.  Magyar and  Z.  Ronay. 2000. Improved feathering on apple nursery trees by BA application. Acta Hortic. 514: 113–119.
•  Kanno,  Y.,  T.  Oikawa,  Y.  Chiba,  Y.  Ishimaru,  T.  Shimizu,  N.  Sano,  T.  Koshiba,  Y.  Kamiya,  M.  Ueda and  M.  Seo. 2016. AtSWEET13 and AtSWEET14 regulate gibberellin-mediated physiological processes. Nat. Commun. 7: 13245. DOI: 10.1038/ncomms13245.
•  Kender,  W. J. and  S.  Carpenter. 1972. Stimulation of lateral bud growth of apple trees by 6-benzylamino purine. J. Amer. Soc. Hort. Sci. 97: 377–380.
•  Komatsu,  H. 2018. Breeding of ‘Shinano Sweet’ and ‘Shinano Gold’ apples and development of their management techniques to stably produce high-quality fruits. Hort. Res. (Japan) 17: 269–277 (In Japanese).
•  Lanar,  L.,  M.  Meszaros,  J.  Namestek and  J.  Sus. 2018. Feathering ability of apple, pear and cherry nursery trees treated with different branch-inducing methods. Acta Hortic. 1206: 189–196.
•  Lanar,  L.,  M.  Meszaros,  K.  Kyselova,  J.  Namestek,  J.  Sus,  H.  Belikova and  P.  Conka. 2020. Branching of nursery apples and plums using various branching inducing methods. Journal of Central European Agriculture 21: 113–123.
•  Looney,  N. E.,  J. S.  Taylor and  R. P.  Pharis. 1988. Relationship of endogenous gibberellin and cytokinin levels in shoot tips to apical form in four strains of ‘McIntosh’ apple. J. Amer. Soc. Hort. Sci. 113: 395–398.
•  Lordan,  J.,  T. L.  Robinson,  M. M.  Sazo,  W.  Cowgill,  B. L.  Black,  L.  Huffman,  K.  Grigg-McGuffin,  P.  Francescatto and  S.  McArtney. 2017. Use of plant growth regulators for feathering and flower suppression of apple nursery trees. HortScience 52: 1080–1091.
•  Miller,  S. S. and  B. J.  Eldridge. 1986. Use of 6-benzylamino purine and Promalin for improved canopy development in selected apple cultivars. Sci. Hortic. 28: 355–368.
•  Ni,  J.,  C.  Gao,  M. S.  Chen,  B. Z.  Pan,  K.  Ye and  Z. F.  Xu. 2015. Gibberellin promotes shoot branching in the perennial woody plant Jatropha curcas. Plant Cell Physiol. 56: 1655–1666.
•  Ni,  J.,  M. L.  Zhao,  M. S.  Chen,  B. Z.  Pan,  Y. B.  Tao and  Z. F.  Xu. 2017. Comparative transcriptome analysis of axillary buds in response to the shoot branching regulators gibberellin A3 and 6-benzyladenine in Jatropha curcas. Sci. Rep. 7: 11417. DOI: 10.1038/s41598-017-11588-0.
•  Okada,  K.,  M.  Wada,  Y.  Takebayashi,  M.  Kojima,  H.  Sakakibara,  M.  Nakayasu,  M.  Mizutani,  M.  Nakajima,  S.  Moriya,  T.  Shimizu and  K.  Abe. 2020. Columnar growth phenotype in apple results from gibberellin deficiency by ectopic expression of a dioxygenase gene. Tree Physiol. 40: 1205–1216.
•  Reig,  G.,  J.  Lordan,  M. M.  Sazo,  S. A.  Hoying,  M. J.  Fargione,  G. H.  Reginato,  D. J.  Donahue,  P.  Francescatto,  G.  Fazio and  T. L.  Robinson. 2019. Effect of tree type and rootstock on the long-term performance of ‘Gala’, ‘Fuji’ and ‘Honeycrisp’ apple trees trained to Tall Spindle under New York State climatic conditions. Sci. Hortic. 246: 506–517.
•  Robinson,  T. L. 2007. Recent advances and future directions in orchard planting systems. Acta Hortic. 732: 367–381.
•  Robinson,  T. L.,  S. A.  Hoying and  G. H.  Reginato. 2006. The Tall Spindle apple production system. NY Fruit Quarterly 14: 21–28.
•  Rossi,  A. D.,  L.  Rufato,  C. L.  Giacobbo,  F. R. C.  Gomes and  J. C.  Fachinello. 2004. Use of Promalin on one-year old trees of the apple cv. ‘Catarina’. Acta Hortic. 636: 145–149.
•  Sadowski,  A.,  M.  Mackiewicz and  R.  Dziuban. 2007. Growth and early bearing of apple trees as affected by the type of nursery trees used for planting. Acta Hortic. 732: 447–455.
•  Soejima,  J.,  Y.  Yoshida,  T.  Haniuda,  H.  Bessho,  S.  Tsuchiya,  T.  Masuda,  S.  Komori,  T.  Sanada,  Y.  Ito,  S.  Sadamori and  Y.  Kashimura. 2010. New dwarfing apple rootstocks ‘JM1’, ‘JM7’ and ‘JM8’. Bull. Natl. Inst. Fruit Tree Sci. 11: 1–16 (In Japanese with English abstract).
•  Tan,  M.,  G.  Li,  X.  Chen,  L.  Xing,  J.  Ma,  D.  Zhang,  H. J.  Ge,  M.  Han,  G.  Sha and  N  An. 2019. Role of cytokinin, strigolactone, and auxin export on outgrowth of axillary buds in apple. Front. Plant Sci. 10: 616. DOI: 10.3389/fpls.2019.00616.
•  Wertheim,  S. J. and  E. N.  Estabrooks. 1994. Effect of repeated sprays of 6-benzyladenine on the formation of sylleptic shoots in apple in the fruit-tree nursery. Sci. Hortic. 60: 31–39.
•  Williams,  M. W. and  H. D.  Billingsley. 1970. Increasing the number and crotch angles of primary branches of apple trees with cytokinins and gibberellic acid. J. Amer. Soc. Hort. Sci. 95: 649–651.