Ystwyth Valley Apple Breeders

The long juvenile stage of apple seedlings, during which they will not flower, is the main cause of the extended generation times in apple breeding. Field-grown apple seedlings rarely flower before they are three years old and often not until they are aged eight years or more.

When growing seedlings the aim is to encourage them to pass through the juvenile phase of development as quickly as possible, and then to flower.

Zimmerman (1973) observed that the lowest bud on a seedling indicating the transition point from the juvenile to the adult phase occured at a height of 1.8 - 2 m on crab apple seedlings grown under greenhouse conditions. However, stage of development is better measured in terms of the number of nodes produced (i.e. points on the stem from which a leaf grows) than by height of the seedling. Accordingly (e.g. Hanke et al. 2007), the transition points from juvenile to adult vegetative and from adult vegetative to adult reproductive phases occur, respectively, around nodes 77 and 122 on the leading shoot.

Most of the traditional techniques for inducing and increasing flowering are associated with retarding vegetative growth of the shoot. Treatment of apple seedlings in this way is counter-productive until the adult vegetative phase is reached. During the actual juvenile phase the aim is to reach around 77 nodes on the leading shoot as quickly as possible, thereby completing this phase. The techniques listed below for reducing the time to flowering are compiled from Janick et al. (1996) and Hanke et al. (2007).

Actions effective during the juvenile phase:

• Selection and propagation of naturally occurring early flowering genotypes or mutants.
• Rapid growth of seedlings from germination to transition to the ‘adult reproductive phase’ at 122 nodes.
• Exposure of seedlings to a longer growing season (increases growth).
• Treatments increasing apical dominance of the leading shoot (increases growth).

 Actions effective during and/or after the adult vegetative phase:

• Treatments that reduce apical dominance in the shoot can promote flower formation.
• Inhibition of vegetative growth by trunk ringing, scoring, bark inversion, root pruning, horizontal shoot orientation, and shoot bending.
• Defoliation.
• Application of certain plant growth regulators/hormones.
• Grafting seedling scions onto dwarfing rootstocks. 

The most effective single strategy for shortening the time to flowering is to grow the seedlings as rapidly and vigorously as possible, maximising growth of a single leader in order to increase the rate of node production, without check. According to Janick et al. (1996) closely planted seedlings grown under optimum conditions in the greenhouse can reach 3 m in height in the first season, compared to I m in height when grown in the field. They can also flower in one year. Given that the youngest growth at the top of the main shoot emerges first from the juvenile condition, the leader should not be pruned until flowers and fruit are produced.

How this approach is applied commercially is illustrated by the following account (MAIA, 2010): 'Three to five thousand seedlings are started every winter in greenhouses, grown under optimal light, nutrients and water, reaching 2 m height (about 70 internodes) by early September. Once past 70 internodes of growth extension, they leave juvenility behind and additional growth is capable of flower bud initiation. Mature buds are then inserted into the base of M9 rootstocks. By late winter these trees are cutback and grown during year 2 into field-ready finished trees that will fruit during years 3, 4 and 5 in a standard commercial M9 (rootstock) format.’

Small-scale and home-based breeders are unlikely to afford the high investment and running costs associated with growing large numbers of seedlings in controlled environments. Traditionally, seedlings have been grown outdoors in nursery beds. Although these suffer from the vagaries of the weather and pest/disease attacks, reasonably high growth rates under field conditions can be supported by following appropriate regimes of nutrient and water supply.

Grafting, or budding, seedling scions onto dwarfing rootstocks is an established means of speeding up time to flowering. The method developed by Tydeman and Alston (1965) involves taking buds from the upper part of the main shoot in late summer of the second year’s growth, grafting these onto dwarfing rootstocks, growing them on as closely planted cordons and pruning back all lateral growth annually in late summer. Their results showed that 88% of 902 budded seedlings had fruited nine years after germination compared with 49% of those on their own roots.

Hanke M-V, Flachowsky H, Peil A, Hättasch C. 2007. No Flower no Fruit – Genetic Potentials to Trigger Flowering in Fruit Trees. Genes, Genomes and Genomics 1(1), 1-20.

Janick J, Cummins JN, Brown SK and Hemmat M. 1996. Apples. In: Janick J, Moore JN (eds), Fruit Breeding, Volume I: Tree and Tropical Fruits. John Wiley & Sons, Inc. 1-77.

MAIA, 2010. Midwest Apple Improvement Association Newsletter, 2010,

Volume 12, Issue 1.

http://midwestapple.com/midwest-apple-improvement-association-newsletters.htm)

Tydeman HM. and Alston FH. 1965. The influence of dwarfing stocks in shortening the juvenile phase of apple seedlings. In: Report of East Malling Research Station, 1964. p. 97-98.

Zimmerman RH. 1973. Juvenility and flowering of fruit trees. Acta Horticulturae 34, 139-142.