According to Bangerth (2000) there are, currently, two hypotheses advanced to explain the abscission of young apple fruits:
- Abscission is caused by insufficient supply of assimilate to fruitlets as a result of limited assimilate production and/or allocation to the fruit.
- Abscission is caused by a regulatory hormonal mechanism by which the plant protects specific fruits from assimilate limited growth later in the season.
Although there is some support for the first hypothesis (Stopar, 1998), abscinding apples often have equal or higher amounts of carbohydrates in them compared with the persisting fruits (Abruzzese et al., 1995).
In recent years, therefore, research has focused on understanding how hormones influence fruitlet abscission, although the causes of abscission are probably due to a combination of hormonal and assimilate supply factors.
The general hypothesis concerning abscission is that various factors influence the balance of auxins and ethylene around the abscission zone, and also influence the receptivity of the tissues in this zone to either auxin (prevents abscission) or ethylene (stimulates abscission).
- For instance, reductions in the supply of foodstuffs (assimilates produced by photosynthesis) to the fruit, will in turn reduce the amounts of auxins that are produced and transported from the seeds out of the fruitlets.
- This renders the abscission zone more sensitive to ethylene stimulated abscission.
- Low light levels in the orchard can reduce photosynthesis, as can applications of the synthetic auxins NAA or NAAm or inhibitors of photosynthesis such as terbacil.
- The processes involved are much more complicated than described here and new information continues to improve our knowledge.
- For a fuller account of the biochemical and molecular processes controlling fruitlet abscission the reader should consult Bangerth (2000) and Bonghi et al., (2000).
Where trees are carrying a very heavy crop, the most vulnerable fruitlets to abscission are those that set slightly later and are, therefore, slightly smaller (Bangerth, 1989).
- Problems can be experienced occasionally where most fruitlets are of similar size and ‘sink strength’.
- In this situation, when conditions reducing assimilate supply are experienced, fruit drop may be excessively severe.
- Where trees develop vigorous growth of extension or bourse shoots, natural fruit drop may also be severe.
- This is thought to be due to the strong competition between the shoots and the fruits for vital assimilates (Bangerth, 1986).
- Competition also seems to play an earlier role, in that when floral abundance is very high, the levels of fruit set (per 100 flowers) are invariably lower than when floral abundance is lower (Jones and Koen, 1986).
- Indeed, work at East Malling showed that diminishing competition between flowers by removal of some of them increased the set of the remaining flowers (Knight et al., 1987).
Research conducted at East Malling has shown that carbon assimilation and its partitioning either from dormant season reserves held in the woody tissues of the tree or from current leaf photosynthesis is influential in fruit set and development.
- Fruit set can be depressed by the removal of the first leaves to emerge on the tree in the spring (the spur leaves) (Ferree and Palmer, 1982).
- The same research showed that the early removal of the bourse shoot could also reduce initial fruit set but increased final yields.
- This apparent paradox is probably explained by the removal of bourse leaves initially reducing assimilate supply and inducing fruit drop but, later, lack of bourse shoots reduces potential competition for assimilates by the bourse shoot as the persisting fruits grow.
Whilst spur leaf removal may appear an attractive method of early thinning, this is not recommended as these leaves are vital for the uptake of calcium into the fruits, as shown by complementary work at East Malling (Jones and Samuelson, 1983).
- The most severe reductions in fruit set on Cox were brought about by removal of spur leaves in the period between full bloom and two weeks later (Proctor and Palmer, 1981).
- Leaf removal in these trials also reduced fruit calcium levels at harvest and removal of bourse shoots reduced return bloom in the subsequent season.