Several natural plant hormones have been implicated in the tree processes leading to flower and/or fruitlet abscission (drop).
- Most of the literature cites ethylene as the main promoter of flower, fruit or leaf abscission in trees, including the apple.
- Applications of chemicals, such as ethephon, that result in the production of ethylene, have been shown to increase abscission, whilst applications of chemicals which inhibit production of ethylene, such as silver, 2,5‑norbornadiene or aminoethoxyvinylglycine (AVG or Retain) have all been shown to reduce fruitlet abscission.
- However, more recent literature reviews suggest that abscission may be better understood as controlled by a balance of ethylene and auxin levels at the zone of abscission (Sexton, 1997).
- Also of importance is the responsiveness of the tissues associated with the abscission zone to either auxins or ethylene. Auxins move from the seeds or other tissues of fruits back into the fruit stalk and spur; this is referred to as basipetal movement.
During the opening of apple flowers the unfertilised ovules show very little growth and the production and export of auxins is low (Gruber and Bangerth, 1990).
- The flowers are, therefore, vulnerable to ethylene-induced abscission during this phase (Wertheim, 1997).
- Pollination, pollen tube growth and fertilisation of the ovules and subsequent seed growth all lead to a significant increase in hormone activity in the ovary.
- The auxins diffusing from the ovary help prevent abscission.
- Thereafter, the vulnerability of fruits to abscission changes as the balance of auxins and ethylene changes.