It is important to that reducing crop loads not only improves the final fruit size of apples.
- At lower crop loads, the apples are often of improved quality; they ripen earlier, are sweeter, often firmer and have higher levels of dry matter.
- Work conducted at East Malling many years ago showed the effects on fruit size, quality and storage potential of thinning the variety Cox’s Orange Pippin (Sharples, 1968).
- More recent work on the same variety (Johnson, 1995) has shown that the ripening maturity of fruits is advanced by thinning treatments conducted at 5 days after full bloom but not by similar thinning treatments carried out 27 or 39 days after full bloom.
- Advances in maturity, as judged by internal ethylene concentration, were as much as 16 days.
- Similar effects of thinning on fruit quality at harvest time have also recently been demonstrated for Braeburn apples growing in Spain (Kelner et al., 1999).
Cell division
Thinning apples can significantly increase cell division in the fruits remaining on the tree.
- However, only early thinning, either of flower buds or flowers, can achieve this objective.
- Delaying thinning to the 12 mm fruitlet stage or later will often have only minimal effects on cell division in the persisting fruits, although it will result in increased cell expansion.
- This is explained by the fact that cell division in fruits such as Cox is completed by about 6 or 7 weeks after full bloom (Denne, 1960).
- Removal of excess flower buds in the winter months will also have benefits to cell division in the fruits forming on the remaining flower buds.
Fruit firmness
More recently, studies conducted at East Malling on the apple variety ‘Cox’s Orange Pippin’ (Johnson, 1992 and 1994) showed that the firmness of fruits at harvest was increased significantly if the trees were thinned, even when fruit size was increased by the thinning.
- Subsequent studies showed that thinning to single fruits/cluster in the period between 5 and 15 days after full bloom gave the best results.
- Thinning to similar levels at 25 days after full bloom gave no increase in firmness at harvest, but later thinning did improve harvest firmness.
- All the recorded increases in fruit firmness were associated with increases in the percentage dry matter in the fruits.
- The increases in harvest firmness were maintained after storage of the fruits in CA conditions (2% O2 and <1% CO2) but not in CA with lower O2 levels or in air storage.
- In the low O2 regime (1.25%) the enhanced softening is thought to have been attributable to increased sensitivity to core flush and senescent breakdown, which may have been related to the higher K and lower Ca status of these fruits.
Early thinning of apples by blossom thinning or very early fruitlet thinning will reduce the competition within the tree for assimilates and improve cell division in the persisting fruitlets.
- Early thinning will improve fruit size and firmness and advance ripening.
- However in some situations early thinned fruits may be more sensitive to storage disorders, such as senescent breakdown and core flush.