Apple fruits grow by cell division and cell expansion. Most cell division occurs in the first 3‑4 weeks following fruit set and by 7 weeks cell division has almost finished.
Thereafter, fruits grow by the cells expanding.
- The best large fruits, in terms of texture and storage potential are those with many cells rather than fewer larger cells.
- Increasing the size of fruits by applications of irrigation and nitrogen, so as to increase cell expansion, may produce fruits of poor texture, which store poorly.
The principal factors influencing cell division in young fruitlets are:
- Initial flower quality
- Crop loading
- Climatic conditions in the first month after fruit set.
Initial flower quality
- Poor ‘quality’ flowers usually result in poor quality fruits, which may be smaller than average and of poor texture.
- The production of flowers of high ‘quality’ is aided by:
- Optimising crop loads in the previous season.
- Maintaining a good balance of new shoot growth and flower production by appropriate pruning/training, nutrition and irrigation.
- Ensuring that the sites of flower production (e.g. the spurs) are well exposed to light.
- Ensuring that the concentrations of gibberellins in sprays applied to aid skin finish, are kept to a minimum.
- Biennial bearing varieties often produce poorer quality flowers, as well as fewer flowers in their ‘off’ year.
Crop loading
Crop loading can have a significant effect on fruit size and quality.
Early thinning of apples by blossom thinning or very early fruitlet thinning will reduce the competition within the tree for assimilates (carbohydrates produced by photosynthesis) 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.
Climatic conditions in the first month after fruit set
Warm temperatures in the 4 to 6 weeks after fruit set will have a very beneficial effect in encouraging cell division in the young developing fruits.
Such conditions will also aid the development of bourse and extension shoot leaves which will begin to contribute to the vital carbohydrates produced by photosynthesis which fuel the growth of young fruitlets.
Endeavour to increase temperatures in orchards during the first six weeks following fruit set, so as to maximise cell division in the young apple fruits.