After fruit set, the growth of the persisting fruitlets is brought about by a combination of cell division and cell expansion.
- Research undertaken many years ago showed that the majority of cell division ceases within 4 weeks of pollination (Bain and Robertson, 1951), whereas cell expansion continues throughout the growth period of the fruit.
- In this Australian research conducted on Granny Smith apples, final differences in fruit size were associated more with cell number than with cell size.
- Even earlier research on the variety Bramley’s Seedling similarly showed cell division to cease only a few weeks after fruit set (Tetley, 1930 and 1931).
Research in New Zealand on the variety Cox showed that fruit weight increased slowly for the first few days following pollination then increased exponentially at a very rapid rate for three weeks and, thereafter, increased at diminishing rates (Denne, 1960).
- Cell division was very rapid in the three week phase of exponential growth and then declined but some cell division occurred until 6 or 7 weeks after pollination.
- Cell expansion was also very rapid until 7 weeks after pollination but then occurred at diminishing rates until harvest.
Studies on the variety Cox in the early 1980s showed that differences in cell size at harvest were not an important factor influencing final fruit size (Skene et al., 1984).
- The indications were that fruit size was more influenced by cell numbers and that few new cells were produced after the fruitlets had reached approximately 18 mm in diameter.
The main bulk of any apple fruit, which is thought to develop from the receptacle in the flower, consists of a parenchymatous tissue forming the flesh, of which the cortex constitutes the greatest percentage by volume. The cells of the cortex are of spongy tissue, large, thin-walled with a thin lining of cytoplasm and a large vacuole.
A number of factors, other than crop loading are known to influence cell division and expansion. These are: