Apple Fruit Growth
Alan N. Lakso and Martin C. Goffinet Department of Horticulture, NYSAES, Cornell University Geneva, NY 14456 This work was supported by the New York State Apple Research and Development Program
The apple fruit goes through a complex developmental sequence over a growing season. Understanding the processes involved, what supports fruit growth and what limits it, helps to support good crop management. This article describes the basic processes of fruit growth and the factors that support or limit fruit growth.
The apple fruit derives from the base of the apple flower after pollination and fertilization of the egg cells in each of the 10 ovules in the base of the flower (Figure 1). After petal fall the base ovary of the flower begins to expand to make the fruit we harvest, as the diagram shows.
Pattern of Apple Growth Once the flower is fertilized, the fruitlet grows initially by exponential cell division; that is, cells divide
to produce twice as many cells, then divide again to give 4 times as many, then 8 times, etc. This gives an ever-increasing rate of growth. For the first week or so growth is only by cell division as cells do not get bigger (Figure 2).
Then from about 1 week after bloom until about 4 or 5 weeks after bloom, growth occurs by both cell division and cell expansion.
Finally, growth for the rest of the season occurs essentially only by cells expanding. During this cell expansion period the
fruit adds a similar amount of weight per day until harvest, although with very heavy crops or cold weather the rate of fruit
growth may decline before harvest. We have found that large fruit have higher growth rates than smaller fruit (for example, 2 grams per day versus 1.2 grams per day respectively). When we measured cell numbers per fruit, we found that the difference in growth rate was directly controlled by cell numbers, as each cell grew the same amount per day in all fruits.
Because the apple tree, with a heavy bloom, will produce 10-15 times more flowers and potential fruit than desirable at harvest, 90-95% of the fruits must fall off to avoid over cropping. If too many fruit compete for too long, cell division is curtailed. In a detailed anatomical study we found that fruit size variation at harvest from thinning trials was about 85% explained by cell numbers and only a small amount due to cell size variation.
The bottom line is that the final fruit size potential, and generally the actual size at harvest, depends primarily on the number of cells in the fruit. And since cell numbers are set in only the first few weeks after bloom, that is a critical time for the whole season. Fruit with low cell numbers from excess competition after bloom, due to late or inadequate thinning, can never catch up later to become large fruit.
Good fruit size requires a lot of cells. Since cell numbers are a result of cell division that occurs only in the first several weeks after bloom, the critical time to adjust fruit numbers by thinning is as early as possible after bloom. If thinning is done early, the retained fruit will have time for further cell division, thus improving size potential. However, if thinning is done late, the fruit will have suffered from competition for too long, have too few cells and have little to no time to catch up in cell numbers.
Additionally, next year’s flower buds are already developing at the same time fruitlets are setting in the weeks after bloom. If thinning is delayed too long, not only is fruit size decreased, but return flowering and next year’s crop potential will be reduced as well. Both these factors place an emphasis on early crop adjustment.
As fruit grow in weight, the apple fruit accumulates large amounts of starch during the season. We have found that the starch is not available to support fruit growth, but it appears that the starch is saved to be converted to sugars as harvest approaches. Presumably this was a natural selection for making fruits in the wild sweeter and more attractive to animals to increase seed dispersal. This change from starch to sugar provides a useful common indicator of fruit maturity.
Why Do Some Fruit Drop and Some Stay on the Tree ?
Since often 90% or more of the young apple fruitlets need to fall, either naturally or by chemical or hand thinning, we wondered why some fruits drop but others stay on the tree? It turns out that fruit that can maintain a continuously high growth rate stay on the tree. However, fruit that have a slow growth or slow their growth for several days will drop.
We have monitored the growth of thousands of fruit over many years and find a consistent general relationship between fruit growth rate and drop (Figure 3). Although varying a bit, it is the same for many varieties as well as for fruit drop caused by different thinners, natural stresses of low light or just excess competition among fruits. A decrease in growth rate that leads to fruit drop can also occur for only a few days, but once it happens the fruit is destined to drop in the next week or so. So the bottom line is that anything that reduces fruit growth will increase drop. That seems to explain why so many different things (stresses, different chemical, cloudy or hot periods, etc.) can increase fruit drop.
In summary, there are many aspects of apple development that have been learned. Many of the key points are:
• Fruits develop from the base of the apple flower after pollination and fertilization and the flower walls around the seed cavity
expand to become the fruit flesh.
• The apple fruit grows initially by cell division for about a week, then by both cell division and cell expansion for 3-4 more weeks,
then predominantly by cell expansion.
• Final potential fruit size depends primarily on cell numbers, which are produced shortly after bloom. So, for good fruit
size thinning effectively and early is critical (that is, small fruits at 1 month after bloom will never make big fruit).
• Carbohydrate reserves support flower development but are apparently not supporting the fruit growth after bloom. Post-bloom fruit growth is supported by the current photosynthesis
of the leaves.
• Spurs support fruit growth in the first few critical weeks after bloom. Extension shoots do not support fruit
initially as they support themselves until a few weeks after bloom.
• If light is limited (cloudy, etc.), shoot growth seems to have priority and fruit growth and set will suffer if there are too many shoots to support.
• Yields of apple orchards depend strongly on the sunlight captured by the spurs in the critical weeks after bloom.
• All of these results emphasize the importance of open canopies to get light to spurs, light pruning to have fewer vigorous shoots, and early thinning to allow maximum cell numbers to be produced.