Bigger fruit, bigger profit
With more insights into how fruit grows, researchers are hoping to learn how to genetically manipulate fruit size.
Fruit size has a great impact on the value of the crop, and researchers are working to understand what influences the size of apples in order to help growers maximize returns.
“If we can increase the size of every apple in your orchard by 20 percent, you have doubled your money,” Dr. Peter Hirst, horticulturist at Purdue University, Indiana, told growers at the Washington State Horticultural Association’s annual meeting. “There’s a pretty strong incentive to focus on fruit size.”
Fruit size is governed by three components: the number of cells, the size of cells, and the space between the cells. There is more air inside a larger apple than a small one, Hirst said. Since apples are sold by weight, cell size and number are more important than the space between them.
Fruit size is affected by the fruit’s genetic makeup (the cultivar), cultural practices in the orchard, and the environment. All those factors need to be optimized to produce larger fruit.Genetic traits have a very strong influence, Hirst said. For example, a crab apple will always be small, even with optimum pollination, fertilization, pruning, and light.
Cultural factors include pruning and thinning (the most important ones), but also tree training, nutrition, cropping history, soil management, and pollination. Environmental factors include soil, temperature, light, and moisture.
To study the processes that control fruit size, Hirst used the crab apple Hopa along with Pixie Crunch, Gala, Empire, and Golden Delicious. Typically, a crab apple weighs about 8 grams, and a Golden Delicious more than 200 grams. The other varieties are in between at around 120 to 130 grams.
Hirst discovered that the size of the fruit cells has a much smaller influence on final fruit size than the number of cells. Most varieties had a similar cell size. The crab apple had cells 128 microns in diameter, and the rest were around 150 microns in diameter. There was a much greater difference in the number of cells in the fruit. Golden Delicious had between 100 and 110 cell layers in the outer flesh, while the crab apples had only 30. The other varieties had between 75 and 90 layers.
The number of cells increases rapidly during the first four weeks of growth, and then the growth rate levels off, Hirst found. To compare the growth rates, he calculated what proportion of the cells were actively dividing at any one time.
Golden Delicious has a high rate of cell production for a fairly long period of time, whereas Gala cells divide at a lower rate for about the same period of time, resulting in a smaller apple. Pixie Crunch has a high rate of cell division for a shorter period of time, which also results in small fruit size.
These findings refute two commonly held beliefs regarding fruit growth, Hirst said. One is that cell division lasts for about 30 days and then stops. His team’s data show a slow and gradual decline in the proportion of cells that are actively dividing. The other is that fruit grows first by cell division and then by cell expansion with no overlap. Hirst’s data show a considerable overlap in cell division and expansion. Cells start to enlarge about 14 days after bloom, but the number of cells keeps increasing until at least 25 days after full bloom.
When Hirst compared Gala and Red Delicious apples, he found that the growth rates were the same. Red Delicious apples were larger at harvest because it was a later-maturing variety, and they were on the tree longer. So, harvest date can also be a determinant of fruit size.
Hirst studied the effect of temperature on fruit growth during the first 40 days after fruit set. To do this, he enclosed trees in poly houses at petal fall which raised the temperature inside the houses by 5° to 10°F.
He found that trees with a low crop load that were exposed to higher temperatures had the largest fruit, while the smallest fruit came from heavily cropped trees exposed to lower temperatures. Small differences in temperatures early in the season translated to big differences in fruit size at harvest. He deduced that the higher temperature affected fruit size by increasing the size of cells, not the number. Large and small fruit had the same number of cells.
The number of cell-division cycles is controlled genetically, but higher temperatures speed them up, he said. When it’s warm, the cell-division phase is completed sooner, leaving more time for cell expansion to take place.
Hirst said this phenomenon is different in apples than in stone fruits. In stone fruits, fruit size at harvest is usually smaller when temperatures are high after bloom.
Asked whether fruit size is influenced by seed count, Hirst said there is a weak relationship, but he is not sure if the seed number affects the cell number or cell size.
Hirst said now that more is known about what influences fruit size, he and his colleagues are working to identify the genes involved. It won’t be just one gene, as fruit size is influenced by more than one factor.
“Now we have made some headway in understanding how these changes in fruit size are played out in terms of cell dynamics, we’re in good shape to understand all the genetic influences and figure out how to manipulate them to get larger fruit size in your orchard,” he said.