Good Fruit Grower / May-15th 2012 / What makes a cherry big?

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What makes a cherry big?

Cell growth is regulated genetically, but it can be influenced by crop load.

What determines the ultimate size of a cherry? Is it the number of cells packed inside the fruit or the size of the cells that determines whether a cherry grows big or small? And are there differences in the cell numbers or sizes between fruit from high or low crop loads?

Dr. Dave Gibeaut, Oregon State University scientist stationed at OSU’s research center in Hood River, is working to gain insight into the growth and development of the fruit.

quipped with an electron microscope that greatly magnifies slices of cherry tissue, Gibeaut has been counting and sizing fruit cells of Chelan, Bing, and Sweetheart to learn more about the timing of cell division and growth of cherry cells.

“We know there are millions of cells in fruit and that ultimate fruit size is dependent on the number of cells and the size of those cells,” he said during a sweet cherry research symposium held in early February at The Dalles, Oregon. “Cell growth is regulated genetically, but it can be influenced by environment, such as crop load.”

Very little is known about individual cherry fruit growth and the role that the number, size, and position of cells play in the final fruit size, he said, adding that little is known about fruit size differences between cultivars and crop loads.

Number, size, position

In comparing fruit growth in trees with high or low crop loads, Gibeaut found that average fruit size in the high crop load was about 15 percent smaller than the low, although some large fruit were present. Also, size was more variable in the high crop load.

Gibeaut tagged fruit from high and low crop loads and examined fruit tissue slices under an electron microscope, counting cell numbers and measuring cell sizes at different growth stages. He counted all cells on a line from the pit to skin, starting at prebloom and ending when fruit was harvested.

Gibeaut found that cell divisions were completed early, within the first two weeks of growth. By the pit-hardening stage, he found no more gains in the numbers of cells. Additionally, he found no apparent difference in the number of cells between fruit from low and high crop loads.

He did observe that the size of cells continued to grow throughout the season until maturity, as one might expect, in fruit from both low and high crop loads.

For cell position, he noticed that cell shape varies depending on where the cell is located. “Cells are not all uniform in size. Some are big and round, and some are small,” said Gibeaut. Cells located in the inner mesocarp were elongated at maturity, while cells in the outer mesocarp were round at maturity. He saw no differences in cell position between low and high crop loads.

Cell distribution

With no differences found between total cell number, size, and position, Gibeaut then looked at cell size distribution or the proportion of large and small cells between the high and low crop loads.

“At bloom time, the low crop load fruit had a greater number of small cells compared to the high crop load,” he said. By midgrowth stage and harvest, the small cells in the low crop load had grown into large cells, and there was a greater number of large cells in comparison to the cells of the high crop load fruit.

“That’s what gives the larger fruit its size—a greater ­proportion of large cells, not more total cells,” he said.

Return bloom

In following an 8.5-row cherry from bloom to harvest, he found that average cell size increases as fruit size increases and that larger cells are dispersed throughout the inner and outer mesocarp. “The larger cells lead to ­bigger fruit size. Low crop loads tend to have a greater ­number of large cells, which lead to larger fruit.”

He will be exploring ways to manipulate the cell number—using plant growth regulators in the postharvest and/or prebloom stages—to try to increase cell numbers of fruit. He also plans to look at return bloom for the effect of bud thinning.

“With half of the cells in the final fruit already present at bloom, and the remaining cell divisions completed within the first two weeks of growth, any manipulation must be done early in the growth cycle,” he said. “But if we could increase the number of cells in the fruit through some means, we may be able to increase the size.”

Gibeaut is working with OSU’s Dr. Todd Einhorn and Washington State University’s Dr. Matt Whiting to improve year to year yield and quality of sweet cherries. Research includes timing and rate of ­gibberellic acid to influence fruit size and growth.