The sugar-like compound called sorbitol may to play a key role in determining the characteristics of apples and may be a good target for plant breeders and geneticists trying to improve fruit quality in apple trees, according to a recent study by researchers at the University of California, Davis.

Findings of the study appear this week in the Proceedings of the National Academy of Sciences of the U.S.A.

During photosynthesis, apple trees produce sorbitol, starch and sucrose -- common table sugar -- in their leaves. Sorbitol, which is actually a sugar alcohol, is frequently used in food processing as an alternative sweetener because it is lower in calories than sucrose and does not promote tooth decay. It is also used as a sugar substitute in candies made for diabetics.

The sorbitol and sucrose are transported from the apple tree's leaves to the developing fruit, as well as to other growing tissues like immature leaves, root tips, and developing flowers and seed.

To investigate how sorbitol distribution in the tree affects fruit quality, the researchers used transgenic apple trees, which had been genetically modified to produce less of the enzyme necessary for synthesizing sorbitol in the trees' leaves. As a result, sorbitol concentrations in the leaves of these test trees were reduced by 90 percent.

The apples produced by the transgenic trees also had different carbohydrate concentrations than the non-transgenic trees in the control group. The transgenic apples accumulated more than twice the glucose of apples in the control group, but less fructose, starch and malic acid.

"We also found that there were no unfavorable effects on fruit firmness or appearance associated with the altered sorbitol-to-sucrose ratio," said Abhaya Dandekar, a plant science professor and lead researcher on the study. "What was unexpected in the apples from the transgenic trees was their low starch, low acidity and higher soluble solids content."

Taken together, the results of this study suggest that sorbitol distribution in an apple tree is critical to how the tree metabolizes carbon and to determining fruit quality characteristics such as sugar-acid balance and starch accumulation.

The study was funded by the Washington Tree Fruit Research Commission.