Dr. Rongcai Yuan made important contributions to understanding fruit abscission processes. He died a year ago of cancer at age 45.

Dr. Rongcai Yuan made important contributions to understanding fruit abscission processes. He died a year ago of cancer at age 45.

Photo by Steve McArtney

Scientists have made great progress in recent years homing in on answers to ­mysteries surrounding why apples fall off trees—so they can help growers make more do it when there are too many on the trees in the spring or to stop them from doing it before harvest.

One leading figure in this work is Dr. Steve McArtney, a North Carolina State University horticulturist who is the Southeast Apple Specialist for that university working jointly with University of Georgia, Clemson University in South Carolina, and the University of Tennessee. He spoke at several state horticultural shows this winter, including those in Michigan and New York.

“Every apple grower knows that warmer temperatures and, in some cases, cloudy weather during the thinning period can increase the activity of a chemical thinner,” McArtney said. “But have you ever wondered about the processes occurring in an apple tree under those conditions that ultimately result in increased thinner activity? Why is it that an application of NAA soon after bloom will cause the fruit to drop, but application of the same material just before harvest will have the exact ­opposite effect, causing the fruit to stick?”

During the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December, McArtney recognized the work of his colleague Dr. Rongcai Yuan, who died of cancer in April of 2010, only 45 years old. Born in China and with a doctorate from University of Massachusetts, Yuan directed a lab and graduate students at the Alson H. Smith, Jr., Agricultural Research and Extension Center in Winchester, Virginia.

Molecular techniques

McArtney said that Yuan was the first to use molecular techniques to quantify how treatments with the chemicals NAA (napthaleneacetic acid), ReTain (aminoethoxyvinylglycine), and 1-MCP (1-methylcyclopropene), and shading alter the expression of genes controlling natural fruit drop.

“This approach identified changes in the expression of more than 700 genes, many of which are involved in regulation of photosynthesis, carbohydrate metabolism and ­transport, auxin transport, and ethylene biosynthesis,” he said.

Yuan was the first to observe that when NAA and ReTain are combined to stop pre­harvest drop, the rate of ReTain, a costlier chemical, can be cut in half.  McArtney has confirmed, and recommends, the best treatment for maintaining fruit firmness while delaying harvest dates of up to three weeks on spur Red Delicious is a half rate of ReTain plus 20 parts per million NAA. Full rates of both can delay harvest four weeks.

Yuan and his graduate student Hong Zhu also worked with fruit thinning. They found the chemical NAA works in much the same way as shading (as on cloudy days).

“Shading and NAA result in remarkably similar changes in carbohydrate and hormone metabolism,” McArtney said.

“Dr. Yuan’s research into the mechanisms responsible for fruit drop in apple reveal a close relationship between carbohydrates, various plant hormones, and enzymes responsible for cell wall breakdown in the fruit cortex and fruit abscission zone. The contributions he made in this area of research are significant and will provide insight and direction for those of us who are trying to manage the fruit drop process for years to come,” he added.