Agricultural Research Service (ARS) scientists are a step closer to improving the hardiness of grape varieties that can be damaged and destroyed by fall frosts and cold winters in North American vineyards.

Grapes, like many other woody plants, stop growing and drop their leaves in the fall, entering a dormant phase that allows them to prepare for and survive the winter. Understanding the timing of this seasonal growth cessation in grapes is a key objective for scientists. If they can find genes that control it, they can help breeders develop more marketable cold-tolerant grapes.

The wild grape species Vitis riparia is unusual among wild grapes because it stops growing when the day length drops below 13 hours, which occurs in late August in upstate New York, according to Amanda Garris, a geneticist at the ARS Grape Genetics Research Unit in Geneva, N.Y. Most other grape species are not sensitive to day length.

Garris, ARS molecular biologist Christopher Owens, James Luby of the University of Minnesota, and Anne Fennel of South Dakota State University want to identify genes for day length sensitivity in V. riparia.

The researchers crossed V. riparia with Seyval, a hybrid grape insensitive to day length. They raised some of the offspring in fields, with natural fluctuations of day length, rainfall and temperature. Others they raised in greenhouses where water and temperature levels remained constant, but artificial "day" lengths were gradually reduced to mimic conditions outside.

They evaluated growth cessation patterns and mapped differences found in 120 DNA markers and six genes. They discovered that in greenhouse-grown grapevines, a region of chromosome 13 is responsible for day-length-induced growth cessation. But in the field, the interaction of multiple cues such as day length, rainfall and temperature fluctuations is more likely to explain the timing of growth cessation.

The study, considered a preliminary step toward identification of genes for day length sensitivity and growth cessation, was recently published in the Journal of the American Society of Horticultural Science.

ARS is the principal intramural scientific research agency of the U.S. Department of Agriculture.