By studying the nutrient content of leaves and the DNA of apple trees, U.S. Department of Agriculture (USDA) scientists in Geneva, N.Y., have identified key genes that could boost the nutrient uptake, productivity and dwarfing characteristics of rootstocks critical to the nation's $3 billion apple industry.

Gennaro Fazio, an Agricultural Research Service (ARS) geneticist, produces new rootstock varieties in collaboration with Cornell scientists at the ARS Plant Genetic Resources Unit in Geneva. ARS is USDA's chief intramural scientific research agency.

Most commercial apple trees are bred in two parts: the fruit-bearing scion above ground, and the rootstock that forms the roots and foundation below ground. The scion determines the variety of apple, but the rootstock determines the tree's size, when it will begin to bear fruit, and how well it resists soil diseases and transports nutrients to the scion.

Fazio and his Cornell partners analyzed genetic markers in two populations of rootstocks that had been studied and characterized for more than seven years to see if genes associated with certain markers play a role in dwarfing, early bearing and fruit productivity. Dwarfing produces trees that are smaller, increasing yields per acre. Early bearing reduces the wait required for a young tree to begin producing fruit. In recent years, Geneva rootstocks have reduced the time from five to two years.

The results showed the location of a new genetic marker (Dw2) and described its relationship to a previously discovered marker (Dw1), shedding light on how genes associated with both markers and their locations affect dwarfing, early bearing and productivity. The results, published in 2014 in the Journal of the American Society for Horticultural Science, will boost efforts by researchers and breeders to improve apple rootstocks.

In another study, the scientists measured nutrients in the leaves of Gala apple trees grafted to one set of rootstocks, and the nutrients in leaves of Golden Delicious trees grafted to another set. Nutrient uptake is a key to tree health and productivity. They also analyzed the DNA of the rootstocks to search for genes associated with enhanced nutrient uptake. The results, published in 2013 in Aspects of Applied Biology, showed that certain genes are likely to enhance the uptake of key nutrients, such as calcium and phosphorus.