Good Fruit Grower / July 2008 / Pitless stone fruit

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Pitless stone fruit

USDA researchers are working to develop pitless plums and should be able to transfer the technology to other stone fruits.

Plant breeders with the U.S. Department of Agriculture in West Virginia are hoping to accomplish for the stone fruit industry what seedless grapes have done for the table grape industry. They're developing pitless plums and plan soon to be working on pitless cherries, peaches, nectarines, and apricots.

"We think there would be a hot market for pitless cherries," said Dr. Chris Dardick, plant molecular biologist at the Appalachian Fruit Research Station in Kearneysville, noting that the pit is a nuisance for both consumers and processors.

"We believe that if we can create varieties that truly are pitless and seedless, there would be a lot of new possibilities in terms of fresh consumption, and we believe it would reduce costs for processing tremendously."

Dardick said the station's fruit breeding team has spent a lot of time considering what kind of fruit enhancements could help growers in the United States. "A lot of the work that goes on is to make the fruit a little sweeter, maybe a little better color or better size, and we decided that what would really make a bigger impact would be brand new traitsthings that open up new markets and give growers new possibilities in terms of the kinds of things they grow. Pitless was one of the obvious things that jumped out."

Stone

The current plant breeders are not the first to come up with the idea of pitless stone fruits. The pit consists of the seed and the hard, woody material surrounding it, which is known as the stone. Luther Burbank, a botanist and horticulturist who revolutionized agriculture in the early 1900s, attempted to develop a stoneless plum, using a naturally occurring wild plum variety that was partially stoneless.

Burbank, who lived from 1849 to 1926, developed more than 800 strains and varieties of fruits, grains, grasses, and vegetables during his career, including the Burbank potato and the plumcot, an interspecific cross of plum and apricot. He was known as "the wizard of horticulture."

Although his work on pitless plums was one of his greatest achievements, it was not commercially successful, according to the Agricultural Research Service, because the crosses he developed still had a fragment of pit and a seed. Dardick said pit fragments are a problem for processors because they still have to be removed.

According to Burbank's writings, he was getting the size of the pit fragment down to the size of a grain of rice and some of the crosses did not have a seed, Dardick said. However, after Burbank's work ended, the idea of the pitless plum appears to have been dropped for close to a century.

The Appalachian Fruit Research Station has some of Burbank's plum trees growing in its orchard. Dardick said the fruit ranges from almost pitless to having a complete stone. Scientists there recently started a molecular breeding project to improve the pitless trait in Burbank's cultivars. The goals are to identify the genes responsible for the pitless trait and develop molecular markers to quickly select for trees in the crosses that carry the pitless gene before they even produce fruit.

The scientists are working to understand exactly what the pit is, how it forms, what it is made of, and how to prevent the stone from hardening, Dardick said. They are analyzing the functions of individual genes in plum, but are also studying peaches and cherries to determine if the timing of stone development and the composition of the stone are the same.

Dardick said any stoneless fruit they develop would have to go through a breeding program to ensure that a new variety would have the fruit quality traits that commercial growers would need.

Early flowering

Other new developments at the Appalachian station should make the fruit-breeding process more efficient. One is the ability to bring stone fruit trees into bearing much more quickly than in the past.

Dardick and his colleagues have built on work that scientists did with poplar trees to identify the gene that controls the timing of flowering. When the gene is overexpressed, trees produce lots of flowers while still young. Dr. Chinnathambi Srinivasan, plant physiologist at Kearneysville, has inserted the gene into plum and developed plum trees that flower and begin fruit development within three months.

"Within a few months of doing the transformation, we can look at fruit in the greenhouse to see if it doesn't have a pit in it," said breeder Dr. Ralph Scorza.

Typically, breeders nowadays use molecular markers that can identify fruit traits in a plant before it fruits. Dardick said that rather than making molecular markers redundant, the early flowering trait will work well with markers and speed up the development of markers for specific traits. "In combination with the markers, it makes the technology faster and more efficient," he said.

Scorza said the early flowering trait would speed up any process that requires fruit to be grown and evaluated, including traditional breeding. If a tree with the early flowering gene is crossed with a nontransformed tree that has desirable characteristics and only the seedlings that don't have the early flowering gene are selected as potential commercial varieties, those progeny would not be considered as genetically modified.

Researchers at Kearneysville have also developed a genetic engineering system in plum that does not require the use of antibiotic resistance genes to identify the plantlets or cells in which the gene has been successfully inserted. This means that it should be easier to obtain regulatory approval for genetically modified crops, Dardick said. "We also hope from the standpoint of consumer acceptance, they would be better and more appealing to people," he said.

Scorza said the variety development program at Kearneysville is an integrated program involving scientists in breeding, molecular biology, and tissue culture who are all working as a single group with the same goals in mind. Also on the team are plant molecular biologists Dr. Zongrang Liu and Dr. Cesar Petri, and plant geneticist Dr. Ann Callahan.

"We all have our particular responsibilities, but we all work together as a group," Scorza said.