A California researcher is identifying the genes in peaches that control internal quality, in an attempt to increase consumer satisfaction with peaches.
Dr. Cameron Peace, molecular geneticist at the University of California-Davis, hopes to find a long-term way to maximize the quality of peaches at harvest and maintain the quality until they’re consumed. He has located a gene in peach that has major effects on fruit softening and ripening.
Simple cold storage can help delay ripening for several weeks, but can lead to internal breakdown in many cultivars, resulting in poor flavor and mealy texture. Preconditioning (two days of ripening) before cold storage can offset the internal breakdown problem, but at added cost to the producer.
A long-term solution is to have cultivars that are resistant to internal breakdown and/or have delayed ripening, Peace told members of the International Fruit Tree Association at their annual conference in Hershey, Pennsylvania.
Some cultivars are resistant to internal breakdown, while others are susceptible. For example, “Georgia Belle” is susceptible, but “Dr. Davis” is not. Because such susceptibility tends to be consistent year after year and wherever a cultivar is grown, a strong genetic influence is likely. “This means it’s easier in the lab to find the controlling genes,” Peace said.
Using a peach chromosome map based on those two cultivars, Peace identified regions that control whether peaches develop mealiness, internal flesh browning, or internal bleeding, the major visual symptoms of internal breakdown. One of these regions also determines whether peaches have melting or nonmelting flesh, and are freestone or clingstone.
Melting flesh and freestone are dominant traits. Both freestone and clingstone melting-flesh peach cultivars are common. Nonmelting clingstone cultivars are the mainstay of the peach canning industry, while true freestone nonmelting cultivars apparently do not exist. Peace said one of the side goals of his research is to help fruit breeders develop freestone nonmelting cultivars, which would be useful for both the fresh and canning industries.
He plans to develop DNA tests that can be used by stone fruit breeders to identify fruit softening characteristics in potential selections, as well as to categorize existing cultivars.
While researching mealiness in peaches, Peace and co-workers located the gene for an enzyme called EndoPG (endopolygalacturonase) that cuts up pectin in the cell walls, reducing cell-to-cell adhesion. Its activity is high in melting fruit, but very low or absent in non-melting fruit. EndoPG was previously known to be somehow involved in the development of mealiness, but the mechanism was unclear. It turns out that EndoPG is the gene that controls both the melting/non-melting and freestone/clingstone traits. A DNA test was developed in Peace’s lab that can identify this genetic fruit type of any tree.
EndoPG is turned on by ethylene during normal ripening.
A recent UC Davis study showed that in melting-flesh fruit, the ethylene level is kept low but eventually rises while the fruit are in cold storage, and there is some EndoPG activity. But after a certain time—others have found the critical threshold to typically be about ten days for most cultivars—the ethylene level and EndoPG activity drop and do not switch back on even when fruit are brought to room temperature. The short EndoPG activity results in incomplete degradation of cell-wall pectin by the time fruit are removed from cold storage for ripening. This seems to expose the fruit to the action of other enzymes that cause unusual cell-wall changes in the absence of EndoPG. There is a gradual rather than rapid decline in firmness, and the fruit become mealy and dry rather than smooth and juicy.
In contrast, nonmelting fruit do not turn mealy. Apparently, this is because without EndoPG activity, they do not get the partial pectin degradation that exposes them to faulty ripening. Preconditioned fruit maintain their ethylene levels and EndoPG activity throughout cold storage, sufficient for normal ripening afterwards.
Now that the role of EndoPG is understood, scientists can explore new ways to preserve fruit quality, Peace said.
If a way could be found to keep EndoPG turned off during cold storage of melting cultivars then switched back on later, perhaps mealiness could be avoided.
Numerous natural variations of the EndoPG gene are under investigation for their effect on softening and texture characteristics of peach and nectarine fruit. Interactions are also being examined between EndoPG and other softening genes and traits, such as stony hard. Fruit that are firm at harvest, resistant to mealiness, and soften when desired, are desired outcomes of this research. Such fruit would have greater flavor, size, and color at harvest, and keep this quality until eaten.