The pawpaw [Asimina triloba (L.) Dunal] is a tree fruit native to many areas of the eastern and midwestern United States from USDA hardiness zones 5 through 8 (Layne, 1996; Pomper et al., 1999). The fruit has a tropical-like flavor resembling a combination of mango, banana, and papaya, and a smooth, custard-like flesh. The high fruit quality and attractive ornamental value give the pawpaw great potential as a commercial tree fruit or as a component in “edible landscapes” (Layne, 1996; Pomper and Layne, 2005). The fruit is highly nutritious and has been found to be higher in many vitamins, minerals, and amino acids than other fruits, including apples, bananas, and oranges (Peterson et al., 1982). The twig, leaf and bark tissues also produce natural defense compounds called annonaceous acetogenins, which have shown potential as a botanical pesticide and as an anti-cancer agent (McLaughlin, 1997).
In 1994, Kentucky State University (KSU) was designated as the U.S. Department of Agriculture (USDA), National Clonal Germplasm Repository for Asimina spp. Germplasm evaluation, preservation, and dissemination have been a high priority at KSU since that time. The repository orchards currently contain over 1700 accessions collected from the wild in 17 states and more than 40 cultivars (Pomper et al., 2003). One of the goals of the repository is to assess levels of genetic diversity in native populations. Another goal is to acquire and preserve unique germplasm in our repository collection. Vegetative and reproductive characteristics may be present in the pawpaw repository accessions that would be helpful in future breeding efforts. Molecular marker studies have been conducted to examine the level of genetic variation found in KSU repository accessions (Huang et al., 1997 and 2000; Pomper et al., 2003). These studies found that pawpaw exhibits a moderate to high level of genetic diversity compared to other woody plant species. An examination of variation in vegetative and reproductive characteristics in pawpaw repository accessions has not been conducted.
In the wild, pawpaw has been found to have very low fruit set (Peterson, 1991). There may be a number of reasons for this low fruit set. Pawpaw flowers are protogynous, so the stigmatic surface is receptive before pollen is shed from the anthers.