Pawpaw Fruit Variety Annonaceous Acetogenin Activity Is Reflected in Twig Tissue.

Pawpaw [Asimina triloba (L.) Dunal] is a native tree fruit in eastern North America. Annonaceous acetogenins are long chained fatty acids contained in pawpaw fruit and vegetative tissues which display pesticidal activity, possibly representing a new botanical pesticide for agronomic pest control. Kentucky State University (KSU) is the site of the USDA Repository for pawpaw species and germplasm evaluation and collection are program priorities. Fruit is a major biomass source for acetogenin extraction; however, 5 to 8 years are required for a tree to mature and produce fruit.  Early detection of high acetogenin fruit selections as first year seedlings would shorten the pawpaw breeding cycle. The objective of this study was to determine if there is a correlation between annonaceous acetogenin activity in ripe fruit and twig tissues. Twigs were collected from eight pawpaw varieties (Mitchell, Overleese, NC-1, Susquehanna, Zimmerman, Wells, Wabash, and Sunflower,) known to vary from high to low in fruit acetogenin activity.  Two and one half grams of dried twig tissue were extracted with 95% ethanol and the brine shrimp test (BST) was employed to assess acetogenin activity in pulp extracts. Concentrated extract was transferred to vials to correspond to 0, 0.5, 1.0, 5.0, and 10 ppm concentrations with three replicate vials per concentration. Twenty brine shrimp larvae, taken 48 h after initiation of hatching in artificial sea water were added to each vial, and the final volume of each vial was adjusted to 5 ml using artificial sea water. After 24 h, survivors were counted. There was a strong negative correlation between the LC50 calculated for pawpaw twig and fruit pulp activity, with an R squared of 0.57, indicating that twig extract activity can be used as an early indicator for high acetogenin fruiting genotypes.