First quantitative trait loci detected for bearing habit and nut quality traits in eastern black walnut (Juglans nigra L.)

Eastern black walnut (Juglans nigra L.) offers potential as an orchard nut crop throughout the Midwest and eastern parts of the US, where the species has a long history of use by indigenous peoples as food and medicine. Currently the nuts and edible kernels of eastern black walnut support a regional industry led by Hammons Products Company, which purchases up to 9.5 million kg of nuts from harvesters in mast years. With most of the industry's black walnuts originating from unimproved wild trees, production is limited by relatively poor-quality nuts with low percent kernel. The relatively undomesticated nature of the species means its production is not comparable to the over 1-billion-dollar annual value of the California Persian walnut (Juglans regia L.) industry. However, the adaptability of eastern black walnut to harsher climate conditions offers the potential to develop an orchard nut crop suitable to a wider area throughout the United States, not just the temperate climate of California. First, literature is reviewed related to the initial selection efforts and foundation of the eastern black walnut breeding program at the University of Missouri Center for Agroforestry, while also examining advances in genetics, metabolomics, and breeding research in the related Persian walnut. In addition, two studies were conducted with data collected in 2021 to identify quantitative trait loci (QTLs) for bearing habit/yield and nut quality traits utilizing a 'Sparrow' x 'Schessler' F1 full-sibling mapping population. Two hundred forty-eight individuals from this population were previously utilized to construct the population's genetic map consisting of 62 EST-SSR and 356 SNP markers. In the first study, three QTLs explaining the degree of spur-bearing (P < 0.01) were found on linkage groups (LG) 8, LG11, and LG16, which are responsible for 7.2 percent, 8.7 percent, and 10 percent of trait variation, respectively. QTL analysis of nut quality traits revealed a total of 11 QTLs associated with in-shell nut mass, kernel mass, nut suture width, nut cheek width, and nut length (P < 0.01). Two additional QTLs were identified for percent kernel (P <0.05). These studies provide the first exploration of genetic control of bearing habit and nut quality traits in eastern black walnut, providing insight into the inheritance of traits for breeding practices, and directing future studies to refine QTL regions for developing DNA markers for assisted seedling selection.