A species wide genetic and horticultural characterization of wild collected Hydrangea quercifolia

Depending on the authority, oakleaf hydrangea (Hydrangea quercifolia Bartr.) is one of two to five North American native hydrangea species. Despite its unique phenotypic characteristics and the enthusiasm surrounding hydrangeas in the horticulture industry, relatively little breeding and genetics work have been done with the species. These circumstances created a timely opportunity to conduct a comprehensive horticultural and genetic evaluation of the species. An understanding of the complete range of horticultural trait variation, genetic diversity and the structuring of that diversity across its range of occurrence will allow breeders to select the best germplasm for breeding and inform land managers in efforts to preserve wild populations for the future. Seeds and leaf tissue were sampled from 75 populations throughout the 6-state native range of occurrence in the southeastern United States. Genetic diversity was initially examined using a subset of 188 samples from the populations with genotyping by sequencing. Population structure was investigated using Structure software and statistical support was found for 6 genetic clusters. One large cluster occurs throughout eastern Louisiana, Mississippi and the western half of Tennessee. The populations west of the Mississippi River are in their own genetic cluster. The other 4 clusters are distributed throughout Alabama, Georgia, Florida and the eastern half of Tennessee. Phenotypic variation at the population level has been studied in four traits to date. Significant variation was found among populations for seed germination rate, averaging at 61.1%. Significant variation was found in traits relating to plant architecture including height, width, node number, internode length, and number of branches. These traits also exhibited a significant environmental interaction. Midwinter cold hardiness was estimated in January 2019 using a controlled freezing assay and significant variation was found to exist in a latitudinal cline with northern populations being more cold hardy (r=0.72; p=0.003). Tolerance to leafspot (Xanthomonas campestris) is the subject of ongoing investigation. Additional traits such as flowering related traits will be studied as the plants continue to mature. Representative seed samples from each population will be made available to researchers and breeders through the USDA Genetic Resources Information Network.