Genetic and Horticultural Characterization of Hydrangea quercifolia Bartr. (Oakleaf Hydrangea) Throughout its Natural Range of Occurrence
2020-06
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Genetic and Horticultural Characterization of Hydrangea quercifolia Bartr. (Oakleaf Hydrangea) Throughout its Natural Range of Occurrence
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2020-06
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Oakleaf hydrangea (Hydrangea quercifolia Bartr.) is an understory shrub native to the southeastern United States. The species occupies a small native range, and little is known about its genetic diversity, needs for conservation or range of phenotypic variation for horticultural traits. This study had two primary objectives. 1) To characterize the structure of the genetic diversity throughout the species range and begin to understand what factors contribute to this structure. 2) To characterize variation for horticulturally important traits. Samples were collected from 188 plants in 73 locations throughout the species range and were genotyped using genotyping by sequencing. A Structure analysis identified 6 genetic clusters which are geographically structured. Although these clusters are weakly differentiated, each has unique alleles. An environmental association analysis determined that environmental variables explain 11.3% of genetic diversity while population structure explains 13.5%. Further, 231 putative adaptive alleles were identified, the majority of which are correlated with precipitation related variables, indicating that precipitation has an impact on genetic diversity in H. quercifolia. Many historically documented populations were found to be either extirpated or at risk of extirpation. The genetic clusters on the southern extent of the species range are relatively small and contain putative adaptive alleles at relatively high allele frequencies. This highlights the importance of preserving representative germplasm from throughout the species range. Seed was collected from 55 populations throughout the species range for the horticultural characterization. Seed germination percentage was characterized in a greenhouse and growth chamber. Plant architecture was characterized as plant height, number of nodes, internode length, number of branches and plant width. Plant architecture was measured in potted and field grown plants in two locations. Tolerance to leaf spot (Xanthomonas campestris L.) was characterized in wild collected seedlings and cultivars by measuring disease severity under natural exposure to inoculum. Cold hardiness was characterized in two winters with a controlled freezing experiment. The first winter, seedlings were tested in January only and the second winter seedlings and cultivars were tested monthly throughout winter. Significant variation among wild populations and cultivars was found for all traits measured in all environments. Mean population seed germination percent ranged from 11% to 93% (mean=61% in greenhouse, 74% in growth chamber). Plant architecture varied by environment, with plants growing larger in Tennessee than in Minnesota. Plant height was correlated with collection site latitude (r=-0.66) with populations from the northeastern extent of the species range being the most compact and populations from Florida being the largest. Leaf spot severity varied significantly among populations and cultivars and was also correlated with latitude in the wild seedlings (r=0.70). Two populations in Florida were identified as sources of resistance to leaf spot while ‘Flemygea’ and ‘Alice’ were identified as having moderate tolerance to leaf spot. Cold hardiness varied among populations and cultivars and among months of the winter. Overall maximum cold hardiness was observed in February (mean LT50=-33.7°C), and several populations maintained an extreme level of cold hardiness into late winter. Midwinter cold hardiness also varied by latitude (r=-0.65). These results indicate that certain wild oakleaf hydrangea populations will be useful for introgressing novel variation into breeding programs.
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University of Minnesota M.S. thesis. June 2020. Major: Applied Plant Sciences. Advisors: Stan Hokanson, Matthew Clark. 1 computer file (PDF); viii, 103 pages.
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