Ecology and horticultural potential of Dirca palustris
Despite emerging interest in the use of native plants for horticulture, phenotypic traits and physiological tolerances of many indigenous species have not been formally evaluated. Because natural genetic resources represent both the historical and modern foundations for horticultural improvements, the status of such resources should receive greater consideration in the discipline. My first objective in the work presented in this thesis was to assess the horticultural potential and evolutionary diversity of five populations of D. palustris that span its range from Florida to North Dakota. The other populations assessed in this project were located in Wisconsin, Illinois, and Alabama. Plants in the populations in Florida and North Dakota were phenotypically distinct, with the former unique in its white-pubescent bud scales and the latter having more inflorescences, more flowers per inflorescence, and greater annual stem elongation than plants in the other populations. We found evidence that limited solar resources and tradeoffs in allocation of resources to floral and vegetative development may account for some of the phenotypic differences we observed. Nonetheless, the populations were genotypically distinct; genotyping by using inter-simple sequence repeats (ISSR) markers yielded 63% polymorphism and showed a range in population-level polymorphism from 20% for North Dakota to 36% for Alabama. The number of loci from each population varied from 230 for North Dakota to 264 for Alabama and Florida, whereas the number of population-specific loci varied clinally from none in North Dakota to 14 in Florida. Given the unique phenotypes of the populations in North Dakota and Florida and the distinct genotypes of the population in Florida, I conclude that these endangered peripheral populations represent valuable priorities for conservation and interesting targets for horticultural evaluation. My second objective was to evaluate the responses of seedlings from Florida, North Dakota, and Maine to root-zone pH in soilless media, as the species has been reported both to favor alkaline or acidic soils. Although the provenances from which seeds were collected represented diverse soil pH, seedlings of D. palustris were nonetheless sensitive to root-zone pH and preferred acidic media. Seedlings from the three provenances differed in some ways in response to root-zone pH, but the overall effects of provenance on development were more pronounced. These results demonstrate both that horticultural gains may be made by selection of genotypes for increased shoot or root growth, and that horticultural production using acidic media offers the best root-zone environment for culture of D. palustris.