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ARS Home » Northeast Area » Washington, D.C. » National Arboretum » Floral and Nursery Plants Research » Research » Research Project #425386

Research Project: Taxonomy of Landscape Trees and Shrubs

Location: Floral and Nursery Plants Research

2017 Annual Report

The long-term objectives of this project are 1) to improve the understanding of interrelationships and the distribution of genetic variation (especially genome size and ploidy level) in understudied groups of specialty crops (primarily woody landscape plants), 2) to further develop the National Arboretum as a scientific resource, and 3) to investigate emerging and suspected invasive woody plants among groups of interest to the nursery industry and to National Arboretum plant introduction programs. These goals will be achieved through the following specific objectives: Objective 1: Identify and characterize genetic resources in understudied priority groups of woody landscape plants, including, but not limited to, Ulmus, Celtis, and Carya, to elucidate their genetic relationships and variation within species. Sub-objective 1.A. Determine taxonomic and genetic relationships among selected species of elms (Ulmus). Sub-objective 1.B. Determine ploidy level and comparative genome size in Carya using flow cytometry. Sub-objective 1.C. Determine the distribution of apomixis and polyploidy in North American species of hackberry (Celtis) and their taxonomy and relationships. Sub-objective 1.D. Resolve taxonomic problems and determine the amount and apportionment of genetic diversity in selected white oaks (Quercus sect. Quercus). Sub-objective 1.E. Carry out research projects to meet needs for taxonomic research expressed by stakeholders, such as the production of regional floras and botanical manuals in collaboration with other scientists and horticulturists. Objective 2: Maintain and enhance the National Arboretum Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Sub-objective 2.A. Maintain and enhance the U.S. National Arboretum Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Sub-objective 2.B. Maintain and enhance the U.S. National Seed Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Objective 3: Determine the identity and basic biological characteristics of selected invasive woody plants in the United States. Apply herbarium records of woody plant introduction and naturalization in the mid-Atlantic area to test statistical models for predicting characteristics contributing to invasive behavior in woody plants.

Investigations will utilize a variety of different data types, primarily morphological data, DNA sequence variation, and variation in single-locus DNA markers within variable sites in the genome, analyzed using phylogenetic analyses and other multivariate statistical methods. Organisms will be studied in the field, herbarium, laboratory, and garden.

Progress Report
In elm, we continued to identify and collect promising elm populations, conducted an analysis of genome size variation among elm species, and we are working with a collaborator to analyze DNA sequence data in the genus. This work allows us to identify additional promising germplasm to test for disease tolerance and use in future breeding work, and add it to our living collection. We also collected oak samples and began DNA diversity analysis to investigate additional promising germplasm in this genus. We arranged a collaboration with a scientist who was working independently on Celtis, collected material for extraction, extracted DNA and begin sequencing (in the collaborator's lab). This will allow us to identify promising germplasm in these very drought- and weather-tolerant trees and shrubs. We collected material of Fraxinus (ash) with collaborators, determined the ploidy using flow cytometry, collected material for DNA extraction, and began genetic analysis (in the collaborator's lab). This work is allowing us to improve sampling of this valuable but critically imperiled germplasm (see Accomplishments). We also surveyed escaped populations of ivy, a genus important in horticulture that has become invasive in the United States, for genome size and morphology, in order to determine the distribution and degree of invasiveness for different species; identify naturally occurring triploids (putative hybrids) and bring them into cultivation, so we can test them to determine whether they are able to produce seed. This work allows us to determine the relative invasiveness of different cultivated ivies, and look for less invasive alternatives for the species of ivy now on the market.

1. Determining the taxonomy of threatened ash trees in eastern U.S. Ash trees in North America are under serious threat from the emerald ash borer, an introduced insect that is decimating ash stands in landscapes and forests. ARS and other agencies are responding to this crisis by collecting seed to preserve critical ash germplasm, but efforts have been hampered by disagreements about how many species of ash should be recognized in the eastern United States, and where they are found. An ARS scientist at the U.S. National Arboretum studied morphological traits and DNA content of ash to determine that the taxonomy currently being used in these conservation efforts is inaccurate, and there are additional valid species that must be included in the conservation efforts. These results will be used by the ARS National Plant Germplasm System, USDA Forest Service, and others to target germplasm for emergency conservation efforts.

Review Publications
Whittemore, A.T., Xia, Z. 2017. Genome size variation in elms (Ulmus spp.) and related genera. HortScience. 52(4):547-553.
Whittemore, A.T. 2017. Typification of infrageneric names in Ulmus L.. Phytotaxa. 297:291-294.