College of Agriculture & Natural Resources
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item INVESTIGATION INTO THE ROLE OF UVR8 IN BALANCING GROWTH AND ACCLIMATION TO UV-B RADIATION IN NATURAL AND TRANSGENIC POPULUS VARIANTS(2021) Wong, Tiffany Marie; Eisenstein, Edward; Sullivan, Joseph; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Research on woody plants offers promise for the development of next-generation biofuel feedstocks with reduced lignin recalcitrance and enhanced saccharification for ethanol production. Natural variants of Populus trichocarpa with diverse lignin content and saccharification differences, and transgenic Populus deltoides constructed for reduced lignin levels for improved cellulose extraction, offer clues to enhance biofuel production but with a tradeoff to overall fitness and biomass. One concern of engineering lignin relates to the protection of plants against environmental stress such as UV-B radiation. Secondary metabolite biosynthesis initiated by UV-B, particularly phenylpropanoids (lignin precursors) and flavonoids, plays an important role in managing and protection of UV stress. Genetic modifications affecting the production of these compounds may have significant physiological consequences. Thus, the goal of this research was to develop a model for biosynthetic compensation of low-lignin Populus to UV-B stress. The effect of UV-B on Populus was evaluated by spectroscopic and metabolomic measurements on leaves. UV-B promoted shifts in physiological and metabolomic responses of natural and transgenic Populus with varying levels of lignin were complex, reflecting compensation from variety of biosynthetic alterations. Therefore, the impact of modulating the expression of the photoreceptor, UVR8, in regulating the response of Populus to UV-B was pursued. Modulation of UVR8 expression in Populus hybrid was achieved by constructing transgenic plants using CRISPR and RNAi, in wild-type, and an RNAi-constructed cinnamyl alcohol dehydrogenase knockdown line. UV-B response of UVR8 modulated Populus indicated that flavonoids were upregulated in UVR8 overexpression lines, and that in a CAD knockdown background, these effects were slightly enhanced. Salicylates were upregulated in UVR8 knockout poplars, suggesting metabolic flux in the pathway, but little difference was seen relative to wild-type plants in CAD lines, and UV-B treatment had little effect. An interesting and unexpected finding was that UVR8 modulated Populus exhibited more rapid growth than wild-type plants. The findings underscore the key role of UVR8 in synchronizing protective metabolic responses to UV-B and suggest an additional function of the photoreceptor in regulating growth and development of Populus through shifts in the chemical equilibria of UVR8 monomers and dimers and interactions with other regulatory factors.Item The comparative effects of three Sedum species on green roof stormwater retention(2013) Starry, Olyssa; Lea-Cox, John; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Green roofs are typically dominated by Sedum species because they can tolerate hot, xeric environments. However, due to their high water use efficiency, some have questioned the selection of these species for stormwater management. We investigated (1) how three common Sedum species contribute to overall stormwater retention efficiency by green roofs in the mid-Atlantic region, and (2) whether species-specific differences in water use could be explained by morphological and physiological characteristics. Water use and CO2 exchange were continuously monitored in growth chamber studies under increasing drought stress for S. album, and S. kamtschaticum, two species known to variably cycle between CAM and C3 metabolisms. Under fall temperature conditions, S. kamtschaticum had gas exchange rates akin to C3 photosynthesis and used 35% more water compared to S. album. Interestingly, S. album conserved water and had malic acid accumulation confirming CAM metabolism for the duration of the experiment, even under well-watered conditions. In field studies, sixteen replicate green roof platforms (n=4 per species) were planted with S. album, S. kamtschaticum, S. sexangulare, or left unplanted during summer 2010. The platforms were monitored intensively for canopy growth, leaf area, root biomass, substrate moisture and runoff for two years (2011 and 2012). Plant treatment effects on stormwater runoff were significant, but most discernible for small and intermediate-sized rainfall events less than 62.5mm. The two species with the greatest stormwater retention efficiencies, S. kamtschaticumand S. sexangulare, also had the highest rates of evapotranspiration (ET), and higher ET rates resulted in less total runoff. Because evapotranspiration was identified as important for predicting performance by plants in the field study, I investigated how ET data from this study, combined with environmental data collected from a weather station at the study site, could be used to improve the application of the FAO56 Penman-Monteith evapotranspiration equations to green roofs. The incorporation of specific seasonal crop coefficients were found to improve correlations between predicted and measured rates of ET and these coefficients were related to plant characteristics. The refinement of ET equations can lead to more accurate hydrologic models of green roofs and design and management tools.Item Genetic consequences of habitat fragmentation and restoration(2012) Lloyd, Michael Warren; Neel, Maile C.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective my dissertation was to assess the effects of habitat loss and fragmentation on genetic diversity and landscape connectivity. I focused on Vallisneria americana Michx. (Hydrocharitaceae), a submersed aquatic plant species found in the Chesapeake Bay. Vallisneria americana has undergone dramatic changes in abundance and distribution throughout its range and has been targeted for restoration, which makes it ideal for examining the effects habitat loss and fragmentation. I examined the naturally occurring genetic diversity across the Chesapeake Bay and its major tributaries. Sites were genetically diverse, but had a range of genotypic diversities. There were four genetic regions, corresponding with geographic regions in the Bay. Vallisneria americana has been the target of restoration, and restoration techniques could be influencing genetic diversity and potentially lowering overall success. I examined various restoration techniques across eight restoration sites, and found that technique did not greatly influence genetic diversity. However, small population size, significant inbreeding coefficients, and low overlap of allele composition among sites provide cause for concern. Measures of functional and potential connectivity provide insights into the degree of contemporary gene flow occurring across a landscape. Pollen dispersal distance was measured using indirect paternity analysis, and is spatially restricted to only a few meters. Dispersal at this scale imposes small genetic neighborhoods within sites, evidenced by high seed relatedness within mothers. I used a graph theoretic approach to examine the distribution and potential connectivity of historic and current patches of V. americana. There was a high turnover in the distribution of patches, and connectivity varied through time, but even if all habitat were occupied, increases in overall network connectivity would not necessarily be observed. I developed an individual based model that I used to test the ability of measures of genetic differentiation to detect changes in landscape connectivity. Genetic differentiation measures became significant after two generations, but the magnitude of change in each was small in all cases and extremely small when population sizes are greater than 100 individuals. These results suggest that genetic differentiation measures alone are inadequate to rapidly detect changes in connectivity.