Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    SEASONAL NITROGEN CYCLING AND HOMEOSTASIS IN POPULUS: SOURCE-SINK COMMUNICATION
    (2018) Li, Gen; Coleman, Gary D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nitrogen (N) is an essential nutrient for plant growth, development and reproduction. Seasonal N cycling is an adaption to nutrient limitation and a feature of the perennial lifestyle of trees. Poplar (Populus) is a model system used to study forest tree genetics and molecular biology, including seasonal N cycling. The accumulation of Bark storage proteins (BSP) is a central feature of seasonal N cycling in poplar, yet our understanding of the contribution of the BSP storage pool to N remobilization during growth and mechanisms that regulate BSP accumulation, catabolism and N remobilization is limited. The research presented in this dissertation is directed towards advancing knowledge of the regulation of seasonal N cycling in poplar using a combination of experimental approaches. The role of the N storage pool to N remobilization was examined through N sink manipulations and the specific role of BSP storage was investigated by N source reduction via RNAi mediated knockdown of BSP accumulation. From this it was found that N remobilization from BSP significantly contributes to shoot growth following dormancy and initial shoot growth is source limited. Poplar bark transcriptome analyses during regrowth following dormancy revealed an enrichment for up-regulated genes associated with auxin transport and signaling. Based on the transcriptome analysis experiments that manipulate auxin production or polar auxin transport were performed and the results indicate that BSP catabolism and N remobilization likely involves polar auxin transport from expanding buds and shoots and auxin-mediated regulation of protease gene expression. Analysis of DNA microarrays of bark RNA during short-day (SD) induction of BSP gene expression was used to identify putative regulatory factors that may play a role of BSP accumulation. The transcriptome analyses indicated that SD represses the expression of genes involved in ethylene production as well as a reduction in bark ethylene biosynthesis. Additionally, treatment of excised stems with ACC or ethephon repressed BSP gene expression while AVG induced BSP gene expression. This repression was reduced in ethylene-insensitive poplars expressing Arabidopsis dominant gain-of-function allele etr1-1. Furthermore, transient expression of ERF12 and ERF41, two transcription factors with the greatest induction in SD treatment, in transgenic tobacco stably transformed with BSPA promoter fused with GUS resulted in enhanced GUS activity suggesting ERF12 and ERF41 may act as positive regulators of BSP gene expression. Since glutamine plays a pivotal role in N partitioning during N storage and remobilization, the possible role of the PII glutamine sensor was studied and it was found that the transcript levels of PII increased in bark during SD-induced leaf senescence and BSP accumulation. PII knockdown poplars using RNAi showed reduced glutamine-induced BSP gene expression. Moreover, glutamine-induced BSP gene expression was also inhibited by 2-OG, a PII-NAGK interaction antagonist, suggesting a possible role of PII-glutamine sensing in BSP accumulation. Taken together, this study provides important insights into the mechanism of seasonal N accumulation and remobilization in poplar.
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    Nitrogen Mineralization from Brassica Cover Crops
    (2006-07-27) Kremen, Amy; Weil, Ray; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The potential of forage radish (Raphanus sativus L.), rape (Brassica napus L.), and rye (Secale cereale L.) cover crops to capture residual nitrogen and then provide early season N to subsequent main crops via mineralization from their residues was compared. At four field experiments established in Maryland (2003-2005), N uptake by radish and rape equaled or exceeded that by rye. No differences in soil inorganic N due to cover crop type were observed during spring 2004. In spring 2005, greatest N release from forage radish residues (March-May) was followed by that from rape residues (May-June). Brassica decay significantly increased growth of immature corn and soybean plants. In a 48-day incubation study comparing N mineralization in fine and coarse textured soils from Brassica and rye root or shoot residues, N mineralization was greatest from forage radish and rape shoots. Compared with rye, the Brassica cover crops showed environmental and agronomic promise.