Horticulture Monographs and Papers

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This is a collection of monographs, dissertations, theses and papers on specific topics related to the discipline of Horticulture.


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    Phenolic root exudate and tissue compounds vary widely among temperate forest tree species and have contrasting effects on soil microbial respiration
    Zwetsloot, Marie J.; Kessler, André; Bauerle, Taryn L. (New Phytologist Trust, 2018)
    Root-soil interactions fundamentally affect the terrestrial carbon (C) cycle and thereby ecosystem feedbacks to climate change. This study addressed the question whether the secondary metabolism of different temperate forest tree species can affect soil microbial respiration. We hypothesized that phenolics can both increase and decrease respiration depending on their function as food source, mobilizer of other soil resources, signaling compound, or toxin. We analyzed the phenolic compounds from root exudates and root tissue extracts of six tree species grown in a greenhouse using high-performance liquid chromatography (HPLC). We then tested the effect of individual phenolic compounds, representing the major identified phenylpropanoid compound classes, on microbial respiration through a five-day soil incubation. Phenolic root profiles were highly species-specific. Of the eight classes identified, flavonoids were the most abundant with flavanols being the predominating sub-class. Phenolic effects on microbial respiration ranged from a 26% decrease to a 46% increase, with reduced respiration occurring in the presence of compounds possessing a catechol ring. Tree species variation in root phenolic composition influences the magnitude and direction of root effects on microbial respiration. Our data support the hypothesis that functional group rather than biosynthetic class determines the root phenolic effect on soil C cycling.
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    Breeding Research and Education Needs Assessment for Organic Vegetable Growers in the Northeast
    Hultengren, Rachel; Glos, Michael; Mazourek, Michael (2016-08-31)
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    Peck, Gregory (Washington State University, 2004-05)
    The first of two studies undertaken in this thesis analyzed Washington State?s organic apple (Malus domestica Borkh.) exports to the European Union (EU) as a case study of the internationalization of the organic marketplace. Washington?s organic apple plantings have grown exponentially and as a result price premiums, which traditionally offset the greater costs of production and motivated many Washington growers to certify their apple orchards, have shrunk. At the same time, demand for organic apples in the EU has been outpacing production, thus making EU member states the most important export market for Washington?s organic apples. However, an entanglement of regulatory bodies from around the world are involved in the certification of organic products, therefore making international sales very difficult. In this paper, I explored the expansion in the organic marketplace and the adjustments undertaken by growers, marketers, and regulatory agencies. As part of a long-term comparison of organic, conventional, and integrated apple farm management systems in the Yakima Valley of Washington State, the second study investigated the productivity and fruit quality of ?Gala? apples during the ninth and tenth growing seasons. We found that the technology available for the organic system limited suitable crop load management and, therefore, consistent yields. Pest and weed control and fertility management were more difficult to manage in the organic system, as they all appeared to contribute to its limited productivity. However, organic apples had 6-10 N higher flesh firmness than conventional apples, and 4-7 N higher firmness than integrated apples. Additionally, consumers consistently rated organic apples to be firmer and to have better textural properties. Few consistent results were found for fruit flavor as measured by soluble solids concentration or titratable acidity, and this was also reflected in consumer panels. Total antioxidant activity was 10-15% higher in organic apples than conventional apples and 5-12% higher than integrated apples. The conventional and integrated apple farm management systems were more similar to each other than either was to the organic system throughout this study. Although organic apple production provided more management challenges than conventional systems, the superior quality of organic apples was a notable finding.