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 SPATIO-TEMPORAL ANALYSIS OF PHOTOTROPISM IN ARABIDOPSIS SEEDLINGS(2019) Pritchard, Candace; Murphy, Angus S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optimization of light capture during seedling development is a major determinant of plant fitness. As seedlings emerge from the soil, the processes of photomorphogenesis and phototropism optimize deployment of structures that capture light for photosynthesis. Photomorphogenesis produces hypocotyl thickening, cotyledon expansion, and chloroplast maturation. Concurrent phototropic responses initiated by blue light position the expanding cotyledons to maximize photosynthesis. The mechanisms underlying both processes have been explored for more than 140 years, but are still not fully understood. This dissertation seeks to provide a better understanding of phototropism by exploring the timing and localization of the constituent mechanisms downstream of the well-characterized perception of blue light by the PHOTOTROPIN photoreceptors. The experiments described herein characterize temporally and spatially distinct processes involved in asymmetric auxin accumulations that lead to differential hypocotyl elongation. To better identify the link between early perception and later auxin transport and elongation events, an open-air system was used to remove seedling hindrance and provide better spatio-temporal resolution. These experiments confirmed the more rapid bending conferred by loss of the ATP Binding Cassette class B (ABCB) 19 auxin efflux transporter and loss of differential elongation in the mid hypocotyl elongation zone in higher order pinformed mutants. However, apart from the enhancement of phototropic bending observed in abcb19 and pin4 mutants, no auxin transport mutants tested showed alterations in early phototropic responses, and no mutant exhibited a delay in the onset of phototropic bending. Recently identified CBC1 and CBC2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO2 1/2) have been shown to act in downstream signaling during phot1-mediated regulation of stomatal conductance. Similarly, during phototropism cbc1cbc2 double mutants show early defects in phot1-mediated phototropism. Further, CBC1 and CBC2 have been shown to regulate S-type anion channels. Analysis of S-type anion channel mutants also reveals defects in early bending responses. These results point to blue light-dependent regulation of anion channel activity having an important role during the earliest stages of phototropism.Item The Economics of Fallow: Evidence from the Eastern Amazon(2007-08-01) Klemick, Heather; Lopez, Ramon; Agricultural and Resource Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With tropical deforestation a major contributor to greenhouse gas emissions and biodiversity loss, the land use decisions of small-scale farmers at the forest margins have important implications for the global environment. In some tropical forests, such as the Eastern Brazilian Amazon, farmers practice a shifting cultivation system that maintains large amounts of land under forest fallow. I examine whether local benefits of fallowing such as soil restoration, erosion mitigation and hydrological regulation are of sufficient value to farmers to stem the expansion of permanent cropland at the expense of forest. I quantify the value of ecosystem services provided by fallow to agriculture and test whether local forest externalities are economically significant, using farm survey and GIS data from the Eastern Amazon. I estimate a production function to determine the contribution of on-farm and upstream fallow to income, using an instrumental variables approach to address endogeneity. I find that on-farm and upstream fallow are both associated with higher farm income. This result both confirms the agronomic evidence that fallow boosts yields and suggests that fallow provides positive hydrological externalities to downstream farms. I also examine whether farmers respond strategically to their neighbors' land use, taking advantage of ecosystem services provided by upstream farms. I use a spatial econometric model to estimate the effect of upstream farms' fallow on downstream land allocation. I find no evidence that farmers alter their fallowing based on land use upstream. I then investigate whether market failures encourage fallowing. If farmers cannot purchase inputs used in cultivation due to liquidity constraints, they may keep more land under fallow than optimal. I use the estimated production function parameters to determine whether each farm's allocation of land between cropping and fallow is efficient from an individual perspective. I then estimate the effect liquidity indicators on land use efficiency. I find that over-fallowing is negatively associated with commercial credit use and off-farm income, suggesting that liquidity constraints do hinder agricultural intensification. Because I find evidence to support the existence of positive externalities to fallow, the loosening of liquidity constraints that encourage fallowing has ambiguous implications for community-level welfare.