Ashe juniper (Juniperus ashei Bucholz) has encroached into the historical grasslands of the Edwards Plateau. This area is environmentally sensitive as it serves as the recharge zone for the Edwards aquifer, providing large municipalities such as Austin, San Antonio, and San Marcos with water. The increased tree density may impact local water budgets, but the trees may have the capability of sequestering a greater amount of carbon than the historic grasslands. An understanding of what regulates gas exchange and water relations at the leaf level of the two dominant tree species, Ashe juniper and live oak (Quercus virginiana P. Mill. Var. fusiformis), is important to assess the impact of juniper encroachment on the aquifer. Photosynthesis and transpiration were measured in four juniper and four oak trees throughout an entire year. Juniper consistently had lower carbon assimilation rates, transpiration, and conductance values than oak. Oak exhibited greater seasonal variation and seemed less dependent on precipitation to maintain gas exchange. Canopy position in live oak regulates leaf level photosynthesis to a higher degree than in Ashe juniper. Gas exchange of both species decline as water becomes limited, but juniper consistently exhibits lower and steadier rates throughout the year than oak. Juniper does not respond quickly to erratic precipitation events. The consistent low rates of gas exchange and stomatal responses in juniper could indicate shallower rooting structure and/or limitation of hydraulic conductivity, as well as photosynthetic capacity. The higher rates of photosynthesis, transpiration, and stomatal conductivity exhibited by live oak during drought suggest a deeper rooting pattern than Ashe juniper. Light response curves were computed for three juniper and three oak trees that were marked and sampled at three different seasons. Juniper light-compensation and light-saturation estimates, at different canopy positions, were poor indicators of differences in photosynthetic capacity between the two species. Patterns of light responses of juniper and oak did not follow responses of previous studies assessing differences in photosynthetic capacity through light-response curves. Computing the light curves was difficult due to low tree activity and lack of responses to changes in light, especially in juniper.

Thesis (Ph. D.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains xiv, 182 p. : ill. Vita. Includes abstract. Includes bibliographical references.

Thesis--Helsingfors. / "The object of the present work was to study the transformations which different sugars undergo in living plant tissue, independently of the photosynthetic process."--Summary, p. [98]. On cover: Reprinted from Annales Academias scientiarum fennicae. Series A. Tom. XLIV, no: 8. Bibliography: p. [101]-105.

Thesis (M.S.)--Ohio State University, 1951. / Includes bibliographical references (leaves 17-18). Available online via OhioLINK's ETD Center.

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In this experiment western milfoil, a common fresh water plant, was used to determine the rate of oxygen production during photosynthesis. The rate was determined based on the amount of gaseous oxygen produced by a given number of plants over a period of time. The combined results of ten trials yielded an average oxygen production rate of 0.57μmol/m²/s ± 0.016μmol/ m²/s. This rate is comparable to those of various other aquatic plants, which vary from 0.1μmol/m²/s to upwards of 5μmol/m²/s [2].

The methodology of phytoplankton production measurements in the Laurentian Great Lakes and other freshwater lakes has remained largely unchanged in the past 40 years. In most studies photosynthesis from a single water sample is measured across an in vitro light gradient usually using an artificial light source then extrapolating to the in situ environment. These traditional methods are laborious, thus limiting the amount of observations in space and time, and may not accurately represent in situ photosynthesis. Active chl a fluorescence, intrinsically linked to photosynthesis, can be measured in situ and instantaneously. Various bio-optical models that scale these fluorescence measurements to phytoplankton production are gaining widespread attention in the marine environment but have not been extensively tested in freshwater ecosystems. The methodology and efficacy of the various bio-optical models are tested in this thesis using a large dataset of active fluorescence profiles and ancillary water chemistry parameters against synchronously derived in vitro phytoplankton production collected across mixing, trophic and taxonomic gradients in Lake Erie. From this analysis, the most common bio-optical model parameterization yields photosynthetic rates that are largely incongruent with in vitro measurements. Bio-optical models are largely a function of two parameters, the absorption spectrum of photosystem II (aPSII) and the photochemical efficiency of PSII (fPSII). In Lake Erie fPSII is relatively constrained suggesting that even nutrient limited phytoplankton achieve balanced growth by adjusting the supply of energy through changes in light harvesting (aPSII) to match the demand for photosynthetic energy. This thesis goes on to demonstrate the success of bio-optical models depends largely on the formulation of aPSII. Alternative methods to derive aPSII, largely ignored in published bio-optical models, are reviewed, formulated, and when incorporated into a bio-optical model and compared to synchronous in vitro production measurements, this novel bio-optical model outperforms all other comparative studies performed across a taxonomic gradient.

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