Rising atmospheric concentrations of carbon dioxide could dramatically influence the performance of crops, but experimental results to date have been highly variable. For example, when C3 plants are grown under carbon dioxide enrichment, productivity increases dramatically at first. But over time, organic nitrogen in the plants decreases and productivity diminishes in soils where nitrate is an important source of this nutrient. We have discovered a phenomenon that provides a relatively simple explanation for the latter responses: in C3 plants, elevated carbon dioxide concentrations inhibit photorespiration, which in turn inhibits shoot nitrate assimilation. Agriculture would benefit from the careful management of nitrogen fertilizers, particularly those that are ammonium based.

The responses of higher plants to rising carbon dioxide concentration in the atmosphere are strongly dependent on their ability to acquire mineral nitrogen, ammonium and nitrate. Elevated atmospheric carbon dioxide limits both sources and sinks of plant mineral nitrogen. With regard to sources, elevated carbon dioxide stimulates microbial immobilization and inhibits nitrogen fixation. With regard to sinks, elevated carbon dioxide inhibits nitrate assimilation into amino acids within the shoots of C3 plants, including gymnosperms, monocotyledons, and dicotyledons. The overall result is that the organic nitrogen content of plants tends to drop precipitously under exposure to elevated carbon dioxide. Key to these observations is the relationship between nitrate assimilation and photorespiration. Conditions that diminish photorespiration in C3 plants—namely, elevated carbon dioxide or low oxygen atmospheres—diminish nitrate assimilation as assessed by gas exchange, nitrogen isotopes, and accumulation of organic nitrogen. Photorespiration appears to be involved in the export of malate from the chloroplast that empowers nitrate reduction in the cytoplasm. C4 plants generate ample amounts of malate in the cytoplasm of their mesophyll cells, and nitrate assimilation in C4 plants does not show sensitivity to atmospheric carbon dioxide level. These results provide new insight into photorespiration, C4 carbon fixation, and the fate of higher plants under the conditions anticipated by the end of the century.

The steady increase in atmospheric CO2 since the industrial revolution is well established as the major driver of global climate change but its direct impacts on biochemical processes like photosynthesis are also of major concern. Some crop plant species have been shown to be less effective at assimilating nitrate, the most abundant form of nitrogen available to plants in most agricultural soils, when grown under elevated CO2. This indicates that rising CO2 concentrations could negatively impact the nitrogen use efficiency of many important crop species and may explain why protein content in some plant species decreases with CO2 enrichment. Improved techniques to measure nitrogen metabolism in plants are needed to further study the underlying cause of this phenomenon and mitigate the impact of rising atmospheric CO2 on our food supply. The three projects described here are a part of this effort. Chapter 1 describes a rosette area measurement tool that was developed as part of a larger study investigating the genetic basis of plant response to different nitrogen forms and concentrations. This tool was able to accurately estimate rosette area from photographs of Arabidopsis seedlings grown on agar plates even when the photographs did not provide a scale. We present a general-purpose ultra-high-resolution data acquisition system in chapter 2. The system interfaces a 32-bit analog to digital converter with a Raspberry Pi single board computer. We provide a simple open-source hardware design and easy to use software libraries enabling this system to be easily built and adapted for any purpose. Finally, we demonstrate the high performance of the system by making precise temperature measurements. Many elements of this data acquisition system were used in the Oxygen analyzer described in chapter 3. Chapter 3 documents the design and characterization of a novel high-resolution oxygen analyzer designed to measure small O2 fluxes against the large ambient background. The original motivation for this instrument was to estimate nitrate assimilation in plants through the ratio of CO2 assimilated to O2 evolved. The design is based around a zirconia O2 sensor that has not traditionally been used for high precision applications. The O2 measurement repeatability of the analyzer was better than 70 ppm, nearly 2 orders of magnitude better than what had previously been reported for an analyzer using this type of sensor. Further improvements to temperature control may yield even higher precision enabling measurements of photosynthetic O2 fluxes.

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