Physiological and environmental controls over isoprene emission

Isoprene is produced in plant chloroplasts and is the most abundant biogenic volatile organic compound (VOC) emitted from many temperate and tropical forests. Understanding the regulation of isoprene emission from plants is necessary because isoprene is closely coupled to basic plant metabolic processes and also has important implications for atmospheric chemistry. The regulation of isoprene production on very short time scales is well understood. However, regulation at larger spatial and temporal scales constitutes a major gap in our understanding of isoprene production. Field and laboratory experiments were performed to examine some of the long-term physiological and environmental mechanisms regulating isoprene emission from plants. The data generated in the dissertation were used to evaluate current leaf-level models of isoprene emission.; Isoprene emission capacity was examined within trees, among trees, between two populations, and over diurnal, seasonal, and annual time scales. In the absence of drought, differences in isoprene emission capacity among plants represented the largest source of variation. Moderate drought caused four-fold decreases in emission capacity. Diurnal variation in emission capacity was also large (>100%), but the magnitude of diurnal isoprene increases was species-specific. Although a positive relationship was found between leaf nitrogen content and isoprene emission capacity, fertilized trees had greater leaf area, which reduced light penetration into the canopy and consequently reduced modeled canopy-level emission in fertilized stands. As photosynthesis declined in response to water stress and high leaf temperature, plants increasing incorporated stored carbon sources (imported from the cytosol) into isoprene production (up to 40% derived from stored carbon). The isotopic fractionation during isoprene production increased during conditions of water and temperature stress.; In light of observed emission data, the development of a mechanistic model of isoprene emission will benefit most from research aimed at understanding the kinetics and temperature dependency of the enzymes in the MEP pathway. Also essential is the development of an appropriate function to describe substrate limitation of isoprene production, which involves both recently assimilated and stored carbon sources.