On the physiology and evolution of volatile isoprenoid emission in plants

Isoprenoid emission by plants (1 PgC/yr) via the chloroplast localised methyl-erythritol phosphate (MEP) pathway is a significant source of carbon that influences atmospheric oxidation chemistry and global carbon cycle. Despite significant progress in isoprenoid research, many deeper questions surround the biochemical regulation of the MEP pathway and the physiological adaptations of emitters that determine the observed diversity in emission rates, hindering our ability to estimate and forecast global emissions. In this thesis, we present the results of a comprehensive set of experiments on the physiology of isoprenoid emission in selected species of Eucalyptus, a diverse native Australian tree genus (>900 species) that is adapted to a wide ranging climate and habitats in Australia and emits both isoprene and monoterpenes at significant measurable quantities. Addressing some of the outstanding questions in complex interactive effects of abiotic stresses on isoprenoid emission, our experiments show how increased isoprenoid emission rate by plants under abiotic stresses (low CO2, heat and drought) is sustained by increased ratio of electron transport rate to net carbon assimilation rate. We also show how the MEP pathway competes with photorespiration for the residual reducing power not invested in carbon assimilation. Our results highlight the importance of species-specific tolerance for drought and provide a basis to observed variability and uncertainty in emission responses to drought. Tackling another important question pertaining annual cycles in emission, we establish an independent role for photoperiod in seasonal oscillations in emission. Experiments show that temperature entrains seasonal rhythms in isoprenoid emission rates and it is likely gated by a photoperiodic clock. Sensitivity of emission to photoperiod means that increasing global mean temperature could interfere with the photoperiod signaling pathway in major emitting tree genera, altering their seasonal emission responses. Using a comprehensive data set, we trace the patterns in the origin and evolution of isoprene emission in land plants and propose a novel hypothesis. We discuss various levels of natural selection acting on isoprenoid emission whilst we put the most important of our experimental results in a global perspective.