Final Activity Report Summary - GLOBALVOC (Understanding global change effects on VOC emission in Populus from eco-physiological to molecular level) Emission of hydrocarbons by trees has a crucial role in the oxidising potential of the atmosphere. In particular, biogenic isoprene makes up one-third to one-half of the total amount of Volatile organic compounds (VOCs) entering the atmosphere. Isoprene oxidation leads to the formation of tropospheric ozone and other secondary pollutants. It is expected that changes in the composition of the atmosphere will influence the emission rate of isoprene which may in turn feedback on the accumulation of pollutants and greenhouse gases. We investigated the Isoprene synthase (ISPS) gene expression and the ISPS protein levels in aspen trees exposed to elevated ozone (O3) or elevated carbon dioxide (CO2) in field grown trees at the AspenFACE experimental site in northern Wisconsin. Elevated O3 reduced ISPS messenger Ribonucleic acid (mRNA) and the amount of ISPS protein in aspen leaves whereas elevated CO2 had no significant effect. Aspen clones with different O3 sensitivity showed different levels of inhibition under elevated O3 conditions. The drop in ISPS protein levels induced a drop in the isoprene emission rate under elevated O3. However, the data indicated that other mechanisms also contributed to the observed strong inhibition of isoprene emission under elevated O3. The level of Dimethylallyl pyrophosphate (DMAPP), i.e. the substrate for isoprene synthesis, was inhibited by both elevated O3 and elevated CO2 even though the effect of the latter was stronger than that of the former. These results, if confirmed by follow-up studies, induced to think that the emission rates of isoprene might decrease on a leaf unit basis in a future atmosphere characterised by elevated levels of CO2 and O3. These findings, associated with the variations in emission assuming changes in air temperature and changes in total leaf area index, were crucial for the modellers to estimate the total emission potential by different forestry ecosystems in the future world.