Incomplete C-13 labelling of alpha-pinene content of Quercus ilex leaves and appearance of unlabelled C in alpha-pinene emission in the dark

alpha-Pinene is formed in and emitted by Quercus ilex leaves. The carbon em itted as alpha-pinene is rapidly and totally labelled by C-13 in CO2 in air , but alpha-pinene contained in the leaf shows a fraction of completely unl abelled carbon even after long exposures to air containing only (CO2)-C-13. When the labelled leaf is darkened, alpha-pinene emission drops but is sti ll measurable for about 10 h, and carbon becomes partially unlabelled. Afte r an 11 h darkening the alpha-pinene content is still as high as in the lig ht but the carbon is mostly unlabelled. If the leaf is re-illuminated but p hotosynthesis is inhibited by removing CO2 and lowering O-2, a burst of emi ssion occurs and the content of alpha-pinene is depleted. Our experiments s uggest that a pool of alpha-pinene which is not directly generated by photo synthesis intermediates exists. Since this pool does not contribute relevan tly to the emission in the light, we hypothesize that it is inhibited in th e light and spatially located in a different compartment from chloroplasts. As we discuss, glycolysis in the cytoplasm and leucine catabolism in the m itochondria are both possible extra-chloroplasts sources of carbon for isop renoids.