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

α ‐Pinene is formed in and emitted by Quercus ilex leaves. The carbon emitted as α ‐pinene is rapidly and totally labelled by ¹³C in CO₂ in air, but α ‐pinene contained in the leaf shows a fraction of completely unlabelled carbon even after long exposures to air containing only ¹³CO₂. When the labelled leaf is darkened, α ‐pinene emission drops but is still measurable for about 10 h, and carbon becomes partially unlabelled. After an 11 h darkening the α ‐pinene content is still as high as in the light but the carbon is mostly unlabelled. If the leaf is re‐illuminated but photosynthesis is inhibited by removing CO₂ and lowering O₂, a burst of emission occurs and the content of α ‐pinene is depleted. Our experiments suggest that a pool of α ‐pinene which is not directly generated by photosynthesis intermediates exists. Since this pool does not contribute relevantly to the emission in the light, we hypothesize that it is inhibited in the light and spatially located in a different compartment from chloroplasts. As we discuss, glycolysis in the cytoplasm and leucine catabolism in the mitochondria are both possible extra‐chloroplasts sources of carbon for isoprenoids.