Figure 8: Incorporation of 13C atoms derived from labelling experiments with 13CO2 into various metabolites during the plant metabolism. The possible positions of 13C atoms are highlighted as colored bars and small boxes, respectively; the color depends on the metabolic pathway which leads to a certain metabolite. 3-Phosphoglycerate (PGA) and glyceraldehyde 3-phosphate (GAP) are built up in the Calvin cycle which generate either molecules with two 13C atoms (pink bar) or molecules carrying three 13C atoms (red bars). A label at position 1 exclusively is also possible. The synthesis of monoterpenes is possible via the MEP pathway (left side) based on pyruvate and GAP (see Figure 1) which leads to a labelled C2 block originating from pyruvate (dark red bar and dark green bar, respectively) and/or a C2 or C3 block originating from GAP carrying either two 13C atoms (pink and light green bar, respectively) or three 13C atoms (red/green bar and red/green box, respectively), respectively. Red color marks carbon atoms coming from IPP and green color marks carbon atoms coming from DMAPP. On the right side the predicted labelling pattern of thymol arising from the mevalonate pathway based on acetyl-CoA is shown. This path does not generate monoterpenes carrying three 13C atoms. Boxed thymol is displayed with the labelling pattern observed by NMR spectroscopy.