Fig. 9.

Kinetics of labelling of oak leaf (Quercus agrifolia) isoprene following transfer to a ¹³CO₂ environment and then back to ¹²CO₂. The figure shows PTR-MS data for masses from 69 to 74, which represent isoprene with zero to five ¹³C carbons, respectively. As can be seen, ¹²C isoprene (m/z 69) disappears rapidly after switching to ¹³CO₂, giving rise to isoprene molecules up to five ¹³C atoms. This result is in agreement with the understanding that isoprene is synthesized in the chloroplasts. The species with one or two ¹³C atoms disappear rapidly, after which in steady state fully labelled isoprene (m/z 74) summed only up to 60 % of the detected species. The experiment shows that isoprene synthesis is closely tied to the photosynthesis and photorespiration cycle (Karl et al. 2002).