Rapid appearance of 13C in biogenic isoprene when 13CO2 is fed to intact leaves

Biogenic isoprene substantially affects atmospheric chemistry, but it is not known how or why many plants, especially trees, make isoprene. We fed ¹³CO₂ to leaves of Quercus rubra and monitored the incorporation of ¹³C into isoprene by mass spectrometry. After feeding ¹³CO₂ for 9 min we found all possible labelling patterns from completely unlabelled to fully labelled isoprene. By 18 min, 84% of the carbon atoms in isoprene were ¹³C. Labelling of the last 20% of the carbon atoms was much slower than labelling of the first 80%. The rate of labelling of isoprene was similar to that reported for phosphoglyceric acid indicating that there is a close linkage between the carbon source for isoprene synthesis and the photosynthetic carbon reduction pathway.