•Joerg-Peter Schnitzler¹, Martin Graus², Juergen Kreuzwieser³, Ulrike Heizmann³, Heinz Rennenberg³, Armin Wisthaler² und Armin Hansel^2
1FZ Karlsruhe, Institut fuer Meteorologie und Klimaforschung, Atmosphaerische Umweltforschung, Garmisch-Partenkirchen
²Institut fuer Ionenphysik, Universitaet Innsbruck
³Institut fuer Forstbotanik und Baumphysiologie, Universitaet Freiburg

The present study (part of AFO 2000) was performed to test if alternative carbon sources besides recently photosynthetically fixed CO2 are used for isoprene formation in the leaves of young poplar trees. In a 13CO2 atmosphere under steady state conditions, only about 75 percent of isoprene became 13C-labeled within minutes. A considerable part of the unlabeled carbon may be derived from xylem transported carbohydrates, as may be shown by feeding leaves with [U-13C]glucose. As a consequence of this treatment app. 8 to 10 percent of the carbon emitted as isoprene was 13C-labeled. In order to identify further carbon sources, poplar leaves were depleted of leaf internal carbon pools and the carbon pools were refilled with 13C-labeled carbon by exposure to 13CO2. Results from this treatment showed that about 30 percent of isoprene carbon became 13C labeled, clearly suggesting that, in addition to xylem transported carbon and CO2, leaf internal carbon pools, e.g. starch, are used for isoprene formation.