Transient releases of acetaldehyde from tree leaves−products of a pyruvate overflow mechanism?

Emissions of acetaldehyde from tree leaves were investigated by proton-transfer-reaction mass spectrometry (PTR-MS), a technique that allows simultaneous monitoring of different leaf volatiles, and confirmed by derivatization and high-performance liquid chromatography analysis. Bursts of acetaldehyde were released by sycamore, aspen, cottonwood and maple leaves following light–dark transitions; isoprene emission served as a measure of chloroplastic processes. Acetaldehyde bursts were not accompanied by ethanol, but exposure of leaves to inhibitors of pyruvate transport or respiration, or anoxia, led to much larger releases of acetaldehyde, accompanied by ethanol under anoxic conditions. These same leaves have an oxidative pathway for ethanol present in the transpiration stream, resulting in acetaldehyde emissions that are inhibited in vivo by 4-methylpyrazole, an alcohol dehydrogenase (Adh) inhibitor. Labelling of leaf volatiles with 13 CO 2 suggested that the pools of cytosolic pyruvate, the proposed precursor of acetaldehyde bursts, were derived from both recent photosynthesis and cytosolic carbon sources. We hypothesize that releases of acetaldehyde during light–dark transitions result from a pyruvate overflow mechanism controlled by cytosolic pyruvate levels and pyruvate decarboxylase activity. These results suggest that leaves of woody plants contribute reactive acetaldehyde to the atmosphere under different conditions: (1) metabolic states that promote the accumulation of cytosolic pyruvate, triggering the pyruvate decarboxylase reaction; and (2) leaf ethanol oxidation resulting from ethanol transported from anoxic tissues. Key-words : acetaldehyde; ethanol; isoprene; leaves; lightenhanced-dark-respiration; proton-transfer-reaction mass spectrometry; pyruvate; pyruvate decarboxylase; volatile organic compound emissions.