Metabolic origin of acetaldehyde emitted by poplar (Populus tremula x P-alba) trees

The metabolic origin and emission by the leaves of the tropospheric trace g as acetaldehyde were examined in 4-month-old poplar trees (Populus tremula x P. alba) cultivated under controlled environmental conditions in a greenh ouse. Treatments which resulted in increased ethanol concentration of the x ylem Sap caused significantly enhanced rates of acetaldehyde and ethanol em ission by the leaves. Leaves fed [C-14]ethanol via the transpiration stream emitted [C-14]acetaldehyde. These findings suggest that acetaldehyde in th e leaves is synthesized by a metabolic pathway that operates in the opposit e direction of alcoholic fermentation and results in oxidation of ethanol. Enzymatic studies showed that this pathway is mediated either by alcohol de hydrogenase (ADH;; EC 1.1.1.1) or catalase (CAT; EC 1.11.1.6), both constit utively present in the leaves of poplar trees. Labelling experiments with [ C-14]-glucose indicated that the ethanol delivered to the leaves by the tra nspiration stream is produced in anaerobic zones of submersed roots by alco holic fermentation, Anoxic conditions in the rhizosphere caused by flooding of the root system resulted in an activation of alcoholic fermentation and led to significantly increased ethanol concentrations In the xylem sap. Th ese results support the hypothesis that acetaldehyde emitted by the leaves of trees is derived from xylem transported ethanol which is synthesized dur ing alcoholic fermentation in the roots.