PLANT PHYSIOLOGY , Vol 102, Issue 1 173-179, Copyright © 1993 by American Society of Plant Biologists
|
METABOLISM AND ENZYMOLOGY |
Metabolism of Transpired Ethanol by Eastern Cottonwood (Populus deltoides Bartr.)
R. C. MacDonald and T. W. Kimmerer
University of Kentucky, Department of Forestry and Graduate Program in Plant Physiology, Biochemistry, and Molecular Biology, 205 T.P. Cooper Building, Lexington, Kentucky 40546-0073
Ethanol has previously been shown to be present in the xylem sap of flooded
and nonflooded trees. Because of the constitutive presence of alcohol
dehydrogenase in the mature leaves of woody plants, we hypothesized that
the leaves and shoots of trees had the ability to metabolize ethanol
supplied by the transpiration stream. 1-[14C]Ethanol was supplied to
excised leaves and shoots of eastern cottonwood (Populus deltoides Bartr.)
in short- and long-term experiments. More than 99% of the radiolabel was
incorporated into plant tissue in short-term experiments, with more than
95% of the label remaining in plant tissue after 24 h. In all experiments,
less than 5% of the label was transpired as ethanol and less than 1% was
emitted as CO2. In excised leaf experiments, less than 0.5% of the
radiolabel escaped from the leaf. Fifty percent of the label was
incorporated into the petioles of excised leaves; 56% was incorporated into
the stems of excised shoots. Very little label reached the leaf mesophyll
cells of excised shoots, as revealed by autoradiography. Radiolabel
appeared primarily in the water- and chloroform-soluble fractions in
short-term experiments, whereas in long-term experiments, label was also
incorporated into protein. These results demonstrate that the leaves and
stems of trees appear to have substantial ability to scavenge ethanol from
the transpiration stream, allowing efficient recovery of ethanol produced
elsewhere by hypoxic tissues. When labeled ethanol was supplied to excised
petioles in a 5-min pulse, 41% of the label was incorporated into organic
acids. Some label was also incorporated into amino acids, protein, and the
chloroform-soluble fraction, with very little appearing in neutral sugars,
starch, or the insoluble pellet. Labeled organic acids were separated by
high performance liquid chromatography and were composed of acetate,
isocitrate, [alpha]-ketoglutarate, and succinate. There was no apparent
incorporation of label into phosphorylated compounds. We conclude that, in
higher plants, ethanol is metabolized to acetaldehyde and then to acetate
by alcohol and aldehyde dehydrogenases, and then into general metabolism.
