Abstract
Trees produce a wide spectrum of volatile organic compounds (VOCs) including isoprene
and monoterpenes, as well as oxygenated compounds like aldehydes, alcohols and carboxylic
acids. In recent years, much progress has been made regarding the elucidation of metabolic
pathways leading to the production of these compounds. This is particularly true for
the biosynthesis of the isoprenoid precursors, isopentenyl diphosphate (IDP) and dimethyl
allyl diphosphate (DMADP). In addition to the classical mevalonate pathway which leads
to the biosynthesis of these compounds, recent studies indicate the presence of a
non-mevalonate pathway originating from pyruvate and glyceraldehyde-3-phosphate (GAP),
also leading to isoprenoid precursors. This new 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway is probably responsible for the formation of
all plastid-derived isoprenoid compounds in plants, including carotenoids, plastochinones,
the prenyl side chains of chlorophyll, as well as monoterpenes and diterpenes. Because
all plastidic isoprenoids studied so far are formed via this new pathway, it is assumed
that isoprene synthesized in the chloroplasts is also produced via this metabolic
route. Among the oxygenated hydrocarbons which are emitted by the leaves of trees,
C-1 and C-2 aldehydes, alcohols and carboxylic acids are of great importance. C-1
compounds are synthesized during many growth and developmental processes such as seed
maturation, cell expansion, cell wall degradation, leaf abscission and senescence
of plant tissues. The production of C-2 compounds, however, seems mainly to be associated
with changing environmental conditions, particularly during stress. Acetaldehyde,
for example, is produced in the leaves of trees if the roots are exposed to anaerobic
conditions which in nature may be caused by flooding. As a consequence of anaerobiosis,
roots produce ethanol through alcoholic fermentation. Ethanol is loaded into the xylem,
transported to the leaves and oxidized there under aerobic conditions, thereby releasing
acetaldehyde and acetic acid.
Key words
Acetaldehyde - acetic acid - ethanol - formaldehyde - formic acid - isoprene - monoterpenes
