A walk through the forest is a good way to become
aquainted with the variety of terpenes emitted by plants.
The most fragrant of these are the monoterpenes, which
lend a distinctive smell to trees (e.g. α- and β-pinene from
pines), mints (e.g. menthol from peppermint), fruits (e.g.
limonene from citrus) and flowers (e.g. geraniol from
roses). In addition to adding fragrance, phytogenic
hydrocarbons play an important role in atmospheric
chemistry. This was recognized as early as 1960 by Fritz
Went who noted that blue hazes often formed above
forested areas (Went, 1960). This idea was picked up by
US President Ronald Reagan who accused trees of
polluting the atmosphere, although in truth phytogenic
hydrocarbons only contribute to pollution in air that is
already contaminated by industry or automobile exhaust
(Feshenfeld et al., 1992).
Terpenes serve a variety of functions in plants including
deterring herbivores and attracting pollinators. Until
recently, the function of terpene emission from some
groups of plants, which do not store the terpenes in their
tissues, has been a mystery. Following the discovery that
isoprene, a hemiterpene, increases the thermotolerance of
photosynthesis in some species (Sharkey & Singsaas, 1995;
Singsaas et al., 1997), Loreto et al. (1998) discovered that
thermotolerance of the monoterpene-emitting oak, Quercus ilex,
increases when leaves are fumigated with monoterpenes.
Now, a report by Delfine et al. in this issue (pp.
27–36) provides further support for the hypothesis that
terpenes play a role in thermotolerance of leaves and
suggests, intriguingly, that the mechanism by which this
occurs is not limited to plants that make these compounds.