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.