A list of 1719 chemical compounds identified from headspace samples of floral scent is presented. The list has been compiled from some 270 published papers, including analyses of 991 species of flowering plants and a few gymnosperms, a sample including seed plants from 90 families and 38 orders. The compounds belong to seven major compound classes, of which the aliphatics, the benzenoids and phenylpropanoids, and, among the terpenes, the mono- and sesquiterpenes, occur in most orders of seeds plants. C5-branched compounds, irregular terpenes, nitrogen-containing compounds, and a class of miscellaneous cyclic compounds have been recorded in about two-thirds of the orders. Sulfur-containing compounds occur in a third of the orders, whereas diterpenes have been reported from three orders only. The most common single compounds in floral scent are the monoterpenes limonene, (E)-β-ocimene, myrcene, linalool, α- and β-pinene, and the benzenoids benzaldehyde, methyl 2-hydroxybenzoate (methyl salicylate), benzyl alcohol, and 2-phenyl ethanol, which occur in 54–71% of the families investigated so far. The sesquiterpene caryophyllene and the irregular terpene 6-methyl-5-hepten-2-one are also common and occur in more than 50% of the families. Orchidaceae are by far the best investigated family, followed by several families known to have many species with strongly scented flowers, such as Araceae, Arecaceae, Magnoliaceae, and Rosaceae. However, the majority of angiosperm families are still poorly investigated. Relationships between floral scent and pollination, chemistry, evolution, and phylogeny are briefly discussed. It is concluded that floral scent chemistry is of little use for phylogenetic estimates above the genus level, whereas the distribution and combinations of floral scent compounds at species and subspecific levels is a promising field of investigation for the understanding of adaptations and evolutionary processes in angiosperms.

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Diversity and Distribution of Floral Scent

Since mimicry was first defined in 1862 by HW Bates, a great number of biologists have been inspired to search for new examples of this phenom­ enon They have directed their attention mostly to visual mimicry, which is not surprising in that humans readily perceive these signals (see 61, 82, 86, 87) By contrast, deception of a signal-receiver by means of chemical stimuli was ignored for a long time, although chemical communication might dominate in the majority of organisms This ommission partly resulted from the fact that chemical communication often operates in low-light environments (eg soil, nocturnal) where field work is difficult However, in the past few years more and more mimetic relationships have been discovered and chemically analyzed because of the progress in trace analysis of organic natural compounds In our search of the literature we found only one review on chemical mimicry phenomena: Stowe (76) provided a com­ prehensive collection of such systems with remarks on their possible evo­ lutionary patterns Some important examples of chemical deception are also discussed in the reviews cited above, but these papers mainly emphasize visual cues The goal of this review is not to present the whole array of examples in detail but to describe, analyze, and explain some strategies used by organisms to gain access to food, mates, or other important resources Following a discussion of terminology, the different strategies described have been arranged jnto seven categories: (a) integration into colonies of social insects, (b) exploitation of mutualisms between ants and other organisms, (c)

Chemical mimicry and camouflage

PLANTS have evolved traits that enable them to influence directly the behaviour and movement of their pollinators. Here I show that flowers in at least 74 diverse angiosperm families undergo dramatic, often localized, colour changes which direct the movements of a variety of pollinators to the benefit of both participants. Floral colour change was first noted almost 200 years ago1 and is known in a variety of species2–17, but the prevalence and significance of the phenomenon have gone largely unrecognized. I find that retention of older flowers increases a plant's attractiveness to pollinators from a distance, that pollinators discriminate between floral colour phases at close range, and that the discrimination involves learning. The phenomenon of floral colour change is taxonomically widespread, morphologically variable (Fig. 1), and physiologically diverse. It has evolved independently in the angiosperms many times and provides a striking example of functional convergence.

Floral colour changes as cues for pollinators

Despite recent interest in the non-sugar components of floral nectar, nearly nothing is known about the ecological importance and phylogenetic distribution of scented nectar. If present, the scent of nectar would provide an honest signal to nectar-feeding animals. Nectar odors may directly impact plant reproductive fitness, through pollinator attraction or deterrence of nectar robbers and florivores. In addition, nectar odors may indirectly impact plant fitness through antimicrobial activity, pleiotropic interactions with plant defense, and communication with predators and parasitoids. The literature provides only circumstantial evidence for scented nectar, through the study of bee honey odors. Here I confirm the presence of scent in the nectar of four out of seven angiosperm species sampled with solid-phase micro-extraction and gas chromatography–mass spectrometry. In Abelia x grandiflora and Hedychium coronarium, nectar odors are a hydrophilic subset of the compounds emitted by surrounding floral tissues, suggesting passive absorption by the nectar standing crop. Sucrose solution applied to the petals of a nectarless flower, Magnolia grandiflora, absorbed a hydrophilic subset of scent compounds after one hour, lending support to this hypothesis. Nectar from Oenothera primiveris and Agave palmeri contained unique scent compounds compared to the floral tissues. The presence of fermentation volatiles in A. palmeri nectar suggests a dynamic role for yeast in its floral biology. These data highlight the need for systematic studies on the distribution and mechanistic importance of scented floral nectar to plant–animal interactions.

Why are some floral nectars scented

The monoterpene alcohol, linalool, is present in the soral fragrance of diverse plant families and is attractive to a broad spectrum of pollinators, herbivores and parasitoids. Floral emission of linalool has evolved de novo in the fragrant, moth-pollinated annual Clarkia breweri (Gray) Greene (Onagraceae) through a combination of up-regulation and ectopic expression of its biosynthetic enzyme, linalool synthase (LIS), in conjunction with allometric size increases in all soral organs. Linalool synthase activity and linalool emissions are 1000-fold lower in a sibling species, C. concinna (Fischer & Meyer) Greene, that is diurnally pollinated. Linalool synthase expression is spatially and temporally regulated during C. breweri sower development, immediately precedes free linalool emission and is absent from nonsoral tissues. Its activity is highest in the style, but most of the linalool product appears to be converted to the pyranoid and furanoid linalool oxides. The LIS structural gene is a member of the terpene synthase gene family, sharing sequence identity with two discrete classes, represented by limonene synthase (LMS) and copalyl pyrophosphate synthase (CPS). Genetic crosses between C. breweri and C. concinna indicate that strong linalool emission segregates as a dominant mendelian trait, whereas the inheritance of linalool oxide formation is more complex, suggesting epistatic biosynthetic pathway interactions. We discuss areas for future research, including comparative studies of linalool biosynthesis in different plant families, entrainment of linalool emission to nocturnal circadian rhythms and the induction of vegetative linalool as an indirect herbivore defense.

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New Perspectives in Pollination Biology: Floral Fragrances. A day in the life of a linalool molecule: Chemical communication in a plant‐pollinator system. Part 1: Linalool biosynthesis in flowering plants

relationships between plants and insect pollinators challenge biologists ross-pollination is the transfer of pollen from anthers where it is produced to a flower stigma on a different plant. The value of cross-pollination is that it promotes gene mixing and results in greater vigor in the offspring. Because crosspollination is riskier than self-pollination, flowering plants have evolved many ways of ensuring its success. Plants that let the wind carry their pollen indiscriminantly to distant ovules must produce copious amounts of pollen to correct for inaccuracy of delivery. These plants, however, need produce only small, inconspicuous flowers. Other plants use biological agents to deliver their pollen, but at a cost of advertising for and paying the agents. Floral aroma is one example of flower advertisement, and in some cases may even act as compensation for pollination services. Wind-pollinated flowers may not emit aromas, nor may bird-pollinated flowers that rely on vivid red colors, which appear black to many insects (Faegri and van der Pijl 1979, Heinrich 1979b, von Frisch 1967). The origin and function of floral aroma have been of recent interest. The fossil record is incomplete but shows that plants with flowerlike structures and insects capable of evolving into pollinators began their radiations at about the same time

Floral AromaHow far will plants go to attract pollinators

SummaryA bioassay for comparing attractiveness of plants to honeybees (Apis mellifera) is described. Regression of data obtained from the bioassay showed that the method indicated greater attractiveness for plants with more flowers, greater amounts of nectar per flower, more intensely coloured flowers, larger flowers, and more fully open flowers. The bioassay relies on the distribution of foragers and not on preferences of individual bees and can be used to compare plants having temporally offset flowering periods, since comparisons are relative to a standard.

Eric H. Erickson

Papers

Floral AromaHow far will plants go to attract pollinators

A Bioassay for Comparing Attractiveness of Plants to Honeybees