Floral scent constitutes an ancient and important channel of communication between flowering plants, their pollinators, and enemies. Fragrance is a highly complex component of floral phenotype, with dynamic patterns of emission and chemical composition. The information content of specific volatile compounds is highly context dependent, and scent can function in direct and indirect ways from landscape to intrafloral scales. Floral scent promotes specialization in plant–pollinator relationships through private channels of unusual compounds, unique ratios of more widespread compounds, or through multicomponent floral filters. Floral scent also promotes outcrossing and reproductive isolation through floral constancy, via appetitive conditioning and discrimination on the basis of diverse mechanisms, including pheromone mimicry, odor intensity, complexity, composition, and synergy with visual stimuli. Finally, floral scent is a sexual signal and should be subject to the same selective pressures and modes of signal evolution as animal display, including signal honesty, sensory drive, and sensory exploitation.

No abstract

All plants synthesize a suite of several hundred terpenoid compounds with roles that include phytohormones, protein modification reagents, anti-oxidants, and more. Different plant lineages also synthesize hundreds of distinct terpenoids, with the total number of such specialized plant terpenoids estimated in the scores of thousands. Phylogenetically restricted terpenoids are implicated in defense or in the attraction of beneficial organisms. A popular hypothesis is that the ability of plants to synthesize new compounds arose incrementally by selection when, as a result of gradual changes in their biotic partners and enemies, the 'old' plant compounds were no longer effective, a process dubbed the 'coevolutionary arms race'. Another hypothesis posits that often the sheer diversity of such compounds provides benefits that a single compound cannot. In this article, we review the unique features of the biosynthetic apparatus of terpenes in plants that facilitate the production of large numbers of distinct terpenoids in each species and how facile genetic and biochemical changes can lead to the further diversification of terpenoids. We then discuss evidence relating to the hypotheses that given ecological functions may be enhanced by the presence of mixtures of terpenes and that the acquisition of new functions by terpenoids may favor their retention once the original functions are lost.

No abstract

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.

Summary• Fragrance is a putatively important character in the evolution of flowering plants, but natural selection on scent is rarely studied and thus poorly understood. We characterized floral scent composition and emission in a common garden of Penstemon digitalis from three nearby source populations.• We measured phenotypic selection on scent as well as floral traits more frequently examined, such as floral phenology, display size, corolla pigment, and inflorescence height.• Scent differed among populations in a common garden, underscoring the potential for scent to be shaped by differential selection pressures. Phenotypic selection on flower number and display size was strong. However, selection favoured scent rather than flower size or colour, suggesting that smelling stronger benefits reproductive success in P. digitalis. Linalool was a direct target of selection and its high frequency in floral-scent bouquets suggests that further studies of both pollinator-and antagonist-mediated selection on this compound would further our understanding of scent evolution.• Our results indicate that chemical dimensions of floral display are just as likely as other components to experience selective pressure in a nonspecialized flowering herb. Therefore, studies that integrate visual and chemical floral traits should better reflect the true nature of floral evolutionary ecology.

SummaryVolatile esters impart distinct characteristics to the floral scent of many plants, and are important in attracting insect pollinators. They are also important flavor compounds in fruits. The ester benzylacetate is a major constituent of the floral scent of Clarkia breweri, an annual plant native to California. The enzyme acetyl-CoA:benzylalcohol acetyltransferase (BEAT), which catalyzes the formation of benzylacetate, has been purified from C. breweri petals, and a cDNA encoding this enzyme has been isolated and characterized. The sequence of the 433-residue BEAT protein does not show high similarity to any previously characterized protein, but a 35-residue region from position 135-163 has significant similarity (42-56% identity) to several proteins known or suspected to use an acylCoA substrate. E. coli cells expressing C. breweri BEAT produced enzymatically active protein, and also synthesized benzylacetate and secreted it into the medium. Of the different parts of the C. breweri flower, petals contained the majority of BEAT transcripts, and no BEAT mRNA was detected in leaves. The levels of BEAT mRNA in the petals increased as the bud matured, and peaked at anthesis, paralleling changes in BEAT activity. However, three days after anthesis, mRNA levels began a steep decline, whereas BEAT activity remained high for the next two days, suggesting that the BEAT protein is relatively stable.

No abstract