Synthesis of ‘cineole cassette’ monoterpenes in Nicotiana section Alatae: gene isolation, expression, functional characterization and phylogenetic analysis

The scent bouquets of flowers of Nicotiana species, particularly those of section Alatae, are rich in monoterpenes, including 1,8-cineole, limonene, β-myrcene, α- and β-pinene, sabinene, and α-terpineol. New terpene synthase genes were isolated from flowers of Nicotiana bonariensis, N. forgetiana, N. longiflora, and N. mutabilis. The recombinant enzymes synthesize simultaneously the characteristic 'cineole cassette' monoterpenes with 1,8-cineole as the dominant volatile product. Interestingly, amino acid sequence comparison and phylogenetic tree construction clustered the newly isolated cineole synthases (CIN) of section Alatae together with the catalytically similar CIN of N. suaveolens of section Suaveolentes, thus suggesting a common ancestor. These CIN genes of N. bonariensis, N. forgetiana, N. longiflora, and N. mutabilis are distinct from the terpineol synthases (TERs) of the taxonomically related N. alata and N. langsdorfii (both Alatae), thus indicating gene diversification of monoterpene synthases in section Alatae. Furthermore, the presence of CINs in species of the American section Alatae supports the hypothesis that one parent of the Australian section Suaveolentes was a member of the present section Alatae. Amino acid sequences of the Nicotiana CINs and TERs were compared to identify relevant amino acids of the cyclization reaction from α-terpineol to 1,8-cineole.     

Fragrance chemistry,nocturnal rhythms and pollination "syndromes" in Nicotiana

GC-MS analyses of nocturnal and diurnal floral volatiles from nine tobacco species (Nicotiana; Solanaceae) resulted in the identification of 125 volatiles, including mono- and sesquiterpenoids, benzenoid and aliphatic alcohols, aldehydes and esters. Fragrance chemistry was species-specific during nocturnal emissions, whereas odors emitted diurnally were less distinct. All species emitted greater amounts of fragrance at night, regardless of pollinator affinity. However, these species differed markedly in odor complexity and emission rates, even among close relatives. Species-specific differences in emission rates per flower and per unit fresh or dry flower mass were significantly correlated; fragrance differences between species were not greatly affected by different forms of standardization. Flowers of hawkmoth-pollinated species emitted nitrogenous aldoximes and benzenoid esters on nocturnal rhythms. Four Nicotiana species in section Alatae sensu strictu have flowers that emit large amounts of 1,8 cineole, with smaller amounts of monoterpene hydrocarbons and alpha-terpineol on a nocturnal rhythm. This pattern suggests the activity of a single biosynthetic enzyme (1,8 cineole synthase) with major and minor products; however, several terpene synthase enzymes could contribute to total monoterpene emissions. Our analyses, combined with other studies of tobacco volatiles, suggest that phenotypic fragrance variation in Nicotiana is shaped by pollinator- and herbivore-mediated selection, biosynthetic pathway dynamics and shared evolutionary history.     

Characteristic alatoid ‘cineole cassette’ monoterpene synthase present in Nicotiana noctiflora

Nicotiana species of the section Alatae emit a characteristic floral scent comprising the? cineole cassette’ monoterpenes 1,8-cineole, limonene, myrcene, β-pinene, α-pinene, sabinene and α-terpineol. All previously isolated ‘cineole cassette’-monoterpene synthase genes are multi product enzymes that synthesize the seven compounds of the ‘cineole cassette’. Interestingly, so far this ‘alatoid’ trait was only shared with the eponymous species Nicotiana suaveolens of the sister section Suaveolentes. To determine the origin of the ‘cineole cassette’ monoterpene phenotype other potential parent species of section Noctiflorae or Petunoides as well as of the distantly related section Trigonophyllae were analysed. A monoterpene synthase producing the set of ‘cineole cassette’ compounds was isolated from N. noctiflorae. N. obtusifolia emitted solely 1,8-cineole and no monoterpenes were found in floral scents of N. petunoides and N. palmeri. Interestingly, the phylogenetic analysis clustered the new gene of N. noctiflora closely to the terpineol synthase genes of e.g. N. alata rather than to cineole synthase genes of e.g. N. forgetiana.     

Qualitative and quantitative analyses of flower scent in Silene latifolia

The quantitative and qualitative variability in floral scent of 98 specimens of the dioecious species Silene latifolia belonging to 15 European and 19 North American populations was determined. Floral scent was collected from single flowers using dynamic headspace methods, and analysed by Micro-SPE and GC-MS methods. The flowers showed a nocturnal rhythm, and scent was emitted only at night. The amount of emitted volatiles varied greatly during the season, from 400 ng/flower/2 min in June to 50 ng/flower/2 min in August and September. The qualitative variability in the floral scent was high and different chemotypes, characterised by specific scent compounds, were found. Female and male flowers emitted the same type and amount of volatiles. The differences in floral scent composition between European and North American populations were small. Typical compounds were isoprenoids like lilac aldehyde isomers, or trans-beta-ocimene, and benzenoids like benzaldehyde, phenyl acetaldehyde, or veratrole. Some of these compounds are known to attract nocturnal Lepidoptera species. The high qualitative variability is discussed in relation to the pollination biology of S. latifolia, and the results are compared with other studies investigating intraspecific variability of flower scent.     

Sexual and temporal variations in floral scent in the subdioecious shrub Eurya japonica Thunb

In many flowering plants, floral scents are a significant trait for visitors, playing an important role in attracting pollinators and/or detracting herbivores. The evolution of flowering plants from hermaphroditism to dioecy is often accompanied by sexual dimorphism in floral scent. In this study, floral scents emitted by different sexual morphs of the subdioecious shrub Eurya japonica Thunb. were collected using a dynamic headspace method, and sexual and temporal variations were evaluated by gas chromatography–mass spectrometry (GC–MS). Two volatiles, α‐pinene and linalool, were identified as the major components of floral scents in females, hermaphrodites, and males. The males emit higher amounts of floral scents, particularly α‐pinene, compared to females or hermaphrodites. Floral scents emitted by males generally decrease as flowers enter senescence, whereas those from females or hermaphrodites do not significantly differ. Intraspecific variations in floral scents of subdioecious species provided by this study would contribute to better understanding of sexual dimorphism in floral scent.     

Stereoisomeric pattern of lilac aldehyde in Silene latifolia, a plant involved in a nursery pollination system

The monoterpene lilac aldehyde is found in floral scent of several plants species, among them Silene latifolia. This plant is involved in a nursery pollination system, because a noctuid moth, Hadena bicruris, is not only pollinator but also seed predator. Lilac aldehyde is the key floral scent compound of S. latifolia for attracting Hadena. This monoterpene has three stereogenic centers, and eight different isomers are possible. Here, we analysed the ratio of lilac aldehyde isomers from plants originating from 18 different populations of S. latifolia using enantioselective multidimensional GC-MS (enantio-MDGC-MS), and compared resulting variability with variability found in total scent emitted by specimen under study. Though variability in total emitted scent was high, ratio of lilac aldehyde isomers was a more conservative trait. There was no correlation between the ratio of lilac aldehyde isomers and the total emitted floral scent pattern. Both, ratio of stereoisomers and total emitted scent were independent from the geographic origin of the plants. In conclusion, the ratio of lilac aldehyde stereoisomers in S. latifolia is a reliable trait, and may used by the nursery pollinator H. bicruris for host-plant detection.     

Investigation of Nicotiana tabacum (+) N. suaveolens cybrids with carpelloid stamens

To investigate cytoplasmic effects on homeotic floral morphology, Nicotiana tabacum and N. suaveolens protoplasts were fused and cybrids obtained to contrast with the sexual alloplasmic line Nta(sua)S. Nta(sua)S contains the nucleus of N. tabacum and cytoplasm of N. suaveolens while cybrids derive from fused cells where the cytoplasms can interact. The three male-sterile somatic cybrid plants analyzed contained mitochondria with N. tabacum and N. suaveolens mtDNA sequences, but not all the N. tabacum or all the N. suaveolens mtDNA sequences were present. The flowers were N. tabacum-like but with a split corolla (not observed in Nta(sua)S) and the whorl of stamens replaced by a whorl of carpel-like structures. Based on scanning electron microscopy the carpelloid stamens had a characteristic N. tabacum stigma, a style of variable length and a pseudo-ovary with ovule-like structures. The Southern blot data were consistent with mtDNA recombination. These genomic changes were maternally inherited. Chloroplasts were either of the N. tabacum or N. suaveolens type. AFLP analysis showed transfer of variable amounts of N. suaveolens nuclear DNA. However, it is the presence of the N. suaveolens sequences and/or absence of N. tabacum sequences in the mitochondria that correlates with the homeotic floral morphology. These cybrids will facilitate the analysis of the role of mitochondrial DNA sequences in floral organ identity; which has received limited attention in genetic flowering models based primarily on Arabidopsis research.     

Scent chemistry and pollinator attraction in the deceptive trap flowers of Ceropegia dolichophylla

Ceropegia species (Apocynaceae, Asclepiadoideae) have pitfall flowers and are pollinated by small flies through deception. It has been suggested that these flies are attracted by floral scent. However, the scent that is emitted from Ceropegia flowers has not been studied using headspace and gas chromatography mass spectrometry methods. It has also been unclear whether or not the flowers are mimics of particular models that attract flies. In the present study, we determined the composition as well as the spatial and temporal patterns of floral scent emitted by C. dolichophylla. Furthermore, we determined the pollinators in the native (China) and non-native (Germany) range of this species, and tested the capability of the floral scent to attract flies in the non-native range. Our data demonstrate that the floral scent, which is emitted from morning until evening, primarily from the tips of the corolla lobes, consists mainly of spiroacetals and aliphatic compounds. Milichiid flies were common visitors/pollinators in the native as well as non-native range, and were attracted by floral scent in bioassays performed in the non-native range. The compounds emitted by C. dolichophylla are unusual for flowers, but are well known from insect pheromones and occur in the glandular secretions of insects. The milichiid flies that visit and pollinate the flowers are kleptoparasites that feed on the prey (haemolymph or other secretions) of predatory arthropods, e.g. spiders, to which they are attracted by scent. Our data thus suggest that the floral scent of C. dolichophylla mimics the feeding sites of kleptoparasitic flies.     

Evolution and diversity of floral scent chemistry in the euglossine bee-pollinated orchid genus Gongora

•Background and Aims Animal-pollinated angiosperms have evolved a variety of signalling mechanisms to attract pollinators. Floral scent is a key component of pollinator attraction, and its chemistry modulates both pollinator behaviour and the formation of plant–pollinator networks. The neotropical orchid genus Gongora exhibits specialized pollinator associations with male orchid bees (Euglossini). Male bees visit orchid flowers to collect volatile chemical compounds that they store in hind-leg pouches to use subsequently during courtship display. Hence, Gongora floral scent compounds simultaneously serve as signalling molecules and pollinator rewards. Furthermore, because floral scent acts as the predominant reproductive isolating barrier among lineages, it has been hypothesized that chemical traits are highly species specific. A comparative analysis of intra- and inter-specific variation of floral scent chemistry was conducted to investigate the evolutionary patterns across the genus.•Methods Gas chromatography–mass spectrometry (GC-MS) was used to analyse the floral scent of 78 individuals belonging to 28 different species of Gongora from two of the three major lineages sampled across the neotropical region. Multidimensional scaling and indicator value analyses were implemented to investigate the patterns of chemical diversity within and among taxonomic groups at various geographic scales. Additionally, pollinator observations were conducted on a sympatric community of Gongora orchids exhibiting distinct floral scent phenotypes.•Key Results A total of 83 floral volatiles, mainly terpenes and aromatic compounds, were detected. Many of the identified compounds are common across diverse angiosperm families (e.g. cineole, eugenol, β-ocimene, β-pinene and terpinen-4-ol), while others are relatively rare outside euglossine bee-pollinated orchid lineages. Additionally, 29 volatiles were identified that are known to attract and elicit collection behaviour in male bees. Floral scent traits were less variable within species than between species, and the analysis revealed exceptional levels of cryptic diversity. Gongora species were divided into 15 fragrance groups based on shared compounds. Fragrance groups indicate that floral scent variation is not predicted by taxonomic rank or biogeographic region.•Conclusions Gongora orchids emit a diverse array of scent molecules that are largely species specific, and closely related taxa exhibit qualitatively and quantitatively divergent chemical profiles. It is shown that within a community, Gongora scent chemotypes are correlated with near non-overlapping bee pollinator assemblies. The results lend support to the hypothesis that floral scent traits regulate the architecture of bee pollinator associations. Thus, Gongora provides unique opportunities to examine the interplay between floral traits and pollinator specialization in plant–pollinator mutualisms.     

Trends in floral scent chemistry in pollination syndromes: floral scent composition in moth-pollinated taxa

KNUDSEN, J. T. & TOLLSTEN, L., Trends in floral scent chemistry in pollination syndromes: floral scent composition in moth-pollinated taxa. Floral scent from 15 moth-pollinated species in nine families was collected by head-space adsorption. The chemical composition was determined by coupled gas chromatography-mass spectrometry (GC-MS). The typical floral scent of moth-pollinated flowers contains some acyclic terpene alcohols, their corresponding hydrocarbons, benzenoid alcohols and esters and small amounts of some nitrogen compounds. The floral scent composition of sphingophilous flowers can be distinguished from that of phalaenophilous flowers by the presence of oxygenated sesquiterpenes. The flowers of three of the studied species had the general appearance and floral scent composition of moth-pollinated flowers, but contained no nectar reward. These species probably rely on deceptive pollination by naive visitors, which are deceived by the similarity of the flowers' morphological and scent chemistry to that of rewarding moth flowers. The finding of similar or structurally closely related floral scent compounds in both temperate and tropical species from both the Old and New worlds suggests that floral scent composition has been selected by a specific group of pollinators, moths that have similar sensory preferences. The functions of floral scent in moth-pollinated flowers are discussed in relation to an often observed over-representation of male moth visitors.     

Is the timing of scent emission correlated with insect visitor activity and pollination in long‐spurred Satyrium species?

Plants are expected to emit floral scent when their pollinators are most active. In the case of long‐tubed flowers specialised for pollination by crepuscular or nocturnal moths, scent emissions would be expected to peak during dawn. Although this classic idea has existed for decades, it has rarely been tested quantitatively. We investigated the timing of flower visitation, pollination and floral scent emissions in six long‐spurred Satyrium species (Orchidaceae). We observed multiple evening visits by pollinaria‐bearing moths on flowers of all study species, but rarely any diurnal visits. The assemblages of moth pollinators differed among Satyrium species, even those that co‐flowered, and the lengths of moth tongues and floral nectar spurs were strongly correlated, suggesting that the available moth pollinator fauna is partitioned by floral traits. Pollinarium removal occurred more frequently during the night than during the day in four of the six species. Scent emission, however, was only significantly higher at dusk than midday in two species. Analysis of floral volatiles using gas chromatography coupled with mass spectrometry yielded 168 scent compounds, of which 112 were species‐specific. The scent blends emitted by each species occupy discrete clusters in two‐dimensional phenotype space, based on multivariate analysis. We conclude that these long‐spurred Satyrium species are ecologically specialised for moth pollination, yet the timing of their scent emission is not closely correlated with moth pollination activity. Scent composition was also more variable than expected from a group of closely related plants sharing the same pollinator functional group. These findings reveal a need for greater understanding of mechanisms of scent production and their constraints, as well as the underlying reasons for divergent scent chemistry among closely related plants.     

Biochemical and structural characterization of benzenoid carboxyl methyltransferases involved in floral scent production in Stephanotis floribunda and Nicotiana suaveolens

Flower-specific benzenoid carboxyl methyltransferases from Stephanotis floribunda and Nicotiana suaveolens were biochemically and structurally characterized. The floral scents of both these species contain higher levels of methyl benzoate and lower levels of methyl salicylate. The S. floribunda enzyme has a 12-fold lower K(m) value for salicylic acid (SA) than for benzoic acid (BA), and results of in silico modeling of the active site of the S. floribunda enzyme, based on the crystal structure of Clarkia breweri salicylic acid methyltransferase (SAMT), are consistent with this functional observation. The enzyme was therefore designated SAMT. The internal concentration of BA in S. floribunda flowers is three orders of magnitude higher than the SA concentration, providing a rationale for the observation that these flowers synthesize and emit more methyl benzoate than methyl salicylate. The N. suaveolens enzyme has similar K(m) values for BA and SA, and the in silico modeling results are again consistent with this in vitro observation. This enzyme was therefore designated BSMT. However, the internal concentration of BA in N. suaveolens petals was also three orders of magnitude higher than the concentration of SA. Both S. floribunda SAMT and N. suaveolens BSMT are able to methylate a range of other benzenoid-related compounds and, in the case of S. floribunda SAMT, also several cinnamic acid derivatives, an observation that is consistent with the larger active site cavity of each of these two enzymes compared to the SAMT from C. breweri, as shown by the models. Broad substrate specificity may indicate recent evolution or an adaptation to changing substrate availability.     

Circadian rhythm of volatile emission from flowers of Nicotiana sylvestris and N. suaveolens

The volatile profiles from flowers of Nicotiana sylvestris and N. suaveolens were investigated by means of dynamic headspace sampling and capillary gas chromatography. Under conditions of light/dark entrainment both species emitted phenylpropanoid-derived volatiles (e.g. benzyl alcohol, methyl benzoate) with maximum emission occurring during the dark period. Emission of these compounds was demonstrated to be circadian by continuance of rhythmicity under conditions of constant light and subsequent re-entrainment to a new light/dark cycle. In contrast, emission of the sesquiterpene hydrocarbon, caryophyllene, from N. sylvestris followed no apparent pattern. The emission of monoterpene hydrocarbons from flowers of N. suaveolens showed diurnal differences only under conditions of light/dark entrainment.     

Floral scent in bird- and beetle-pollinated Protea species (Proteaceae): Chemistry,emission rates and function

Evolutionary shifts between pollination systems are often accompanied by modifications of floral traits, including olfactory cues. We investigated the implications of a shift from passerine bird to beetle pollination in Protea for floral scent chemistry, and also explored the functional significance of Protea scent for pollinator attraction. Using headspace sampling and gas chromatography–mass spectrometry, we found distinct differences in the emission rates and chemical composition of floral scents between eight bird- and four beetle-pollinated species. The amount of scent emitted from inflorescences of beetle-pollinated species was, on average, about 10-fold greater than that of bird-pollinated species. Floral scent of bird-pollinated species consists mainly of small amounts of “green-leaf volatiles” and benzenoid compounds, including benzaldehyde, anisole and benzyl alcohol. The floral scent of beetle-pollinated species is dominated by emissions of linalool, a wide variety of other monoterpenes and the benzenoid methyl benzoate, which imparts a fruity odour to the human nose. The number of compounds recorded in the scent of beetle-pollinated species was, on average, greater than in bird-pollinated species (45 versus 29 compounds, respectively). Choice experiments using a Y-maze showed that a primary pollinator of Protea species, the cetoniine beetle Atrichelaphinis tigrina, strongly preferred the scent of inflorescences of the beetle-pollinated Protea simplex over those of the bird-pollinated sympatric congener, Protea roupelliae. This study shows that a shift from passerine bird- to insect-pollination can be associated with marked up-regulation and compositional changes in floral scent emissions.     

Floral scent in natural hybrids of Ipomopsis (Polemoniaceae) and their parental species

Background and Aims

Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.

Methods

Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography–mass spectrometry.

Key Results

The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.

Conclusions

The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.     

Evolution of funnel-revolver flowers and ornithophily in nasa (loasaceae)

Floral morphology, distribution, and flower visitors for 60 taxa of Nasa are investigated and compared to molecular trees inferred both from a combined marker analysis (ITS1 and trnL (UAA)) and from a single marker (ITS1). Flowers conform to two different floral types: Firstly, "tilt-revolver flowers", with spreading to reflexed, white to yellow petals and small, brightly coloured floral scales contrasting with the petals and firmly enclosing the nectar (Saccatae and Carunculatae); secondly, "funnel-revolver flowers", with half-erect to erect, orange to red petals and floral scales not contrasting with the petals, or enclosed in the corolla, and nectar freely accessible by funnel-shaped floral scales ( Alatae, Grandiflorae, and N. venezuelensis species group). Phylogenetic analysis shows that "tilt-revolver flowers" represent the plesiomorphic condition by outgroup comparison. The two groups with tilt-revolver flowers in Nasa are not monophyletic ( Saccatae are paraphyletic, Carunculatae are polyphyletic). Most Saccatae fall into two monophyletic assemblages, the N. poissoniana species group and the N. triphylla species group. The remainder of Saccatae group either with Grandiflorae ( N. insignis species group) or with Alatae ( N. laxa species group). The clades retrieved in the molecular analysis contradict the traditional classification, but are congruent with vegetative morphology, details of the flower morphology, and biogeography. "Funnel-revolver flowers" represent the derived condition, but molecular data suggest a convergent development (at least twice independently), since the corresponding species do not constitute a monophyletic group. "Tilt-revolver flowers" are visited and pollinated by bees (especially Colletidae), whereas "funnel-revolver flowers" are mostly visited by hummingbirds. The transition from melittophily to ornithophily may have been the license for the colonization of, and the diversification in, both cloud forest and high Andean habitats.     

Flower color-flower scent associations in polymorphic Hesperis matronalis (Brassicaceae)

Floral scent emission rate and composition of purple and white flower color morphs of Hesperis matronalis (Brassicaceae) were determined for two populations and, for each, at two times of day using dynamic headspace collection and GC-MS. The floral volatile compounds identified for this species fell into two main categories, terpenoids and aromatics. Principal component analysis of 30 compounds demonstrated that both color morphs emitted more scent at dusk than at dawn. Color morphs varied in chemical composition of scent, but this differed between populations. The white morphs exhibited significant differences between populations, while the purple morphs did not. In the white morphs, one population contains color-scent associations that match expectations from classical pollination syndrome theory, where the flowers have aromatic scents, which are expected to maximize night-flying moth pollinator attraction; in the second population, white morphs were strongly associated with terpenoid compounds. The potential impact that pollinators, conserved biosynthetic pathways, and the genetics of small colonizing populations may have in determining population-specific associations between floral color and floral scent are discussed.     

The emission of floral scent from Lilium ‘siberia’ in response to light intensity and temperature

Floral scent is an important part of volatile compounds emitted from plants, and is influenced by many environmental factors. In this study, the floral scent emitted from Lilium ‘siberia’, a common breed of lily, was collected by dynamic headspace at different levels of light intensity (0, 100, 300, 600, 1,000, and 1,500 μmol m^?2 s^?1) and temperature (10, 20, 30, and 40 °C). Using the automated thermal desorption-gas chromatography/mass spectrometry (ATD-GC/MS) technique, the components and release amounts were subsequently identified to investigate the influence of light and temperature on the emission of floral scent. The results revealed that the numbers and release amounts of floral scent components were significantly influenced by light intensity and temperature, showing the similar pattern: first increasing and then decreasing. After light intensity treatment, the maximum numbers and release amounts mainly appeared at 600 and 1,000 μmol m^?2 s^?1. For temperature treatment, 30 °C resulted in the highest numbers and release amounts of the floral scent components. At different levels of light intensity and temperature, terpenoid compounds showed the highest numbers and release amounts among the component categories. α-Ocimene and linalool were the two terpenoid compounds with the highest release amounts, and accounted for the highest proportion. The results obtained provide evidence that both light intensity and temperature trigger the emission of floral scent. The particular response mechanisms must be investigated in future research.     

Smells like debauchery: The chemical composition of semen-like,sweat-like and faintly foetid floral odours in Xysmalobium (Apocynaceae: Asclepiadoideae)

Floral volatiles play an important role in plant communication with both pollinators and antagonists, but remain poorly explored for many plant groups. Asclepiads (Apocynaceae: Asclepiadoideae subtribe Asclepiadinae) represent a diverse group in South African grasslands, but the scents of most species remain unexplored and few genera are sufficiently sampled to allow comparisons between congeners. I used dynamic headspace extraction methods and coupled gas chromatography-mass spectrometry (GC–MS) to examine the scent chemistry of three unusually scented asclepiads in the genus Xysmalobium and then combined these data with previously published data to explore inter- and intraspecific variation in the genus. A total of 74 compounds (33–44 per species) from various compound classes were detected in the species examined here. The sweet but faintly foetid scent of Xysmalobium asperum was dominated by epoxy oxoisophorone in combination with various other terpenoids and aromatics, and small amounts of p-cresol. The sweat-like scent of Xysmalobium tysonianum was dominated by a few aromatics in combination with isovaleric acid and several aliphatic compounds normally associated with microbial degradation or fermentation. The semen-like scent of Xysmalobium parviflorum flowers examined here contained large relative amounts of 1-pyrroline, and comparison with previously published data for dung-scented flowers from a different population revealed clear divergence in the relative amounts of this compound and p-cresol. I also detected 25 compounds that were not shared between the two X. parviflorum populations. Comparison of scent data for eight Xysmalobium species revealed very distinct chemical profiles with limited overlap between species. These results are discussed in relation to the possible roles of these volatiles as pollinator attractants and the evolution of floral scents within the genus.