Sources of floral scent variation: Can environment define floral scent phenotype?

Studies of floral scent generally assume that genetic adaptation due to pollinator-mediated natural selection explains a significant amount of phenotypic variance, ignoring the potential for phenotypic plasticity in this trait. In this paper, we assess this latter possibility, looking first at previous studies of floral scent variation in relation to abiotic environmental factors. We then present data from our own research that suggests among-population floral scent variation is determined, in part, by environmental conditions and thus displays phenotypic plasticity. Such an outcome has strong ramifications for the study of floral scent variation; we conclude by presenting some fundamental questions that should lead to greater insight into our understanding of the evolution of this trait, which is important to plant-animal interactions.Key words: abiotic factors, aromatics, floral scent, GxE interaction, phenotypic plasticity, pollination, terpenoids, volatilesFloral scent is thought to function as a major non-visual attractive cue for many pollinators in a large number of plant systems^1,2 and therefore most research on this plant trait has proceeded in the context of pollination ecology. Such studies have revealed the physiological and behavioral responses of pollinators to various floral volatiles (reviewed in refs. ³ and ⁴), convergent evolution of odor phenotypes attractive to specific pollinator classes (reviewed in refs. ⁵ and ⁶), reproductive isolation of plant species due to differences in pollinator attraction by scent,⁷ and instances of deception in which flowers mimic insect pheromones to effect pollination.⁸ Together, this body of evidence suggests that specific floral scent profiles can have important implications for the reproductive potential of many plant species.This pollinator-centered viewpoint has carried through to research on floral scent variation, including our most recent work on the insect-pollinated species Hesperis matronalis (Brassicaceae).⁹ Such studies usually suggest that the floral scent variation commonly found within and among individuals, populations and species (reviewed in ref. ²) is due to genetic differentiation as a result of selection by pollinators over time (reviewed in ref. ¹⁰). But an organism''s genes are only one factor determining phenotype. Both biotic (living) and abiotic (non-living) environmental conditions can profoundly affect phenotype expression, leading to significant variation. For plants, abiotic factors such as climate and soil chemistry can have particularly strong effects on phenotypes. When these environmental conditions cause changes in phenotype, we would say that a trait displays phenotypic plasticity.^11–13 A number of studies have uncovered phenotypic plasticity for many different plant traits.¹² However, while phenotypic variation in floral scent has been well-documented^1,2 and correlated with variation in biotic factors like pollinator behavior,^14–17 these studies were decidedly focused on natural selection, rather than phenotypic plasticity, as an organizational framework.However, in examining the scientific literature on floral scent, we found four studies in which the effects of naturally variable abiotic factors on floral scent profiles were examined, three of which were performed by the same research group (18^–21 (21). Moreover, these studies are decidedly not analyzed and interpreted using standard protocols for phenotypic plasticity studies.¹³

Table 1

A survey of previous studies examining changes in floral scent phenotype due to abiotic factors

How do visitation patterns vary among pollinators in relation to floral display and floral design in a generalist pollination system?

Diverse pollinator assemblages may impose complex selection and thus limit specialisation to particular pollinators. Previous work has concentrated on how visitation rates of different pollinators vary in space and time and how pollinators may vary in efficiency. In this study I quantify variation in visitation rates and foraging behaviour of different insect types (1) in space and time and (2) in relation to variation in floral design (flower size and form) and floral display (number of open flowers) for the distylous clonal shrub Jasminum fruticans. Mean visitation rate showed a significant interaction between insect type and population for seven populations in one year, and between insect types and years for a single population over 3 years. There was also a significant interaction between insect type and population for the proportion of flowers visited. In general the number of visits was positively related to the number of open flowers in a patch, but analyses by insect type showed that this was only true for bee flies and butterflies. Short-tongued bees showed a positive relationship between visitation rate and the number of open flowers on the focal stem, and hawkmoths and butterflies made more visits to plants with larger flowers. Hawkmoths were the only insect type to show a positive relation between the number of flowers visited per foraging bout and flower size. The significant differences between different insect types in patterns of variation in visitation rates in response to floral design and display may act to diversify selection on floral traits, and thereby constrain specialisation of the plant to particular pollinators.     

How does insect visitation trigger floral colour change?

Abstract.  1. Visitation by the key pollinator, Bombus terrestris , was implicated in inducible flower colour change in Lupinus pilosus . Behaviour at the flower and rate of visitation by these bumble bees had specific effects; exclusion of this flower visitor led to retarded onset, and reduced rate, of colour change.
2. The foraging behaviour of B. terrestris was influenced by floral colour change in L. pilosus . Choice of pre-change flowers was greater than random in relation to the proportion of colour phases available within the plant population.
3. Levels of floral manipulation that mimicked the flower handling characteristics of visiting bumble bees confirmed that triggering of the pollen release mechanism is necessary for the instigation of colour change.
4. Moreover, this study suggests that, in L. pilosus , an aspect of pollination (pollen deposition by bees and/or subsequent pollen tube growth within the style) is linked with colour change and may act as the trigger for such change.     

Does floral trait variability enhance reproductive success in deceptive orchids?

Unusually high intra-specific floral trait variability has often been described within deceptive orchid populations, as opposed to rewarding ones. Such variability is traditionally thought to have consequences on reproduction in this orchid group, i.e. phenotypically variable deceptive species may have a reproductive success advantage compared to those with a constant floral display. The proposed reason for this hypothetic pattern is that floral trait variability decreases pollinator avoidance learning in dealing with nectarless flowers, hence increasing their visitation rate. However, despite an intuitive and appealing hypothesis and a possible mechanism to explain it, the often-cited higher reproductive success induced by floral trait variability still remains unsupported.Here, we review the literature and consider eight studies that have experimentally or correlatively tested this hypothesis in deceptive orchids. In all these experiments, we have found no difference in average reproductive success between populations with high versus low flower trait variability, either in scent variable or colour polymorphic species. We discuss possible explanations for the lack of this pattern including the incapability of pollinators in perceiving the variability, the scarce relevance of polymorphic traits in the choice of species to forage on, or a different pollinator behaviour than the one proposed. We suggest that the high phenotypic variability is not likely to enhance deceptive orchids’ reproductive success, but is more likely to be a consequence of relaxed selection by pollinators. Nonetheless, information regarding orchid pollination strategy or pollinator cognitive abilities is often superficial, hence calling researchers for additional investigations that can contribute to a better understanding of this debated and yet unsupported hypothesis.     

用floral dip法对黄瓜遗传转化的初步研究

用floral dip方法进行遗传转化已经在模式植物拟南芥、部分十字花科及豆科植物苜蓿中取得成功,但在黄瓜转化中未见报道.本研究首次采用floral dip法转化黄瓜,结果表明,采用0.05%Silwet-77和含质粒pX6-GAD-GLP-1的工程农杆菌EHA105转化黄瓜自交系P2及2M1,分别收获种子544和1022粒:将收获种子经60 mg/L的卡那霉素筛选,其抗性植株移栽成活后分别进行PCR及斑点杂交检测,在2个自交系中均获得了阳性转化植株,转化率分别为0.55%和0.68%.本研究首次将floral dip法用于黄瓜遗传转化并获得成功,为扩展floral dip法的应用领域、研究floral dip法在葫芦科植物上的应用奠定了基础,同时也进一步完善了黄瓜转基因手段.     

Are pollination syndromes useful predictors of floral visitors in Tasmania?

Abstract Diurnal visitors to the flowers of many native plant species were identified in a wide range of Tasmanian sclerophyllous vegetation between September 1996 and April 1997. These foraging profiles were analysed to determine whether they were characteristic of various floral morphologies in predictable ways. It was found that although visitor profiles were sometimes consistent with classic pollination syndromes, these syndromes were unreliable predictors of floral visitors. Very few flowers were exclusively bird‐pollinated, and none were strictly fly‐, beetle‐, wasp‐, or butterfly‐pollinated. The majority of flowering plants were unspecialized in their morphology, and consequently hosted a diverse array of visitors. In addition, visitor profiles to congeners with similar floral morphologies, and even to conspecifics, differed between habitats. Altitude was a major factor in determining visitors, with flies being the most abundant visitors above 700 m. However, congeners in several genera of Epacridaceae, as well as the genus Correa, which differed in floral morphology also differed in visitor profiles. Tubular flowers were associated with birds, while flowers with more accessible nectar were visited by insects. The only taxa exhibiting a bee‐pollination syndrome that were largely visited by bees were the Fabaceae and Goodenia ovata Sm. Several species with purple or pink flowers were also predominantly visited by bees, but did not strictly conform to the melittophilous syndrome. In contrast, other flowers exhibiting an ostensibly mellitophilous syndrome hosted very few bees. Of these, species that occurred at high altitude were mainly visited by flies, while others received very few potential pollen vectors.     

Life history, floral development, and mating system in Clarkia xantiana (Onagraceae): do floral and whole-plant rates of development evolve independently?

Autogamously self-fertilizing taxa have evolved from outcrossing progenitors at least 12 times in the annual wildflower genus, Clarkia (Onagraceae). In C. xantiana, individuals of the selfing subspecies (ssp. parviflora) flower at an earlier age, produce successive flowers more rapidly, and produce flowers that complete their development more rapidly than their outcrossing counterparts (ssp. xantiana). Two hypotheses have been proposed to explain the joint evolution of these whole-plant and individual floral traits. The accelerated life cycle hypothesis proposes that selection favoring a short life cycle in environments with short growing seasons (such as those typically occupied by parviflora) has independently favored genotypes with early reproduction, synchronous flower production, and rapidly developing, self-fertilizing flowers. The correlated response to selection hypothesis similarly proposes that selection in environments with short growing seasons favors early reproduction, but that rapid floral development and increased selfing evolve as correlated responses to selection due to genetic linkage (or pleiotropy) affecting both whole-plant and floral development. We conducted a greenhouse experiment using maternal families from two field populations of each subspecies to examine covariation between floral and whole-plant traits within and among populations to seek support for either of these hypotheses. Our results are consistent with the accelerated life cycle hypothesis but not with the correlated response to selection hypothesis.     

Antagonistic effects of floral scent in an insect–plant interaction

In southwestern USA, the jimsonweed Datura wrightii and the nocturnal moth Manduca sexta form a pollinator–plant and herbivore–plant association. Because the floral scent is probably important in mediating this interaction, we investigated the floral volatiles that might attract M. sexta for feeding and oviposition. We found that flower volatiles increase oviposition and include small amounts of both enantiomers of linalool, a common component of the scent of hawkmoth-pollinated flowers. Because (+)-linalool is processed in a female-specific glomerulus in the primary olfactory centre of M. sexta, we hypothesized that the enantiomers of linalool differentially modulate feeding and oviposition. Using a synthetic mixture that mimics the D. wrightii floral scent, we found that the presence of linalool was not necessary to evoke feeding and that mixtures containing (+)- and/or (−)-linalool were equally effective in mediating this behaviour. By contrast, females oviposited more on plants emitting (+)-linalool (alone or in mixtures) over control plants, while plants emitting (−)-linalool (alone or in mixtures) were less preferred than control plants. Together with our previous investigations, these results show that linalool has differential effects in feeding and oviposition through two neural pathways: one that is sexually isomorphic and non-enantioselective, and another that is female-specific and enantioselective.     

Generalized pollination system: Are floral traits adapted to different pollinators?

Flowers that are pollinated both during the day and at night could exhibit two different groups of pollinators and produce two different sets of attractants and rewards. We explored the patterns of emission of flower scents and production of nectar in the cactus Echinopsis chiloensis ssp. chiloensis, in relation to the patterns of activity of its diurnal and nocturnal pollinators. We measured frequency of flower visitors, analyzed floral scents, measured nectar production and sugar concentration, and performed pollination exclusion experiments. Bees were the main visitors at daytime and hawkmoths at nighttime. Diurnal scents were dominated by several compounds that can attract a wide range of pollinators, whereas nocturnal scents were less diverse and were dominated by (E)-nerolidol, a compound eliciting antennal responses in hawkmoths. Nectar volume and sugar concentration at night were similar to those recorded in hawkmoth-pollinated flowers. Daytime nectar volume was higher than those commonly found in bee-pollinated flowers, but similar to those found in flowers pollinated by several pollinators. Daytime sugar concentration was similar to those recorded in bee-pollinated flowers. Flowers of E. chiloensis ssp. chiloensis seem morphologically adapted to hawkmoth pollination, but diurnal and nocturnal pollinators contribute to similar extents to reproductive success. Additionally, diurnal and nocturnal pollinators showed a synergic effect on the product of fruit set and seed set. The results are discussed in terms of the linkage between floral traits and perception abilities and requirements of pollinators.     

Do changes in floral odor cause speciation in sexually deceptive orchids?

 We investigated differences in floral odor between two sympatric, closely related sexually deceptive orchid species, Ophrys fusca and O. bilunulata, which are specifically pollinated by Andrena nigroaenea and A. flavipes, respectively. We identified biologically active compounds by gas chromatography with electroantennographic detection using antennae of the pollinator bees. Alkanes, alkenes, aldehydes, and farnesyl hexanoate released electroantennographic reactions. The relative amounts of alkanes were mostly the same between the two orchid species, whereas the relative amounts of most alkenes were significantly different. On the grounds of these findings and behavioral experiments conducted in earlier studies, we suggest that the difference in relative amounts of alkenes is responsible for the selective attraction of pollinators in the two orchids. Speciation in this group of Ophrys orchids may be brought about by changes in pattern of alkenes, which lead to attraction of a different pollinator species and therefore reproductive isolation. Received November 22, 2001; accepted February 21, 2002 Published online: November 7, 2002 Addresses of the authors: Florian P. Schiestl* and Manfred Ayasse, Department of Evolutionary Biology, Institute of Zoology, University of Vienna, Althanstrasse 14, A-1090 Vienna. *Present address: Geobotanical Institute ETH, Zollikerstrasse 107, CH-8008 Zürich. (e-mail: schiestl@geobot.umnw.ethz.ch)     

Is the nectar redox cycle a floral defense against microbial attack?

Many angiosperms use a remarkable reproductive strategy that relies on attracting animals (insect, avian or mammalian pollinators) to transfer pollen between plants. Relying on other organisms for sexual reproduction seems evolutionarily untenable, but the great diversity of angiosperms illustrates how highly successful this strategy is. To attract pollinators, plants offer a variety of rewards. Perhaps the primary floral reward is floral nectar. Plant nectar has long been considered a simple sugar solution but recent work has demonstrated that nectar is a complex biological fluid containing significant and important biochemistry with the potential function of inhibiting microbial growth. These results lead the way to novel insights into the mechanisms of floral defense and the co-evolution of angiosperms and their pollinators.     

Morfología floral de la subtribu Leiboldiinae (Vernonieae,Asteraceae)

Morphological features of flowers and inflorescences from 13 of 14 species of subtribe Leiboldiinae are analyzed, with the goal of identifying characters with taxonomic value for the delimitation of genera and species. The principal characters to recognize genera in the subtribe are: presence of palaea on the receptacle (Bolanosa), number of series in the involucre and pappus (Stramentopappus), shape of the apices of the phyllaries (Lepidonia), and colour and duration of the pappus. At the specific level, the more important characters are diversity and distribution of trichomes, colour of florets and differences in the sculpturing of the surfaces of the petals.     

A survey of floral traits, breeding systems, floral visitors, and pollination systems of the angiosperms of the Juan Fernández Islands (Chile)

A survey of the reproductive features of the angiosperm flora of the Juan Fernández Archipelago (Chile) is presented to provide a species-based review of reproduction and pollination, to identify generalizations associated with these systems, to understand the evolution of these features, and to utilize these data to promote conservation. The collection of original data was extensive, based on our own fieldwork, and was combined with data from existing literature. Data recorded include habit, sexual system, flower size, shape, and color, and the hypothesized pollination system of the first colonizers. In addition, the data on compatibility, presence and type of dichogamy, observed floral visitors, presence of floral rewards, and currently known pollination systems are summarized. Pairwise comparisons of different features are tested for statistical association. The flora is typically composed of perennials. The majority of the species have very small or small flowers. Inconspicuous flowers (i.e., a shape character describing flowers with no optical attraction) are widespread, as are dish-shaped flowers. Green is the most frequent flower color, followed by white and yellow. Most species are hermaphroditic, 9% are dioecious, and 9% are monoecious. Some 30% of the species are protandrous and 7% protogynous. Detailed studies of compatibility of about 14% of the flora indicate that 85% of these species are self-compatible (SC). Although most species studied are SC, their level of autogamy is low. Nevertheless, selfing mediated via geitonogamy is the most frequent mechanism of pollen transfer. Outcrossing is mainly achieved through dioecy and self-incompatibility (SI), promoted by dichogamy in the hermaphroditic flowers, and facilitated by wind pollination. About 55% of the species offer nectar rewards, and only 2% offer pollen rewards. Floral visitors are rare to uncommon. Two hummingbird species, one of them endemic, are considered as pollinators for 14 plant species. Flies, moths, and beetles are the native insect visitors to flowers, but they have been documented on only 11 plant species (7%). Even insect visits to these few species were rarely observed. Given the infrequent, irregular, and imprecise nature of native insect association with flowers, there is no certainty that any of the species are truly insect pollinated. Two species of introduced ants and a new endemic bee were recorded as well; however, neither is likely currently important to the pollination of the native flora species. About 9% of the extant flora is currently bird pollinated, and we hypothesize that 47% is wind pollinated. However, we propose that most of the colonizers were ancestrally either insect or wind pollinated. There is association between a number of current floral features and the hypothesized pollination of colonizers. Therefore, to a large extent the flower color, shape, and size of the extant flora may express the pollination syndromes of colonizers rather than representing extant pollination. In addition, the presence of nectar in many species of extant flora does not necessarily indicate biotic pollination. Thus, studies of the reproductive biology on oceanic island plants need to be conducted species by species before broad generalizations can be made, because the observed features can be misleading. Possible changes in the pollination system were assessed by comparison of species for which there are reliable data with the hypothesized pollination of their colonist progenitors. The wind-or bird-pollinated species have retained the pollination system of the colonizers. In other instances, species seem to bear a different pollination system: from ancestral insect systems to current hummingbird-or wind-pollination systems. The lack of alternative means of biotic pollination seems to have led in a number of instances to anemophily—in essence a default pollination system. The lack of strong selection pressure for wind pollination and the relative youth of the archipelago may help explain why the features associated with wind pollination in these species are not so obvious. Because there are many recorded extinctions of vascular plants from islands versus those from continental areas, it is imperative to invest additional effort in protecting the remaining island species. Conservation or restoration programs cannot be effective without a deep and broad understanding of the reproductive biology of the plants. In order to conserve these plants, programs must involve a combination of reproductive and environmental measures. The ultimate fate of some species may depend on preserving the plant-hummingbird relationship, including the web of organisms that affect both plant and pollinator. The populations of introduced animals and weeds must be controlled. Experimentally produced allogamous seeds would enhance diversity in restoration programs. In addition, the preservation of habitat seems to be the central challenge to indirectly protect the unique island species.     

Relationships between species’ floral traits and pollinator visitation in a temperate grassland

Knowledge about plant–plant interactions for pollinator service at the plant community level is still scarce, although such interactions may be important to seed production and hence the population dynamics of individual plant species and the species compositions of communities. An important step towards a better understanding of pollination interactions at the community level is to assess if the variation in floral traits among plant species explain the variation in flower visitation frequency among those species. We investigated the relative importance of various floral traits for the visitation frequency of all insects, and bumblebees and flies separately, to plant species by measuring the visitation frequency to all insect-pollinated species in a community during an entire flowering season. Visitation frequency was identified to be strongly positive related to the visual display area and the date of peak flowering of plant species. Categorical variables, such as flower form and symmetry, were important to the visitation frequency of flies only. We constructed floral similarity measures based on the species’ floral traits and found that the floral similarity for all species’ traits combined and the continuous traits separately were positively related to individual visitation frequency. On the other hand, plant species with similar categorical floral traits did not have similar visitation frequencies. In conclusion, our results show that continuous traits, such as flower size and/or density, are more important for the variation in visitation frequency among plant species than thought earlier. Furthermore, differences in visitation frequency among pollinator groups give a poor support to the expectations derived from the classical pollination syndromes.     

Linné's floral clock is slow without pollinators--flower closure and plant-pollinator interaction webs

Temporal patterns of flower opening and closure within a day are known as Linné's floral clock. Time of flower closure has been explained mainly by light in the traditional botanical literature. We show with a set of experiments that Asteraceae flower heads can close within three hours after pollination, whereas un-pollinated flower heads stay open until the late afternoon. This suggests that closing time strongly depends on pollinators. Using plant-pollinator interaction webs we further demonstrate that the daily pattern of flower opening and the rapid response to pollination can impose strong temporal dynamics on interspecific interactions within a single day. We observed pollinator species turnover and changes in facilitation vs. competition among plants. Our results show for the first time that pollination induces rapid flower closure on the community level. This causes imprecision in Linné's floral clock with far-reaching consequences for plant-pollinator interactions.     

Electroantennogram responses of male Sphinx perelegans hawkmoths to floral and ‘green-leaf volatiles’

The effects of sublethal dosages of the chloronicotinyl insecticide imidacloprid on different strains of the tobacco whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae), have been studied after leaf dip and systemic application. All bioassays were performed with the insecticide susceptible strain, SUD-S, and two Spanish biotypes, ALM-2 and LMPA-2, both resistant to conventional insecticides and with a lower susceptibility towards imidacloprid. Honeydew, excreted by all strains feeding on treated and untreated cotton leaf discs was quantified by photometric analysis of its carbohydrate content. EC50-values for the depression of honeydew excretion in female adults after systemic application of imidacloprid were calculated at 0.037 ppm, 0.027 ppm and 0.048 ppm for strains SUD-S, ALM-2 and LMPA-2, respectively, indicating no significant differences between strains in feeding behaviour throughout an 48 h testing period. Depending on the strain these EC50-values were 150- to 850-times lower than LC50-values calculated for mortality in the same bioassay. Starvation tests revealed mean survival times of >48 h for female adults placed on agar without leaf discs, indicating that sublethal dosages of imidacloprid which caused antifeedant responses, were probably not covered in common 48 h systemic bioassays, used to monitor resistance to imidacloprid. Effects of sublethal dosages on honeydew excretion after leaf dip application seem to be minor. In choice situations with systemically treated and untreated leaf discs in a single container, female adults of B. tabaci showed a clear preference for the untreated leaf discs. However, when using leaf discs treated by painting the surface with imidacloprid in the same bioassay, feeding activities on treated and untreated leaf discs were not significantly different. The results of the present study demonstrate the antifeedant properties of imidacloprid on B. tabaci, which might play an essential role after soil application or seed treatment under field conditions.