Deciphering the Biotic and Climatic Factors That Influence Floral Scents: A Systematic Review of Floral Volatile Emissions

Global change aggravates drought, with consequences for plant reproduction

BACKGROUND

The frequency and intensity of droughts are expected to increase under global change, driven by anthropogenic climate change and water diversion. Precipitation is expected to become more episodic under climate change, with longer and warmer dry spells, although some areas might become wetter. Diversion of freshwater from lakes and rivers and groundwater pumping for irrigation of agricultural fields are lowering water availability to wild plant populations, increasing the frequency and intensity of drought. Given the importance of seasonal changes and extremes in soil moisture to influence plant reproduction, and because the majority of plants are flowering plants and most of them depend on pollinators for seed production, this review focuses on the consequences of drought on different aspects of reproduction in animal-pollinated angiosperms, emphasizing interactions among drought, flowering and pollination.

SCOPE

Visual and olfactory traits play crucial roles in attracting pollinators. Drought-induced floral changes can influence pollinator attraction and visitation, together with pollinator networks and flowering phenology, with subsequent effects on plant reproduction. Here, we review how drought influences these different aspects of plant reproduction. We identify knowledge gaps and highlight areas that would benefit from additional research.

CONCLUSIONS

Visual and olfactory traits are affected by drought, but their phenotypic responses can vary with floral sex, plant sex, population and species. Ample phenotypic plasticity to drought exists for these traits, providing an ability for a rapid response to a change in drought frequency and intensity engendered by global change. The impact of these drought-induced changes in floral traits on pollinator attraction, pollen deposition and plant reproductive success does not show a clear pattern. Drought affects the structure of plant-pollinator networks and can modify plant phenology. The impact of drought on plant reproduction is not always negative, and we need to identify plant characteristics associated with these more positive responses.

Floral Volatile Organic Compounds of Mitchella repens (Rubiaceae)

Mitchella repens (partridgeberry; family Rubiaceae) is a creeping, understory plant native to eastern North America. The twinned, tubular flowers of this distylous plant are bright white and produce volatile organic compounds (VOCs). Partridgeberry has intermorph incompatibility and thus requires pollinators to move pollen from one morph to the other. Despite partridgeberry being a common member of forest communities, little is known about its pollination syndrome. Using headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) analysis the floral VOCs were identified, with the four predominant molecules being α-pinene, camphene, D-limonene, and verbenone. The VOC profile contained 27 molecules consisting mostly of monoterpenes. Two independent sample t-tests confirmed that each morph produced statistically similar floral VOC profiles (p > 0.1). Additionally, two of the predominant VOC molecules, α-pinene and D-limonene, were measured throughout the 5-day flowering cycle. Simple linear regressions of these compound levels versus days after flowering (DAF) confirmed that α-pinene and D-limonene both decreased with flower age. Insect visits were observed to correlate with α-pinene and D-limonene concentrations, peaking at 1-2 DAF and then declining through 5 DAF.

Diurnal patterns of floral volatile emissions in three species of Narcissus

PREMISE

Plants generate a wide array of signals such as olfactory cues to attract and manipulate the response of pollinators. The present study addresses the temporal patterns of scent emission as an additional dimension to the scent composition. The expectation is that divergent floral function is reflected in divergent qualitative and temporal emission patterns.

METHODS

We used GC-ion mobility spectrometry with an integrated pre-concentration for automated acquisition of the temporal trends in floral volatile emissions for N. viridiflorus, N. papyraceus, and N. cantabricus subsp. foliosus.

RESULTS

We found a considerable increase in scent emissions and changes in scent composition for N. viridiflorus at night. This increase was particularly pronounced for aromatic substances such as benzyl acetate and p-cresol. We found no diurnal patterns in N. papyraceus, despite a similar qualitative composition of floral volatiles. Narcissus cantabricus subsp. foliosus showed no diurnal patterns either and differed considerably in floral scent composition.

CONCLUSIONS

Scent composition, circadian emission patterns, and floral morphology indicate divergent, but partially overlapping pollinator communities. However, the limited pollinator data from the field only permits a tentative correlation between emission patterns and flower visitors. Narcissus papyraceus and N. cantabricus show no clear diurnal patterns and thus no adjustment to the activity patterns of their diurnal pollinators. In N. viridiflorus, timing of scent emission indicates an adaptation to nocturnal flower visitors, contradicting Macroglossum as the only reported pollinator. We propose that the legitimate pollinators of N. viridiflorus are nocturnal and are still unidentified.

Chemical Composition of Volatile and Extractive Organic Compounds in the Inflorescence Litter of Five Species of Woody Plants

The decomposition of plant litter, most of which is found in forests, is an important element of the global carbon cycle, as a result of which carbon enters the atmosphere in the form of not only CO2 but also volatile organic compounds (VOCs). Although the formation of litter is associated with autumn cooling, in the spring, there is a very intense fall of faded inflorescences of woody plants. This study examined the chemical composition of the litter and VOCs emitted from decaying inflorescences of four species of forest-forming trees: silver birch, European hornbeam, black alder and aspen. All litter emissions consisted of 291 VOCs, mainly terpenes actively participating in atmospheric processes. The detection of a number of typical mushroom metabolites, such as 1-octen-3-ol, known as "mushroom alcohol", and alkyl sulphides, suggests that inflorescence-derived VOCs are a mixture of components of plant and microbial origin. In methanol extracts of the fallen inflorescences of all types, 263 organic compounds were identified, the majority of which were related to carbohydrates. Their share in the extracts was 72-76%. In general, the composition of the extractive compounds indicates the easy availability of this material for assimilation by various types of destructors.

Convergence and molecular evolution of floral fragrance after independent transitions to self-fertilization

Studying the independent evolution of similar traits provides valuable insights into the ecological and genetic factors driving phenotypic evolution.1 The transition from outcrossing to self-fertilization is common in plant evolution2 and is often associated with a reduction in floral attractive features such as display size, chemical signals, and pollinator rewards.3 These changes are believed to result from the reallocation of the resources used for building attractive flowers, as the need to attract pollinators decreases.2,3 We investigated the similarities in the evolution of flower fragrance following independent transitions to self-fertilization in Capsella.4,5,6,7,8,9 We identified several compounds that exhibited similar changes in different selfer lineages, such that the flower scent composition reflects mating systems rather than evolutionary history within this genus. We further demonstrate that the repeated loss of β-ocimene emission, one of the compounds most strongly affected by these transitions, was caused by mutations in different genes. In one of the Capsella selfing lineages, the loss of its emission was associated with a mutation altering subcellular localization of the ortholog of TERPENE SYNTHASE 2. This mutation appears to have been fixed early after the transition to selfing through the capture of variants segregating in the ancestral outcrossing population. The large extent of convergence in the independent evolution of flower scent, together with the evolutionary history and molecular consequences of a causal mutation, suggests that the emission of specific volatiles evolved as a response to changes in ecological pressures rather than resource limitation.

Functional characterization of terpene synthases SmTPS1 involved in floral scent formation in Salvia miltiorrhiza

Plants attract beneficial insects and promote pollination by releasing floral scents. Salvia miltiorrhiza, as an insect-pollinated flowering plant, which has been less studied for its floral aroma substances. This study revealed that S. miltiorrhiza flowers produce various volatile terpenoids, including five monoterpenes and ten sesquiterpenes, with the sesquiterpene compound (E)-β-caryophyllene being the most abundant, accounting for 28.1% of the total volatile terpenoids. Y-tube olfactometer experiments were conducted on the primary pollinator of S. miltiorrhiza, the Apis ceranas. The results indicated that (E)-β-caryophyllene compound had an attractive effect on the Apis ceranas. By comparing the homologous sequences with the genes of (E)-β-caryophyllene terpene synthases in other plants, the SmTPS1 gene was selected for further experiment. Subcellular localization experiments showed SmTPS1 localized in the cytoplasm, and its in vitro enzyme assay revealed that it could catalyze FPP into β-Elemene, (E)-β-caryophyllene and α-Humulene. Overexpression of SmTPS1 in S. miltiorrhiza resulted in a 5.29-fold increase in gene expression. The GC-MS analysis revealed a significant increase in the concentration of (E)-β-caryophyllene in the transgenic plants, with levels 2.47-fold higher compared to the empty vector plants. Furthermore, Y-tube olfactometer experiments showed that the transgenic plants were significantly more attractive to Apis ceranas compared to the empty vector plants. Co-expression analysis suggested that four SmMYCs (SmMYC1, SmMYC5, SmMYC10, and SmMYC11) may be involved in the transcriptional regulation of SmTPS1. The yeast one-hybrid screen and the Dual luciferase assay indicated that SmMYC10 positively regulates the expression of SmTPS1. In conclusion, this study lays a foundation for the functional analysis and transcriptional regulation of terpene synthase genes in S. miltiorrhiza.

Physiological and Biochemical Analysis Revealing the Key Factors Influencing 2-Phenylethanol and Benzyl Alcohol Production in Crabapple Flowers

Floral scent (FS) plays a crucial role in the ecological functions and industrial applications of plants. However, the physiological and metabolic mechanisms underlying FS formation remain inadequately explored. Our investigation focused on elucidating the differential formation mechanisms of 2-phenylethanol (2-PE) and benzyl alcohol (BA) by examining seven related enzyme concentrations and the content of soluble sugar, soluble proteins, carbon (C) and nitrogen (N), as well as the C/N ratio. The findings revealed that the peak content of 2-PE in M. 'Praire Rose' and BA in M. 'Lollipop' occurred during the end flowering stage (S4) and flowering stage (S3) periods, respectively. The enzyme concentration change trends of phenylpyruvate decarboxylase (PDL), phenylacetaldehyde reductase (PAR), soluble protein, C, N, and C/N ratio changes during the S3-S4 period in M. 'Praire Rose' and M. 'Lollipop' were entirely opposite. Correlation and PCA analysis demonstrated that the content of CYP79D73 (a P450) and N, and the C/N ratio were key factors in 2-PE production in M. 'Praire Rose'. The production of BA in M. 'Lollipop' was more influenced by the content of phenylacetaldehyde synthase (PAAS), CYP79D73, and soluble sugar. As CYP79D73 exits oppositely in correlation to 2-PE (M. 'Praire Rose') and BA (M. 'Lollipop'), it is hypothesized that CYP79D73 was postulated as the primary factor contributing to the observed differences of 2-PE (M. 'Praire Rose') and BA (M. 'Lollipop') formation. These results carry significant implications for crabapple aromatic flower breeding and the essential oil industry etc.

Olfaction in the Anthropocene: NO3 negatively affects floral scent and nocturnal pollination

There is growing concern about sensory pollutants affecting ecological communities. Anthropogenically enhanced oxidants [ozone (O3) and nitrate radicals (NO3)] rapidly degrade floral scents, potentially reducing pollinator attraction to flowers. However, the physiological and behavioral impacts on pollinators and plant fitness are unknown. Using a nocturnal flower-moth system, we found that atmospherically relevant concentrations of NO3 eliminate flower visitation by moths, and the reaction of NO3 with a subset of monoterpenes is what reduces the scent's attractiveness. Global atmospheric models of floral scent oxidation reveal that pollinators in certain urban areas may have a reduced ability to perceive and navigate to flowers. These results illustrate the impact of anthropogenic pollutants on an animal's olfactory ability and indicate that such pollutants may be critical regulators of global pollination.

Macroevolution of floral scent chemistry across radiations of male euglossine bee-pollinated plantsMacroevolución de olores florales a través de radiaciones de plantas polinizadas por abejas euglosinas machosMacroevolução dos voláteis florais em radiações de plantas polinizadas por machos de abelhas Euglossini

Floral volatiles play key roles as signaling agents that mediate interactions between plants and animals. Despite their importance, few studies have investigated broad patterns of volatile variation across groups of plants that share pollinators, particularly in a phylogenetic context. The "perfume flowers," Neotropical plant species exhibiting exclusive pollination by male euglossine bees in search of chemical rewards, present an intriguing system to investigate these patterns due to the unique function of their chemical phenotypes as both signaling agents and rewards. We leverage recently developed phylogenies and knowledge of biosynthesis, along with decades of chemical ecology research, to characterize axes of variation in the chemistry of perfume flowers, as well as understand their evolution at finer taxonomic scales. We detect pervasive chemical convergence, with many species across families exhibiting similar volatile phenotypes. Scent profiles of most species are dominated by compounds of either the phenylpropanoid or terpenoid biosynthesis pathways, while terpenoid compounds drive more subtle axes of variation. We find recapitulation of these patterns within two independent radiations of perfume flower orchids, in which we further detect evidence for the rapid evolution of divergent floral chemistries, consistent with the putative importance of scent in the process of adaptation and speciation.

Global warming impairs the olfactory floral signaling in strawberry

BACKGROUND

Global warming is expected to impact the chemical communication between flowering plants and their pollinators. Surprisingly, it is unknown whether and how temperature-induced changes in scent emission affect pollinator behavior. Strawberry (Fragaria x ananassa) is a plant primarily pollinated by bees and hoverflies, with the former group being particularly attracted to the floral scent they emit.

RESULTS

Using chemical analytical, electrophysiological, and behavioral approaches we tested whether temperature-induced shifts in floral scent of strawberry affect chemical communication with its main bee pollinators (Apis mellifera, Bombus terrestris, Osmia bicornis). While strawberry flowers in the optimum scenario released 10.4 ng/flower/hour, mainly p-anisaldehyde (81%) and seven other scent compounds, in the warmer scenario, the flowers did not emit any detectable scent. In the behavioral experiments, the pollinators were attracted by the scents of the optimum scenario.

CONCLUSIONS

We predict that the absence of detectable scent emissions from strawberry plants grown under heat stress will reduce the attractiveness of the flowers to the bee pollinators. Our study raises important ecological and agricultural questions, as decreased attractiveness of flowers to pollinators might potentially lead to insufficient bee pollination, with potential negative consequences for ecosystem functioning and crop yields, particularly in regions reliant on bees as primary pollinators. Given that our study centered on bee pollinators, it is needed to conduct further research to evaluate the impact on hoverflies.

GLV reveal species differences and responses to environment in alpine shrub Rosa sericea complex

Plant Volatile components are an ecological adaptation mechanism of plants that can reflect species differences and environment information where it is located. The alpine shrub Rosa sericea complex consists of several allied species, which are morphologically similar and difficult to distinguish, they are typical distribution along the elevation in the Himalayas and the Transverse Ranges. We selected two typical areas to find that the different species could be distinguished by their "green leaf volatile components" (GLV) composition as well as their geographical location, and it was evident that species with glands had higher sesquiterpene content. Correlation analysis revealed the relation between volatile components and ecology factors (climate factors, soil factors, phyllospheric microorganisms). Our study adds a new perspective and basis for the environmental adaptations of different species in the alpine shrub Rosa sericea complex from a chemical ecology perspective.

Floral volatiles evoke partially similar responses in both florivores and pollinators and are correlated with non-volatile reward chemicals

BACKGROUND

Plants often use floral displays to attract mutualists and prevent antagonist attacks. Chemical displays detectable from a distance include attractive or repellent floral volatile organic compounds (FVOCs). Locally, visitors perceive contact chemicals including nutrients but also deterrent or toxic constituents of pollen and nectar. The FVOC and pollen chemical composition can vary intra- and interspecifically. For certain pollinator and florivore species, responses to these compounds are studied in specific plant systems, yet we lack a synthesis of general patterns comparing these two groups and insights into potential correlations between FVOC and pollen chemodiversity.

SCOPE

We reviewed how FVOCs and non-volatile floral chemical displays, i.e. pollen nutrients and toxins, vary in composition and affect the detection by and behaviour of insect visitors. Moreover, we used meta-analyses to evaluate the detection of and responses to FVOCs by pollinators vs. florivores within the same plant genera. We also tested whether the chemodiversity of FVOCs, pollen nutrients and toxins is correlated, hence mutually informative.

KEY RESULTS

According to available data, florivores could detect more FVOCs than pollinators. Frequently tested FVOCs were often reported as pollinator-attractive and florivore-repellent. Among FVOCs tested on both visitor groups, there was a higher number of attractive than repellent compounds. FVOC and pollen toxin richness were negatively correlated, indicating trade-offs, whereas a marginal positive correlation between the amount of pollen protein and toxin richness was observed.

CONCLUSIONS

Plants face critical trade-offs, because floral chemicals mediate similar information to both mutualists and antagonists, particularly through attractive FVOCs, with fewer repellent FVOCs. Furthermore, florivores might detect more FVOCs, whose richness is correlated with the chemical richness of rewards. Chemodiversity of FVOCs is potentially informative of reward traits. To gain a better understanding of the ecological processes shaping floral chemical displays, more research is needed on floral antagonists of diverse plant species and on the role of floral chemodiversity in visitor responses.

The genetic structuring in pollinating wasps of Ficus hispida in continental Asia

The interaction between figs and fig wasps provides a striking example of obligate brood site pollination mutualism. Monoecious figs, constituting independent radiations in each tropical biome, are present in significant proportions worldwide, but in continental Asia, dioecious figs have diverged into various niches, making the region's assemblage remarkably diverse. However, the reproductive success of figs and fig wasps largely depends on the fig wasp dispersal process. Monoecious fig pollinators in continental Asian tropical rain forests exhibit high gene flow of the plant, while many dioecious fig pollinators have a more restricted gene flow. However, there are limited studies on the genetic structure of dioecious Ficus pollinators that pollinate figs with intermediate gene flow. Here, we used molecular methods to investigate the genetic structure of pollinating wasps of the widely distributed dioecious Ficus hispida in China and Southeast Asia. Sequence data from two gene regions were used: the mitochondrial protein-coding gene cytochrome c oxidase subunit I (COI) and the nuclear 28S genes. Both molecular and morphological results support two fig wasp species at our sampling sites. Our findings suggest that for widely sympatric Ficus species in continental Asia, monoecious figs presenting long gene glow have the fewest fig wasp species, followed by dioecious figs presenting intermediate gene flow, and dioecious figs presenting local gene flow have the most fig wasp species.

Ecological convergence in phytochemistry and flower-insect visitor interactions along an Andean elevation gradient

The diversity of specialized molecules produced by plants radiating along ecological gradients is thought to arise from plants' adaptations to local conditions. Therefore, closely related species growing in similar habitats should phylogenetically converge, or diverge, in response to similar climates, or similar interacting animal communities. We here asked whether closely related species in the genus Haplopappus (Asteraceae) growing within the same elevation bands in the Andes, converged to produce similar floral odors. To do so, we combine untargeted analysis of floral volatile organic compounds with insect olfactory bioassay in congeneric Haplopappus (Asteraceae) species growing within the same elevation bands along the Andean elevational gradient. We then asked whether the outcome of biotic interactions (i.e., pollination vs. seed predation) would also converge across species within the same elevation. We found that flower odors grouped according to their elevational band and that the main floral visitor preferred floral heads from low-elevation band species. Furthermore, the cost-benefit ratio of predated versus fertilized seeds was consistent within elevation bands, but increased with elevation, from 6:1 at low to 8:1 at high elevations. In the light of our findings, we propose that climate and insect community changes along elevation molded a common floral odor blend, best adapted for the local conditions. Moreover, we suggest that at low elevation where floral resources are abundant, the per capita cost of attracting seed predators is diluted, while at high elevation, sparse plants incur a higher herbivory cost per capita. Together, our results suggest that phytochemical convergence may be an important factor driving plant-insect interactions and their ecological outcomes along ecological gradients.

Biosynthesis and the Transcriptional Regulation of Terpenoids in Tea Plants (Camellia sinensis)

Terpenes, especially volatile terpenes, are important components of tea aroma due to their unique scents. They are also widely used in the cosmetic and medical industries. In addition, terpene emission can be induced by herbivory, wounding, light, low temperature, and other stress conditions, leading to plant defense responses and plant-plant interactions. The transcriptional levels of important core genes (including HMGR, DXS, and TPS) involved in terpenoid biosynthesis are up- or downregulated by the MYB, MYC, NAC, ERF, WRKY, and bHLH transcription factors. These regulators can bind to corresponding cis-elements in the promoter regions of the corresponding genes, and some of them interact with other transcription factors to form a complex. Recently, several key terpene synthesis genes and important transcription factors involved in terpene biosynthesis have been isolated and functionally identified from tea plants. In this work, we focus on the research progress on the transcriptional regulation of terpenes in tea plants (Camellia sinensis) and thoroughly detail the biosynthesis of terpene compounds, the terpene biosynthesis-related genes, the transcription factors involved in terpene biosynthesis, and their importance. Furthermore, we review the potential strategies used in studying the specific transcriptional regulation functions of candidate transcription factors that have been discriminated to date.

Comprehensive analysis of floral scent and fatty acids in nectar of Silene nutans through modern analytical gas chromatography techniques

The aim of this work was to show the potential of multidimensional gas chromatography combined with mass spectrometry and suitable chemometrics means based on untargeted and profiling data analysis to strengthen the information provided by floral scent and nectar fatty acids of four genetically differentiated lineages (E1, W1, W2, and W3) of the nocturnal moth-pollinated herb Silene nutans. Volatile organic compounds emitted by flowers were trapped for a total of 42 samples by in-vivo sampling dynamic head space for analysing floral scent by untargeted approach, while 37 samples of nectar were collected for analysing fatty acids through profiling analysis. The resulting data from floral scent analysis were aligned and compared using a tile-based methodology followed by data mining to access high-level information. Based on floral scent and nectar fatty acid results, it was possible to distinguish E1 from the W lineages, and W3 from W1 and W2. This work puts the bases for a larger study aiming to clarify the existence of prezygotic barriers involved in speciation among lineages of S. nutans, and thus the possible implication of different flower scents and nectar compositions in this phenomenon.

Insect exuviae as soil amendment affect flower reflectance and increase flower production and plant volatile emission

Soil composition and herbivory are two environmental factors that can affect plant traits including flower traits, thus potentially affecting plant-pollinator interactions. Importantly, soil composition and herbivory may interact in these effects, with consequences for plant fitness. We assessed the main effects of aboveground insect herbivory and soil amendment with exuviae of three different insect species on visual and olfactory traits of Brassica nigra plants, including interactive effects. We combined various methodological approaches including gas chromatography/mass spectrometry, spectroscopy and machine learning to evaluate changes in flower morphology, colour and the emission of volatile organic compounds (VOCs). Soil amended with insect exuviae increased the total number of flowers per plant and VOC emission, whereas herbivory reduced petal area and VOC emission. Soil amendment and herbivory interacted in their effect on the floral reflectance spectrum of the base part of petals and the emission of 10 VOCs. These findings demonstrate the effects of insect exuviae as soil amendment on plant traits involved in reproduction, with a potential for enhanced reproductive success by increasing the strength of signals attracting pollinators and by mitigating the negative effects of herbivory.

Variation in scent amount but not in composition correlates with pollinator visits within populations of deceptive Arum maculatum L. (Araceae)

Floral scent is vital for pollinator attraction and varies among and within plant species. However, little is known about how inter-individual variation in floral scent affects the abundance and composition of floral visitor assemblages within populations. Moreover, for deceptive plants it is predicted that intra-population variation in scent can be maintained by negative frequency-dependent selection, but empirical evidence is still lacking. To investigate the ecological and evolutionary relations between inter-individual scent variation (i.e., total emission and composition) and floral visitors in deceptive plants, we studied floral scent, visitor assemblages, and fruit set in two populations of fly-pollinated (Psychodidae, Sphaeroceridae; Diptera) and deceptive Arum maculatum from Austria (JOS) and northern Italy (DAO). By correlating individual data on floral scent and visitor assemblages, we show that inter-individual variation in floral scent partly explains variation in visitor assemblages. The quantity of floral scent emitted per individual correlated positively with visitor abundance in both populations but explained visitor composition only in DAO, where strongly scented inflorescences attracted more sphaerocerid flies. However, in each population, the composition of floral scent did not correlate with the composition of floral visitors. There was also no evidence of negative frequency-dependent selection on floral scent. Instead, in JOS, more frequent scent phenotypes attracted more pollinators and were more likely to set an infructescence than rarer ones. Our results show that floral scent, despite being key in pollinator attraction in A. maculatum, only partly explains variation in pollinator abundance and composition. Overall, this study is the first to shed light on the importance of inter-individual variation in floral scent in explaining floral visitor assemblages at the population level in a deceptive plant species.

Floral scent divergence across an elevational hybrid zone with varying pollinators

Divergence in floral traits attractive to different pollinators can promote reproductive isolation in related species. When isolation is incomplete, hybridization may occur, which offers the opportunity to explore mechanisms underlying reproductive isolation. Recent work suggests that divergence in floral scent may frequently contribute to reproductive barriers, although such divergence has seldom been examined in species with generalized pollination. Here, we used two closely related Penstemon species, P. newberryi and P. davidsonii, and their natural hybrids from an elevational gradient with pollinator communities that are predicted to vary in their reliance on floral scent (i.e., primarily hummingbirds at low elevation vs. bees at high elevation). The species vary in a suite of floral traits, but scent is uncharacterized. To address whether scent varies along elevation and potentially contributes to reproductive isolation, we genetically characterized individuals collected at field and identified whether they were parental species or hybrids. We then characterized scent amount and composition. Although the parental species had similar total emissions, some scent characteristics (i.e., scent composition, aromatic emission) diverged between them and may contribute to their isolation. However, the species emitted similar compound sets which could explain hybridization in the contact area. Hybrids were similar to the parents for most scent traits, suggesting that their floral scent would not provide a strong barrier to backcrossing. Our study suggests floral scent may be a trait contributing to species boundaries even in plants with generalized pollination, and reinforces the idea that evolutionary pollinator transitions may involve changes in multiple floral traits.

Electrophysiological and Behavioral Responses of Apis mellifera and Bombusterrestris to Melon Flower Volatiles

As important pollinators, honeybees and bumblebees present a pollination behavior that is influenced by flower volatiles through the olfactory system. In this study, volatile compounds from melon flowers were isolated and identified by headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS), and their effects on Apis mellifera and Bombus terrestris were investigated by electroantennogram (EAG) and behavior tests (Y-tube olfactometer). The results showed that 77 volatile compounds were detected in melon flowers, among which the relative content of aldehydes was the highest (61.34%; 82.09%). A. mellifera showed a strong EAG response to e-2-hexenal, e-2-octenal, and 1-nonanal. B. terrestris showed a strong EAG response to e-2-hexenal, e-2-octenal, 2,5-dimethyl-benzaldehyde, benzaldehyde and benzenepropanal. In behavior tests, the volatiles with the highest attractive rate to A. mellifera were e-2-hexenal (200 μg/μL, 33.33%) and e-2-octenal (300 μg/μL, 33.33%), and those to B. terrestris were e-2-hexenal (10 μg/μL, 53.33%) and 2,5-dimethyl-benzaldehyde (100 μg/μL, 43.33%). E-2-hexenal and e-2-octenal were more attractive to A. mellifera than B. terrestris, respectively (10 μg/μL, 10 μg/μL, 200 μg/μL). In conclusion, the volatiles of melon flowers in facilities have certain effects on the electrophysiology and behavior of bees, which is expected to provide theoretical and technical support for the pollination of A. mellifera and B. terrestris in facilities.

An analytical pipeline to support robust research on the ecology, evolution, and function of floral volatiles

Research on floral volatiles has grown substantially in the last 20 years, which has generated insights into their diversity and prevalence. These studies have paved the way for new research that explores the evolutionary origins and ecological consequences of different types of variation in floral scent, including community-level, functional, and environmentally induced variation. However, to address these types of questions, novel approaches are needed that can handle large sample sizes, provide quality control measures, and make volatile research more transparent and accessible, particularly for scientists without prior experience in this field. Drawing upon a literature review and our own experiences, we present a set of best practices for next-generation research in floral scent. We outline methods for data collection (experimental designs, methods for conducting field collections, analytical chemistry, compound identification) and data analysis (statistical analysis, database integration) that will facilitate the generation and interpretation of quality data. For the intermediate step of data processing, we created the R package bouquet, which provides a data analysis pipeline. The package contains functions that enable users to convert chromatographic peak integrations to a filtered data table that can be used in subsequent statistical analyses. This package includes default settings for filtering out non-floral compounds, including background contamination, based on our best-practice guidelines, but functions and workflows can be easily customized as necessary. Next-generation research into the ecology and evolution of floral scent has the potential to generate broadly relevant insights into how complex traits evolve, their genomic architecture, and their consequences for ecological interactions. In order to fulfill this potential, the methodology of floral scent studies needs to become more transparent and reproducible. By outlining best practices throughout the lifecycle of a project, from experimental design to statistical analysis, and providing an R package that standardizes the data processing pipeline, we provide a resource for new and seasoned researchers in this field and in adjacent fields, where high-throughput and multi-dimensional datasets are common.

Genetic mapping of sexually dimorphic volatile and non-volatile floral secondary chemistry of a dioecious willow

Secondary chemistry often differs between sexes in dioecious plant species, a pattern attributed to its possible role in the evolution and/or maintenance of dioecy. We used GC-MS to measure floral volatiles emitted from, and LC-MS to quantitate non-volatile secondary compounds contained in, female and male Salix purpurea willow catkins from an F2 family. Using the abundance of these chemicals, we then performed quantitative trait locus (QTL) mapping to locate them on the genome, identified biosynthetic candidate genes in the QTL intervals, and examined expression patterns of candidate genes using RNA-seq. Male flowers emitted more total terpenoids than females, but females produced more benzenoids. Male tissue contained greater amounts of phenolic glycosides, but females had more chalcones and flavonoids. A flavonoid pigment and a spermidine derivative were found only in males. Male catkins were almost twice the mass of females. Forty-two QTL were mapped for 25 chemical traits and catkin mass across 16 of the 19 S. purpurea chromosomes. Several candidate genes were identified, including a chalcone isomerase associated with seven compounds. A better understanding of the genetic basis of the sexually dimorphic chemistry of a dioecious species may shed light on how chemically mediated ecological interactions may have helped in the evolution and maintenance of dioecy.

Population-specific responses of floral volatiles to abiotic factors in changing environments

PREMISE

Shifts in abiotic factors can affect many plant traits, including floral volatiles. This study examined the response of floral volatiles to water availability and whether phenotypic plasticity to water availability differs among populations. It also investigated genetic differentiation in floral volatiles, determined the effect of temperature on phenotypic plasticity to water availability, and assessed temporal variation in floral scent emission between day and evening, since pollinator visitation differs at those times.

METHODS

Rocky Mountain columbine plants (Aquilegia coerulea), started from seeds collected in three wild populations in Colorado, Utah, and Arizona, were grown under two water treatments in a greenhouse in Madison, Wisconsin, United States. One population was also grown under the two water treatments, at two temperatures. Air samples were collected from enclosed flowers using dynamic headspace methods and floral volatiles were identified and quantified by gas chromatography (GC) with mass spectrometry (MS).

RESULTS

Emission of three floral volatiles increased in the wetter environment, indicating phenotypic plasticity. The response of six floral volatiles to water availability differed among populations, suggesting genetic differentiation in phenotypic plasticity. Five floral volatiles varied among populations, and emission of most floral volatiles was greater during the day.

CONCLUSIONS

Phenotypic plasticity to water availability permits a quick response of floral volatiles in changing environments. The genetic differentiation in phenotypic plasticity suggests that phenotypic plasticity can evolve but complicates predictions of the effects of environmental changes on a plant and its pollinators.

Water stress and nitrogen supply affect floral traits and pollination of the white mustard, Sinapis alba (Brassicaceae)

Changes in environmental conditions are likely to have a complex effect on the growth of plants, their phenology, plant-pollinator interactions, and reproductive success. The current world is facing an ongoing climate change along with other human-induced environmental changes. Most research has focused on the impact of increasing temperature as a major driving force for climate change, but other factors may have important impacts on plant traits and pollination too and these effects may vary from season to season. In addition, it is likely that the effects of multiple environmental factors, such as increasing temperature, water availability, and nitrogen enrichment are not independent. Therefore, we tested the impact of two key factors-water, and nitrogen supply-on plant traits, pollination, and seed production in Sinapis alba (Brassicaceae) in three seasons defined as three temperature conditions with two levels of water and nitrogen supply in a factorial design. We collected data on multiple vegetative and floral traits and assessed the response of pollinators in the field. Additionally, we evaluated the effect of growing conditions on seed set in plants exposed to pollinators and in hand-pollinated plants. Our results show that water stress impaired vegetative growth, decreased flower production, and reduced visitation by pollinators and seed set, while high amount of nitrogen increased nectar production under low water availability in plants grown in the spring. Temperature modulated the effect of water and nitrogen availability on vegetative and floral traits and strongly affected flowering phenology and flower production. We demonstrated that changes in water and nitrogen availability alter plant vegetative and floral traits, which impacts flower visitation and consequently plant reproduction. We conclude that ongoing environmental changes such as increasing temperature, altered precipitation regimes and nitrogen enrichment may thus affect plant-pollinator interactions with negative consequences for the reproduction of wild plants and insect-pollinated crops.

Variation in floral volatiles across time, sexes, and populations of wind-pollinated Schiedea globosa

PREMISE

Floral scent is a key aspect of plant reproduction, but its intraspecific variation at multiple scales is poorly understood. Sexual dimorphism and temporal regulation of scent can be shaped by evolution, and interpopulation variation may be a bridge to species differences. We tested whether intraspecific chemical diversity in a wind-pollinated species where selection from biotic pollination is absent is associated with genetic divergence across the Hawaiian archipelago.

METHODS

Floral volatiles from females, males, and hermaphrodites of subdioecious Schiedea globosa grown in a common environment from 12 populations were sampled day and night and analyzed by gas chromatography-mass spectrometry. Variation among groups was analyzed by constrained ordination. We also examined the relationships of scent dissimilarity to geographic and genetic distance between populations.

RESULTS

Flowers increased total emissions at night through higher emissions of several ketones, oximes, and phenylacetaldehyde. Females emitted less total scent per flower at night but more of some aliphatic compounds than males, and males emitted more ketones and aldoximes. Scent differed among populations during day and night. Divergence in scent produced at night increased with geographic distance within 70-100 km and increased with genetic distance for males during the day and night, but not for females.

CONCLUSIONS

Schiedea globosa exhibits diel and sex-based variation in floral scent despite wind pollination and presumed loss of biotic pollination. In males, interpopulation scent differences are correlated with genetic differences, suggesting that scent evolved with dispersal within and across islands.

Effects of elevated ozone on the emission of volatile isoprenoids from flowers and leaves of rose (Rosa sp.) varieties

Tropospheric ozone (O₃) affects isoprenoid emissions, and floral emissions in particular, which may result in potential impacts on the interactions of plants with other organisms. The effects of ozone (O₃) on isoprenoid emissions have been investigated for many years, while knowledge on O₃ effects on floral emissions is still scarce and the relevant mechanism has not been clarified so far. We investigated the effects of O₃ on floral and foliar isoprenoid emissions (mainly isoprene, monoterpenes and sesquiterpenes) and their synthase substrates from three rose varieties (CH, Rosa chinensis Jacq. var. chinensis; SA, R. hybrida 'Saiun'; MO, R. hybrida 'Monica Bellucci') at different exposure durations. Results indicated that the O₃-induced stimulation after short-term exposure (35 days after the beginning of O₃ exposure) was significant only for sesquiterpene emissions from flowers, while long-term O₃ exposure (90 days after the beginning of O₃ exposure) significantly decreased both foliar and floral monoterpene and sesquiterpene emissions. In addition, the observed decline of emissions under long-term O₃ exposure resulted from the limitation of synthase substrates, and the responses of emissions and substrates varied among varieties, with the greatest variation in the O₃-sensitive variety. These findings provide important insights on plant isoprenoid emissions and species selection for landscaping, especially in areas with high O₃ concentration.

A meta-analysis of responses in floral traits and flower-visitor interactions to water deficit

Alterations in water availability and drought events as predicted by climate change scenarios will increasingly impact natural communities with effects already emerging at present. Water deficit leads to increasing physiological stress in plants, likely affecting floral development and causing changes in floral morphology, nectar and pollen production or scent. Understanding how these floral traits are altered by water deficit is necessary to predict changes in plant-pollinator interactions and how communities are impacted in the future. Here we employ a meta-analysis approach to synthesize the current evidence of experimental water deficit on floral traits and plant-pollinator interactions. Furthermore, we explore experimental factors potentially increasing heterogeneity between studies and provide ideas how to enhance comparability between studies. In the end, we highlight future directions and knowledge gaps for floral traits and plant-pollinator interactions under water deficit. Our analysis showed consistent decreases in floral size, number of flowers and nectar volume to reduced water availability. Other floral traits such as the start of flowering or herkogamy showed no consistent pattern. This indicates that effects of reduced water availability differ between specific traits that are potentially involved in different functions such as pollinator attraction or efficiency. We found no general decreasing visitation rates with water deficit for flower-visitor interactions. Furthermore, the comparison of available studies suggests that increased reporting of plant stress severity and including more hydraulic and physiological measurements will improve the comparability across experiments and aid a more mechanistic understanding of plant-pollinator interactions under altered environmental conditions. Overall, our results show that water deficit has the potential to strongly affect plant-pollinator interactions via changes in specific floral traits. Linking these changes to pollination services and pollinator performance is one crucial step for understanding how changing water availability and drought events under climate change will alter plant and pollinator communities.

Flower Production, Headspace Volatiles, Pollen Nutrients, and Florivory in Tanacetum vulgare Chemotypes

Floral volatiles and reward traits are major drivers for the behavior of mutualistic as well as antagonistic flower visitors, i.e., pollinators and florivores. These floral traits differ tremendously between species, but intraspecific differences and their consequences on organism interactions remain largely unknown. Floral volatile compounds, such as terpenoids, function as cues to advertise rewards to pollinators, but should at the same time also repel florivores. The reward composition, e.g., protein and lipid contents in pollen, differs between individuals of distinct plant families. Whether the nutritional value of rewards within the same plant species is linked to their chemotypes, which differ in their pattern of specialized metabolites, has yet not been investigated. In the present study, we compared Tanacetum vulgare plants of five terpenoid chemotypes with regard to flower production, floral headspace volatiles, pollen macronutrient and terpenoid content, and floral attractiveness to florivorous beetles. Our analyses revealed remarkable differences between the chemotypes in the amount and diameter of flower heads, duration of bloom period, and pollen nutritional quality. The floral headspace composition of pollen-producing mature flowers, but not of premature flowers, was correlated to that of pollen and leaves in the same plant individual. For two chemotypes, florivorous beetles discriminated between the scent of mature and premature flower heads and preferred the latter. In semi-field experiments, the abundance of florivorous beetles and flower tissue miners differed between T. vulgare chemotypes. Moreover, the scent environment affected the choice and beetles were more abundant in homogenous plots composed of one single chemotype than in plots with different neighboring chemotypes. In conclusion, flower production, floral metabolic composition and pollen quality varied to a remarkable extend within the species T. vulgare, and the attractiveness of floral scent differed also intra-individually with floral ontogeny. We found evidence for a trade-off between pollen lipid content and pollen amount on a per-plant-level. Our study highlights that chemotypes which are more susceptible to florivory are less attacked when they grow in the neighborhood of other chemotypes and thus gain a benefit from high overall chemodiversity.

The neuroecology of insect-plant interactions: the importance of physiological state and sensory integration

Natural behaviorally important stimuli are combinations of cues that are integrated by the nervous system to elicit behavior. Nonetheless, these cues dynamically change in time and space. In turn, the animal's internal state can cause changes in the encoding and representation of these stimuli. Despite abundant behavioral examples, links between the neural bases of sensory integration and the internal state-dependency of these responses remains an active study area. Recent studies in different insect models have provided new insights into how plasticity and the insect's internal state may influence odor representation. These studies show that complex stimuli are represented in unique percepts that are different from their sensory channels and that the representations may be modulated by physiological state.

Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polyploidy

Volatile organic compounds (VOCs) are emitted by plants as a consequence of their interaction with biotic and abiotic factors, and have a very important role in plant evolution. Floral VOCs are often involved in defense and pollinator attraction. These interactions often change rapidly over time, so a quick response to those changes is required. Epigenetic factors, such as DNA methylation and histone modification, which regulate both genes and transcription factors, might trigger adaptive responses to these evolutionary pressures as well as regulating the rhythmic emission of VOCs through circadian clock regulation. In addition, transgenerational epigenetic effects and whole genome polyploidy could modify the generation of VOCs’ profiles of offspring, contributing to long-term evolutionary shifts. In this article, we review the available knowledge about the mechanisms that may act as epigenetic regulators of the main VOC biosynthetic pathways, and their importance in plant evolution.