Pollination biology in the dioecious orchid Catasetum uncatum: How does floral scent influence the behaviour of pollinators?

From Leaves to Reproductive Organs: Chemodiversity and Chemophenetics of Essential Oils as Important Tools to Evaluate Piper mollicomum Kunth Chemical Ecology Relevance in the Neotropics

Piper mollicomum Kunth (Piperaceae) plays a vital role in the preservation of the Brazilian Atlantic Forest by contributing to the regeneration of deforested areas. Recent scientific investigations have analyzed the chemical constituents and seasonal dynamics of essential oils (EO) from various Piper L. species, highlighting the need to elucidate their chemical-ecological interactions. This study aims to expand the chemical-ecological knowledge of this important taxon in neotropical forests, using P. mollicomum as a model. The methodologies employed include the collection of plant material, EO extraction by hydrodistillation, analysis of EO by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detector (GC-FID), recording the frequency of visits by potential pollinators and microclimatic variables, and by conducting calculations of chemodiversity and chemophenetic indices. Chemical analyses indicated that the diversity of EO and environmental factors are linked to the activities of potential pollinators. In the Tijuca Forest, P. mollicomum revealed significant interactions between its volatile constituents and microclimatic variables, showing that the chemodiversity of the leaves and reproductive organs correlates with pollinator visitation. Additionally, a notable difference in chemical evenness was observed between these vegetative structures. The chemophenetic indices by Ramos and Moreira also revealed correlations with chemical diversity.

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.

To be or not to be fragrant: floral scent of some bat-pollinated cacti

Floral scent is a key olfactory cue in both diurnal and nocturnal pollination systems. In the case of nocturnal systems, such as bat-pollinated flowers, odour seems to play a more important role than visual cues. Cactaceae include many bat-pollinated species; however, few studies have investigated the olfactory cues in this family. We analysed and compared the chemical composition of the floral bouquet of three chiropterophilous cactus species, among which are a pair of congeners that differ considerably in scent intensity. Our research presents novel findings regarding the floral scent chemistry of chiropterophilous cactus species. We documented the first case of a bat-pollinated cactus whose flowers lack perceptible floral scent and in which no volatile compounds were detected in our chemical analyses. Additionally, we provide a comprehensive analysis of the chemical composition of the floral bouquet of the other two bat-pollinated species, revealing a resemblance among closely related species within the same genus. We highlight the need for further studies using biotests to investigate the mechanisms through which bats find flowers lacking scent.

Volatile Constituents from Catasetum (Orchidaceae) Species with Occurrence in the Brazilian Amazon

BACKGROUND

Catasetum Rich. ex Kunth is a genus of Neotropical orchids distributed in Central and South American regions. In the Brazilian Amazon, there are more than 60 species of Catasetum. The floral aromas of orchids are little known, particularly of Catasetum species. This work aimed to analyze the chemical constituents of the volatile concentrates of eight Catasetum specimens from the Amazon: C. alatum (1), C. albovirens (2), C. barbatum (1), C. ciliatum (2), C. galeritum (1), and C. gnomus (1).

METHODS

Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyzed and identified the constituents of the volatile concentrates, and principal component analysis (PCA) and hierarchical cluster analysis (HCA) were used in the multivariate statistical analysis.

RESULTS

The Catasetum main constituents in descending order and above 10% were trans-geranylgeraniol, 1,4-dimethoxybenzene, linalool, 2-phenylethyl acetate, geraniol, 7-epi-1,2-dehydro-sesquicineole, 1,8-cineole, benzyl acetate, limonene, methyl salicylate, (E)-β-farnesene, anisyl butyrate, cis-carvone oxide, cadin-4-en-10-ol, indole, α-pinene, and δ-cadinene.

CONCLUSIONS

Multivariate statistical analysis of Catasetum species showed that C. barbatum, C. albovirens, and C. gnomus are distinct from the other studied species, while C. alatum, C. ciliatum, and C. galeritum presented the same primary classes of compounds. These results contribute to a better understanding of the genus Catasetum chemotaxonomy.

The volatile chemistry of orchid pollination

Covering: up to September 2022Orchids are renowned not only for their diversity of floral forms, but also for their many and often highly specialised pollination strategies. Volatile semiochemicals play a crucial role in the attraction of a wide variety of insect pollinators of orchids. The compounds produced by orchid flowers are as diverse as the pollinators they attract, and here we summarise some of the chemical diversity found across orchid taxa and pollination strategies. We focus on compounds that have been experimentally demonstrated to underpin pollinator attraction. We also highlight the structural elucidation and synthesis of a select subset of important orchid pollinator attractants, and discuss the ecological significance of the discoveries, the gaps in our current knowledge of orchid pollination chemistry, and some opportunities for future research in this field.

Floral Scent Chemistry and Pollinators of a Sexually Dimorphic Neotropical Orchid

Catasetum is a speciose Neotropical orchid genus of which male and female flowers emit scents acting both as attractant and reward for their exclusive pollinators, male orchid bees (Euglossini: Apidae). In Catasetum, it is well known that flowers display a remarkably morphological sexual dimorphism. However, it remains poorly investigated whether this is also true for floral scents. Here, we investigated the pollination ecology and floral scent traits (chemistry and total emission) of C. maranhense, a species endemic to the Brazilian N/NE region. Males of Euglossa securigera are the only pollinators of C. maranhense. The floral scent of C. maranhense is composed of 29 volatile compounds, with eucalyptol, indole, (E)-Methyl p-methoxycinnamate, and (Z)-Methyl p-methoxycinnamate accounting for more than 80% of the scent bouquet. No sexual dimorphism was detected in any of the traits investigated. We discuss the ecological and evolutionary significance of our findings to Catasetum species and other unisexual perfume plants.

Anatomy and ultrastructure floral osmophores of Catasetum fimbriatum (Orchidaceae)

Catasetum fimbriatum is a dioecious species whose flowers fully adapted to an euglossinophilic mode of pollination. Euglossini male bees collect the volatile fragrances which are produced in osmophores on the flowers. In order to understand the mechanism of scent secretion and floral interaction with the pollinator, we describe the location, histochemistry, anatomy, and ultrastructure of osmophores in pistillate and staminate flowers of this species. Fresh flowers were submerged in neutral red solution to locate the position of the osmophores. Other histochemical test performed includes the NADI reaction to detect terpenoids, Sudan IV for lipids, and Lugol's iodine solution to detect starch. Anatomical and ultrastructural traits were studied with bright field and transmission electron microscopes, respectively. The location of osmophores differs between pistillate and staminate flowers. In pistillate flowers, secretory tissues were observed on the ribbed adaxial surface of the labellum, but not on its margins, whereas in staminate flowers, they were found throughout the adaxial surface of the labellum and especially in the fimbriae. Anatomy and ultrastructure of the osmophores in the labellum of both types of flowers were similar. They present characteristics of metabolically active cells, such as abundant mitochondria, rough endoplasmic reticulum, vesicles, plastids with starch grains, and lipid globules. Granulocrine secretion and cycles of cytoplasmic contraction and expansion appear to allow the release of products without involving the rupture of the cuticle. Individuals of Eufriesea auriceps and Euglossa sp. were captured in staminate and pistillate flowers but, it seems likely, that only the former pollinates this orchid species.

Pollination of the strongly scented Sarcoglottis acaulis (Orchidaceae) by male orchid bees: nectar as resource instead of perfume

The Neotropical orchid genus Sarcoglottis comprises ~40 species, which emit strong floral scents, presumably involved in pollinator attraction. Information on basic aspects of its natural history is scant, with the few studies available so far pointing to nectar-seeking orchid bees as pollinators. Here, we investigated the reproductive biology of Sarcoglottis acaulis, addressing the ecological meaning of its floral scent. In Atlantic Forest fragments of NE Brazil, we described the floral biology, determined the breeding system and recorded the pollinators of S. acaulis. Additionally, we chemically characterized its floral scent and assessed its role on pollinator attraction. Although self-compatible, S. acaulis depends on pollinators to boost fruit set. Male orchid bees of Eulaema atleticana and E. niveofasciata were the only recorded pollinators. They foraged for nectar only, in spite of the strong scent emitted by S. acaulis flowers. The floral scent is composed of six compounds, of which geraniol and nerol elicited electroantennographic responses in Eulaema bees. A synthetic mixture of these compounds attracted Eulaema bees in field assays but did not trigger the stereotyped scent-gathering behaviour. The floral scent of S. acaulis acts in signalling and nectar is the sole reward for pollinators. Despite the low pollinator frequency, S. acaulis shows a high fruit set (77%), particularly when compared to other orchids. Attributes such as pollinia that release small pollen loads, allowing a single pollinia-carrying bee to pollinate several flowers, low amount of nectar, steady-state flowering and traplining behaviour of pollinators, might act together to assure this outstanding fruit set.

Occurrence and Prevention of Delayed Autonomous Selfing in Salvia umbratica (Lamiaceae)

Delayed autonomous selfing (DAS) provides reproductive assurance under conditions of pollinator and/or pollen-limitation. Few plant species have been investigated to determine if DAS is terminated when a flower is sufficiently pollinated by a pollen vector, thereby saving plant resources for other purposes. We examined this possibility in bumblebee-pollinated Salvia umbratica. We first showed that DAS resulting in high fruit set (100%) and seed set (>80%) per flower occurred in the absence of insect pollinators by means of style recurvature and was completed in 94% of flowers 72 h after they opened. In contrast, in flowers pollinated immediately after opening, DAS was prevented by corollas dropping away before styles recurve toward the upper thecae. We next showed that hand-pollination of flowers immediately after they opened resulted in high fruit set (100%) and seed set (>80%) when 5-10 pollen grains or more were deposited on their stigmas, whereas fruit set and seed set were reduced to 45.00 and 22.50%, respectively, when pollen loads were reduced to 1-3 pollen grains. Finally, we showed that on average single pollinator visits deposited 26 pollen grains on stigmas of flowers that had just opened, which is more than enough to ensure high fruit and seed set. Our results indicate that flower longevity is highly correlated with the pollinator environment and female fitness of S. umbratica, with extended flower longevity allowing DAS to occur being advantageous when pollinators are absent, while reduced floral longevity and prevention of DAS being favored when flowers are pollinated by pollinators. Thus, flower longevity in S. umbratica varies so as to optimize reproductive output and resource efforts, and is dependent on the availability and effectiveness of pollinators to pollinate flowers.

Combining biotechnology and evolution for understanding the mechanisms of pollinator attraction

Many flowering plants rely on pollinators for their reproductive success. Plant-pollinator interactions usually depend on a complex combination of traits based on a fine-tuned biosynthetic machinery, with many structural and regulatory genes involved. Yet, the physiological mechanisms in plants are the product of evolutionary processes. While evolution has been modifying flowers through millions of years, it is also a rapid process that can change plant traits within few generations. Here we discuss both mechanistic and evolutionary aspects of pollinator attraction. We also propose how latest advances in biotechnology and evolutionary studies, and their combination, will improve the elucidation of molecular mechanisms and evolutionary dynamics of pollinator attraction in changing environments.

Storage of orchid pollinia with varying lipid thermal fingerprints

Orchid pollinia have the potential to make a valuable contribution to current techniques of germplasm storage and assisted reproduction, yet information regarding their preservation and their ability to remain viable over time is currently limited. Dactylorhiza fuchsii and Disa uniflora were used as models for investigating potential techniques for storing orchid pollinia. Initially, freshly harvested pollinia of Dact. fuchsii were incubated at 25 °C and 100% RH (relative humidity) for up to 7 days and germinated in vitro. For pollinia from both species, moisture sorption isotherms were constructed and thermal fingerprints generated using differential scanning calorimetry (DSC). Pollinia were stored at three temperatures (5, - 18 and - 196 °C) after equilibration at four different RHs (5, 33, 50 and 75%) and germinated. The isotherms and DSC results varied between species. Compared with D. uniflora, pollinia of Dact. fuchsii consistently equilibrated at higher moisture content (MC) for each RH, had less detectable lipids by DSC and had shorter lifespans, remaining viable after 3-4 months only at - 20 and - 196 °C and at low RH (5 and 33%). Both species' pollinia stored well at - 20 °C and - 196 °C, although there was some evidence of a small loss of viability under cryopreservation. In conclusion, pollen of these two species can be stored successfully for at least 3-4 months, and to maximize the pre-storage quality, it is recommended that fresh pollen is collected from flowers just prior to anthesis.

Pollination ecology of the Neotropical gesneriad Gloxinia perennis: chemical composition and temporal fluctuation of floral perfume

Although common among orchids, pollination by perfume-gathering male euglossine bees is quite rare in other Neotropical families. In Gesneriaceae, for example, it is reported in two genera only, Drymonia and Gloxinia. Flowers of G. perennis are known to emit perfume, thereby attracting male euglossine bees as pollinators. However, detailed reports on the pollination ecology, as well as on chemistry of floral perfume of individuals in natural populations, are still missing. In this study, we report on the pollination ecology of G. perennis, focusing on the ecological significance of its floral perfume. In natural populations in Peru, we documented the floral biology and breeding system of G. perennis, as well as its interaction with flower visitors. We also characterised the chemical composition of floral perfume, as well as its timing of emission. Gloxinia perennis is self-compatible and natural pollination success is high. Spontaneous self-pollination occurs as a 'just in case strategy' when pollinators are scarce. Perfume-collecting males of Eulaema cingulata and El. meriana were identified as pollinators. The perfume bouquet of G. perennis consists of 16 compounds. (E)-Carvone epoxide (41%) and limonene (23%) are the major constituents. Perfume emission is higher at 09:00 h, matching the activity peak of Eulaema pollinators. Flowers of G. perennis have evolved a mixed strategy to ensure pollination (i.e. self- and cross-pollination), but cross-pollination is favoured. The size and behaviour of Eulaema males enables only these bees to successfully cross-pollinate G. perennis. Furthermore, G. perennis floral perfume traits (i.e. chemistry and timing of emission) have evolved to optimise the attraction of these bees.

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.

Ethnobotany of dioecious species: Traditional knowledge on dioecious plants in India

ETHNOPHARMACOLOGICAL RELEVANCE

More than 15,000 angiosperm species are dioecious, i.e., having distinct male and female individual plants. The allocation of resources between male and female plants is different, and also variation in secondary metabolites and sex-biased herbivory is reported among dioecious plants. However, little is known about the ethnobotany of dioecious species and whether preferences exist for a specific gender, e.g., in food, medicine or timber.

AIM OF THE STUDY

The aim of this study was: 1) to study whether Indian folk healers have preference for plant genders, and to document their knowledge and use of dioecious species; 2) to understand the concept of plant gender in Indian systems of medicine and folk medicine, and whether Ayurvedic literature includes any references to gender preference.

MATERIALS AND METHODS

Lists of dioecious plants used in Indian systems of medicine and folk medicine were compiled. Ethnobotanical data was collected on perceptions and awareness of dioecious plants, and preferences of use for specific genders of dioecious species using semi-structured interviews with folk healers in Tamil Nadu, India. In addition, twenty Ayurvedic doctors were interviewed to gain insight into the concept of plant gender in Ayurveda.

RESULTS

Indian systems of medicine contain 5-7% dioecious species, and this estimate is congruent with the number of dioecious species in flowering plants in general. Informants recognized the phenomenon of dioecy in 31 out of 40 species, and reported gender preferences for 13 species with respect to uses as timber, food and medicine. Among informants different plant traits such as plant size, fruit size, and visibility of fruits determines the perception of a plant being a male or female. Ayurvedic classical literature provides no straightforward evidence on gender preferences in preparation of medicines or treatment of illness, however it contains details about reproductive morphology and sexual differentiation of plants.

CONCLUSIONS

A knowledge gap exists in ethnobotanical and ethnopharmacological literature on traditional knowledge of dioecious plants. From this explorative study it is evident that people have traditional knowledge on plant gender and preferential usages towards one gender. Based on this, we propose that researchers conducting ethnobotanical and ethnopharmacological studies should consider documenting traditional knowledge on sexual systems of plants, and test the existence of gender specific usages in their conceptual framework and hypothesis testing. Incorporating such concepts could provide new dimensions of scientific knowledge with potential implications to conservation biology, chemical ecology, ethnoecology and drug discovery.

Olfactory specialization for perfume collection in male orchid bees

Insects rely on the olfactory system to detect a vast diversity of airborne molecules in their environment. Highly sensitive olfactory tuning is expected to evolve when detection of a particular chemical with great precision is required in the context of foraging and/or finding mates. Male neotropical orchid bees (Euglossini) collect odoriferous substances from multiple sources, store them in specialized tibial pouches and later expose them at display sites, presumably as mating signals to females. Previous analysis of tibial compounds among sympatric species revealed substantial chemical disparity in chemical composition among lineages with outstanding divergence between closely related species. Here, we tested whether specific perfume phenotypes coevolve with matching olfactory adaptations in male orchid bees to facilitate the location and harvest of species-specific perfume compounds. We conducted electroantennographic (EAG) measurements on males of 15 sympatric species in the genus Euglossa that were stimulated with 18 compounds present in variable proportions in male hind tibiae. Antennal response profiles were species-specific across all 15 species, but there was no conspicuous differentiation between closely related species. Instead, we found that the observed variation in EAG activity follows a Brownian motion model of trait evolution, where the probability of differentiation increases proportionally with lineage divergence time. However, we identified strong antennal responses for some chemicals that are present as major compounds in the perfume of the same species, thus suggesting that sensory specialization has occurred within multiple lineages. This sensory specialization was particularly apparent for semi-volatile molecules ('base note' compounds), thus supporting the idea that such compounds play an important role in chemical signaling of euglossine bees. Overall, our study found no close correspondence between antennal responses and behavioral preferences/tibial contents, but confirms the utility of EAG profiling for discovering certain behaviorally active compounds.

A volatolomic approach for studying plant variability: the case of selected Helichrysum species (Asteraceae)

The species of Helichrysum sect. Stoechadina (Asteraceae) are well-known for their secondary metabolite content and the characteristic aromatic bouquets. In the wild, populations exhibit a wide phenotypic plasticity which makes critical the circumscription of species and infraspecific ranks. Previous investigations on Helichrysum italicum complex focused on a possible phytochemical typification based on hydrodistilled essential oils. Aims of this paper are three-fold: (i) characterizing the volatile profiles of different populations, testing (ii) how these profiles vary across populations and (iii) how the phytochemical diversity may contribute in solving taxonomic problems. Nine selected Helichrysum populations, included within the H. italicum complex, Helichrysum litoreum and Helichrysum stoechas, were investigated. H. stoechas was chosen as outgroup for validating the method. After collection in the wild, plants were cultivated in standard growing conditions for over one year. Annual leafy shoots were screened in the post-blooming period for the emissions of volatile organic compounds (VOCs) by means of headspace solid phase microextraction coupled with gas-chromatography and mass spectrometry (HS-SPME-GC/MS). The VOC composition analysis revealed the production of overall 386 different compounds, with terpenes being the most represented compound class. Statistical data processing allowed the identification of the indicator compounds that differentiate the single populations, revealing the influence of the geographical provenance area in determining the volatile profiles. These results suggested the potential use of VOCs as valuable diacritical characters in discriminating the Helichrysum populations. In addition, the cross-validation analysis hinted the potentiality of this volatolomic study in the discrimination of the Helichrysum species and subspecies, highlighting a general congruence with the current taxonomic treatment of the genus. The consistency between this phytochemical approach and the traditional morphometrical analysis in studying the Helichrysum populations supports the validity of the VOC profile in solving taxonomic problems.

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

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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.