The importance of key floral bioactive compounds to honey bees for the detection and attraction of hybrid vegetable crops and increased seed yield

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

Critical Pollination Chemistry: Specific Sesquiterpene Floral Volatiles in Carrot Inhibit Honey Bee Feeding

Many plants rely on insect pollination, yet numerous agricultural plant-breeding programs focus on traits that appeal to growers and consumers instead of pollinators, leading to declining pollinator attraction and crop yields. Using hybrid carrot seed production as a model, we investigated low-yielding carrot varieties by analyzing sugars and minerals in nectar and floral volatile composition. While the analysis of nectar sugars and minerals did not reveal any key differences between the carrot varieties, differences between the 112 detected volatiles in 23 samples were observed. Numerous differentiating sesquiterpenes were identified in floral solvent extracts, and subsequent behavioral assays showed that β-ocimene from higher-yielding carrot varieties stimulated nectar feeding (attractant), while α- and β-selinene from lower-yielding lines decreased feeding (deterrents). Sesquiterpenes have previously been implicated in plant defense, suggesting a trade-off between pollination and protection. Our results highlight the importance of volatiles as regulators of pollinator attraction in agricultural settings.

Deciphering Plant-Insect-Microorganism Signals for Sustainable Crop Production

Agricultural crop productivity relies on the application of chemical pesticides to reduce pest and pathogen damage. However, chemical pesticides also pose a range of ecological, environmental and economic penalties. This includes the development of pesticide resistance by insect pests and pathogens, rendering pesticides less effective. Alternative sustainable crop protection tools should therefore be considered. Semiochemicals are signalling molecules produced by organisms, including plants, microbes, and animals, which cause behavioural or developmental changes in receiving organisms. Manipulating semiochemicals could provide a more sustainable approach to the management of insect pests and pathogens across crops. Here, we review the role of semiochemicals in the interaction between plants, insects and microbes, including examples of how they have been applied to agricultural systems. We highlight future research priorities to be considered for semiochemicals to be credible alternatives to the application of chemical pesticides.

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.

Tropospheric Ozone Alters the Chemical Signal Emitted by an Emblematic Plant of the Mediterranean Region: The True Lavender (Lavandula angustifolia Mill.)

Among air pollutants, tropospheric ozone (O3) is one of the most stressful for organisms due to its strong oxidative potential. For instance, high ozone concentration ([O3]) has the potential to affect (i) the emission of volatile organic compounds (VOCs) by plants and (ii) the lifetime of these VOCs in the atmosphere, and consequently disturb crucial signals in the interactions between plants and other organisms. However, despite the determinant role of VOCs emitted by flowers for pollinator attraction, a very limited number of studies have investigated the impact of O3 on floral VOCs. In this study, we investigated the effect of high [O3] episodes on the VOCs emitted by a flowering Mediterranean plant: the true lavender (Lavandula angustifolia Mill., Lamiaceae). To do so, in controlled conditions, we exposed (i) the entire plant to high but realistic [O3] (200 ppb for 5 h) and (ii) only the VOCs emitted by lavender to increasing [O3] (0, 40, 80, 120, and 200 ppb). We sampled VOCs of lavender in both conditions and analyzed them by Gas Chromatography-Mass Spectrometry in order to qualify and quantify the flowering lavender’s emissions and the reaction of VOCs with O3 in the atmosphere. Our results showed that exposure to high [O3] during a short period (5 h) did not affect the emission of VOCs by flowering lavender. Incidentally, we also showed that the chemical signal varied in quantities and proportions over the day. Moreover, we showed that after their emission by the plant, composition of the VOCs changed quantitatively and qualitatively in an atmosphere containing [O3] naturally observed nowadays. Quantities of several of the major terpenes emitted by lavender decreased drastically during O3 exposure, whereas concentrations of some VOCs increased, such as carbonyls and carboxylic acids, which are probably reaction products of terpenes with O3. Exposure to high [O3] thus directly affected the proportions of VOCs in the atmosphere. Because pollinators generally use a blend of VOCs in particular proportions as a signal to localize flowers, the numerous pollinators of lavender may experience difficulty in recognizing specific floral odors during frequent and moderate [O3] episodes in the Mediterranean region.

It Is Not All About Being Sweet: Differences in Floral Traits and Insect Visitation among Hybrid Carrot Cultivars

Cytoplasmically male-sterile (CMS) carrot cultivars suffer from low pollination rates. In this study, insect visitation varied more than eightfold between 17 CMS carrot cultivars in a field-based cultivar evaluation trial. The visitation rates of honey bees, nectar scarabs, muscoid flies, and wasps each significantly differed among these cultivars. No significant difference in visitation rates was observed among cultivars of different CMS type (brown-anther or petaloid) or flower colour, but cultivars of Berlicumer root type had significantly higher insect visitation rates than Nantes. Six cultivars were further compared in regard to selected umbel traits: as umbel diameter increased, so did the visitation of soldier beetles, while that of honey bees decreased. Finally, nectar of these six cultivars was analysed for sugar content, which revealed monosaccharides to be the most common sugars in all. There was high variation in the levels of sugars from individual umbellets but no significant difference in nectar sugar composition among cultivars, suggesting that nectar sugar composition is of minor importance regarding pollinator attraction to hybrid CMS carrot umbels.

The Scent of Individual Foraging Bees

Honey bees (Apis mellifera) forage by using their sense of smell and returning to floral odours that they have previously learned to associate with high-quality food rewards. Foraging bees communicate with other bees in the hive about food sources by exchanging chemical and locational information. It is well established that bees transfer non-volatile information regarding taste and quality of nectar via trophallaxis and communicate location information via directional dances. But to our knowledge, volatiles carried by returning forager bees on their bodies has not been explored as another source of chemical information. We investigated the cuticular-adsorbed odours of bees when foraging on three different crops and compared their odours with the crops' flower headspace. We found that cuticular extracts were in majority correlated with the flower headspace where bees were foraging, specific to the crop and field. Our results support the hypothesis that the scent of returning forager bees can be communicated to hivemates and is associated with information about current floral resources. Some of the floral volatiles that we identified in bee extracts had been previously found to be key compounds learned from the crop, thus supporting a mechanism for the selection of decisive compounds.

Selection of key floral scent compounds from fruit and vegetable crops by honey bees depends on sensory capacity and experience

Flowers have complex odours often comprising hundreds of volatile compounds. Floral scents are species-specific, and vary also among populations, varieties, sexes or lines, as well as with phenology. Honey bees, Apis mellifera, generally associate only a few key compounds among the complex floral scent with the food reward which guides their foraging choices. How these key compounds are selected remains partially unexplained, despite their crucial role in influencing foraging. Using electrophysiological techniques and behavioural assays, we identified the key bioactive compounds that bees detected with their antennae and that were associated with appetitive responses from four fruit crops and three vegetable crops. Three quantities of identified key volatile compounds were assayed with the two methods in each of four different seasons with experienced foragers. Whether the selection of these key compounds is determined by the sensory capability of the bee or influenced by its foraging experience was assessed by comparing experienced and naïve bees. Our results showed that experienced foragers were electrophysiologically-sensitive to a specific set of key compounds for each crop, independent of variation in quantity among several varieties. Experienced foragers responded to these compounds in all seasons, with increased electrophysiological amplitude with increasing quantities. Behavioural appetitive responses varied amongst compounds and seasons, revealing preferences based on associative learning. Naïve bees that were exposed to compounds and subsequently learned them, tended to be overall more sensitive. We discuss our results based on the identity of each bioactive compound and their presence in nature. Preferences for specific floral compounds based on sensory biases exist and associative learning may reinforce behavioural attraction depending on foraging experience in each season.

Flowers of European pear release common and uncommon volatiles that can be detected by honey bee pollinators

Floral scents are important pollinator attractants, but there is limited knowledge about the importance of single components in plant–pollinator interactions. This especially is true in crop pollination systems. The aim of this study is to identify floral volatiles of several European pear cultivars (Pyrus communis L.), and to determine their potential in eliciting physiological responses in antennae of honey bees (Apis mellifera L.), the most important pollinators of pear. Volatiles were collected by dynamic headspace and analysed by (high resolution) gas chromatography coupled to mass spectrometry (GC/MS) and nuclear magnetic resonance spectroscopy. Antennal responses were investigated by GC coupled to electroantennographic detection (GC/EAD). We trapped in the mean 256 ng of scent per flower and hour (flower−1 h−1) from the different cultivars with either linalool + methyl benzoate or methyl 2-hydroxy-3-methylpentanoate as most abundant compounds. Of the 108 detected pear floral scent components, 17 were electrophysiologically active in honey bee antennae. Among these compounds were (E)-N-(2-methylbutyl)- and (E)-N-(3-methylbutyl)-1-(pyridin-3-yl)methanimine, which were not known from nature before to the best of our knowledge. Most other compounds identified as flower scent in pear are widespread compounds, known from flowers of various other species. Our results provide new insights in the floral volatile chemistry of an important insect-pollinated crop and show for the first time that honey bees have the olfactory ability to detect several pear floral volatiles. These data are an important basis for more detailed studies of the olfactory communication between honey bees and European pear flowers and might in the long term be used to manipulate the attractiveness of pear to obtain optimal fruit set.

Ketalization of 2-heptanone to prolong its activity as mite repellant for the protection of honey bees

BACKGROUND

2-Heptanone is a volatile liquid known to be effective in protecting honey bees from parasitic mite infestations in hives. The present study aimed to show that chemical derivatives of 2-heptanone would release the ketone for a significantly longer time than it takes for the pure ketone to evaporate and preferably for as long as two brood cycles of a honey bee (42 days).

RESULTS

A liquid ketal of 2-heptanone with glycerol (Glyc-Ket) and solid ketals of the ketone with polyvinyl alcohol (PVAl-Ket), containing different amounts of the ketone, were synthesized. The fully resolved ¹ H and ¹³ C nuclear magenetic resonance (NMR) spectra of the ketals are discussed. In the case of the polymer, differential scanning calorimetry (DSC) of a ketal was also compared with the unketalized polyvinyl alcohol. The length of time for which 2-heptanone was released by the ketals was determined by gas chromatography-mass spectrometry of the headspace. In the case of Glyc-Ket, the concentration of the 2-heptanone in the liquid phase was also monitored by ¹ H NMR spectroscopy. The deketalization was pH dependent, ranging between 2.0 and 2.5 for Glyc-Ket and between 2.0 and 3.5 for PVAl-Ket.

CONCLUSION

Under bee hive conditions, the release of 55 mmol 2-heptanone from Glyc-Ket lasted for 42 days, whereas the release of the ketone from the PVAl-Ket with a similar amount of the ketone lasted for 23 days, versus a maximum of 17 days for an equivalent amount of the pure ketone. These ketals therefore have the potential to be effective mite repellants for the protection of honey bees. © 2019 Society of Chemical Industry.

Flower scent of Ceropegia stenantha: electrophysiological activity and synthesis of novel components

In specialized pollination systems, floral scents are crucial for flower-pollinator communication, but key volatiles that attract pollinators are unknown for most systems. Deceptive Ceropegia trap flowers are famous for their elaborate mechanisms to trap flies. Recent studies revealed species-specific floral chemistry suggesting highly specialized mimicry strategies. However, volatiles involved in fly attraction were until now identified in C. dolichophylla and C. sandersonii, only. We here present data on C. stenantha for which flower scent and pollinators were recently described, but volatiles involved in flower-fly communication stayed unknown. We performed electrophysiological measurements with scatopsid fly pollinators (Coboldia fuscipes) and identified 12 out of 13 biologically active floral components. Among these volatiles some were never described from any organism but C. stenantha. We synthesized these components, tested them on antennae of male and female flies, and confirmed their biological activity. Overall, our data show that half of the volatiles emitted from C. stenantha flowers are perceived by male and female fly pollinators and are potentially important for flower-fly communication in this pollination system. Further studies are needed to clarify the role of the electrophysiologically active components in the life of scatopsid fly pollinators, and to fully understand the pollination strategy of C. stenantha.

Limited Cross Plant Movement and Non-Crop Preferences Reduce the Efficiency of Honey Bees as Pollinators of Hybrid Carrot Seed Crops

Pollination rates in hybrid carrot crops remain limited after introduction of honey bee hives. In this study, honey bee foraging behaviour was observed in commercial hybrid carrot seed crops. Significantly more visits were made to male-fertile (MF) rather than cytoplasmically male-sterile (CMS) flowers. Pollen was collected from bees returning to a hive, to determine daily variation in pollen loads collected and to what level the bees were foraging for carrot pollen. Honey bees visited a wide range of alternative pollen sources and made relatively few visits to carrot plants throughout the period of flowering. Visitation rates to other individual floral sources fluctuated but visitation to carrot was consistently low. The underlying rate of carrot pollen visits among collecting trips was modelled and estimated to be as low as 1.4%, a likely cause of the limited success implementing honey bee hives in carrot crops.