Nectar in Plant-Insect Mutualistic Relationships: From Food Reward to Partner Manipulation

Salvage of floral resources through re-absorption before flower abscission

Plants invest floral resources, including nectar and pigment, with likely consequent reproductive costs. We hypothesized that plants, whose flowers abscise with age, reabsorb nectar and pigment before abscission. This was tested with flowers of Rhododendron decorum, which has large, conspicuous white flowers that increasingly abscise corollas as flowers age. As this species is pollinated by bees, we also hypothesized that nectar concentration would be relatively high (i.e., > 30% wt/vol) and petals would contain UV-absorbing pigment. Floral nectar volume and concentration were sampled on successive days until abscission (up to ten days old, peak at five days) and for sub-sample of four-day-old flowers. Flowers just abscised were similarly sampled. Flower colours were measured using a modified camera, with recordings of spectral reflectance for abscised and open non-abscised flowers. Pigment content was summed values of red, green, blue channels of false color photos. As expected, flowers reabsorbed almost all nectar before abscission, separately reabsorbing nectar-sugar and nectar-water, and petals contained UV-absorbing pigment. However, flowers did not reabsorb pigment and nectar-concentration was < 30% wt/vol. That flowers reabsorb nectar, not pigment, remains unexplained, though possibly pigment reabsorption is uneconomical. Understanding floral resource reabsorption therefore requires determination of biochemical mechanisms, plus costs/benefits for individual plants.

Australian native flower colours: Does nectar reward drive bee pollinator flower preferences?

Colour is an important signal that flowering plants use to attract insect pollinators like bees. Previous research in Germany has shown that nectar volume is higher for flower colours that are innately preferred by European bees, suggesting an important link between colour signals, bee preferences and floral rewards. In Australia, flower colour signals have evolved in parallel to the Northern hemisphere to enable easy discrimination and detection by the phylogenetically ancient trichromatic visual system of bees, and native Australian bees also possess similar innate colour preferences to European bees. We measured 59 spectral signatures from flowers present at two preserved native habitats in South Eastern Australia and tested whether there were any significant differences in the frequency of flowers presenting higher nectar rewards depending upon the colour category of the flower signals, as perceived by bees. We also tested if there was a significant correlation between chromatic contrast and the frequency of flowers presenting higher nectar rewards. For the entire sample, and for subsets excluding species in the Asteraceae and Orchidaceae, we found no significant difference among colour categories in the frequency of high nectar reward. This suggests that whilst such relationships between flower colour signals and nectar volume rewards have been observed at a field site in Germany, the effect is likely to be specific at a community level rather than a broad general principle that has resulted in the common signalling of bee flower colours around the world.

New Tools for Conservation Biological Control: Testing Ant-Attracting Artificial Nectaries to Employ Ants as Plant Defenders

Knowledge of the role of ants in many agroecosystems is relatively scarce, and in temperate regions the possibility to exploit ants as biocontrol agents for crop protection is still largely unexplored. Drawing inspiration from mutualistic ant–plant relationships mediated by extrafloral nectaries (EFNs), we tested the use of artificial nectaries (ANs) in order to increase ant activity on pear trees and to evaluate the effects on the arthropods, plant health and fruit production. While EFNs secrete a complex solution mainly composed of sugars and amino acids, ANs were filled with water and sucrose only. The results suggest that ANs can be used as manipulative instruments to increase ant activity over long periods of time. High ant activity was significantly linked to lower incidence of the pathogen fungus Venturia pyrina (pear scab) on pear leaves, and of the presence of Cydia pomonella (codling moth) caterpillars on pear fruit production. These results further encourage exploring underrated possibilities in the development of new tools for conservation biological control (CBC).

Patterns of floral nectar standing crops allow plants to manipulate their pollinators

‘Pollination syndromes’ involving floral nectar have eluded satisfactory evolutionary explanation. For example, floral nectars for vertebrate-pollinated plants average low sugar concentrations, while such animals prefer high concentrations, perplexing pollination biologists and arousing recent controversy. Such relationships should result from evolutionary games, with plants and pollinators adopting Evolutionarily Stable Strategies, and nectar manipulating rather than attracting pollinators. Plant potential to manipulate pollinators depends on relationships between neighbouring flowers within plants, for all nectar attributes, but this has not been investigated. We measured nectar volume, concentration and sugar composition for open flowers on naturally-growing Blandfordia grandiflora plants, presenting classic bird-pollinated plant syndrome. To evaluate potential pollinator manipulation through nectar, we analysed relationships between neighbouring flowers for nectar volume, concentration, proportion sucrose, log(fructose/glucose), and sugar weight. To evaluate potential attraction of repeat-visits to flowers or plants through nectar, we compared attributes between successive days. Nearby flowers were positively correlated for all attributes, except log(fructose/glucose) as fructose≈glucose. Most relationships between nectar attributes for flowers and plants on successive days were non-significant. Nectar-feeding pollinators should therefore decide whether to visit another flower on a plant, based on all attributes of nectar just-obtained, enabling plants to manipulate pollinators through adjusting nectar. Plants are unlikely to attract repeat pollinator-visits through nectar production. Floral nectar evolution is conceptually straightforward but empirically challenging. A mutant plant deviating from the population in attributes of nectar-production per flower would manipulate, rather than attract, nectar-feeding pollinators, altering pollen transfer, hence reproduction. However, links between floral nectar and plant fitness present empirical difficulties.

The mechanics of nectar offloading in the bumblebee Bombus terrestris and implications for optimal concentrations during nectar foraging

Nectar is a common reward provided by plants for pollinators. More concentrated nectar is more rewarding, but also more viscous, and hence more time-consuming to drink. Consequently, theory predicts an optimum concentration for maximizing energy uptake rate, dependent on the mechanics of feeding. For social pollinators such as bumblebees, another important but little-studied aspect of foraging is nectar offloading upon return to the nest. Studying the bumblebee Bombus terrestris, we found that the relationship between viscosity (µ) and volumetric transfer rates (Q) of sucrose solutions differed between drinking and offloading. For drinking, Q ∝ µ^-0.180, in good agreement with previous work. Although offloading was quicker than drinking, offloading rate decreased faster with viscosity, with Q ∝ µ^-0.502, consistent with constraints imposed by fluid flow through a tube. The difference in mechanics between drinking and offloading nectar leads to a conflict in the optimum concentration for maximizing energy transfer rates. Building a model of foraging energetics, we show that including offloading lowers the maximum rate of energy return to the nest and reduces the concentration which maximizes this rate by around 3%. Using our model, we show that published values of preferred nectar sugar concentrations suggest that bumblebees maximize the overall energy return rather than the instantaneous energy uptake during drinking.

Gall-Colonizing Ants and Their Role as Plant Defenders: From 'Bad Job' to 'Useful Service'

Galls are neoformed structures on host plant tissues caused by the attack of insects or other organisms. They support different communities of specialized parasitic insects (the gall inducers), and can also provide refuge to other insects, such as moths, beetles and ants, referred to as secondary occupants. This study focuses on galls induced by the oak gall wasp Andricus quercustozae and secondarily colonized by ants in a mixed oak forest. A field survey and two experiments were carried out to a) study ant (species-specific) preferences for different features of the galls, b) describe differences in gall architecture due to ant activity, c) analyse the effects of the presence of gall-dwelling ants on plant health. The results show that there are differences between ant species in gall colonization and in the alteration of gall opening and inner structure. We verified that gall-dwelling ants protect their host plants efficiently, offering them an indirect defence mechanism against enemies (predators and pathogens). The data suggest a new paradigm in ant–plant relationships mediated by the presence of galls on the plants whose ecological and evolutionary implications are discussed.

Diversity of nectar amino acids in the Fritillaria (Liliaceae) genus: ecological and evolutionary implications

Nectar is considered to be a primary food reward for most pollinators. It mostly contains sugars, but also has amino acids. The significance of the concentration and composition of amino acids in nectar is often less understood than that of its volume, sugar concentration and composition. However, there is a trend towards a broader approach in ecological research, which helps to understand nectar properties in an ecological context. The genus Fritillaria, exhibiting great diversity in flower morphology, nectar composition, and dominant pollinators, allows for the possibility to study some of the above. We studied the concentration and composition of amino acids in the nectar of 38 Fritillaria species attracting different groups of pollen vectors (bees, flies, passerines, and hummingbirds). The flowers of fritillaries produced nectar with a varying composition and concentration of amino acids. These differences were mostly associated with the pollinator type. The nectar of passerine bird-pollinated species was rich in amino acids, whereas humming bird-pollinated produced low amino acid nectar. Contrary to previous reports nectar of the insect-pollinated species did not contain a higher amount of proline. Two non-protein amino acids, sarcosine and norvaline, were detected in the floral nectar for the first time.

Diversity of nectar amino acids in the Fritillaria (Liliaceae) genus: ecological and evolutionary implications

Nectar is considered to be a primary food reward for most pollinators. It mostly contains sugars, but also has amino acids. The significance of the concentration and composition of amino acids in nectar is often less understood than that of its volume, sugar concentration and composition. However, there is a trend towards a broader approach in ecological research, which helps to understand nectar properties in an ecological context. The genus Fritillaria, exhibiting great diversity in flower morphology, nectar composition, and dominant pollinators, allows for the possibility to study some of the above. We studied the concentration and composition of amino acids in the nectar of 38 Fritillaria species attracting different groups of pollen vectors (bees, flies, passerines, and hummingbirds). The flowers of fritillaries produced nectar with a varying composition and concentration of amino acids. These differences were mostly associated with the pollinator type. The nectar of passerine bird-pollinated species was rich in amino acids, whereas humming bird-pollinated produced low amino acid nectar. Contrary to previous reports nectar of the insect-pollinated species did not contain a higher amount of proline. Two non-protein amino acids, sarcosine and norvaline, were detected in the floral nectar for the first time.

Fiber-optic refractometer for in vivo sugar concentration measurements of low-nectar-producing flowers

Sugar concentration in floral nectars is an assessment required in several diverse fields of application. The widely used analysis, consisting of nectar extraction with a microcapillary and sugar concentration measurement with a light refractometer, is not reliable when the nectar is secreted in small quantities, unextractable with a microcapillary. Ancillary methods adopted in such cases are destructive, rather complicated and often provide much less precise and accurate results. The microscopic-size, low cost and biocompatibility of optical fibers were exploited to deliver light directly inside the flower with minimal invasiveness and measure instantaneously the refractometric properties of the nectar without extracting it. After comparing the new and old methods using two known nectariferous species, the new approach was validated on Primula palinuri, whose nectar is unextractable with microcapillaries. The fiber-optic probe was able to measure the nectar refractive index in P. palinuri flowers making it possible to highlight a previously undetected significant trend of the sugar concentration throughout the long anthesis of the single flowers. Changes in nectar concentrations are similar in both longistylous and brevistylous flowers. The fiber-optic refractometer is an advancement of light refractometer analysis. Further customization of the laboratory set-up into portable equipment will boost applications.

Nectar composition in moth-pollinated Platanthera bifolia and P. chlorantha and its importance for reproductive success

Sugars (glucose, fructose and sucrose), as well as proteogenic and non-proteogenic amino acids, are present in the nectar of Platanthera bifolia and P. chlorantha. Nectar quantity and quality are floral traits that are subjected to pollinator-mediated selection. Nectar sugar and amino acid (AA) composition in two sister species, P. bifolia and P. chlorantha, was analysed and the interspecies differences in nectar and the importance of these nectar characteristics for reproductive success were investigated. Nectar was collected from four P. bifolia and three P. chlorantha populations that exist in different habitats in three regions of NE Poland. Nectar from about 30 flowers (from each population) was sampled and analysed using high-performance liquid chromatography. We found the same primary sugars and AA components in the nectar of both species, although their content varied between the populations according to habitat properties. The nectar of P. bifolia and P. chlorantha both had low sugar concentrations (9.04-20.68%) and were dominated by hexoses, with sucrose:hexoses ratios between 0.03 and 0.31 across the different populations (the average for the P. bifolia populations was 0.17 and the average for the P. chlorantha populations was - 0.05). Total sugar content did not influence reproductive success and we found positive selection on fructose content. In general, 23 different AAs were detected in both Platanthera species. Cysteine and γ-aminobutyric acid were present in only one population of P. chlorantha. Sarcosine dominated among the non-proteogenic AAs. To our knowledge, this is the first report that characterizes the sugar and AA profiles in the nectar of P. bifolia and P. chlorantha in natural populations in the context of effectiveness of reproduction. Total AAs negatively influenced male reproductive success (r = - 0.79). Pollinators of the investigated species were found to be sensitive to the AAs' taste, from taste classes I and IV. Correlation between male reproductive success and the content of AAs from these groups was 0.79 in both cases. In this manuscript, we investigated the characteristics of P. bifolia and P. chlorantha nectar, and compared these characteristics to the available data in the context of their adaptations to the requirements of pollinators and with regard to the importance of nectar quality for reproductive success of the studied species.

Brassicaceae flowers: diversity amid uniformity

The mustard family Brassicaceae, which includes the model plant Arabidopsis thaliana, exhibits morphological stasis and significant uniformity of floral plan. Nonetheless, there is untapped diversity in almost every aspect of floral morphology in the family that lends itself to comparative study, including organ number, shape, form, and color. Studies on the genetic basis of morphological diversity, enabled by extensive genetic tools and genomic resources and the close phylogenetic distance among mustards, have revealed a mosaic of conservation and divergence in numerous floral traits. Here I review the morphological diversity of the flowers of Brassicaceae and discuss studies addressing the underlying genetic and developmental mechanisms shaping floral diversity. To put flowers in the context of the floral display, I describe diversity in inflorescence morphology and the variation that exists in the structures preceding the floral organs. Reconstructing the floral morphospace in Brassicaceae coupled with next-generation sequencing data and unbiased approaches to interrogate gene function in species throughout the mustard phylogeny offers promising ways to understand how developmental mechanisms originate and diversify.

Random search with resetting as a strategy for optimal pollination

The problem of pollination is unique among a wide scope of search problems, since it requires optimization of benefits for both the searcher (pollinator) and its targets (plants). To address this challenge, we propose a pollination model which is based on a framework of first passage under stochastic restart. We derive equations for the search time and number of visited plants as functions of the distribution of nectar in the plant population and of the probability that a pollinator will leave the plant after examining a flower, thus effectively restarting the search. We demonstrate that nectar variation in plants serves as a driving force for pollination and establish conditions required for optimal pollination, which provides an efficient pollinator search strategy and the maximum number of plants visited by the pollinator.

Pathogen-induced changes in floral scent may increase honeybee-mediated dispersal of Erwinia amylovora

Honeybees are well recognised for their key role in plant reproduction as pollinators. On the other hand, their activity may vector some pathogens, such as the bacterium Erwinia amylovora, the causative agent of fire blight disease in pomaceous plants. In this research, we evaluated whether honeybees are able to discriminate between healthy and E. amylovora-infected flowers, thus altering the dispersal of the pathogen. For this reason, honeybees were previously trained to forage either on inoculated or healthy (control) apple flower. After the training, the two honeybee groups were equally exposed to inoculated and control flowering apple plants. To assess their preference, three independent methods were used: (1) direct count of visiting bees per time frame; (2) incidence on apple flowers of a marker bacterium (Pantoea agglomerans, strain P10c) carried by foragers; (3) quantification of E. amylovora populations in the collected pollen loads, proportional to the number of visits to infected flowers. The results show that both honeybee groups preferred control flowers over inoculated ones. The characterisation of volatile compounds released by flowers revealed a different emission of several bioactive compounds, providing an explanation for honeybee preference. As an unexpected ecological consequence, the influence of infection on floral scent increasing the visit rate on healthy flowers may promote a secondary bacterial spread.

Chitin: Structure, Chemistry and Biology

Chitin is a linear polysaccharide of the amino sugar N-acetyl glucosamine. It is present in the extracellular matrix of a variety of invertebrates including sponges, molluscs, nematodes and arthropods and fungi. Generally, it is an important component of protective or supportive extracellular matrices that cover the tissue that produces it or the whole body of the organism. Chitin fibres associate with each other adopting one of three possible crystalline organisations, i.e. α-, β- or γ-chitin. Usually, chitin fibre bundles interact with chitin-binding proteins forming higher order structures. Chitin laminae, which are two-dimensional sheets of α-chitin crystals with antiparallel running chitin fibres in association with β-folded proteins, are primary constituents of the arthropod cuticle and the fibrous extracellular matrix in sponges. A tri-dimensional composite material of proteins coacervates and β-chitin constitute hard biomaterials such as the squid beak. The molecular composition of γ-chitin-based structures that contribute to the physical barrier found in insect cocoons is less well studied. In principle, chitin is a versatile extracellular polysaccharide that in association with proteins defines the mechanical properties of tissues and organisms.

Crop Domestication Alters Floral Reward Chemistry With Potential Consequences for Pollinator Health

Crop domestication can lead to weakened expression of plant defences, with repercussions for herbivore and pathogen susceptibility. However, little is known about how domestication alters traits that mediate other important ecological interactions in crops, such as pollination. Secondary metabolites, which underpin many defence responses in plants, also occur widely in nectar and pollen and influence plant-pollinator interactions. Thus, domestication may also affect secondary compounds in floral rewards, with potential consequences for pollinators. To test this hypothesis, we chemically analysed nectar and pollen from wild and cultivated plants of highbush blueberry (Vaccinium corymbosum L.), before conducting an artificial diet bioassay to examine pollinator-pathogen interactions. Our results indicated that domestication has significantly altered the chemical composition of V. corymbosum nectar and pollen, and reduced pollen chemical diversity in cultivated plants. Of 20 plant metabolites identified in floral rewards, 13 differed significantly between wild and cultivated plants, with a majority showing positive associations with wild compared to cultivated plants. These included the amino acid phenylalanine (4.5 times higher in wild nectar, 11 times higher in wild pollen), a known bee phagostimulant and essential nutrient; and the antimicrobial caffeic acid ester 4-O-caffeoylshikimic acid (two times higher in wild nectar). We assessed the possible biological relevance of variation in caffeic acid esters in bioassays, using the commercially available 3-O-caffeoylquinic acid. This compound reduced Bombus impatiens infection by a prominent gut pathogen (Crithidia) at concentrations that occurred in wild but not cultivated plants, suggesting that domestication may influence floral traits with consequences for bee health. Appreciable levels of genetic variation and heritability were found for most floral reward chemical traits, indicating good potential for selective breeding. Our study provides the first assessment of plant domestication effects on floral reward chemistry and its potential repercussions for pollinator health. Given the central importance of pollinators for agriculture, we discuss the need to extend such investigations to pollinator-dependent crops more generally and elaborate on future research directions to ascertain wider trends, consequences for pollinators, mechanisms, and breeding solutions.