Nectar Sugar Composition Research Articles

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.

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.

Nectar features, diurnal and nocturnal pollinators, and male fitness in Qualea grandiflora (Vochysiaceae)

Nectar production in flowers has been associated with pollination systems and seen as part of plant reproductive strategies. But other factors may have a role on nectar features and its efficacy as reward. We investigated pollination system of Qualea grandiflora, the most widespread woody species of Cerrado, the Neotropical savannas in Central Brazil, focusing on nectar features, their relationship with nocturnal and diurnal pollinators and how nectar affects male fitness, measured through pollen removal. Qualea grandiflora flowers during rainy season; anthesis starts at dusk, with a slightly sweet scent, pollen release and receptive stigma. Flowers remained opened until next evening and were visited and pollinated by a wide range of nocturnal and diurnal animals. Hawk moths were more frequent and responsible for most Q. grandiflora male fitness. Nectar was secreted during the night and early morning, with maximum accumulated volume up to 20.9 μl. Sugar concentration (23.12%) and amount (1.79 mg) remained constant during flower lifespan. No signals of active nectar resorption were observed. Nectar sugar composition was sucrose dominant during the night, and sucrose and hexose rich during the day. Standing crop measurements indicated an important proportion of nectar intake by flower visitors. Qualea grandiflora seemed to synchronize its floral and nectar features with hawk moths. However, nectar composition encompasses general preferences by both diurnal and nocturnal pollinators, assuring a mixed pollination system which seems well adapted to the species continental-scale distribution.

The Impact of Yeast Presence in Nectar on Bumble Bee Behavior and Fitness

The presence of yeasts in pollen and floral nectar is rather the norm than the exception. Due to the metabolic activities of yeasts, sugar and amino acid composition of nectar often drastically change and may negatively impact the nutritional value of nectar for pollinators and hence insect fitness. On the other hand, the presence of yeasts in floral nectar may also increase its nutritional value due to yeast's probiotic effect and the release of yeast's metabolites. In this study, we investigated whether the presence of defined flower‐ and insect‐associated yeasts affected individual and colony fitness of the bumble bee pollinator Bombus terrestris. Specifically, we tested whether the presence of yeasts in nectar affected bumble bee foraging behavior and nectar consumption, individual growth and colony development, larval and queen mortality, and mating success. Quantitative analyses of sugar and amino acid profiles showed that nectar yeasts significantly affected the chemical composition of nectar. However, dual‐choice experiments indicated that yeast inoculation did not significantly affect foraging behavior or consumption rates. Nest development, on the other hand, was significantly affected by the presence of yeasts, but effects largely depended on species identity, with Candida bombiphila, Metschnikowia gruessii, and Rhodotorula mucilaginosa having the largest positive impact on colony growth. Interestingly, the effects at the colony level were more pronounced than at the individual level. In vitro growth tests further showed that yeasts have the potential to suppress the growth of the bumble bee gut pathogen Crithidia bombi. Overall, these results demonstrate that nectar‐inhabiting yeasts can have diverse effects on bumble bee fitness and therefore may mediate plant–pollinator mutualisms.

The impact of yeast presence in nectar on bumble bee behavior and fitness

AbstractThe presence of yeasts in pollen and floral nectar is rather the norm than the exception. Due to the metabolic activities of yeasts, sugar and amino acid composition of nectar often drastically change and may negatively impact the nutritional value of nectar for pollinators and hence insect fitness. On the other hand, the presence of yeasts in floral nectar may also increase its nutritional value due to yeast's probiotic effect and the release of yeast's metabolites. In this study, we investigated whether the presence of defined flower‐ and insect‐associated yeasts affected individual and colony fitness of the bumble bee pollinatorBombus terrestris. Specifically, we tested whether the presence of yeasts in nectar affected bumble bee foraging behavior and nectar consumption, individual growth and colony development, larval and queen mortality, and mating success. Quantitative analyses of sugar and amino acid profiles showed that nectar yeasts significantly affected the chemical composition of nectar. However, dual‐choice experiments indicated that yeast inoculation did not significantly affect foraging behavior or consumption rates. Nest development, on the other hand, was significantly affected by the presence of yeasts, but effects largely depended on species identity, withCandida bombiphila,Metschnikowia gruessii, andRhodotorula mucilaginosahaving the largest positive impact on colony growth. Interestingly, the effects at the colony level were more pronounced than at the individual level. In vitro growth tests further showed that yeasts have the potential to suppress the growth of the bumble bee gut pathogenCrithidia bombi. Overall, these results demonstrate that nectar‐inhabiting yeasts can have diverse effects on bumble bee fitness and therefore may mediate plant–pollinator mutualisms.

Floral micromorphology and nectar composition of the early evolutionary lineage Utricularia (subgenus Polypompholyx, Lentibulariaceae)

Utricularia (Lentibulariaceae) is a genus comprising around 240 species of herbaceous, carnivorous plants. Utricularia is usually viewed as an insect-pollinated genus, with the exception of a few bird-pollinated species. The bladderworts Utricularia multifida and U. tenella are interesting species because they represent an early evolutionary Utricularia branch and have some unusual morphological characters in their traps and calyx. Thus, our aims were to (i) determine whether the nectar sugar concentrations and composition in U. multifida and U. tenella are similar to those of other Utricularia species from the subgenera Polypompholyx and Utricularia, (ii) compare the nectary structure of U. multifida and U. tenella with those of other Utricularia species, and (iii) determine whether U. multifida and U. tenella use some of their floral trichomes as an alternative food reward for pollinators. We used light microscopy, histochemistry, and scanning and transmission electron microscopy to address those aims. The concentration and composition of nectar sugars were analysed using high-performance liquid chromatography. In all of the examined species, the floral nectary consisted of a spur bearing glandular trichomes. The spur produced and stored the nectar. We detected hexose-dominated (fructose + glucose) nectar in U. multifida and U. tenella as well as in U. violacea. In both U. multifida and U. tenella, there were trichomes that blocked the entrance into the throat and spur. Because these trichomes were rich in chromoplasts and contained lipid droplets, they may form an additional visual attractant. Bearing in mind the phylogenetic hypothesis for the genus, we suggest that an early ancestor of Utricularia had a nectariferous spur flower with a lower lip that formed a wide landing platform for bee pollinators.

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

Main conclusionSugars (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.

Floral micromorphology of the bird-pollinated carnivorous plant species Utricularia menziesii R.Br. (Lentibulariaceae).

Bird pollination is rare among species in the genus Utricularia, and has evolved independently in two lineages of this genus. In Western Australia, the Western Spinebill, Acanthorhynchus superciliosus, visits flowers of Utricularia menziesii (section Pleiochasia: subgenus Polypompholyx). This study aimed to examine the micromorphology of U. menziesii flowers to assess traits that might be linked to its pollination strategy. Light microscopy, histochemistry and scanning electron microscopy were used. Nectar sugar composition was analysed using high-performance liquid chromatography. The flowers of U. menziesii fulfil many criteria that characterize bird-pollinated flowers: red colour, a large, tough nectary spur that can withstand contact with a hard beak, lack of visual nectar guides and fragrance. Trichomes at the palate and throat may act as tactile signals. Spur nectary trichomes did not form clearly visible patches, but were more frequently distributed along vascular bundles, and were small and sessile. Each trichome comprised a single basal cell, a unicellular short pedestal cell (barrier cell) and a multicelled head. These trichomes were much smaller than those of the U. vulgaris allies. Hexose-dominated nectar was detected in flower spurs. Fructose and glucose were present in equal quantities (43 ± 3.6 and 42 ± 3.6 g L-1). Sucrose was only detected in one sample, essentially at the limit of detection for the method used. This type of nectar is common in flowers pollinated by passerine perching birds. The architecture of nectary trichomes in U. menziesii was similar to that of capitate trichomes of insect-pollinated species in this genus; thus, the most important specializations to bird pollination were flower colour (red), and both spur shape and size modification. Bird pollination is probably a recent innovation in the genus Utricularia, subgenus Polypompholyx, and is likely to have evolved from bee-pollinated ancestors.

Nectar production dynamics and daily pattern of pollinator visits in Brazil nut (Bertholletia excelsa Bonpl.) plantations in Central Amazon: implications for fruit production

We investigated composition and secretion patterns of nectar in the Brazil nut tree (Bertholletia excelsa) and visitation patterns and glossa length of the main flower visitors along the anthesis, aiming to understand the implications for pollination and fruit production. Nectar sugar composition was dominated by sucrose and nectar secretion was continuous until 15:30 h, although flowers secreted, respectively, almost 50 and 80% of the total nectar volume and solutes in the hours immediately following flower opening, which coincides with peak flower visitation by bees. We observed a total of 19 bee species visiting the flowers to collect nectar throughout the day that can be considered pollinators. The three most abundant bee species were Xylocopa frontalis, Eufriesea flaviventris, and Eulaema mocsaryi that accounted for about 90% of the visits. In open flowers, nectar was generally scarce, encouraging bees to move among trees, and likely increasing xenogamous pollen transfer in natural habitats. However, in the large-scale Brazil nut tree plantation studied here, where genetically identical (clone) individuals are planted together in high densities, even where bees move between trees, they seem to promote functional geitonogamy, determining pollen limitation.

Contrasting effects of yeasts and bacteria on floral nectar traits.

Flowers can be highly variable in nectar volume and chemical composition, even within the same plant, but the causes of this variation are not fully understood. One potential cause is nectar-colonizing bacteria and yeasts, but experimental tests isolating their effects on wildflowers are largely lacking. This study examines the effects of dominant species of yeasts and bacteria on the hummingbird-pollinated shrub, Mimulus aurantiacus, in California. Wildflowers were inoculated with field-relevant titres of either the yeast Metschnikowia reukaufii or the bacterium Neokomagataea sp. (formerly Gluconobacter sp.), both isolated from M. aurantiacus nectar. Newly opened flowers were bagged, inoculated, harvested after 3 d and analysed for microbial abundance, nectar volume, and sugar and amino acid concentration and composition. Yeast inoculation reduced amino acid concentration and altered amino acid composition, but had no significant effect on nectar volume or sugar composition. In contrast, bacterial inoculation increased amino acid concentration, enhanced the proportion of nectar sugars comprised by monosaccharides, and reduced nectar volume. The results presented suggest that microbial inhabitants of floral nectar can make nectar characteristics variable among flowers through divergent effects of yeasts and bacteria on nectar chemistry and availability, probably modifying plant-pollinator interactions.

Response of extrafloral nectar production to elevated atmospheric carbon dioxide

Extrafloral nectar attracts ants, whose presence provides protection for the plant against herbivores. Extrafloral nectar is thus a critical component of many plant–insect mutualisms worldwide, so environmental perturbations that alter extrafloral nectar production or composition could be disruptive. The carbon–nutrient balance hypothesis predicts that under elevated CO2 the total volume of extrafloral nectar will increase but the proportion of the foliar carbohydrate pool secreted as extrafloral nectar will decrease, without any change in the sugar composition of the extrafloral nectar. We investigated the impact of elevated atmospheric CO2 on extrafloral nectar in an Australian wild cotton species, Gossypium sturtianum J.H.Willis. Under elevated CO2 there was an increase in the proportion of leaves actively producing nectar and a decrease in the nectar volume per active leaf. Elevated CO2 did not affect the total volume or composition of extrafloral nectar, but there was a change in how the nectar was distributed within the leaf canopy, as well as evidence of increased turnover of leaves and earlier onset of flowering. By the end of the study, there was no difference in the total resources allocated to extrafloral nectar under elevated CO2, which contrasts with the predictions of the carbon-nutrient balance hypothesis. Developmental changes, however, could affect the timing of extrafloral nectar production which could, in turn, alter the foraging patterns of ants and their defence of plants.

Floral Scent and Nectar Sugar Composition of Temnadenia odorifera (Apocynoideae, Apocynaceae)

Temnadenia odorifera is an endemic species from the Brazilian Atlantic Forest. This study was developed in order to identify the volatile compounds emitted by the living flowers and nectar, to evaluate the temporal pattern of scent emission, and the sugars composition of its nectar. Analyses of the flower scent by dynamic headspace in vivo, of nectar sugar composition and studies on floral biology were performed. Twenty-three volatile compounds were identified in the flowers scent. The total amount of odor emitted by flowers varied significantly throughout anthesis, ranging from 10232.7 ng g-1 (9 to 12 h) to 620.2 ng g-1 (15 to 18 h). 2-Phenylethanol and (E)-cinnamyl alcohol were the major compounds. Concentration ratio between disaccharides and the sum of hexoses ranged from 2.3 to 3.04, which can be correlated to the physiological needs inherent to big bees, the most frequent insects in T. odorifera. Analysis of the chemical composition of T. odorifera flowers provided a broader understanding of the mechanisms responsible for plant-insect interactions.

Nectar-living yeasts of a tropical host plant community: diversity and effects on community-wide floral nectar traits.

We characterize the diversity of nectar-living yeasts of a tropical host plant community at different hierarchical sampling levels, measure the associations between yeasts and nectariferous plants, and measure the effect of yeasts on nectar traits. Using a series of hierarchically nested sampling units, we extracted nectar from an assemblage of host plants that were representative of the diversity of life forms, flower shapes, and pollinator types in the tropical area of Yucatan, Mexico. Yeasts were isolated from single nectar samples; their DNA was identified, the yeast cell density was estimated, and the sugar composition and concentration of nectar were quantified using HPLC. In contrast to previous studies from temperate regions, the diversity of nectar-living yeasts in the plant community was characterized by a relatively high number of equally common species with low dominance. Analyses predict highly diverse nectar yeast communities in a relatively narrow range of tropical vegetation, suggesting that the diversity of yeasts will increase as the number of sampling units increases at the level of the species, genera, and botanical families of the hosts. Significant associations between specific yeast species and host plants were also detected; the interaction between yeasts and host plants impacted the effect of yeast cell density on nectar sugars. This study provides an overall picture of the diversity of nectar-living yeasts in tropical host plants and suggests that the key factor that affects the community-wide patterns of nectar traits is not nectar chemistry, but rather the type of yeasts interacting with host plants.

Floral signals and filters in a wasp- and a bee-pollinated Gomphocarpus species (Apocynaceae: Asclepiadeae)

Floral traits may function as attractants for some flower visitors and also as mechanisms that filter out non-beneficial flower visitors. Our current understanding of floral filter mechanisms is limited as this particular aspect of floral function has received less attention than floral attractants. Here we report on a case of two closely related Gomphocarpus species, G. physocarpus and G. fruticosus, that are similar in their general floral shape and architecture but differ in their main pollinators, which are vespid wasps and honeybees, respectively. To identify the floral traits that best explain the observed pollinators we analysed and compared the floral scent composition, the flower colour, flower morphology, and nectar sugar composition of the two species. Furthermore, the response to floral scent was tested in behavioural experiments with Belanogaster and Polistes vespid wasps. The floral scent composition differed significantly between the plant species. Acetic acid was a main component in the wasp-pollinated G. physocarpus, which is highly unusual for a floral scent bouquet. Choice experiments suggest that the species-specific odour might be responsible for the selective attraction of either wasps or bees. Furthermore, wasp-pollinated G. physocarpus flowers offer relatively large amounts of nectar that is freely accessible to insect visitors, whereas in G. fruticosus the site of nectar accumulation is covered by a specific morphological structure that makes it difficult for short-tongued insects such as wasps to access the nectar. Altogether, pollinator specialisation in the two Gomphocarpus species seem to be due to a combination of floral filter mechanisms and olfactory cues that cause flower visitors to learn to differentiate between the two species.

Floral and pollination biology, breeding system and nectar traits of Callistemon citrinus (Myrtaceae) cultivated in India

This study reports the influence of habitat on floral (flower and nectar characteristics, phenology) and pollination biology (flower visitors and breeding system) on Callistemon citrinus (syn. Callistemon lanceolatus), an Australian species of family Myrtaceae. In India, this small ornamental tree is cultivated in gardens, avenues and road sides and flowers throughout the year. At Agra (Uttar Pradesh, northern India) this species flowers twice a year (February–May and August–November). Flowers open early in the morning and can be characterized as protandrous for a brief period. Fresh open flowers presented ca. 25% of total produced nectar. Nectar sugar composition consisted of only glucose and fructose. A wide array of visitors (honey bees (Apis dorsata), butterflies, wasps, ants, moth, hover flies, several birds and Indian palm squirrel) visit flowers either for pollen or nectar or for facilitating self- and cross-pollination by their intra- and inter-tree movements. Among these, honey bees are dominant in number and in the amount of pollen on their body, while butterflies, ants, wasps, sunbirds, parrots, oriental white-eye sparrow and squirrels forage only for nectar and can be considered occasional pollinators or nectar thieves. Although flowers are highly visited, nectar in standing crop showed that flower visitors did not consume the total nectar produced. Nectar replenishment decreased with age in both bagged- and exposed flowers, showing the capability of secreting nectar after removal during the entire flower lifetime. Experimental hand-pollinations showed that naturalized C. citrinus fruits matured through autogamy, geitonogamy or xenogamy as was reported for native Australian populations. C. citrinus displays an interesting reproductive strategy, shows phenotypic plasticity of flowering periodicity and interacts with a generalized pollinator system, attracts a wide array of animal species with a conspicuous amount or replenished nectar after removals, and uses both compatibility strategies – xenogamy and autogamy – to ensure successful pollination and seed production at native or naturalized habitats.

Nutrient enrichment is associated with altered nectar and pollen chemical composition in Succisa pratensis Moench and increased larval mortality of its pollinator Bombus terrestris L.

Pollinators are declining worldwide and possible underlying causes include disease, invasive pest species and large scale land use changes resulting in habitat loss and degradation. One particular cause of habitat degradation is the increased inflow of nutrients due to anthropogenic combustion processes and large scale application of agricultural fertilizers. This nutrient pollution has been shown to affect pollinators through the loss of nectar and pollen-providing plant species. However, it may also affect pollinators through altering the nectar and pollen chemical composition of plant species, hence influencing pollinator food quality. Here, we experimentally investigated the effect of nutrient enrichment on amino acid and sugar composition of nectar and pollen in the grassland plant Sucissa pratensis, and the subsequent colony size and larval mortality of the pollinating bumblebee Bombus terrestris. We found less of the essential amino acids glycine and arginine in the pollen of fertilized plants, and more arginine, ornithine and threonine in the pollen of control plants. Nectar glucose and pollen fructose levels were lower in fertilized plants as compared to control plants. Furthermore, bumblebee colonies visiting fertilized plants showed more dead larvae than colonies visiting control plants. Our results suggest that the fitness of bumblebees can be negatively affected by changes in their food quality following nutrient pollution. If similar patterns hold for other plant and pollinator species, this may have far reaching implications for the maintenance of pollination ecosystem services, as nutrient pollution continues to rise worldwide.

Nectar properties and the role of sunbirds as pollinators of the golden-flowered tea (Camellia petelotii).

Properties of floral nectar have been used to predict if a plant species is pollinated by birds. To see whether winter-flowering plants evolve nectar properties corresponding to bird pollinators, nectar properties of several Camellia species (including the golden-flowered tea), as well as the role of floral visitors as effective pollinators, were examined. Potential pollinators of Camellia petelotii were identified at different times of day and under various weather conditions. A bird exclusion experiment was used to compare the pollination effectiveness of birds and insects. Nectar sugar components (fructose, glucose, and sucrose) from C. petelotii growing wild and another seven Camellia species and 22 additional cultivars (all in cultivation) were examined by high-performance liquid chromatography (HPLC). The sunbird Aethopyga siparaja and honeybees were the most frequent floral visitors to C. petelotii. Honeybee visits were significantly reduced in cloudy/rainy weather. The fruit and seed set of flowers with birds excluded were reduced by 64%, indicating that bird pollination is significant. For the wild populations of C. petelotii, a bagged flower could secrete 157 μL nectar; this nectar has a low sugar concentration (19%) and is sucrose-dominant (87%). The eight Camellia species and 22 cultivars had an average sugar concentration of around 30% and a sucrose concentration of 80%, demonstrating sucrose-dominant nectar in Camellia species. The nectar sugar composition of Camellia species was characterized by sucrose dominance. In addition, the large reduction in seed set when birds are excluded in the golden-flowered tea also supports the suggestion that these winter-flowering plants may have evolved with birds as significant pollinators.

Pollinator type and secondarily climate are related to nectar sugar composition across the angiosperms

Pollinators are important agents of selection on floral traits, including nectar sugar composition. Although it is widely assumed that the proportion of sugars (mainly sucrose, glucose and fructose) in nectar reflects pollinators’ physiological limitations and digestive efficiency, the relative impact of pollinators and abiotic factors on nectar sugar composition, as well as the generality of these associations across the angiosperms, remain unknown. We compiled data on nectar sugar composition for >1000 plant species, along with information on flower visitors, plant growth form and latitudinal climatic zone, to provide the first comprehensive assessment of correlates of variation in sugar nectar composition in the angiosperms. After assembling a phylogeny linking all species in the dataset, we estimated the amount of phylogenetic signal in the percentage of sucrose and, by applying phylogenetically-informed multiple regressions, we evaluated whether nectar composition was influenced either by the main pollinator group, plant growth form, or latitudinal climatic zone. The relative importance of each of these factors was then assessed through model selection based on Akaike information criteria and deviance partitioning analysis. Nectar was dominated by sucrose in 56.8% of all the species, glucose in 16.7%, and fructose in 5.5%. Nectar in the remaining species was characterized by similar proportions of the three sugars. Variation in the proportion of sucrose was highest (~70%) at the intrafamily level, and had a significant but low phylogenetic signal, which partially reflects phylogenetic conservatism of the pollinator niche. After controlling for phylogenetic effects, the proportion of sucrose was mainly related to pollinator type and secondarily to climate. Accordingly, this study indicates that nectar sugar composition shows high evolutionary lability and its variation reflects plant-pollinator associations.

Bee visitation rates to cultivated sunflowers increase with the amount and accessibility of nectar sugars

AbstractPollinators make foraging decisions based on numerous floral traits, including nectar and pollen rewards, and associated visual and olfactory cues. For insect‐pollinated crops, identifying and breeding for attractive floral traits may increase yields. In this study, we examined floral trait variation within cultivated sunflowers and its effects on bee foraging behaviours. Over 2 years, we planted different sunflower inbred lines, including male‐fertile and male‐sterile lines, and measured nectar volume, nectar sugar concentration and composition, and corolla length. During bloom, we recorded visits by both managed honey bees and wild bees. We then examined consistency in relative nectar production by comparing field results to those from a greenhouse experiment. Sunflower inbred lines varied significantly in all floral traits, including the amount and composition of nectar sugars, and in corolla length. Both wild bee and honey bee visits significantly increased with nectar sugar amount and decreased with corolla length, but appeared unaffected by nectar sugar composition. While wild bees made more visits to sunflowers providing pollen (male‐fertile), honey bees preferred plants without pollen (male‐sterile). Differences in nectar quantity among greenhouse‐grown sunflower lines were similar to those measured in the field, and bumble bees preferentially visited lines with more nectar in greenhouse observations. Our results show that sunflowers with greater quantities of nectar sugar and shorter corollas receive greater pollination services from both managed and wild bees. Selecting for these traits could thus increase sunflower crop yields and provide greater floral resources for bees.

Pollinator adaptation and the evolution of floral nectar sugar composition.

A long-standing debate concerns whether nectar sugar composition evolves as an adaptation to pollinator dietary requirements or whether it is 'phylogenetically constrained'. Here, we use a modelling approach to evaluate the hypothesis that nectar sucrose proportion (NSP) is an adaptation to pollinators. We analyse ~2100 species of asterids, spanning several plant families and pollinator groups (PGs), and show that the hypothesis of adaptation cannot be rejected: NSP evolves towards two optimal values, high NSP for specialist-pollinated and low NSP for generalist-pollinated plants. However, the inferred adaptive process is weak, suggesting that adaptation to PG only provides a partial explanation for how nectar evolves. Additional factors are therefore needed to fully explain nectar evolution, and we suggest that future studies might incorporate floral shape and size and the abiotic environment into the analytical framework. Further, we show that NSP and PG evolution are correlated - in a manner dictated by pollinator behaviour. This contrasts with the view that a plant necessarily has to adapt its nectar composition to ensure pollination but rather suggests that pollinators adapt their foraging behaviour or dietary requirements to the nectar sugar composition presented by the plants. Finally, we document unexpectedly sucrose-poor nectar in some specialized nectarivorous bird-pollinated plants from the Old World, which might represent an overlooked form of pollinator deception. Thus, our broad study provides several new insights into how nectar evolves and we conclude by discussing why maintaining the conceptual dichotomy between adaptation and constraint might be unhelpful for advancing this field.