Nectar sugar concentration contributes to structuring bumblebee and plant interactions

In a controlled environment, we artificially induced drought during flowering of Epilobium angustifolium, an animal-pollinated plant. Leaf water potential (ψ(l)) and floral traits were monitored over a 12-d period of soil moisture depletion. Soil moisture depletion induced drought stress over time, as revealed by significant treatment × day interactions for predawn and midday ψ(l). Nectar volume and flower size showed significant negative responses to drought stress, but nectar sugar concentration did not vary between treatments. Floral traits were more buffered from drought than leaf water potentials. We used path analysis to examine direct and indirect effects of ψ(l) on floral traits for plants in well-watered (control) vs. drought treatments. According to the best-fit path models, midday ψ(l) has significant positive effects on flower size and nectar volume in both environments. However, for controls midday ψ(l) also had a significant negative effect on nectar sugar concentration. Results indicate that traits influencing floral attractiveness to pollinators in E. angustifolium vary with plant water status, such that pollinator-mediated selection could indirectly target physiological or biochemical controls on ψ(l). Moreover, under mesic conditions selection for greater nectar sugar reward may be constrained by the antagonistic effects of plant water status on nectar volume and sugar concentration.

Flowers that are pollinated both during the day and at night could exhibit two different groups of pollinators and produce two different sets of attractants and rewards. We explored the patterns of emission of flower scents and production of nectar in the cactus Echinopsis chiloensis ssp. chiloensis, in relation to the patterns of activity of its diurnal and nocturnal pollinators. We measured frequency of flower visitors, analyzed floral scents, measured nectar production and sugar concentration, and performed pollination exclusion experiments. Bees were the main visitors at daytime and hawkmoths at nighttime. Diurnal scents were dominated by several compounds that can attract a wide range of pollinators, whereas nocturnal scents were less diverse and were dominated by (E)-nerolidol, a compound eliciting antennal responses in hawkmoths. Nectar volume and sugar concentration at night were similar to those recorded in hawkmoth-pollinated flowers. Daytime nectar volume was higher than those commonly found in bee-pollinated flowers, but similar to those found in flowers pollinated by several pollinators. Daytime sugar concentration was similar to those recorded in bee-pollinated flowers. Flowers of E. chiloensis ssp. chiloensis seem morphologically adapted to hawkmoth pollination, but diurnal and nocturnal pollinators contribute to similar extents to reproductive success. Additionally, diurnal and nocturnal pollinators showed a synergic effect on the product of fruit set and seed set. The results are discussed in terms of the linkage between floral traits and perception abilities and requirements of pollinators.

Nectar volume and sugar composition of male and female flowers of Cucurbita pepo L. (squash), a vine native to tropical Mexico, were studied in an Italian botanical garden. Flowers opened at dawn and closed at noon. Both sexes were extremely rewarding compared with most bee‐pollinated flowers, producing 22–40 mg sugar/flower in 6 h. Female flowers produced significantly more nectar sugar than did males, mainly because of a higher concentration of sugars in nectar (440 vs. 325 mg/mL). The temporal pattern of secretion was similar in the two sexes, and both nectars were sucrose rich. Sugar composition did not vary during anthesis. By 6 h after flower closure, nectar volume and sugar concentration had decreased drastically, especially in female flowers. Cucurbita pepo has the ability to reabsorb most or all unconsumed nectar.

Nectar reabsorption in flowering plants seems to be a rare phenomenon and to our knowledge it has not been reported previously in the Orchidaceae. In this study we present data that show statistically significant differences in nectar sugar concentrations before and after pollination. Virgin flowers of the African epiphytic orchid Aerangis verdickii showed mean sugar concentrations in the nectar of 14.4 and of 18.3% in 1994 and 1995, respectively (sucrose mass by mass equivalent). Sugar concentration in nectar from naturally pollinated flowers had a mean of 3.2 and of 8.7% for the same period. In an experiment in the wild 60 flowers were protected against nectar theft by the ants. Half of those flowers were hand-pollinated. Control (virgin) flowers had nectar with a mean value of 13.8%, while flowers measured 48 h after pollination had a mean sugar concentration of 4.3%. The mean volume of nectar based on measurements of 23 virgin flowers from nine different plants was 19 uL. We estimated an average energy value for the nectar produced of 684 J per plant per season. At the study site, over 60% of the unmanipulated flowers were robbed of all their nectar by arboreal ants, Polyrachis spp. (Formicinae). The adaptive significance of nectar reabsorption for A. verdickii is probably a function of the environmental stresses to which it has been exposed and the relative costs of nectar production.

Climate change is intensifying droughts across the globe, challenging species to adapt to novel conditions. While plant physiological and phenological responses to drought are well-documented, less is known about how water scarcity affects the evolution of selfing across species ranges. According to the selfing syndrome hypothesis, in environments where selfing confers a fitness advantage, selection should favour floral traits associated with increased selfing relative to outcrossing. We used a field experiment near the northern range edge of the scarlet monkeyflower (Mimulus cardinalis) to test this hypothesis both spatially (among leading-edge, central, and trailing-edge populations), and temporally (between cohorts separated by a period of historic drought). Although populations from different range positions showed genetic differentiation in some floral traits, these differences did not consistently support predictions of the selfing syndrome hypothesis. Contrary to the predictions of reduced investment in floral rewards and increased selfing ability at range edges, the sugar content of nectar was greater and autogamous seed set was smaller in leading-edge than central populations, herkogamy tended to be greater in trailing-edge populations relative to leading-edge and central ones, and nectar volume did not vary predictably among regions. There was no support for the evolution of selfing syndrome from the predrought ancestors to the postdrought descendants. Instead, in leading-edge populations, descendants evolved greater sugar content relative to ancestors, and there were no other differences between ancestors and descendants in any other trait or region. Overall, these findings suggest that mating system evolution in M. cardinalis likely reflects a complex interplay of regional factors including range position, historical adaptation, and local environmental variability, rather than simple stress-induced shifts towards selfing.

This study was conducted to prove possibility of honey tree, which analyzed the visited number of honeybees, secreted nectar volume, nectar sugar content and amino acid in addition to estimating honey quantities that can ultimately reap in flowers of Styrax japonicus Sieb. & Zucc.. The surveyed tree’s flowers bloom during 12 days and maximum blooming period of flowers was on 23 rd to 25 th May in 2013. Honeybee visited flowers in priority and visited number of honeybee per flowering lateral bunch can be assumed 290 honeybees for a day. Honeybee visiting was concentrated at around 10~11 a.m.. On average, nectar volume secreted by nectary was 1.13 ul from one flower and nectar concentration (w/w %) presented 39.6%. Sugar content were calculated at 71.0 ug per flower which was estimated by multiplying secreted nectar volume (ul/flower) by nectar sugar (ug/ul). The minimum estimate of honey harvest for official tree (15 years, Height = 3m, Diameter at breast height = 12cm) in this study was 3,032 mg. Result of amino acid content ratio showed that Proline, Glutamate, Hydroproline, Serine, Asparagine were more abundant than others. Especially, Proline percentages (40.4%) were highest among the other amino acids. Finally, the surveyed tree is considered as possible honey plant because of its nectar characteristics and honeybee visiting. Honey is made of nectar sugar as well as by digestive system of honeybee, therefore a more associated study is necessary to analyze free sugar component in nectar and accurately presume to harvest honey quantity.

Floral adaptation to local pollinator guilds in a terrestrial orchid

Reward quality predicts effects of bird-pollinators on the reproduction of African Protea shrubs

Floral nectar plays a critical role in shaping the ecology and evolution of plant-pollinator interactions. Effective and efficient methods that allow for broad-scale sampling of nectar volume and sugar concentration across a diversity of taxa are needed to improve our understanding of many dimensions of mutualistic plant-pollinator interactions—including their basic ecology and evolution, their responses to environmental change, and their conservation and restoration. Despite the key importance of nectar for mediating plant-pollinator interactions, quantifying floral nectar in the field from many different plant species is challenging because there is often no one-size-fits-all sampling method that is effective across a diversity of floral structures and nectar traits. Different methods require different preparation, and sampling from many species involves a variety of logistical challenges. Here we provide a methodological roadmap for sampling floral nectar in the field from many different plant species. We describe our nectar collection methods in detail, including necessary equipment, calculations, and approaches appropriate for different floral morphologies. We also provide a troubleshooting guide for common problems encountered while collecting nectar in the field. To demonstrate the utility and effectiveness of our methods for collecting nectar from many different species, we present results on nectar trait variation from 53 species in an ecosystem. Our method illustrates that nectar traits vary considerably within and among plant species, indicating that large-scale nectar sampling projects are an important consideration for many basic and applied questions in pollination ecology and evolution. We hope that across many plant communities and ecosystems, our paper provides a practical roadmap for how to navigate the complexities of quantifying floral nectar traits.

Colony foraging activity of four Melipona species (Apidae: Meliponinae, tribe Meliponini) was studied during the dry season, when many plants flower in central Panama. The efficiency of sucrose solution uptake by Melipona was compared to that of domesticated European Apis mellifera. Dynamics of nectar foraging were also recorded for 3 of the Melipona visiting the forest shrub, Hybanthus prunifolius (Violaceae). 1. Sugar concentration in nectar brought to nests averaged from 21 to 60% sugar for 15 colonies of M. fasciata, M. compressipes triplaridis, M. fuliginosa and M. marginata micheneri. Concentrations ranged from 19 to 72%, and all species collected nectars ranging at least between 24 and 63% sugar. However, M. compressipes and M. marginata preferred higher concentrations and foraged less on dilute nectars. Peak colony nectar harvest occured in late morning or early afternoon; peak pollen harvest was in early morning. 2. Imbibing rates of bees given 20, 30, 45, 60 or 70% sucrose solutions were highest at ≦45% sucrose, but caloric intake was most rapid at 60% sucrose for all species. All but M. marginata displayed greater net intake rates than domesticated European Apis mellifera. A foraging choice model incorporating caloric reward and imbibing rates of bees suggests M. compressipes and M. marginata should specialize on richer nectars. Rate of caloric intake per forager weight was higher for all Melipona (0.03-0.13g) than for A. mellifera (0.10 g). 3. The nectar of Hybanthus prunifolius (Violaceae), a shrub pollinated exclusively by Melipona, progressed from 35 to 60% sugar during the day. Bees foraged most when nectar was below 60% concentration, a pattern best explained as the result of intercolony competition and greater availability of lower quality nectar. 4. Sugar concentration in nectar harvested by colonies rose from lower to higher values through the day for Melipona. The increasing caloric reward of nectar is adaptive in exploiting foraging preferences of such bees. As standing nectar crop is depleted by competing bees, a gradual shift to more rewarding nectar should promote increased bee foraging range, more flower visits during a foraging trip, floral constancy, and genetic outcrossing. 5. The nectar load capacity of A. mellifera is greater than that of Melipona. Other factors being equal, Africanized A. mellifera, now a permanent resident of neotropical forests, should visit more flowers during a foraging trip. Additional species differences in foraging behavior are analyzed.

Pollinator declines caused by forage habitat loss threaten insect pollination services. Pollinating insects depend on adequate floral resources, and their ability to track these resources. Variability of these resources and the effect on insect foraging choice is poorly understood. We record patterns of visitation to six wildflower species and test the hypotheses that: pollinators preferentially visit the most rewarding flowers; nectar diurnal variations affect foraging preferences; pollinators respond most strongly to nectar rewards. Nectar volume and sugar concentration were negatively correlated within plant species over time of day where greater concentration and lower volume was evident in the afternoon, but this did not correspond to pollinator visitation. Both floral abundance and nectar quality (total sugar per inflorescence) positively affect insect visitation. For some foragers, the positive effects of high‐quality rewards were only evident when floral abundance was high (>50 inflorescences per patch), perhaps reflecting the low probability of pollinators detecting scarce rewards. Pollen quality (total protein per inflorescence) was negatively related to visitation of Apis mellifera and Bombus pascuorum . Fewer pollinators visiting flowers of higher pollen quality could reflect plant allocation trade‐offs or the presence of secondary metabolites in pollen, meaning pollen foraging is likely affected by factors other than protein concentration. Nectar rather than pollen appeared to be the main driver of floral choice by insects in this system. Conservation schemes for bees in farmland or gardens might benefit from ensuring that rewarding plant species are present at high density and/or are aggregated in space.

The sugar concentration of floral nectar is a key metric for describing nectar composition and a major factor influencing pollinator visitation to flowers. Across pollination biology research there are multiple approaches in use for describing nectar sugar concentration. With these different approaches there are several potential sources of confusion which, if not accounted for, can lead to errors. Further potential for error arises if researchers wish to make comparisons between the energetic content of nectars containing different ratios of sucrose, fructose and glucose. Regardless of whether concentration is measured per mole or per unit mass, the energetic content differs between the hexose sugars (glucose and fructose) and sucrose. Appropriate conversion is needed for direct comparison. Here we address these two issues with the following aims. We consolidate the literature on this topic with examples of the different methods for reporting nectar sugar concentrations, provide insight into potential sources of error, and derive equations for converting between the different ways of expressing sugar concentration for the three primary nectar sugars: sucrose, glucose and fructose. Second, we discuss the relative energetic content of sucrose, glucose, and fructose, and rationalise adjustment of ‘energetic value’ rather than reporting concentration directly. In this way, we hope to harmonise ongoing work in pollination ecology.

Flowers morphology and nectar concentration determine the preferred food source of stingless bee, Heterotrigona itama

Urbanization is a leading threat to biodiversity, but scientifically informed management of urban ecosystems can mitigate negative impacts. For wild bees, which are declining worldwide, careful consideration of flower choice in public and private green spaces could help preserve their diversity. While floral density and species richness are both linked to wild bee diversity, the mechanisms underlying these relationships are not fully understood. Here, we tested two hypotheses relating the influence of floral trait composition to bee species richness, which we have termed the within-trait diversity and optimal floral trait hypotheses. Specifically, we assessed whether variation in bee richness relates to variation in the weighted variance (trait diversity) and mean (optimal trait) of floral traits observed in urban green spaces across the city of Montreal, Canada. Our analyses focused on two floral traits relating to pollinator feeding success: nectar sugar concentration and corolla length. After accounting for variation in floral density among sites, bee richness was positively related to community-weighted variance in corolla length, supporting the within-trait diversity hypothesis. These findings suggest that management practices that increase the diversity of flower morphologies in urban green spaces can promote the persistence of wild bee communities in cities.

Comparettia falcata is an epiphytic, neotropical orchid that produces nectar as a pollinator reward. In Puerto Rico, C. falcata is allogamous and pollinated by the endemic hummingbird Chlorostilbon maugaeus. Autogamous pollinations are possible, but may result in reduced fruit set. For the 1989 and 1990 flowering seasons, the probability of pollinarium removals and natural pollinations increased with individual inflorescence display size. However, the frequency of effective pollinator visits was independent of flowering phenology in both years. A positive correlation between inflorescence size and reproductive success occurred in 1990 but not in 1989. In 1990 plants produced longer spurs, a higher standing crop of nectar, and a more concentrated nectar than in 1989. There was no relation between nectar availability (= standing crop of nectar) and sugar concentration in either year. Nectar availability and sugar concentration did not vary among the first four flowers of an inflorescence in either season. Nectar availability was not a good predictor of effective visitation. Comparettia falcata has a higher natural fruit set than tropical deceptive orchids, suggesting that pollinator visitation may be enhanced by nectar reward. The small, dilute nectar volumes secreted by C. falcata may benefit the plant by increasing interplant pollinator movement and pollen dispersal.