Total Nectar Production Research Articles

Phenology of farmland floral resources reveals seasonal gaps in nectar availability for bumblebees

Abstract Floral resources are known to be important in regulating wild pollinator populations and are therefore an important component of agri‐environment and restoration schemes which aim to support pollinators and their associated services. However, the phenology of floral resources is often overlooked in these schemes — a factor which may be limiting their success. Our study characterizes and quantifies the phenology of nectar resources at the whole‐farm scale on replicate farms in Southwestern UK throughout the flowering season. We quantify the corresponding nectar demands of a subset of common wild pollinators (bumblebees) to compare nectar supply and pollinator demand throughout the year, thereby identifying periods of supply‐demand deficit. We record strong seasonal fluctuations in farmland nectar supplies, with two main peaks of nectar production (May and July) and a considerable “June Gap” in between. March and August/September are also periods of low nectar availability. Comparing the phenology of nectar supply with the phenology of bumblebee nectar demand reveals “hunger gaps” during March and much of August/September when supply is unlikely to meet demand. Permanent pasture and woodland produced the greatest share of farmland nectar because of their large area; however, linear features such as hedgerows and field margins provided the greatest nectar per unit area. Fifty percent of total nectar was supplied by just three species (Allium ursinum, Cirsium arvense and Trifolium repens), but some less productive species (e.g. Hedera helix and Taraxacum agg.) were important in ensuring phenological continuity of nectar supply. Synthesis and applications. By comparing the phenology of farmland nectar supply with the phenology of pollinator demand, we demonstrate that the timing of nectar supply may be as important as total nectar production in limiting farmland pollinator populations. Considering phenology in the design of agri‐environment or restoration schemes is therefore likely to improve their suitability for pollinators. Plant species which flower during periods of resource deficit (early spring and late summer) should be prioritized in schemes which aim to conserve or restore pollinator populations. Maintaining a range of semi‐natural habitats with complementary flowering phenologies (e.g. woodland, hedgerows and field margins) will ensure a more continuous supply of nectar on farmland, thereby supporting pollinators for their entire flight season.

Nectar ecology of the endemic epiphytic hummingbird-pollinated bromeliad Vriesea altodaserrae: secretion dynamics and pollinator visitation pattern

ABSTRACT Hummingbirds are the main pollinators of most bromeliad species, whose nectar traits usually respond to the selective pressures imposed by pollinators. Considering the specialization of hummingbird-pollinated bromeliads, we expect a close relationship between nectar ecophysiology and the needs of the main pollinators. In this sense, we studied the nectar ecology of the endemic epiphytic bromeliad Vriesea altodaserrae by assessing its nectar traits to address the following questions: i) do flowers respond to successive experimental removals of nectar? ii) is hummingbird visitation frequency related to nectar secretion pattern? We found that V. altodaserrae depended completely on hummingbirds for sexual reproduction, and nectar composition was consistent with that of most hummingbird-pollinated species. Most of the nectar was secreted at bud stage and, if not removed, flowers reabsorb it at the end of their lifespan. Total nectar production did not change after successive removals, and nectar secretion rhythm did not affect the frequency of hummingbird visits. Vriesea altodaserrae was visited by two-thirds of the hummingbird species recorded at the study site, but especially by those of Trochilinae subfamily, suggesting specialization for this group of hummingbirds and highlighting the importance this endemic bromeliad as a keystone species in areas of highland Atlantic forest.

Nectar ecology of the endemic epiphytic hummingbird-pollinated bromeliad Vriesea altodaserrae: secretion dynamics and pollinator visitation pattern

ABSTRACT Hummingbirds are the main pollinators of most bromeliad species, whose nectar traits usually respond to the selective pressures imposed by pollinators. Considering the specialization of hummingbird-pollinated bromeliads, we expect a close relationship between nectar ecophysiology and the needs of the main pollinators. In this sense, we studied the nectar ecology of the endemic epiphytic bromeliad Vriesea altodaserrae by assessing its nectar traits to address the following questions: i) do flowers respond to successive experimental removals of nectar? ii) is hummingbird visitation frequency related to nectar secretion pattern? We found that V. altodaserrae depended completely on hummingbirds for sexual reproduction, and nectar composition was consistent with that of most hummingbird-pollinated species. Most of the nectar was secreted at bud stage and, if not removed, flowers reabsorb it at the end of their lifespan. Total nectar production did not change after successive removals, and nectar secretion rhythm did not affect the frequency of hummingbird visits. Vriesea altodaserrae was visited by two-thirds of the hummingbird species recorded at the study site, but especially by those of Trochilinae subfamily, suggesting specialization for this group of hummingbirds and highlighting the importance this endemic bromeliad as a keystone species in areas of highland Atlantic forest.

PIN6 is required for nectary auxin response and short stamen development

The PIN family of proteins is best known for its involvement in polar auxin transport and tropic responses. PIN6 (At1g77110) is one of the remaining PIN family members in Arabidopsis thaliana to which a biological function has not yet been ascribed. Here we report that PIN6 is a nectary-enriched gene whose expression level is positively correlated with total nectar production in Arabidopsis, and whose function is required for the proper development of short stamens. PIN6 accumulates in internal membranes consistent with the ER, and multiple lines of evidence demonstrate that PIN6 is required for auxin-dependent responses in nectaries. Wild-type plants expressing auxin-responsive DR5:GFP or DR5:GUS reporters displayed intense signal in lateral nectaries, but pin6lateral nectaries showed little or no signal for these reporters. Further, exogenous auxin treatment increased nectar production more than tenfold in wild-type plants, but nectar production was not increased in pin6 mutants when treated with auxin. Conversely, the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) reduced nectar production in wild-type plants by more than twofold, but had no significant effect onpin6lines. Interestingly, a MYB57 transcription factor mutant, myb57-2, closely phenocopied the loss-of-function mutant pin6-2. However, PIN6 expression was not dependent on MYB57, and RNA-seq analyses of pin6-2 and myb57-2 mutant nectaries showed little overlap in terms of differentially expressed genes. Cumulatively, these results demonstrate that PIN6 is required for proper auxin response and nectary function in Arabidopsis. These results also identify auxin as an important factor in the regulation of nectar production, and implicate short stamens in the maturation of lateral nectaries.

Does spatial aggregation of total nectar production lead to genetic structure?

Abstract Spatial aggregation of plants of high nectar production, receiving an enhanced pollinator service is known to occur in Echium vulgare . Moreover, an emanating effect of nectar production on pollinator visits may occur, i.e. many pollinator visits may be observed around high nectar patches. Consequently, gene flow within patches of plants of high nectar production and their close neighbours may result in genetic structure. In this study, we investigated whether aggregation of total nectar production (nectar production per flower×number of flowers) and its emanating effect resulted in genetic structure in a natural E. vulgare population. We compared the spatial structure of total nectar production, pollinator visits and microsatellite markers using spatial autocorrelation analysis. Increased geitonogamy, caused by longer boutlengths in plants of high nectar production may generate genetic structure. We estimated selfing rates of plants of the highest and lowest total nectar production. Spatial aggregation of total nectar production occurred on a relatively small scale up to 2.83 m. A significant emanating correlation between total nectar production and pollinator visits was observed on a relatively large scale up to a 4.24 m. Thus, around patches of high nectar production numbers of pollinator visits were relatively high, while few visits were observed around patches of low nectar production. Weak genetic structure was present on a small scale up to 2.20 m. This corresponded with the scale of aggregation of total nectar production. High gene flow around the patches of high nectar production seems to weaken genetic structure. This is supported by the relatively low selfing rates. The average selfing rate of the plants of highest nectar production was 8.8% and that of the plants of lowest nectar production 5.0%. Low gene flow within and around low nectar patches sustain a weak genetic structure or, conversely, may have caused it in the first instance. Results indicate the importance of spatial structure of nectar production for pollinator movement.

Pollination ecology of Penstemon roseus (Plantaginaceae), an endemic perennial shifted toward hummingbird specialization?

between populations, we found that total nectar production had no effect on seed production at either site. The daily nectar secretion rate of 0.3‐0.65 mg sugar per flower per 1‐3 days was low relative to other hummingbird-adapted Penstemon species (typical range: 1.5‐5 mg sugar per flower), and it might be intermediate between hummingbird- and bee-adapted Penstemon flowers. Our results support the hypothesis about a shift toward hummingbird pollination, and provide an example of a ‘despecialized’ Penstemon species, which attracts high-energy pollinators (hummingbirds) and profits from outcrossing, but retains bee-syndrome floral traits and low sugar production rates.

The Relationship Between Prey Capture and Characteristics of the Carnivorous Pitcher Plant, Sarracenia Alata Wood

ABSTRACT We examined the relationship between pitcher characteristics (sugar concentration in nectar, percentage of red coloration and three indices of size) and prey capture in three populations of the carnivorous plant, Sarracenia alata Wood. The indices of size (height, funnel diameter and hood area) were highly correlated in all three populations. Pitcher size and mass of prey capture per day differed significantly among populations. Pitcher size was significantly positively related to total mass of prey capture per day, explaining 37–76% of the variation depending on the population. The sugar concentration in nectar and the red coloration as we measured them were not important in determining prey capture. Examining alternative measurements of nectar (e.g., total nectar production and/or presence of amino acids), volatiles and the presence of UV reflectance patterns may yield further insight into the relationship between pitcher characteristics and prey capture.

Nectar removal effects on seed production in Moussonia deppeana (Gesneriaceae), a hummingbird-pollinated shrub

Animal-pollinated angiosperm plants that respond positively to nectar removal by replenishment invest energy that can entail a reproductive cost. Here we use nectar removal manipulation to investigate whether nectar removal increases total nectar production in Moussonia deppeana (Gesneriaceae) and to see if this leads to lower seed production. We found that flowers strongly respond to increased nectar removal by producing 2–4 times as much additional nectar. Other nectar removal treatments had little effect on nectar production and rule out alternative explanations at the flower level. Compared with open-pollinated flowers, hand cross-pollination yielded similar seed production in flowers with experimentally increased nectar removal. Although flower-level effects of nectar replenishment on seed production were not detected in M. deppeana, costs of nectar replenishment need further study as tradeoffs between nectar production and seed production could be expressed at the plant level and/or as long-term effects.

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

Summary The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare . Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.

Differential nectar production between male and female flowers in a wild cucurbit: Cucurbita maxima ssp. andreana (Cucurbitaceae)

In dioecious and monoecious plants that depend on animal vectors for reproduction, pollinators have to be attracted to male and female flowers for pollination to be effective. In the monoecious Cucurbita maxima ssp. andreana, male flowers are produced in greater quantity, are spatially more exposed to pollinators and offer pollen in addition to nectar as floral rewards. Nectar traits were compared between male and female flowers to determine any differences in the characteristics of the main reward offered to pollinators. Nectar chemical composition and sugar proportions were similar between flower types. Total nectar sugar production per female flower was threefold higher than per male flower, and nectar removal did not have any effect on total nectar production in both flower morphs. Pollinators reduced nectar standing crops to similar and very scarce amounts in both flower types. Results indicate indirectly that pollinators are consuming more nectar from female flowers, suggesting that the higher nectar production in female flowers may be a reward-based strategy to achieve the high female reproductive output observed in this species.Key words: Cucurbitaceae, Cucurbita maxima ssp. andreana, nectar production, nectar sugar composition, removal effects, standing crop.

Characteristics of secretion of nectar in Pyrostegia venusta (Ker‐Gawl.) Miers (Bignoniaceae)

summarySecretion patterns and characteristics of nectar of an Argentinian population of Pyrostegia venusta at different times of the day are described with respect to flower age. Flowers are protandrous and hummingbird‐pollinated, producing exudate before and after anthesis. There is a concomitant increase in both concentration and volume in 1‐d‐old flowers, followed by a sharp increase in concentration and decrease in nectar volume in 2‐d‐old flowers.Removal of nectar throughout the flower lifetime increases the volume produced but reduces the concentration. Flowers with nectar removed three times produced 73% more nectar volume than the controls, and those with nectar removed twice produced 20% more. However, the total mass of sugar produced was comparable in all three sets.Two experiments were designed to determine the effect of changes in volume and concentration on nectar production: (a) the concentration was initially maintained while the volume was changed (halved, doubled and untouched as a control), and (b) the concentration was reduced to a minimum replacing nectar by different volumes of distilled water as in the previous experiment. These concentration‐volume changes do not essentially affect total nectar production irrespective of treatment. A powerful homeostatic mechanism would operate by which the plant maintains both these variables in the nectar. The differences observed in the nectar production rate among treatments would indicate its existence.

Nectar secretion pattern and removal effects in three species of Solanaceae

Nectar secretion pattern and effects of nectar removal were compared in three Argentinean Solanaceae: Nicotiana glauca, which is hummingbird pollinated, and Nicotiana longiflora and Petunia axillaris, which are sphinx moth pollinated. Nectar volume, concentration, and sugar production were measured at different time intervals according to the species' flower life span. Nectar volume and total sugar production increased as a function of flower age in both species of Nicotiana analyzed; however, these parameters were quite stable in P. axillaris. This species produced less nectar and nectar sugar than the other two. When all sets with nectar removal were compared with the controls, significant differences were found in nectar volume and quantity of nectar sugar in N. longiflora, and in sugar concentration in P. axillaris. In the latter, the observed difference did not affect the total amount of sugar secreted. Nicotiana glauca did not show any significant difference in the parameters analyzed. Total nectar production was inhibited by periodic removal in N. longiflora, while in N. glauca and P. axillaris it was unaffected. Key words: nectar secretion pattern, nectar removal, Nicotiana, Petunia, Solanaceae.

Insect Dispersal of Fungal Disease: Effects of Ustilago Infection on Pollinator Attraction in Viscaria vulgaris

Transmission efficiency is an important part of plant-disease interactions. Here I report on differences in insect-attracting traits of healthy and fungus-infected plants, in a system whose pollinating insects disperse fungal spores. Viscaria vulgaris (Caryophyllaeae) infected by the anther smut, Ustilago violacea, bloomed earlier and remained open longer than healthy plants. Infected flowers produced significantly less nectar than did healthy ones, but total nectar production per inflorescence was similar in the two types, because infected plants produced more flowers per inflorescence. Sugar content of nectar was similar in infected and healthy flowers. I trapped few airborne anther smut spores, suggesting that pollinating insects are the important disease vector. Bumblebees, the major dispersers of pollen and spores, visited fewer infected flowers than expected by chance. The early flowering of infected flowers forces naive pollinators to visit infected flowers, thus they become disease vectors even though they prefer healthy plants later in the flowering season. Spore dispersal reached its peak before that of Viscaria pollen, suggesting a very efficient dispersal system.

Bumblebee Response to Variation in Nectar Availability

I examined the response of bumblebees to two kinds of spatial variation and two kinds of temporal variation in nectar levels. The spatial variation involved differences in reward value among plant species and differences in nectar availability among patches of flowers of a single species. The temporal variation involved changes in nectar availability over a season and between years. In all cases the response of bees was measured by differences in bee density or abundance. The response of bees of different plant species was determined by the density of foraging bees on the flowers of each species at a given time (bee:flower ratio = B/F). I examined several groups of plant species whose members overlapped in their flowering periods and in their bumblebee visitors. Within a group, the relative magnitudes of the B/F's for species indicate the degree to which their flowers are preferred by bees. If bees are optimal foragers, preference should reflect the per—flower reward value of species. I characterized the potential reward value of species by their nectar production rates (NPR's). The bees' preference for species was found to be very similar to the relative magnitudes of their NPR's. I also considered whether including the time and energy costs of foraging on the different species would produce a better predictor of preference. While these factors did affect the absolute reward value of species they did not influence their reward value relative to one another. Foraging costs are most likely to have no effect on preference when species have similar floral morphologies. To assess the response of bees to differences in patch quality I bagged some of the inflorescenes in an experimental patch. Following this reduction in the number of flowers, the number of bees in the patch decreased proportionately, producing a B/F very similar to that of a control patch. When the bagging was removed the number of bees increased such that the B/F was again similar to that of the control patch. In general these results provide further support for optimal foraging in bumblebees. I examined the response of bees to temporal variation in nectar levels to determine whether resource limitation for bumblebees change over time. Seasonal changes in the abundances of three bumblebee species were compared with seasonal changes in the total nectar production by the plants each bee visited. Preference factors were used to convert floral production to nectar production. In general, seasonal production and utilization were closely matched, indicating no major change in nectar limitation. The cumulative numbers of bees seen during each of two summer seasons (1974 and 1975) were very similar. This indicates that the resource base, which had not changed between years, was capable of supporting only a limited number of bees. During 1975 a natural species removal experiment occurred which allowed an assessment of competition among bees. Apis mellifera, which had been well represented in 1974, was absent. The bumblebees of short and medium tongue length with which honeybees overlapped in resource used exhibited competitive release. Their abundances increased in 1975 resulting in near—perfect density compensation.