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Do flowers removed of either nectar or pollen attract fewer bumblebee pollinators? An experimental test in Impatiens oxyanthera.

ABSTRACT: Pollen and nectar are the primary rewards offered by flowers to pollinators. In floral visitors of some plant species, pollen thieves and nectar robbers cause the reduction in pollen grain number and nectar volume, respectively. However, it remains unclear whether the absence of either of the two rewards in a given flower reduces its attraction to nectar- and pollen-collecting pollinators. We hypothesized that flowers removed of either nectar or pollen would attract fewer pollinators. We studied protandrous Impatiens oxyanthera, whose flowers provide bumblebee pollinators with both nectar and pollen in the male phase. We conducted floral reward manipulation experiments to explore how the removal of either nectar or pollen from flowers influences pollinator behaviour by comparing their visitation rates and visit duration. Compared with the control flowers, the flowers removed of pollen attracted significantly more bumblebee pollinators per 30 min, but the flowers removed of nectar or those removed of both pollen and nectar attracted significantly fewer bumblebee pollinators per 30 min. Moreover, the visit duration of bumblebee pollinators to control flowers or flowers removed of pollen was longer than that to flowers removed of nectar or those removed of both pollen and nectar. Our investigations indicated that compared with control flowers, the flowers removed of nectar attracted fewer bumblebee pollinators, supporting our hypothesis. However, our other hypothesis that pollen removal would reduce pollinator visits was not supported by our results. Instead, compared with control flowers, the flowers that contained only nectar attracted more bumblebee pollinators. Nectar seems to be the main reward, and bumblebee pollinators mainly used the absence of pollen as a visual signal to locate I. oxyanthera flowers with a potentially higher amount of nectar.

SUBMITTER: Li DF

PROVIDER: S-EPMC8255076 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Planted meadows are increasingly used to improve the biodiversity and aesthetic amenity value of urban areas. Although many 'pollinator-friendly' seed mixes are available, the floral resources these provide to flower-visiting insects, and how these change through time, are largely unknown. Such data are necessary to compare the resources provided by alternative meadow seed mixes to each other and to other flowering habitats. We used quantitative surveys of over 2 million flowers to estimate the nectar and pollen resources offered by two exemplar commercial seed mixes (one annual, one perennial) and associated weeds grown as 300m2 meadows across four UK cities, sampled at six time points between May and September 2013. Nectar sugar and pollen rewards per flower varied widely across 65 species surveyed, with native British weed species (including dandelion, Taraxacum agg.) contributing the top five nectar producers and two of the top ten pollen producers. Seed mix species yielding the highest rewards per flower included Leontodon hispidus, Centaurea cyanus and C. nigra for nectar, and Papaver rhoeas, Eschscholzia californica and Malva moschata for pollen. Perennial meadows produced up to 20x more nectar and up to 6x more pollen than annual meadows, which in turn produced far more than amenity grassland controls. Perennial meadows produced resources earlier in the year than annual meadows, but both seed mixes delivered very low resource levels early in the year and these were provided almost entirely by native weeds. Pollen volume per flower is well predicted statistically by floral morphology, and nectar sugar mass and pollen volume per unit area are correlated with flower counts, raising the possibility that resource levels can be estimated for species or habitats where they cannot be measured directly. Our approach does not incorporate resource quality information (for example, pollen protein or essential amino acid content), but can easily do so when suitable data exist. Our approach should inform the design of new seed mixes to ensure continuity in floral resource availability throughout the year, and to identify suitable species to fill resource gaps in established mixes.

Project description:Some pollination systems, such as buzz-pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the "fit" between flower and pollinator influences pollen transfer, particularly among buzz-pollinated species. Here we use Solanum rostratum, a buzz-pollinated plant with dimorphic anthers and mirror-image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz-pollinated species. We speculate that in buzz-pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).

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Do flowers removed of either nectar or pollen attract fewer bumblebee pollinators? An experimental test in Impatiens oxyanthera.

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