Floral nectar, being a primary reward for insect visitors, is a key factor in shaping plant–pollinator interactions. However, little is known about the variability in nectar traits, which could potentially affect pollinators and the reproduction of the species. We investigated intraspecific variation in nectar traits in 14 populations of a Red-listed plant, Polemonium caeruleum. Populations varied in terms of the proportion of self-compatible and self-incompatible individuals, and insect communities visiting flowers. Using HPLC, we determined the nectar sugar and amino acid (AA) composition and concentration. We also recorded some basic habitat parameters, which could influence nectar chemistry. In seven selected populations, we investigated the taxonomic composition of the insects visiting flowers. Our observations revealed significant intraspecific variability in nectar chemistry in P. caeruleum. Nectar production was male-biased, with male-phase flowers secreting sucrose- and AA-rich nectar. An analysis revealed that variability in P. caeruleum nectar may be slightly shaped by environmental factors. The studied nectar characters, especially sugars, had little effect on insects visiting flowers. We argue that variation in nectar traits in this generalist plant is a matter of random genetic drift or “adaptive wandering” rather than directional specialization and adaptation in the most effective and abundant group of pollinators.
Polemonium caeruleum (Polemoniaceae) represents a very interesting system of compatibility transition. Studies of its biological and ecological properties in the context of the breeding system of various populations may help to understand the evolutionary mechanism of this process. We investigated some aspects of the breeding system, diversity and foraging behavior of the visitors, and relationship between population properties and fruit set in three populations from NE Poland. We found distinct compatibility systems in two studied populations and showed that if a population is self-compatible (SC), selfing is mediated by insects via geitonogamous pollen transfer. Despite the population properties (compatibility, visitor diversity and activity, population size, density, or floral display), P. caeruleum is not pollen limited and pollinators are highly important as a key factor determining the high reproductive success. Visitor assemblages (including key pollinators, bumblebees, and honey bees) and their foraging behavior on inflorescences vary between the populations, which may influence differences in the breeding system. The selfincompatible population was visited by a more diverse group of insects from Hymenoptera, Diptera, Lepidoptera, Heteroptera, and Coeloptera, which may favor effective cross-pollen transfer, whereas the SC population was pollinated mainly by Apis mellifera, which may promote mixed-mating. Studies on a wider range of P. caeruleum populations are needed to determine selective factors responsible for compatibility transition.
absTRacT. During the survey of Łempis Nature Reserve (Augustów Forest, NE Poland) 353 species of vascular plants were recorded, including 32 considered threatened with extinction in Poland (7 species from the Polish "red data book" and 32 species from the Polish "red list"). Among them there were 41 species protected under the Polish law, including 19 strictly protected species and four species listed in the Annex II of the EU Habitats Directive. Ten orchid species were recorded, including the most valuable plants of the nature reserve -Cypripedium calceolus, Hammarbya paludosa, Liparis loeselii and Listera cordata. Among 32 noted Cyperaceae species (including 26 members of the Carex genus), there was a number of threatened plants, e.g. Baeothryon alpinum, Carex chordorrhiza, C. loliacea and Eriophorum gracile. Other rare species included Agrimonia pilosa, Cladium mariscus, Laserpitium latifolium, Linnaea borealis, Potamogeton gramineus and Pulsatilla patens. The presence of anthropophytes was significant (4.5% of the flora), probably due to the past forest management practices as well as proximity of settlements. The extremely rich flora, along with pristine wetland ecosystems (mesotrophic lakes, various mire forests and fens), places the Łempis Nature Reserve among the most valuable nature reserves in Poland.key woRDs: red list species, neophytes, ancient forest species, wetland, Cyperaceae, Orchidaceae
Floral nectar is one of the key rewards in the mutualistic interactions between plants and pollinators. However, there is a growing amount of evidence that shows that another group of organisms may be involved in the pollination process, namely the microorganisms, which often inhabit floral nectar. However, little is known about the function and taxonomic diversity of microorganisms inhabiting the nectar of plants. Bacterial communities inhabiting nectar of a rare plant species, <em>Polemonium caeruleum</em>, in one artificial and two natural populations in Poland were analyzed using a metagenomic approach. <em>Polemonium caeruleum</em> is a boreal plant species, and requires appropriate pollinator services for seed production. The reproductive system of this plant may vary between individuals and populations (mixed-mating), as well as with insect visitor assemblages. We considered that nectar-dwelling bacteria of <em>P. caeruleum</em> might affect the insects visiting the plant’s flowers, which in turn can result in changes in the plant’s phenotype and its reproductive system. Bacterial diversity in nectar samples was surveyed using culture-independent 16S rRNA gene amplicon sequencing (MiSeq, Illumina). We found that bacterial communities inhabiting the nectar of <em>P. caeruleum</em> differed between populations, although those differences were mostly quantitative. Many of the identified bacterial genera have been found previously in nectar of other plant species, or in the guts of insect visitors, and are described as tolerant of high sugar concentrations and catalase positive (which allows bacteria to survive in the presence of hydrogen peroxide).
A vast majority of angiosperms are pollinated by animals, and a decline in the number and diversity of insects often affects plant reproduction through pollen limitation. This phenomenon may be particularly severe in rare plant species, whose populations are shrinking. Here, we examined the variability in factors shaping reproductive success and pollen limitation in red-listed Polemonium caeruleum L. During a 5-year study in several populations of P. caeruleum (7–15, depending on year), we assessed the degree of pollen limitation based on differences in seed set between open-pollinated (control) and hand-pollinated flowers. We analysed the effects of flower visitors, population size, and meteorological data on plant reproductive success and pollen limitation. Our study showed that pollen limitation rarely affected P. caeruleum populations, and was present mainly in small populations. Pollen limitation index was negatively affected by the size of population, visitation frequency of all insects, and when considering the visitation frequency of individual groups, also by honeybee visits. Seed production in control treatment was positively influenced by the population size, average monthly precipitation in June and visits of hoverflies, while visits of honeybees, average monthly temperature in September, and average monthly precipitation in August influenced seed production negatively. As generalist plant P. caeruleum can be pollinated by diverse insect groups, however, in small populations their main visitors, the honeybees and bumblebees, may be less attracted, eventually leading to the disappearance of these populations. In pollination of P. caeruleum managed honeybees may play a dual role: while they are the most frequent and efficient flower visitors, their presence decreases seed set in open-pollinated flowers, which is most probably related to efficient pollen collection by these insects.
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