Nectar Chemistry or Flower Morphology—What Is More Important for the Reproductive Success of Generalist Orchid Epipactis palustris in Natural and Anthropogenic Populations?

The aim of this study was to determine the level of reproductive success (RS) in natural and anthropogenic populations of generalist orchid Epipactis palustris and its dependence on flower structure and nectar composition, i.e., amino acids and sugars. We found that both pollinaria removal and female reproductive success were high and similar in all populations, despite differences in flower traits and nectar chemistry. Flower structures were weakly correlated with parameters of RS. Nectar traits were more important in shaping RS; although, we noted differentiated selection on nectar components in distinct populations. Individuals in natural populations produced nectar with a larger amount of sugars and amino acids. The sucrose to (fructose and glucose) ratio in natural populations was close to 1, while in anthropogenic ones, a clear domination of fructose and glucose was noted. Our results indicate that the flower traits and nectar composition of E. palustris reflect its generalist character and meet the requirements of a wide range of pollinators, differing according to body sizes, mouth apparatus, and dietary needs. Simultaneously, differentiation of nectar chemistry suggests a variation of pollinator assemblages in particular populations or domination of their some groups. To our knowledge, a comparison of nectar chemistry between natural and anthropogenic populations of orchids is reported for the first time in this paper.

Floral Nectar Chemistry in Orchids: A Short Review and Meta-Analysis

Nectar is one of the most important flower traits, shaping plant–pollinator interactions and reproductive success. Despite Orchidaceae including numerous nectariferous species, nectar chemistry in this family has been infrequently studied. Therefore, the aim of this study is to compile data about nectar attributes in different orchid species. The scarcity of data restricted analyses to sugar concentration and composition. Our results suggest that the most important factor shaping nectar traits in orchids is the pollinator type, although we also found differentiation of nectar traits according to geographical regions. In spurred orchids, the length of the spur impacted nectar traits. We recommend the development of studies on nectar chemistry in orchids, including a wider range of species (both in taxonomic and geographical contexts), as well as extending the analyses to other nectar components (such as amino acids and secondary metabolites). The nectar biome would be also worth investigating, since it could affect the chemical composition of nectar. This will enrich the understanding of the mechanisms of plants–pollinators interactions.

How Are the Flower Structure and Nectar Composition of the Generalistic Orchid Neottia ovata Adapted to a Wide Range of Pollinators?

Plant-pollinator interactions significantly influence reproductive success (RS) and drive the evolution of pollination syndromes. In the context of RS, mainly the role of flower morphology is touched. The importance of nectar properties is less studied, despite its significance in pollination effectiveness. Therefore, the aim of this study was to test selection on flower morphology and nectar chemistry in the generalistic orchid Neottia ovata. In 2019–2020, we measured three floral displays and six flower traits, pollinaria removal (PR), female reproductive success (FRS), and determined the soil properties. The sugars and amino acids (AAs) were analyzed using the HPLC method. Data were analyzed using multiple statistical methods (boxplots, ternary plot, one-way ANOVA, Kruskal-Wallis test, and PCA). Variation of flower structure and nectar chemistry and their weak correlation with RS confirms the generalistic character of N. ovata. In particular populations, different traits were under selection. PR was high and similar in all populations in both years, while FRS was lower and varied among populations. Nectar was dominated by glucose, fructose, and included 28 AAs (Ala and Glu have the highest content). Sugars and AAs influenced mainly FRS. Among soil parameters, carbon and carbon:nitrogen ratio seems to be the most important in shaping flower structure and nectar chemistry.

Intraspecific Variation in Nectar Chemistry and Its Implications for Insect Visitors: The Case of the Medicinal Plant, Polemonium Caeruleum L.

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.

Inter-Individual Nectar Chemistry Changes of Field Scabious, Knautia arvensis

Nectar is crucial to maintain plant-pollinator mutualism. Nectar quality (nutritional composition) can vary strongly between individuals of the same plant species. The factors driving such inter-individual variation have however not been investigated closer. We investigated nectar quality of field scabious, Knautia arvensis in different grassland plant communities varying in species composition and richness to assess whether nectar quality can be affected by the surrounding plant community. We analyzed (with high performance liquid chromatography) the content of carbohydrates, overall amino acids, and essential amino acids. Amino acid and carbohydrate concentrations and proportions varied among plant individuals and with the surrounding plant community but were not related to the surrounding plant species richness. Total and individual carbohydrate concentrations were lowest, while proportions of the essential amino acids, valine, isoleucine, leucine (all phagostimulatory), and lysine were highest in plant species communities of the highest diversity. Our results show that K. arvensis nectar chemistry varies with the composition of the surrounding plant community, which may alter the taste and nutritional value and thus affect the plant’s visitor spectrum and visitation rate. However, the strong inter-individual variation in nectar quality requires additional studies (e.g., in semi-field studies) to disentangle different biotic and abiotic factors contributing to inter-individual nectar chemistry in a plant-community context.