Abstract
Main conclusionSugars (glucose, fructose and sucrose), as well as proteogenic and non-proteogenic amino acids, are present in the nectar of Platanthera bifolia and P. chlorantha.Nectar quantity and quality are floral traits that are subjected to pollinator-mediated selection. Nectar sugar and amino acid (AA) composition in two sister species, P. bifolia and P. chlorantha, was analysed and the interspecies differences in nectar and the importance of these nectar characteristics for reproductive success were investigated. Nectar was collected from four P. bifolia and three P. chlorantha populations that exist in different habitats in three regions of NE Poland. Nectar from about 30 flowers (from each population) was sampled and analysed using high-performance liquid chromatography. We found the same primary sugars and AA components in the nectar of both species, although their content varied between the populations according to habitat properties. The nectar of P. bifolia and P. chlorantha both had low sugar concentrations (9.04–20.68%) and were dominated by hexoses, with sucrose:hexoses ratios between 0.03 and 0.31 across the different populations (the average for the P. bifolia populations was 0.17 and the average for the P. chlorantha populations was − 0.05). Total sugar content did not influence reproductive success and we found positive selection on fructose content. In general, 23 different AAs were detected in both Platanthera species. Cysteine and γ-aminobutyric acid were present in only one population of P. chlorantha. Sarcosine dominated among the non-proteogenic AAs. To our knowledge, this is the first report that characterizes the sugar and AA profiles in the nectar of P. bifolia and P. chlorantha in natural populations in the context of effectiveness of reproduction. Total AAs negatively influenced male reproductive success (r = − 0.79). Pollinators of the investigated species were found to be sensitive to the AAs’ taste, from taste classes I and IV. Correlation between male reproductive success and the content of AAs from these groups was 0.79 in both cases. In this manuscript, we investigated the characteristics of P. bifolia and P. chlorantha nectar, and compared these characteristics to the available data in the context of their adaptations to the requirements of pollinators and with regard to the importance of nectar quality for reproductive success of the studied species.
Highlights
The wide varieties of flower types that have evolved in plants are adapted in different ways to their pollinators
Fruiting shaped at the lowest level in two P. bifolia populations (17.8% in BC and 26.6% in POB1), while the highest was in BF P. chlorantha and in SMOL P. bifolia populations
Significant differences are reported for the levels of reproductive success between populations of P. bifolia (FRS: F = 60.128; Male reproductive success (MRS): F = 9.274; p < 0.001) and P. chlorantha (FRS: F = 7.624; MRS: F = 11.215; p < 0.001)
Summary
The wide varieties of flower types that have evolved in plants are adapted in different ways to their pollinators. The structure of the flower and other traits that include colour or scent are often subjected to pollinator-mediated selection (Maad 2000; Leiss and Klinkhamer 2005; Nepi et al 2018; Parachnowitsch et al 2019) One of these traits being the evolutionary answer for pollinators’ preferences is nectar. Many studies document that nectar concentration and composition are often linked with pollinator types Hawk moths feed primarily while hovering and mainly at night (at lower temperatures); this type of activity requires more energy (Willmer 2011; Fowler et al 2016)
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