Abstract
Floral nectar contains mainly sugars but also amino acids, organic acids, inorganic ions and secondary compounds to attract pollinators. The genus Nicotiana exhibits great diversity among species in floral morphology, flowering time, nectar compositions, and predominant pollinators. We studied nectar samples of 20 Nicotiana species, composed equally of day- and night-flowering plants and attracting different groups of pollinators (e.g. hummingbirds, moths or bats) to investigate whether sugars, amino acids, organic acids and inorganic ions are influenced by pollinator preferences. Glucose, fructose and sucrose were the only sugars found in the nectar of all examined species. Sugar concentration of the nectar of day-flowering species was 20% higher and amino acid concentration was 2-3-fold higher compared to the nectar of night-flowering species. The sucrose-to-hexose ratio was significantly higher in night-flowering species and the relative share of sucrose based on the total sugar correlated with the flower tube length in the nocturnal species. Flowers of different tobacco species contained varying volumes of nectar which led to about 150-fold higher amounts of total sugar per flower in bat- or sunbird-pollinated species than in bee-pollinated or autogamous species. This difference was even higher for total amino acids per flower (up to 1000-fold). As a consequence, some Nicotiana species invest large amounts of organic nitrogen for certain pollinators. Higher concentrations of inorganic ions, predominantly anions, were found in nectar of night-flowering species. Therefore, higher anion concentrations were also associated with pollinator types active at night. Malate, the main organic acid, was present in all nectar samples but the concentration was not correlated with pollinator type. In conclusion, statistical analyses revealed that pollinator types have a stronger effect on nectar composition than phylogenetic relations. In this context, nectar sugars and amino acids are more strongly correlated with the preferences of predominant pollinators than organic acids and inorganic ions.
Highlights
Floral nectars are aqueous, carbohydrate-rich solutions that are secreted by nectaries of flowering plants
To a lower extent amino acids, organic acids, lipids, proteins, inorganic ions, scents and other secondary compounds are found in nectar [1,2,3], too
Nectar sugar concentration is higher in beepollinated flowers (35% (w/v)) than in flowers pollinated by butterflies or hummingbirds (20– 25% (w/v)) [10], because the optimal nectar concentration is higher for viscous dippers than for suction feeders [11]
Summary
Carbohydrate-rich solutions that are secreted by nectaries of flowering plants. In some studies nectar sugar and/or amino acid compositions have been related to diurnal and nocturnal pollinators [30,31] Since these pollinators visit either day- or night-flowering species those studies have indirectly investigated the influence of flowering time on nectar composition. These investigations are usually limited to sugars and in some cases to amino acids. Other constraints such as phylogenetic relations or ecological conditions could have an impact on nectar composition For this purpose, we investigated primary metabolites, which are involved in fundamental plant biochemistry processes (sugars, amino acids, organic acids), and inorganic ions in the nectar of related species with different pollinators. Such comprehensive studies about the occurrence of amino acids and organic acids among closely related species with different pollination types are rare, but they are necessary for a better understanding of the ecological role of these metabolites in nectar
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