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Abstract
Floral nectar is the most important reward for pollinators and an integral component of the pollination syndrome. Nectar research has mainly focused on sugars or amino acids, whereas more comprehensive studies on the nectar composition of closely related plant species with different pollination types are rather limited. Nectar composition as well as concentrations of sugars, amino acids, inorganic ions, and organic acids were analyzed for 147 species of Bromeliaceae. This plant family shows a high diversity in terms of floral morphology, flowering time, and predominant pollination types (trochilophilous, trochilophilous/entomophilous, psychophilous, sphingophilous, chiropterophilous). Based on the analyses, we examined the relationship between nectar traits and pollination type in this family. Nectar of all analyzed species contained high amounts of sugars with different proportions of glucose, fructose, and sucrose. The total concentrations of amino acids, inorganic cations, and anions, or organic acids were much lower. The analyses revealed that the sugar composition, the concentrations of inorganic cations and anions as well as the concentration of malate in nectar of bat-pollinated species differed significantly from nectar of species with other pollination types. Flowers of bat-pollinated species contained a higher volume of nectar, which results in a total of about 25-fold higher amounts of sugar in bat-pollinated species than in insect-pollinated species. This difference was even higher for amino acids, inorganic anions and cations, and organic acids (between 50 and 100-fold). In general, bat-pollinated plant species invest large amounts of organic and inorganic compounds for their pollinators. Furthermore, statistical analyses reveal that the characteristics of nectar in Bromeliaceae are more strongly determined by the pollinator type rather than by taxonomic groups or phylogenetic relations. However, a considerable part of the variance cannot be explained by either of the variables, which means that additional factors must be responsible for the differences in the nectar composition.
Highlights
The family of the Bromeliaceae is one of the species-richest non-woody plant families in the Neotropics
Nectar samples of plants from different botanical gardens were used and it is conceivable that the growth site has an influence on the nectar composition
The nectar composition of a given species is relatively constant and independent of the locations of the greenhouses and the growth site of the plants (Figure 1). This is in line with a former study that showed that greenhouse and field nectar samples are similar in sugar composition (Krömer et al, 2008; Supplementary Table S6)
Summary
The family of the Bromeliaceae is one of the species-richest non-woody plant families in the Neotropics. It has experienced a remarkable adaptive radiation in the flora and, a wide variety of flower morphology has emerged (Benzing, 2000). The family is divided into eight subfamilies (Brocchinioideae, Lindmanioideae, Tillandsioideae, Hechtioideae, Navioideae, Bromelioideae, Puyoideae, Pitcairnioideae) which subsume approximately 58 genera and more than 3000 species (Givnish et al, 2011). The current taxonomie of Bromeliaceae is in strong flux due to newer morphological and genetic studies, so that new subfamilies or genera are created and species are frequently being assigned to other taxa (Zizka et al, 2013; Barfuss et al, 2016; Gomes-da-Silva and Souza-Chies, 2018). The plant family is diverse in morphological, ecological, or physiological aspects; for example, about 60% of Bromeliaceae are epiphytic (Zotz, 2013) and several species use CAM photosynthesis to produce sugars (Crayn et al, 2015)
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