Abstract
Nectar composition varies between species, depending on flowering time and pollinator type, among others. Various models of the biochemical and molecular mechanisms underlying nectar production and secretion have been proposed. To gain insights into these mechanisms, day- and night-flowering tobacco (Nicotiana) species with high or low proportions of hexoses in the nectar were analyzed. Nectar and nectaries were simultaneously collected, throughout the day and night. Soluble sugars and starch were determined and the activity and expression level of cell wall invertase (CW-INVs) were measured in nectaries. Nectaries and nectar of the five Nicotiana species contained different amounts of sucrose, glucose, and fructose. CW-INV activity was detected in the nectaries of all Nicotiana species and is probably involved in the hydrolysis of sucrose in the nectary tissue and during nectar secretion. The larger differences in the sucrose-to-hexose-ratio between nectaries and nectar in diurnal species compared to nocturnal species can be explained by higher sucrose cleavage within the nectaries in night-flowering species, and during secretion in day-flowering species. However, cell wall invertase alone cannot be responsible for the differences in sugar concentrations. Within the nectaries of the Nicotiana species, a portion of the sugars is transiently stored as starch. In general, night-flowering species showed higher starch contents in the nectaries compared to day-flowering species. Moreover, in night flowering species, the starch content decreased during the first half of the dark period, when nectar production peaks. The sucrose concentrations in the cytoplasm of nectarial cells were extrapolated from nectary sucrose contents. In day-flowering species, the sucrose concentration in the nectary cytoplasm was about twice as high as in nectar, whereas in night-flowering species the situation was the opposite, which implies different secretion mechanisms. The secreted nectar sugars remained stable for the complete flower opening period, which indicates that post-secretory modification is unlikely. On the basis of these results, we present an adapted model of the mechanisms underlying the secretion of nectar sugars in day- and night-flowering Nicotiana.
Highlights
Nectar is a sugar-rich solution which is produced by most angiosperm plants to fulfill extensive functions, e.g., the attraction of pollinators and protection against herbivores (Brandenburg et al, 2009; González-Teuber and Heil, 2009; Adler et al, 2012)
Nicotiana attenuata seeds were provided by the Max Planck Institute for Chemical Ecology (Jena, Germany), N. benthamiana seeds were provided by the University of Rostock (Germany), N. africana and N. sylvestris seeds were provided by the Botanical Garden of Ruhr University Bochum (Germany), and N. tabacum seeds were provided by NiCoTa (Rheinstetten, Germany)
In the case of night-flowering tobacco (N. benthamiana, N. sylvestris), the lowest sugar concentration in nectar was found in the first half of the night period, but the concentration increased during the second half of the night period
Summary
Nectar is a sugar-rich solution which is produced by most angiosperm plants to fulfill extensive functions, e.g., the attraction of pollinators and protection against herbivores (Brandenburg et al, 2009; González-Teuber and Heil, 2009; Adler et al, 2012). In a third proposed secretory mechanism, nectar metabolites are transported symplastically to the outer nectary cells and packed into vesicles, which are produced by the endoplasmic reticulum (ER) or the Golgi complex, to fuse with the plasma membrane and release the nectar metabolites to the nectary surface (Fahn, 1979a,b). These three models for nectar secretion are not necessarily mutually exclusive, and other modes of nectar secretion can occur in different plant species
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