Nectar quality and pollination in three pairs of congeneric species

Many flowering plants reward animal pollinators through production of food rewards, generally in the form of nectar. Animal pollinators are attracted to flowers, and the nectar rewards they contain, by floral traits including color. The concept of pollination syndromes suggests that floral color and shape are often associated with pollination by particular species of animal visitors, e.g. red tubular flowers are considered to be hummingbird flowers. We examined the relationship between flower color, shape, and nectar reward in three congeneric species pairs of plants that produce conspicuous flowers attractive to animal visitors. The species pairs studied included: 1) jewelweed (Impatiens capensis) and pale jewelweed (Impatiens pallida), 2) Japanese honeysuckle (Lonicera japonica) and trumpet honeysuckle (Lonicera sempervirens), and 3) an ornamental Rhododendron (Rhododendron sp.) and rosebay rhododendron (Rhodendron maximum). For each plant species studied, we measured floral size and three attributes of floral nectar: the volume per flower, the per cent sugar concentration, and the per cent composition of component sugars including sucrose, glucose, and fructose.  All species pairs differed in floral color and in floral size.  For two of three species pairs, honeysuckles and Rhododendrons, we found that total sugar concentration did not differ between species, despite differences in floral color and size; the honeysuckles exhibited significant differences in the relative proportion of glucose and fructose in the nectar. For the third species pair, the jewelweeds, we found that the two species differed in total sugar concentration. Early efforts to document the influence of nectar rewards on patterns of insect visitation to the two jewelweed species suggest that bees preferred flowers of pale jewelweed which produced larger quantities of more sugar-rich nectar.  Further studies will document the response of pollinators to floral traits and nectar quality in the two other congeneric pairs.