It’s not news to say that flowers have evolved to attract pollinators. There are many studies on how flower shape and colour can attract visitors. However, a new paper by Filip Vandelook, Steven Janssens and colleagues examines another factor, nectar. Their new research shows that nectar has also evolved to meet the needs of pollinators. Their work, in the Balsaminaceae family, shows that not only have plants optimised their nectar – they have done it rapidly.
The Balsaminaceae family is a little odd. There are just two genera in it, Impatiens and Hydrocera. There is only one Hydrocera species known. Impatiens, on the other hand, has over a thousand species. Filip Vandelook said: “Since most species included in the study were Impatiens, of course, the result and interpretation are heavily biased towards this genus. On the other hand, the very high speciation rates and the very high diversity of flowers in Impatiens made this a very interesting group to study rates of evolution of nectar traits. We wondered whether evolution of nectar traits could keep up this high pace, and the answer was a clear yes.”
Getting the dose of nectar right is critical for a plant. Too little and it won’t attract pollinators. But offer too much, and the visitors will be sated rapidly. Full up, they won’t have any incentive to forage around more flowers, carrying pollen along the way.
The team found nectar varied between Impatiens flowers. Some were suitable for butterflies, laced with amino acids. This aided the butterflies, who lacked protein in their diet. Other flowers were geared more towards bees. In some nectar, the sugars were less concentrated, but the nectar was produced in volume. Nectar like this attracted birds. It showed how Impatiens used nectar as one of a few tools to manage pollination working with other parts of the plant. Vandelook said: “Overall flower morphology (shape of petals and sepals, colour or the petals and sepals) plays a complementary role in attracting pollinators. In case of Impatiens, nectar composition does not play the role of barrier towards pollinators, that will more likely be the overall floral morphology, although it must be stressed that some Impatiens attract more than one pollinator group, so possible pollinator barriers are not always waterproof.
One of the interesting findings of the study is that pollinators have particular requirements. Each kind of pollinator has its favourite nectar cocktail. Vandelook noted: “For most nectar components the ranges were quite narrow and closely associated with the main pollinators. It has been shown in other studies that the viscosity of nectar is closely related to the nectar drinking mechanism, because of biophysics and energy optimization. Since pollinators can be clearly categorized according to for example drinking mechanism, this also results in clear categories of nectar composition. There is very little room for in between.”
The work on nectar composition could be a vital clue to an evolutionary puzzle, how did Impatiens get so diverse so rapidly? Steve Janssens concluded: “We are definitely interested to find out what the general impact is of a changing nectar composition on the extremely rapid radiation of Impatiens. Can nectar composition drive speciation or does it just ride along?” Given colour and shape are known to drive speciation, the possibility of nectar-driven speciation is intriguing.
Vandelook, F., Janssens, S. B., Gijbels, P., Fischer, E., Van den Ende, W., Honnay, O., & Abrahamczyk, S. (2019). Nectar traits differ between pollination syndromes in Balsaminaceae. Annals of Botany. https://doi.org/10.1093/aob/mcz072