What is it that gives mānuka floral nectar its unique composition?

The secret of mānuka honey, is mānuka nectar, but what is the secret of mānuka nectar?

Not all honey is equal. New research by Clearwater and colleagues has been looking how mānuka nectar, an essential ingredient of mānuka honey, is affected by various factors such as temperature, drought and even the genes of mānuka (Leptospermum scoparium, Myrtaceae) plants.

Understanding the composition of the nectar is important because of how bees make honey. They take nectar from flowers and store in their crop, specialised foregut. Here, it’s partially digested. When a bees arrives back in the hive, she passes on the nectar by regurgitating it, and the next bee passes it along and so on, until the fluid is eventually stored in a honeycomb. It’s then fanned to evaporate the excess water from it to become honey. The qualities of the honey therefore depend on the qualities of the nectar. For example, eating a lot of rhododendron honey is a Very Bad Idea.

In contrast, there is a lot of demand for mānuka honey. New Zealand produces 1700 tons of mānuka honey a year, of which 1800 tons is consumed by the UK alone. The driver for this fraud is the possible health benefits of mānuka honey. While the science is not conclusive, there are reasons to think that mānuka honey could have health benefits as an antibiotic treatment.

The non-peroxide antibacterial activity of mānuka honey originates from dihydroxyacetone, a saccharide, present in the floral nectar. When the nectar becomes honey the dihydroxyacetone becomes methylglyoxal – and this is the antibacterial agent. The causes of variation in nectar composition and the origin of the dihydroxyacetone are unknown. Clearwater and colleagues examined how the nectar yield and composition of mānuka varied with temperature, among genotypes, and as flowers develop because of differential secretion and reabsorption of the various nectar components.

Different varieties of mānuka have different genes, so the scientists started by selecting six mānuka genotypes. They planted the plants and grew them without nectar-feeding insects. They measured how the flowers developed and the composition of the nectar in the flowers at various stages of development. They also stressed some of the plants by restricting water and compared them with their better-watered neighbours to see how this affected the nectar.

Stages of mānuka flower development
Stages of mānuka flower development in plan (above) and half flower view (below), for the MI genotype, from flower opening until sepal fall. Source: Clearwater et al. 2018.

What they found was that there was nectar almost as soon as the flowers opened until the petals started falling off the flowers. There wasn’t always the same amount of sugars in the nectar though – which suggests that some of the plants reabsorbed some of their nectar when no insects came for it. They also found that the ratio of sugars to dihydroxyacetone varied according to the genotype of the mānuka plant, so it would seem that not all plants are as good for the honey. They also found that the stage of flower development was important too, meaning that for the best yields you’d want to catch the right plants at the right time. It’s not surprise there are problems with cowboy apiarists roaming New Zealand.

As for the magic ingredient, Clearwater and his team found the amount of dihydroxyacetone per flower only weakly correlated with the amount of other sugars. The authors think this means that the dihydroxyacetone probably has a different source in the flower to the other sugars.

For now mānuka isn’t giving up all its secrets.

Further reading

Clearwater, M. J., Revell, M., Noe, S., & Manley-Harris, M. (2018). Influence of genotype, floral stage, and water stress on floral nectar yield and composition of mānuka (Leptospermum scoparium). Annals of Botany, 121(3), 501–512. https://doi.org/10.1093/aob/mcx183

Cooper, R., & Jenkins, R. (2012). Are there feasible prospects for manuka honey as an alternative to conventional antimicrobials? Expert Review of Anti-Infective Therapy, 10(6), 623–625. https://doi.org/10.1586/eri.12.46

Liu, M., Lu, J., Müller, P., Turnbull, L., Burke, C. M., Schlothauer, R. C., … Harry, E. J. (2015). Antibiotic-specific differences in the response of Staphylococcus aureus to treatment with antimicrobials combined with manuka honey. Frontiers in Microbiology, 5. https://doi.org/10.3389/fmicb.2014.00779

Also published on Medium.