Chemical time capsules: Unlocking the secrets of mānuka's past

On the second European voyage to New Zealand, aboard Captain Cook's Endeavour in 1769-1770, botanists Sir Joseph Banks and Daniel Solander collected the first scientific samples of flora from Aotearoa. Banks and Solander collected mānuka from all eight sites where they landed, but did not record the individual samples' specific sites of origin. 

Nearly 250 years later, leaves from the 18th century mānuka samples have been analysed and compared with modern mānuka herbarium samples. The results demonstrate that volatiles (chemical compounds that can disperse in the air at ambient temperature) are stably stored in mānuka leaves for centuries, contrasting with previous findings in some other plants that show volatiles are rapidly lost within a year at room temperature.

In order to further geographical and historical knowledge about the varying mānuka chemotypes between regions, Professor Nigel Perry, Catherine Sansom and Dr John van Klink (all based at the University of Otago) analysed herbarium samples of known chemotypes from modern and historical samples of mānuka in their recent paper 'Chemical time capsules: bioactive volatiles in eighteenth century herbarium samples of mānuka, Leptospermum scoparium', an article in press on Taylor & Francis Online. The analyses were conducted using a method that only required a few small leaves — a process called headspace solid phase micro-extraction gas chromatography (HS-SPME-GC).  

Mānuka is well known to have many rongoā (traditional Māori medicinal) uses, and the honey produced from its nectar is world-famous. Antimicrobial properties in the honey make it a popular natural treatment for dressing wounds and sore throats. Mānuka oil, steam distilled from the foliage, also possesses antimicrobial properties and has been reported as virucidal, meaning it can deactivate or destroy viruses.

The active ingredients that make mānuka oil an effective medicinal tool are rare molecules called triketones, with the main active medicinal molecule being leptospermone. The triketones are stored within oil glands in mānuka leaves.

Mānuka is widespread throughout New Zealand but growth habits and chemotypes vary greatly in different regions. In mānuka oil, the active ingredient Leptospermone has only been found in high concentrations in samples from the East Cape of New Zealand’s North Island.

The high value and demand for mānuka honey worldwide in recent years has resulted in commercial plantings of mānuka between regions, so historical knowledge of the naturally occurring chemical diversity and regional chemotypes of mānuka offers a chance to see changes in regional chemotypes and also reveal historical indicators of biogeographical and human influence.

Results from the analyses show modern East Cape mānuka seed grown in Palmerston North and in the South Island retained the same triketone chemotype as the East Cape populations, and the total volatiles from the historical sample collected from 1769 were similar to the recent mānuka samples, allowing the researchers to derive the regional origins of the mānuka samples collected by Banks and Solander.

The HS-SPME-GC analyses revealed that intact leaves failed to give a sufficient release of volatiles, whereas finely ground mānuka leaves did, including Banks and Solander's samples. This led the researchers to suggest this occurs due to robust oil gland linings, which form a 'chemical time capsule' capable of preserving its contents for centuries.

The paper 'Chemical time capsules: bioactive volatiles in eighteenth century herbarium samples of mānuka, Leptospermum scoparium' is available to read in full as an article in press on Taylor & Francis Online and is intended for inclusion in the upcoming special issue of New Zealand Journal of Crop and Horticultural Science on mānuka biology.