"Chemical time capsules" from Captain Cook's voyage matched with modern mānuka

Robust oil gland linings in historical mānuka samples collected during one of Captain James Cook’s voyages serve as “chemical time capsules”, linking them to modern mānuka varieties, a Plant & Food Research study has found.

Botanists Sir Joseph Banks and Daniel Solander, aboard HMS Endeavour, collected several mānuka samples around Aotearoa New Zealand in 1769, but they did not record the individual collecting sites. They took the samples back to Britain for botanical classification, where they were assigned the Latin name Leptospermum scoparium. The samples were then stored in the British Museum, but most were returned home in the 1890s and are treasured in the herbarium collections of Auckland War Memorial Museum and Te Papa. 

Plant & Food Research chemist Professor Nigel Perry and his Dunedin-based team have shown that Leptospermum scoparium (mānuka or kahikātoa to Māori) has distinct leaf chemistries (chemotypes) in different areas of Aotearoa, e.g. volatile antimicrobial triketones in Tairāwhiti East Cape; or pine-smelling compounds from Te Tai Tokerau Northland. This chemistry is controlled by mānuka genes rather than by growing conditions: East Cape seed grown in Palmerston North or Christchurch are the same triketone chemotype as their parents. 

“Although most plant volatiles are lost quite quickly, think of those old herbs in your pantry, this is not so for mānuka,”says Professor Perry.

“We’ve developed a miniaturised method to analyse the volatiles in tiny sub-samples of the precious herbarium samples and successfully measured volatiles stored in the robust oil gland linings of the historical mānuka leaves 250 years after Captain Cook’s voyage.”

The total volatiles from the samples collected by Banks and Solander were similar to modern samples, and the scientists could identify chemotypes suggesting the collecting sites of these historical samples.

Another Plant & Food Research team, led by Dr David Chagné, is using a different approach to track the origins of mānuka. They have constructed the first high density DNA linkage map for L. scoparium, using thousands of genetic markers. 

This map links DNA information to specific traits such as disease resistance (possibly including myrtle rust) and drought tolerance, and to chemical composition of the foliage (e.g. the triketones) and nectar (e.g. the Unique Mānuka Factor, UMF).  

Constructing a genetic map is a significant step in managing mānuka, which will translate into tools for DNA fingerprinting and for breeding. Seedlings with commercial potential for high-value mānuka honey and/or foliage chemotypes will be identified. 

Citations:

Sansom, van Klink & Perry (2019) Chemical time capsules: bioactive volatiles in eighteenth century herbarium samples of mānuka, Leptospermum scoparium, N Z J Crop Hort Sci, DOI: 10.1080/01140671.2019.1576745

Chagné, Ryan, Saeed, Van Stijn, Brauning, Clarke, Jacobs, Wilcox, Boursault, Jaksons, Jones, Thrimawithana, Schwinn & Lewis (2018) A high density linkage map and quantitative trait loci for tree growth for New Zealand mānuka (Leptospermum scoparium L.), N Z J Crop Hort Sci, DOI: 10.1080/01140671.2018.1540437