Plants evolving in isolation
New Zealand broke away from the supercontinent Gondwana around 85 million years ago. Some native plants are ancient survivors from Gondwana. Many other plants have arrived by dispersal over the ocean since then, but once they arrived they too were isolated.
The long isolation has seen the evolution of diverse plants, many of them endemic to New Zealand (found nowhere else). They contain a wide range of natural chemicals, many with previously unknown ‘skeletons’ – the frameworks of carbon atoms from which the molecules are built.
The first humans to encounter New Zealand’s plants were the Polynesians who arrived around 1250–1300 CE. They used many plants for medicinal purposes. For example, extracts of the bark of the tall forest tree pukatea (Laurelia novae-zelandiae) were used as painkillers – the bark contains pukateine, which has a chemical structure similar to morphine.
The first chemist to study New Zealand’s native plants was William Skey, starting in the 1860s. Then, in the early 1900s, a fruitful partnership grew between Thomas Easterfield, first professor of chemistry at Victoria College (now Victoria University of Wellington), and Bernard Aston, first chemist at the Department of Agriculture and a competent botanist. In 1900 they isolated the chemical tutin from the tutu tree (Coriaria arborea).
Like explorers, those who discover new molecular ‘skeletons’ (structures of chemicals) have naming rights. Scientist Denis Lauren, working with Ted Corbett at the University of Otago, found that the leaves of the rimu tree contain a chemical with a unique structure – four rings sharing a central carbon atom. His colleagues succeeded in getting it named laurenene. It has subsequently also been found in some tōtara species.
The 20 endemic conifer species were obvious and rewarding targets for these chemical explorers, and those who came later. These trees and shrubs contain several natural chemicals that were found first (and sometimes only) in New Zealand.
Chemicals from plants
Certain plant species have evolved chemicals such as poisons or irritants (for deterring browsing animals), or pigments and scents (for attracting pollinators). They may do this by stimulating the sense of sight, smell, taste or touch.
Natural plant chemicals that can stimulate the senses include:
- bright red pigments in the flowers of pōhutukawa (Metrosideros excelsa), often called the New Zealand Christmas tree
- the smelly sulphur compound released by crushed leaves of stinkwood (Coprosma foetidissima)
- the pungent-tasting antifungal ingredient in horopito, the pepper tree (Pseudowintera colorata)
- the stinging toxins in needles of ongaonga or tree nettle (Urtica ferox).
Why study plant chemicals?
Most natural chemicals are not obvious to human senses and can only be detected by chemical analysis. Scientists have explored the natural chemicals of the New Zealand bush for three reasons: curiosity, commercial applications, and medical uses.