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Plant extracts

by  Nigel Perry

An infusion of mānuka leaves, to ease stomach complaints, was a traditional Māori herbal medicine. Kauri trees yield gum, which was exported in the 19th century for its use as a varnish. Chemists studying the ‘natural pharmacy’ of the forests have found unique compounds – including mānuka’s antibiotic leptospermone – which show promise as drugs and other products.


Unique plants and chemicals

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.

Māori use

The first humans to encounter New Zealand’s plants were the Polynesians who arrived around 1250–1300 AD. 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.

Early chemists

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), 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).

Lauren’s leaf

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 sulfur 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.


Toxic and commercial compounds

By isolating the active chemical in poisonous plants, researchers can work on remedies, treatments or possible beneficial effects of a poison. Poisonous plants may also contain chemicals new to science.

Tutu

One plant that soon came under the microscope in New Zealand was tree tutu (Coriaria arborea) – sheep, cattle and horses died after browsing on its leaves. In 1900 the chemists Thomas Easterfield and Bernard Aston purified and identified tutin, a neurotoxin new to science.

Tutu poisoning via honey

An indirect form of tutu poisoning affected humans, through honey bees from the northern hemisphere. In some parts of New Zealand the bees occasionally produced honey that caused violent convulsions and death. Investigation of honey from the Bay of Plenty in the 1940s led to the discovery of mellitoxin, closely related to tutin. The connection to tutu was made when it was noticed that bees were collecting honeydew from vine hoppers, which were in turn feeding on tutu. Honey production is now controlled when tutu and vine hoppers are common.

Varnish from kauri gum

One of the first New Zealand natural plant products to be used commercially was kauri gum, a resin that oozes from kauri trees (Agathis australis), which grow in the far north.

Blended with linseed oil, the gum was used for varnish, and for the manufacture of floor coverings. A trial batch was sent to London in the 1830s and by 1899 gum production reached a peak of 9,979 tonnes.

Nearly 100 years later, in 1985, a processing plant was built at Kaimaumau north of Awanui to extract resins and waxes from kauri chips and dust from a peat swamp. The plan was to use the extracts for paper coatings and lipsticks that wouldn’t melt in the sun. However, there were technical problems and the factory closed in 1989.

A bite in the bush

Horopito grows throughout most of New Zealand, especially on bush margins. Chewing a tiny bit of the red-blotched leaf will show why this plant is also called pepper tree, and why it was sometimes used to treat toothache – or at least distract from it. The chemical producing the burning, pungent taste is called polygodial.

Steroids from poroporo

In Taranaki in the late 1970s and early 1980s, poroporo shrubs (Solanum laciniatum and S. aviculare) were grown for solasodine, a steroid used in contraceptives. When it proved cheaper to raise such plants overseas or use synthetic substitutes, poroporo was no longer cultivated in New Zealand.

Manool and totarol from conifers

Manool is extracted from dead logs of the small native conifer, pink pine (Halocarpus biformis). Manool is valued by perfumers because it can easily be converted into compounds with a powerful, sweet yet dry aroma similar to ambergris.

Totarol, as indicated by its name, was first found in tōtara (Podocarpus totara). This compound has an antibiotic effect and is being developed as a natural preservative.

The accumulation of compounds such as totarol and manool in the heartwood of long-lived conifers probably helps to protect the trees from rotting. When the timber is used for building, there is no need for additional antifungal treatments.


Medicines

Cures from nature

In modern medicine, a natural chemical is mostly the starting point for the development of synthetic pharmaceuticals. However, some natural chemicals are too complex for this to be economic. One example is the breast cancer drug taxol, which is prepared from plantation-grown yew trees in the United States.

Mānuka

Mānuka (Leptospermum scoparium) is one of the most widespread of the woody plants in New Zealand, growing from Cape Rēinga down to Stewart Island, and from coastal swamps up to the subalpine zones. Māori used it in medicines, such as infusions for stomach complaints.

Antibiotic effect: leptospermone

In the 1990s a group of Māori from the East Cape of the North Island, working with scientist Noel Porter, found that extracts from the leaves of local mānuka shrubs had an antibiotic effect. This is due to a high level of the active ingredient, a natural chemical called leptospermone. It acts against the methicillin-resistant bacterium Staphylococcus aureus – the original ‘super bug’ that is a serious problem in hospitals.

Mānuka is sometimes called tea tree, but New Zealand mānuka oil is distinct, both chemically and in its antibiotic properties, from Australian tea tree oil.

The good oil

Mānuka oil, and creams containing it, are sold as a treatment for skin infections. The leaves are harvested sustainably from natural stands of mānuka, but the development of plantations is under way. To extract the oil, steam is passed through the cut foliage, condensed, and then separated, leaving the essential oil.

Most mānuka does not yield essential oils with the antibiotic properties of East Cape mānuka oil. The related kānuka (Kunzea ericoides) is often confused with mānuka, but it gives a very different essential oil, with no leptospermone and much weaker antibiotic properties.

Mānuka honey

Mānuka honey is made by bees that collect nectar from mānuka flowers. In some areas the honey has antibiotic properties, but this does not appear to be the effect of the chemical leptospermone, as in the leaf oil. Researchers at Waikato University and Industrial Research Ltd (a Crown Research Institute), have tried to identify the active chemicals in mānuka honey, in order to optimise production and quality. In 2007 researchers at the Technical University of Dresden, Germany, identified the active chemical as the compound methyl glyoxal. Later that year the United States Food and Drug Administration approved the marketing and selling of a wound dressing impregnated with mānuka honey.

Poisons as medicines

Plant poisons have potential as medicines if used at the right dose – for example digitalis (which regulates and strengthens the heartbeat, and is poisonous in overdose) is obtained from foxgloves.

Prostratin from Pimelea

In New Zealand, pinatoro or Strathmore weed (Pimelea prostrata) has poisoned animals, and was the original source of the toxin known as prostratin. The same compound has since been isolated from a Samoan plant used in traditional medicine, and found to have activity against human immunodeficiency virus (HIV). New Zealand Pimelea could be another source of prostratin if this compound is developed as a treatment.

Horopito’s antifungal effect

A search for new medicines from New Zealand native plants at the University of Canterbury led to the discovery that an extract of leaves of horopito (Pseudowintera colorata) was active against the yeast Candida albicans, which causes the fungal disease thrush. The natural product that causes the burning, pungent taste of horopito is polygodial, the active antifungal ingredient. A Nelson company is growing this species and selling herbal medicines containing powdered leaves and leaf extracts.


Hononga, rauemi nō waho

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How to cite this page: Nigel Perry, 'Plant extracts', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/plant-extracts/print (accessed 9 December 2019)

He kōrero nā Nigel Perry, i tāngia i te 24 Sep 2007