Plant Scent: What’s That Smell?

The production of a scent by a flower is well-understood by many as a method of attracting birds, bats, butterflies, beetles, ants and various other invertebrates to the flower. The smell produced by a flower acts as an attractant, which is generally combined with a reward of nectar, and has the primary function of assisting the plant with reproduction via pollination.

For some plants, the floral scent can be a delight, with each flower producing a distinctive scent that is attractive to a certain faunal assemblage. For other flowers, the scent can be less appealing to the human nose, but attracts the correct pollinator none-the-less. Flowers that smell like carrion have evolved to attract flies and beetles that would normally lay their eggs in rotting meat and faeces. They are often tempted to the carrion flowers by the smell and their visitation to the flower inadvertently pollinates it, before they depart for a more suitable place to lay their eggs.

But what about the strong scent emitted by leaves, bark and other plant tissues when no flowers are present? The roots of many Acacia have a strong foetid smell when being handled, which is produced by nitrifying root bacteria nodules, indicating that they are active and performing their task. But often, the scent in plant leaves is produced by complex chemicals. The complex chemicals that give plants their odour are often the by-products or waste components of plant metabolism, or photosynthesis. These secondary metabolites are known as volatile organic compounds. They are known as volatile, because they evaporate quickly from the liquid state and enter the air as gas, which causes the sudden detection of a scent. The largest groups of volatile organic compounds are the terpenoids (compounds with an isoprenoid structure) and green leaf volatiles. For other plants the odours are a result of other secondary metabolites called flavonoids and phenols, which are composed of hydroxyl groups attached to an aromatic ring.

Green leaf volatiles are best known as the smell that is produced by freshly mown grass, generally resulting from 6-carbon aldehydes and alcohols. When grass is damaged (e.g. cut by a lawnmower) it triggers enzymes to start breaking down fats and phospholipids, leading to the formation of linolenic and linoleic acids that are oxidised and broken down by another enzyme. The process splits the molecule into fragments that lead to the cut grass smell.

Terpenoids are responsible for contributing to many scents produced by plants. The smell of Native Pine (Callitris glaucophylla) comes from pinene. The smell of native lemongrass (Cymbopogon ambiguus) is a result of limonene and α-terpineol (both commonly found in citrus), as well as eugenol and elemicin (found in nutmeg and clove). Species of Eucalyptus contain a terpenoid called cineole, which gives the leaves their characteristic fresh scent. Cineole can also be found in other local native plants such as Striped Mintbush (Prostanthera striatiflora). Sticky Bluerod (Stemodia viscosa) contains terpenoids such as caryophyllene (pepper-like scent in rosemary), fenchol (found in basil) and limonene.

Some other strongly scented natives are Apple Bush (Pterocaulon sphacelatum), Gidgee (Acacia cambagei), and Curry Wattle (Acacia spondylophylla).

While Sticky Hopbush (Dodonaea viscosa) has a distinctive scent and flower capsules that are visually similar to Hops (Humulus lupulus), used in the production of beer, they are not botanically related. Hopbush (D. viscosa) gets its name, as is was used to make beer by early European Australians, yet there are no taxonomic links to Hops (H. lupulus). Sticky Hopbush produces a scent from a combination of flavonoids such as isorhamnetin, hyperoside and a citrus flavonoid rutin, whereas Hops produces its scent from  myrcene, beta-pinene and alpha-humulene (a sesquiterpene). Their scent is, however, somewhat similar despite the difference composition. On a side note, Hops and Marijuana (Cannabis sativa) have similar organoleptic properties (taste and smell), as they have similar aromatic compounds, owing to their taxonomic relatedness.

Volatile organic compound emissions are affected by factors that include temperature (determines rates of volatilization) and sunlight (determines rates of biosynthesis). Emission occurs almost exclusively from the leaves, the stomata in particular. Hence, the Gidgee around town will smell to varying degrees, depending on the weather. There is a stand of Gidgee near Billygoat Hill in Alice Springs, which commonly will stink out the region on a rainy, misty and high humidity morning during a period of weather depressions.

The production of volatile organic compounds can require extra energy by plants and therefore can come at a cost. So why bother? Strong odours emitted by plants may also be a way of deterring browsing herbivores or insects. Volatile terpenoids released by plants when under attack from herbivorous insects allows predatory insects (or insect parasitoids) to locate prey secondarily through infected hosts (E.g. Pine trees). Volatile organic compounds may even be produced to help kill off other plants in the vicinity, in order to thrive themselves (E.g. Eucalyptus sp.). Some plants give off scent when crushed that induces defence mechanisms in neighbouring plants or promote production of new cells at the site of the wound to repair the damage. Some compounds even act as antibiotics to prevent infection at the site of the crush.

So if you start smelling something strange on the wind following a change in weather, you may be able to sniff it out to a plant upwind!

Plant Stowaways in Camel Harness

By Marg Friedel

Back In March, Marg gave a talk to the Alice Springs Field Naturalists Club, which she called “Where did they come from and how did they get here? Examining the evidence for some familiar weeds of arid central Australia”.  As part of her rummaging in the records of Australia’s Virtual Herbarium (AVH), and lots of follow-up reading and discussion, she found evidence for camel harness being the source of a surprising number of invasive plant species.

Not so surprising was the evidence for Buffel Grass (Cenchrus ciliaris), which was first recorded in AVH south of Wyndham in 1897, near the Ord River.  Camels were in use, supplying the goldfields at Hall’s Creek, and the cameleers commonly rested at waterholes and creeks.  From the 1880s, camels were sourced from India to modern Afghanistan and were brought into Western Australia via Fremantle predominantly, as well as Geraldton, Port Hedland and Albany. They serviced the pastoral industry and mines both inland and along the WA coast.  Joe Moore, storekeeper at Port Headland, persuaded school children to collect the seeds from buffel grass growing around the town from about 1910, and distributed it to stations in the district.

Buffel Grass at Nicker Creek WA, 2014, from 1930s Michael Terry expedition (Image M. Friedel).

Buffel grass also came with camels via Port Augusta from the 1860s, and camel trains and Ghan towns were a feature of much of inland South Australia, Northern Territory and New South Wales, as well as WA. The first herbarium record for NT is Woodforde Well in 1931, but we know from Walter Smith that cameleers were deliberately spreading buffel grass well before that.

Fountain Grass (Cenchrus setaceus) first appears in AVH in 1903 at Eurelia, near Orroroo, South Australia. Cloncurry Buffel (Cenchrus pennisetiformis), supposedly introduced by General Birdwood after WWI, appears in 1915 in the Geraldton-Greenough area. Birdwood Grass (Cenchrus setigera), appears at Roebourne in 1932, in keeping with its introduction by General Birdwood.  Hence it’s likely that three of the Cenchrus species, including buffel grass, came with camels initially, and that subsequently there were deliberate introductions.

Rosy Dock (Acetosa vesicaria) was first collected by naturalist Richard Helms in Perth in 1892, after he left the Lindsay expedition in the Murchison district.  Rosy dock is native to north Africa, southwestern Asia and the Indian sub-continent, so it’s a likely accidental inclusion in camel harness arriving in Fremantle.

Rosy Dock (Acetosa vesicaria) in Palmer Valley, 1979 (Image M. Friedel).

Kapok Bush (Aerva javanica) was found in 1937 on the de Grey River and Roy Hill Station in 1938, according to AVH.  The Ord River Regeneration Project was undertaken from the 1960s, using seed sourced from existing populations on Anna Plains station in the Pilbara and Fitzroy Crossing in the West Kimberley.  These populations were understood at the time to have come from camel harness, and kapok bush was known to be used historically by Arabian people for cushion and saddle padding.

Kapok Bush (Aerva javanica) at Alice Springs, 2017 (Image Weed Management Branch, NTG).

Perhaps more surprisingly Rubber Bush (Calotropis procera) is likely to have arrived with the camels that serviced the railhead at Mungana, in Queensland, for the nearby copper mines.  A railway operated from Mareeba to Mungana from about 1900, and Mungana was the focus for camel teams for about six years. Rubber bush was first reported in AVH in 1935 at Mungana.

Rubber Bush (Calotropis procera) on Barkly Tablelands, 2004 (Weed Management Branch, NTG).

And of course the date palm (Phoenix dactylifera) was distributed by cameleers, all up giving us quite a substantial list of species likely to have arrived with cameleers and their camels.

Marg would like to hear from anyone with any additional information – whether in support or counter to her story.

~ Marg Friedel