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!