The ‘green smell’ is a familiar sensation, often encountered when walking through a garden or mowing a lawn. This aroma, associated with freshness and nature, is a complex blend of volatile organic compounds released by plants. It indicates plant presence, especially when plant tissues are disturbed.
The Chemistry of Green Scents
The compounds primarily responsible for the ‘green smell’ are Green Leaf Volatiles (GLVs). These small, airborne molecules are predominantly six-carbon (C6) compounds, including aldehydes, alcohols, and esters. Key examples are hexanal, with a grassy odor, and (Z)-3-hexenal, known for fresh, green notes like freshly cut grass. Other GLVs include (E)-2-hexenal, a sharper, herbal-green scent, and (Z)-3-hexenol, a ‘leaf alcohol.’ These volatile molecules easily evaporate, allowing detection by our olfactory system.
How Plants Produce Green Scents
Plants rapidly produce these green scents, primarily in response to physical damage or stress. This emission, occurring within seconds to minutes of damage like cutting, crushing, or herbivore feeding, results from enzymatic processes.
Lipases first release free fatty acids from galactolipids. Lipoxygenase enzymes then add oxygen to these fatty acids, forming hydroperoxides. Hydroperoxide lyase (HPL) then cleaves these hydroperoxides, producing C6 aldehydes like (Z)-3-hexenal and hexanal. Further enzymatic reactions convert these aldehydes into alcohols and esters. This rapid, localized production ensures GLVs are among the first compounds encountered when tissue is damaged.
The Biological Role of Green Scents
Green scents play a multifaceted role in the plant kingdom, acting as communication signals. They are important in plant defense against herbivores and pathogens. When a plant is damaged, GLVs can directly deter some herbivores from further feeding.
Beyond direct deterrence, GLVs also serve as indirect defense signals by attracting natural enemies of herbivores, such as predatory insects and parasitic wasps. These compounds can also initiate plant-to-plant communication, warning nearby undamaged plants of potential threats. Upon perceiving GLVs from a distressed neighbor, receiving plants can ‘prime’ their defenses, preparing a faster, stronger response if attacked. This signaling highlights the ecological significance of green scents in plant survival.
Perceiving Green Scents
The human olfactory system is sensitive to green scents, often associating them with freshness, vitality, and natural environments. These associations are deeply ingrained, contributing to psychological and emotional responses. For many, the smell of freshly cut grass evokes feelings of rejuvenation or nostalgia.
GLVs and similar compounds are widely utilized in various industries. In perfumery, they create ‘green’ notes, adding a fresh, leafy, or natural dimension to fragrances. In the food flavoring industry, these compounds enhance the perception of freshness in products, contributing to the characteristic flavors of fruits and vegetables. This industrial application shows the impact these volatile molecules have on human sensory experience.