Many of the volatile organic compounds that are the source of this sensory experience are emitted by terrestrial plants. Flowers or vegetative organs of the plants around us release molecules of a very wide chemical diversity into the air we breathe in. But why does terrestrial vegetation produce this rich variety of small molecules?
Plants know how to defend themselves
First of all, they are defence mechanisms that enable the plant to resist the many stresses or aggressions it faces, especially when it is short of water or sick. The plant can produce these volatile compounds in a constitutive way, and they are then stored in structures such as the trichomesThese glandular hairs will be easily released to repel herbivores or even intoxicate them.
Their biosynthesis can also be triggered by an injury such as an insect bite. They induce other parts of the plant to produce defensive substances such as phytoalexinsIt is made up of a number of different molecules, such as antimicrobial or anti-fungal molecules, which enable it to resist attack by a pathogenic organism or herbivore. Emitted into the atmosphere, these signals are perceived by other plants, which in turn produce defence molecules.
When plants communicate
These communication functions are also important in the positive interactions between the plant and its pollinating insects. Floral aromas attract a cohort of insects, bees, bumblebees, flies, beetles, or even mosquitoes that associate the smell with the presence of nectar. The ripe fruit releases compounds that are attractive to animals, which after eating the fruit will disperse their seeds.
Exchanges of chemical signals are therefore of great importance in the functioning of ecosystems. The result of a very long co-evolution, particularly between higher plants and insects, they modulate essential plant functions such as pollination. They also help to limit herbivore populations by attracting their predators and parasites. Complex communication networks are established between the different trophic levels.
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The attack of a caterpillar induces the emission of volatile molecules that affect the butterfly's egg-laying behaviour but attract parasites that lay eggs in the caterpillar. These signals are often mixtures of volatile compounds whose proportions ensure the specificity of this communication. An insect parasite can thus recognize which species of butterfly has attacked the plant. Insect responses are often dependent on the scent context in which they perceive the signal. An orchid flower can mimic the sexual signal of a pollinator to ensure the specificity of its pollination.
Human activity creates interference
Human activity is putting these complex communication networks at risk, but the risks are still poorly assessed. Agricultural processing industries, agricultural activities and livestock breeding produce large quantities of volatile organic compounds that mix with natural sources. The sensory impact of these emissions has long been known to us when they are the source of odour nuisances such as manure spreading or local composting. However, the study of the impact on the functioning of ecosystems has only just begun.
Volatile organic compounds are naturally degraded in the atmosphere by UV radiation. However, the increase in concentrations of ozone or other reactive groups such as nitrogen monoxide caused by industrial activities or transport significantly reduces their lifetime in the atmosphere. reduces at the same time the distances at which foraging insects can detect floral aromas. But it also modifies the composition of the flowers because all their constituents are not degraded at the same speed, their proportions are no longer the same and the odorous mixture changes in nature, as shown by the bee tests.
Direct effects of pollutants on insect olfaction are also likely because olfactory communication appears particularly sensitive to the interactions between volatile molecules present in the air. in the smelly background.
Towards a modification of olfactory landscapes?
In addition to these pollutants, global climate change, including increases in CO2 and temperature rises, itself affects the metabolisms of the reacting plants by qualitatively and quantitatively modifying their emissions. We can expect the olfactory landscape to change profoundly in the coming decades. While the development of chemical ecology has enabled us to unlock the secrets of olfactory communication, we are still far from being able to assess the overall importance of these olfactory landscapes on biodiversity and the consequences of their alteration. Through a leverage effect, any change in signals essential to the location of a vital resource, or to the synchronization of the cycles of two species, can have significant repercussions on animal populations or plant communities.
The impacts will be all the more important on specialist species that use very specific signals to locate their host. We should therefore question the need to take into account the sensory dimension in biodiversity management programmes. Our data in this area are very recent, with analyses dating back only a few decades. It would therefore be urgent to take stock of the state of fragrant landscapes, monitor their evolution, assess the risks represented by each type of disturbance, and then look for methods to preserve their important components.
We're already trying to remedy the nuisance emissions from composting platforms or livestock buildings by emitting masking odours or by surrounding hedgerow sites that create turbulence that dilutes the emissions and directs them higher into the atmosphere.
In addition to the sensory loss that we would feel during a walk in the forest that has become odourless, the impact could be significant for insect populations already weakened by multiple stresses and plant communities deprived of their means of communication.
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Michel RenouDirector of Research in Insect Biology, Inra
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