When drought changes the smell of flowers

Pollination: crucial to the reproduction of flowering plants

Pollination is an ecosystem service provided by insects, which are essential for the reproduction of most flowering plants, including those we eat. To locate floral resources, pollinators use both visual signals (flower color and size) and chemical signals: odors. Flower scent is a complex mixture of dozens of volatile molecules, and is highly sensitive to stress (e.g. water stress). A change in flower scent could have consequences for their interactions with insects.

This 2018 study was carried out in the context of Mediterranean scrubland. The scientists sought to understand how moderate long-term rainfall reduction (their system excluding an average of 12% of rainwater) affected floral scent, and what the consequences were for plant-pollinator interactions. The results showed a significant change in the floral odors of all the species studied, caused in particular by more intense emissions of volatile stress marker compounds. In other words, the garrigue, under even limited drought conditions, doesn't smell quite the same!

Drought alters the relationship between pollinators and plants

At the same time, they also observed a change in plant-pollinator interactions: honeybees visited non-stressed plants more often, and conversely, stressed plants were visited more by solitary bees. As honeybees have a great ability to discriminate between resources, particularly those of good quality, it is conceivable that they preferred to harvest resources from non-stressed plants. These changes in bee species visits could also be linked to changes in floral resources. For example, although we didn't detect any difference in the amount of nectar produced by the dominant flowers of the garrigue, they did measure a fourfold reduction in the amount of nectar produced by thyme under stress conditions! Finally, although they didn't measure it, drought may also have altered the quality of floral resources, such as the composition of different sugars in nectar.

Although the results do not show any impact on plant seed production, it is still worrying to note that moderate drought can modify floral scents and plant-pollinator interactions. In this context, it is likely that extreme climatic events (such as those observed in 2022) will have far greater adverse consequences on ecosystem functioning. What's more, they have only studied one component of climate change - drought - but it's a safe bet that when combined with temperature changes or atmospheric pollution, these effects will be multiplied tenfold. More research on the subject is needed to predict the impact of these extreme events, and to identify levers for mitigating the harmful effects of climate change.