Air pollution, particularly at night, becomes a more insidious foe. During the day, sunlight helps to break down pollutants like ozone, but once the sun sets, these pollutants accumulate. Among them, ozone and nitrogen dioxide react to form highly reactive nitrate radicals. These radicals have the potential to alter plant fragrances, a phenomenon that scientists had hinted at but never fully understood until now.
Joel Thornton, an atmospheric scientist at the University of Washington in Seattle, led a team to uncover the specifics of this interaction. They focused on the scent molecules emitted by the pale evening primrose (Oenothera pallida), a flower known for its nighttime allure. In a wind tunnel experiment, they released the flower's fragrance and observed how hawk moths, the flower’s primary pollinators, responded. Under normal conditions, the moths adeptly navigated upwind, following the scent trail with ease.
The introduction of pollutants like ozone and nitrogen dioxide changed the scene dramatically. With these pollutants present, the hawk moths' graceful flight turned erratic. They flew in confused zigzags, struggling to home in on the scent. By exposing moth antennae to each component of the flower’s fragrance, the researchers identified the key scent cues: two aromatic compounds known as monoterpenes. When these compounds encountered nitrate radicals, they broke down, drastically diminishing the flower's aroma.
“It was surprising [that] a seemingly subtle change in concentration of only two compounds — out of more than 20 — was sufficient to eliminate the flower’s attractiveness,” noted Jeff Riffell, a biologist on the team. The effect of this chemical alteration on the moths is akin to asking a blindfolded person to find a cup of coffee — a task rendered near impossible by the loss of critical sensory information.
To see if these lab results held true in a natural setting, ecologist Jeremy Chan, now at the University of Naples Federico II in Italy, conducted a field experiment. He planted a mix of real and fake primroses in a field east of Seattle. Some of the fake flowers emitted pure primrose fragrance, while others released a combination of fragrance and pollutants that produced nitrate radicals. Cameras recorded the nocturnal visits of moths to these flowers. The results were stark: moths visited real and unsullied fake flowers two to three times per night, but those with polluted scents attracted less than one visit per night on average.
This study sheds light on a critical but often overlooked impact of air pollution. The blunting of floral scents disrupts the ability of pollinators to locate their targets, threatening not only individual plant species but entire ecosystems and food security. As industrialization continues to escalate, understanding and mitigating these effects become increasingly crucial. If nocturnal pollinators like hawk moths are struggling to find flowers, it could signal broader implications for pollination, crop production, and the health of native plant species. This research underscores the interconnectedness of our environment and the far-reaching consequences of air pollution, extending well beyond human health to the very foundations of our ecological systems.
References: J. K. Chan et al. Olfaction in the Anthropocene: NO3 negatively affects floral scent and nocturnal pollination. Science. Vol. 383, February 8, 2024, p. 607. doi: 10.1126/science.adi0858.