Pollutants mask flower scents from pollinators

The relationship between plant and pollinator is a delicate one, forged over millions of years of evolution. In order to attract insects to spread their pollen, a lot of plants announce their existence by releasing their own particular smell. These smells are created by forming a cocktail of different chemicals. But now, scientists at the University of Washington have discovered that common, nitrogen-containing compounds in air pollution can attack these odours and mask the ability of plants to attract their key pollinators, denting plant productivity. The two leads of the study are Joel Thornton and Jeff Riffell…

Jeff - We were very interested in understanding how the atmospheric pollutant impacts animal behaviour. And we took advantage of this plant pollinator system, this primrose and its moth, to examine how air pollution could impact and affect the ability of these moths to pollinate their flowers.

Chris - Which pollutants were you looking at, Joel?

Joel - So we were specifically interested in pollutants that arise from nitrogen oxides, which are emitted by fuel combustion for transportation and energy production. These can get converted in the atmosphere through chemical reactions to highly reactive radical species, we call them. And the specific one we were targeting in this study was the nitrate radical. It's a nitrogen with three oxygens attached to it.

Chris - And Jeff, biologically speaking, what did you actually do when you were interested in studying this particular chemical? How were you applying that to this moth primrose story?

Jeff - So these flowers that are pollinated by moths, they open up at night. They emit a really high intensity scent to lure the moths from far away. And we actually discovered that some of these compounds, especially these monoterpenes in the floral scent, giving it this kind of piney odour, we're very sensitive to these atmospheric pollutants that are occurring, these nitrate radicals that are occurring in the environment.

Chris - Is the nitrate radical breaking down the scent molecules, Joel? Or is it affecting the ability of the moths to smell it?

Joel - Yeah, so it's affecting the ability of moths to smell it because it's breaking down the monoterpenes through reactions, changing it into slightly different shapes, slightly different elements as being part of the molecule that change the ability of the moths to actually detect and they don't recognise it as being part of a floral scent.

Chris - And what impact does this have on the rates of pollination, Jeff?

Jeff - So these moths require these infochemicals, these monoterpenes to locate these patches of flowers that can be quite distant, kilometres away from one another. And once they're degraded, the moss can no longer recognise the flowers at all. What we found is that both in the laboratory and in the field, their visitation to flowers decreased pretty dramatically, up to 70%. And this had a really strong effect on plant fitness. It impacted, we estimated, about 20% of their ability to produce fruits and seeds, for instance.

Chris - And were the levels at which this was happening relevant to what we see around the world, Joel?

Joel - Yes. That was one of our goals was to replicate as best we can the environment as it is today in terms of the types and amounts of pollutants that are interacting with the floral scents. So it's quite relevant to an urban area, say downwind. So it replicates a large number of areas around the world.

Chris - And Jeff, is this indicative of what's probably happening more broadly? Do more insects use more similar compounds like this and therefore they're likely to be falling victim to the same effect and therefore are more plants losing productivity because of this?

Jeff - The infochemicals that were really degraded by these pollutants are used in a variety of plant pollinator interactions, and they're also involved in a variety of other processes and occurrences like how insects find their host plants. And what we can think then is that, because these are kind of ubiquitous infochemicals, we really strongly believe that what we found in this one plant pollinator system could be generalisable to what's actually occurring worldwide.

Chris - I know we're talking here about effectively plant fertility, which is achieved through pollination, but could the same effects also impact the ability of insects to find their mates? Because insects do, like moths especially, track each other down using pheromone smells. Could they be attacked by atmospheric pollution in the same way and therefore we're also going to see a decimation potentially of insect populations?

Jeff - Yeah, these info chemicals, these airborne chemicals, are used by insects and animals in general for a variety of different processes like finding mates, finding prey, finding plants, and available plants to use for food. And so we just don't know enough about how the effects of these airborne pollutants could have on these other processes. And so this is an important topic that we need to study more.

Chris - So what's the take home message here, Joel?

Joel - Broadly speaking, it shows how deep the impacts of air pollution go into the Earth's systems. So for many years we have been worried about the effects of air pollution, rightly so on the quality of human life. And it's a negative health impact through long-term exposure. But in this study, we think it can be used as a way to extrapolate to other processes in ecosystems that depend on these info chemicals, where air pollution is sort of penetrating deep into the air system and impacting ecosystem functioning and agricultural productivity and food security through the negative effects it's having on pollinators in this way. And so it's really about the broader negative impacts of air pollution going beyond just what we can see and smell, say, in an urban area.