Climate Change Is Making Plants More Toxic

We normally look at the climate crisis as something that’s harmful to the environment, and while that is definitely true, its effect on plants is not entirely straightforward. Rising carbon dioxide levels can actually be good for some plants, because plants use CO2 for photosynthesis. That means that when CO2 levels are high, some plants can grow faster.

But while that might be a good thing for individual plants, overall, it could actually be a problem. And that’s because growing faster isn’t always a good thing. Faster growth and higher levels of CO2 can have unpredictable effects, changing the way they produce useful or dangerous compounds and potentially throwing off the balance in their environments.

CO2 is a relatively small percentage of the atmosphere, making up just 0.04% of its total gases, but it has increased a lot since pre-industrial times. Pre-industrial CO2 levels were about 280 parts per million, while current CO2 levels are hovering between 415 and 420 ppm. This means CO2 levels are one and a half times higher than they were before the Industrial Revolution.

Researchers have begun studying the effect of increased CO2 on plants, and some of the results are pretty surprising. For example, some vegetables become less nutritious at higher levels of CO2. Exactly why this happens is still unclear, but it does seem like higher CO2 levels might reduce the amount of nutrition we get from food. This could have huge repercussions for human health, especially in parts of the world that are already dealing with food insecurity.

Something similar can happen to tobacco plants, which produce less nicotine at higher levels of CO2. It would be easy to predict how rising CO2 levels might affect plants if the effect was always the same. Instead, what happens varies from species to species.

Plants can also produce more of their active ingredients. One example of this is sweet wormwood, which produces artemisinin, a treatment for malaria. Under high CO2 conditions, Artemisia produces more of this compound. And plants like the wild poppy produce even more opium-related chemicals at higher CO2.

So we just don’t know enough about how plants are going to change moving into the future. But what we do know is that in some cases, higher CO2 levels can even make plants more dangerous. For example, take the famine weed. The famine weed is native to Central and South America. In its natural habitat, its growth is controlled by organisms that eat the weed’s leaves, like the Parthenium beetle.

But outside of its native environment, the famine weed is a harmful invader. It’s an invasive species in Australia, Africa, and the Indian subcontinent. It’s particularly harmful because it produces a toxin called parthenin, which prevents other plants from growing around it. The toxin helps the plant spread by killing competing plants, and possibly also making the plant unappealing to herbivores.

Aside from being harmful to surrounding plants, parthenin can also cause asthma and skin irritation in humans, so many of the countries where it’s invasive are very interested in controlling its spread. That was the motivation of a 2021 study published in Nature Plants, which looked into the spread of famine weed in Australia.

The famine weed was introduced into Australia in the 1950s, but although two biotypes of famine weed were introduced at around the same time, only one biotype flourished and became invasive. Biotypes are groups of genetically identical organisms within a species, so they’re what we might also call a strain. So A single species can have multiple biotypes.

The researchers were initially looking into the difference between the two biotypes, trying to understand why one became invasive, while the other did not. They did find one lead: that the invasive biotype produced more parthenin than the non-invasive biotype.

And they also discovered that CO2 levels can change the way the plants produce parthenin. When they grew the plants at pre-industrial levels of CO2, the invasive biotype didn’t produce as much parthenin as it does in the wild. But ramp up the CO2, and the amount of parthenin produced by the invasive biotype also increased.

The non-invasive biotype, on the other hand, didn’t increase the amount of parthenin it produced when grown at higher CO2 levels. This means that the invasive biotype has probably adapted to use the current high CO2 atmosphere to its advantage, using the increased levels of CO2 as a source of carbon for parthenin production.

And so as CO2 levels increase, the famine weed’s invasive biotype could become, well, even more toxic and even more invasive. This could be a concern not just for the famine weed, but also for other toxic plants, which might also take advantage of the extra CO2 to produce more toxins. Many of these toxins help the plants outcompete other plants or protect themselves from predation.

This means that plants that develop stronger toxins could become better at outcompeting other plants. And while this is a problem for invasive species, it could also be a problem for native ecosystems. One plant suddenly adapting to outcompete all the others could be disruptive for the ecosystem as a whole.

In fact, it could be that some plants have already begun changing to adapt to high carbon dioxide conditions, just like famine weed. The fact that this is happening already is a reminder that fighting the climate crisis requires us to pay attention to what’s happening now, not just try to predict what will happen in the future. It’s important to study how rising CO2 levels change the physiology of plants, because as it turns out, just feeding them more CO2 is not automatically beneficial.

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