Rising carbon dioxide concentrations are making “the world’s most destructive toxic weed” more toxic

Parthenium hysterophorus is an annual herbaceous weed, often known as carrot grass, bitter weed, star weed, white top, wild feverfew, the “Scourge of India”, and congress grass. If you’ve not heard of it, consider yourself lucky, as it has been called “the world’s most destructive toxic weed“. Research recently published in Nature Plants shows that it’s getting worse.

Clifford Rice and colleagues compared two P. hysterophorus biotypes, one invasive and one not, to see how the plants responded to rising carbon dioxide concentrations. They grew the plants in chambers to control carbon dioxide levels and see how carbon dioxide concentrations affected their development. They found that the invasive biotype produced much more parthenin, a toxin used by the plant, at higher carbon dioxide concentrations. The worrying thing about these experiments is the concentrations that the botanists used to study the effects of carbon dioxide on toxin production.

“The majority of plant–CO2 research compares responses at ‘ambient’ (~365–415 ppm) and ‘elevated’ (~600–800 ppm or greater) concentrations, but current CO2 concentrations are already elevated relative to the 180–280-ppm range to which terrestrial plants had adapted for 800,000 years before the Industrial Revolution,” write Rice and colleagues. “The rate of increase in atmospheric CO2 has accelerated since the mid-twentieth century (from ~300 ppm in 1950 to 414.5 ppm in August 2020, and this recent change from subambient to modern ambient levels is likely to have the largest impact on C3 photosynthesis and plant secondary chemistry…”

The greater difference at lower carbon dioxide concentrations is because the availability of carbon dioxide is the biggest bottleneck for photosynthesis, say the authors. At higher levels of carbon dioxide, other factors restrict photosynthesis. An example they give is in Ziska et al‘s study of poison ivy, where production of the phenolic defence compound urushiol was 173% higher at 400 than at 300 ppm, but only 91% higher at 600 than at 400 ppm.

More parthenin in Parthenium is a serious problem. CABI’s Invasive Species blog says: “Parthenium weed is incredibly destructive; it kills other plant species within close proximity via allelopathy and can wipe out entire fields of crops… Parthenium growing in agricultural areas can poison livestock which in turn can then affect human health. Symptoms include mouth ulcers, skin lesions and even death if consumed in large enough quantities.”

Rice and colleagues note that it’s rare for studies to compare the carbon dioxide concentrations that plants evolved to use with modern-day concentrations. It’s a demonstration that rising carbon dioxide levels already have effects beyond increasing temperatures on the planet, and it’s not always a benefit. More carbon dioxide isn’t beneficial if your new neighbour can use it to pump out chemicals to poison you.

You can read this paper free via ReadCube.