As Carbon Dioxide Goes Up, Plants’ Nutrient Content Declines

ABOVE: Soybeans (Glycine max) are among the plants known to have reduced nutritional value after exposure to excess carbon dioxide. © ISTOCK.COM, HAUGED

As greenhouse gas concentrations in the air continue to climb, plants are faced with a veritable feast of carbon dioxide, which they use alongside water and sunlight to photosynthesize. While years of research shows that this profusion of carbon allows some plants to grow faster and larger, a literature review published today (November 3) in Trends in Plant Science indicates that the full story is far less encouraging.

The review collates a growing body of evidence that the carbon dioxide-triggered increase in photosynthesis, known as the carbon fertilization effect, presents a mixed bag for plant health, and pieces together the molecular mechanisms affected by high carbon dioxide levels. On one hand, the boosted photosynthesis and consequently heightened carbohydrate production ups the biomass of C3 plants, a group that contains the vast majority of vegetation on Earth. But multiple studies suggest that these plants, which include major agricultural crops, take in less nitrogen as a result of that excess carbon, thereby reducing their nutritional value and their ecosystems’ capacities to act as carbon sinks.

Biogeochemist Samantha Weintraub-Leff, who didn’t work on the study but researches similar questions at the National Ecological Observatory Network (NEON), a network of field sites throughout the US, applauds the review authors for delving into what genetic and molecular mechanisms might be driving these changes in crop and wild plants.

“We’ve got to figure out how to get around this to make sure we’re not only producing more food, but also food that’s as nutritious as we need it to be,” Weintraub-Leff tells The Scientist. “That’s hugely important for human wellbeing. I think it’s very important they’re pointing this out and trying to identify a root cause.”

See “Climate Change May Favor Nitrogen-Fixing Plants”

The review lays out and evaluates four leading hypotheses that have emerged to explain why increased environmental CO₂ exposure results in reduced concentrations of mineral nutrients, especially that of nitrogen, which plants acquire from soil in the form of nitrates and which is a critical building block for molecules that make up both plants and the animals that eat them.

Hypothesized mechanisms include: straightforward dilution, in which an increase in overall biomass and carbon results in a lower concentration of everything else; the closing of stoma, the openings in plant leaves, for longer periods of the day, as well as stomal narrowing, both of which result in less nutrient uptake through the roots and reduced transportation of nitrogen to shoots; disruption of the molecular pathway that fuels the nitrogen reduction reaction, which converts nitrates taken from the soil into usable proteins; and dysregulation of the nitrate uptake system in the roots.

“I don’t think anyone would debate that we are seeing increasing carbon-to-nitrogen ratios,” says Weintraub-Leff. “Plants are not keeping up overall with nitrogen uptake compared to the amount of carbon they’re seeing. The contribution of this paper is trying to get at the mechanisms. . . . that’s important because then we can figure out what to do about this, especially in our agro ecosystems.”

The authors suggest that all four hypotheses play at least a small role in the issue. But based on the published evidence for each hypothesis—primarily made up of experiments in which model plants such as soybeans (Glycine max), rice, and Arabidopsis thaliana are subjected to different levels of CO₂ exposure in a chamber—the review’s authors suggest that the third and fourth hypotheses are better supported, and likely play a more prominent role than the first two. Those hypotheses both indicate that increased CO₂ concentrations in the air and soil interfere with the gene expression and molecular processes that typically grant plants the ability to take in and then make use of nitrates.

We need to speed [adaptation] up in this very high CO₂ world we’ve artificially created.

—Samantha Weintraub-Leff, NEON

“What makes [those two] hypotheses more compelling is the fact that they are focused on N specific-mechanisms, and even on nitrate-related mechanisms,” review coauthor Antoine Martin, a plant biologist at the National Centre for Scientific Research in France, tells The Scientist over email.

Experiments cited in the review suggest that the increased environmental CO₂ downregulates various NRT genes that are involved in transporting nitrogen through and from a plant’s roots, as well as photosynthesis genes such as RBCL. However, the review notes that researchers haven’t yet accumulated much long-term data on these specific genetic mechanisms, and Martin adds that he found it particularly surprising to find a decrease in nitrogen uptake despite increased photosynthesis. The mechanism for this effect hasn’t yet been fully uncovered.

Roslyn Gleadow, a biologist at Monash University in Australia who studies nitrogen metabolism in plants and didn’t contribute to the review, notes that the hypotheses that high carbon dioxide alters nitrate uptake and assimilation were controversial when first proposed about ten years ago, but that evidence for them has since built.

“I’m not at all surprised that there are lower levels of micronutrients in plants grown at elevated carbon dioxide,” she says. “I’ve found this myself, and it has been recorded many times. What was interesting was that this was not simply from a dilution effect.”

See “Plants in UK Bloom a Month Earlier Than in 1980s: Study”

A key takeaway highlighted both by the paper and by experts who spoke with The Scientist is that these changes spell trouble for agriculture and for ecosystems around the world.

“What has been becoming increasingly obvious to those of us working in the photosynthesis field is that photosynthesis will only benefit from increasing carbon dioxide for so long, and that is now starting to plateau,” Gleadow says. “So the huge benefit we’ve seen on plants drawing down carbon dioxide from the atmosphere, increasing crop growth, increasing tree growth—that is going to flatten out in the next ten years or so.”

The review notes that a reduction in plants’ mineral nutrients can lead to depletions of important compounds for the soil and the soil microbiome, and disrupt the cyclic flow of nitrogen and carbon throughout the ecosystem. The authors suggest that genetically engineering crop plants to be more adaptive to high CO₂, or actively selecting for those with more adaptive traits, will become increasingly important.

“Gene editing in crops will be a relevant approach as soon as the genetic determinants of this negative response will be clearly identified,” Martin writes.

See “Climate Change Likely to Slow Plant Growth in Northern Hemisphere”

“I think the place where we can actually do something about this is in croplands,” says Weintraub-Leff. “And that’s not trivial.”

She adds: “We need to speed [adaptation] up in this very high CO₂ world we’ve artificially created.”

However, she says that this strategy becomes far less feasible when considering the plants that live in nonagricultural ecosystems, where it’s harder to manipulate plants and the soil they grow in. “The only thing we can do in forests and grasslands is bring down CO₂.”

Gleadow agrees, arguing that genetic engineering alone can’t counteract the devastation of ongoing climate change. “This is yet another alarm bell that says we absolutely have to reduce emissions,” she says. “I think gene editing is a fabulous tool, [but] whether it’s the answer, I really don’t know, and I just can’t see those sorts of things spreading widely throughout the globe.”