It might seem there’s an upside to the rising levels of carbon dioxide in the atmosphere. Plants are growing faster.
However, in many species of plants, quantity is not quality. Most plants are growing faster, but they have on average more starch, less protein and fewer key vitamins in them, says James Metzger, a professor and chair of the Department of Horticulture and Crop Science in Ohio State University’s College of Food, Agricultural, and Environmental Sciences.
This change is happening because the current level of carbon dioxide in the atmosphere is 400 parts per million, nearly double what it was in the middle of the 18th century, the start of the industrial revolution. And it keeps rising, spurred by the burning of fuels.
Taking in carbon dioxide and light, a plant forms sugars and starches first, then other nutrients such as protein, fat and antioxidants. Although carbon dioxide is necessary for plants to live, too much carbon dioxide can reduce the amount of valuable nutrients the plant produces, such as iron, zinc and vitamin C.
“The loss of nutrients, particularly protein, is serious,” Metzger says. “That does not help in the effort for people to eat more balanced diets and increase their nutrition.”
Animal meat and dairy products are a significant source of protein for humans. So, if animals aren’t getting sufficient protein from plants, that will affect what they can produce as food.
What’s happening is that a higher level of carbon dioxide in the atmosphere reduces the amount of photorespiration that occurs in plants. During photorespiration, plants take in oxygen from the environment, release carbon dioxide and produce waste products including glycolic acid, which a plant can’t use. For the plant to turn the glycolic acid into a product it can use, the plant has to do more photosynthesis, the process through which plants use sunlight, water and carbon dioxide to create glucose, a form of sugar that plants need to survive.
Low stress not always good
Low rates of photorespiration, caused by the higher amounts of carbon dioxide, are associated with low stress levels in plants, which ironically is not a good thing. That’s because stressed plants respond by producing antioxidants such as vitamins C and E, as well as higher protein levels. So, as carbon dioxide levels in the atmosphere rise, there is less photorespiration and therefore less stress on plants. And the reduced stress means increased growth, but at a cost, a decline in the nutritional quality of the plants.
“This has been observed in many different species of plants,” Metzger says.
If the plant is not producing enough antioxidants, that’s not just less healthy for people who later eat the plant, but also for the plant’s ability to fend off diseases, Metzger says. Plants can become more vulnerable to diseases as well as insects. With fewer nutrients in the plants, the insects have to devour more of them to get the same nutritional value.
Not all plants react to the rising carbon dioxide levels in the same way. Some crops, including corn and sugar cane, do not decrease in nutritional value amid higher carbon dioxide levels. That’s because their photosynthesis process differs from that of most other plants.
Temperature is a factor as well. Depending on the temperature, plants can react in different ways to high carbon dioxide levels. The rising carbon dioxide levels that are triggering more photosynthesis can hinder the growth of some plants cultivated in temperatures below 59 degrees F, such as winter wheat, says Katrina Cornish, Ohio research scholar and endowed chair in bio-based emergent materials with CFAES.
Plants grown in hot weather conditions can also be impeded by elevated carbon dioxide. In hot temperatures, many plants stay cool by opening wide the pores on the underside of their leaves. But in an atmosphere with high carbon dioxide, the pores do not open as wide, so plants are not able to keep themselves cool, Cornish says. This could cause “the plants to become crispy critters and die, when they were OK at lower carbon dioxide levels,” she says. “Plants need time to adapt to the increase in carbon dioxide levels. And the increase is happening so quickly, plants are not going to have a chance to adapt.”
In the short term, the additional photosynthesis spurred by higher carbon dioxide levels may bring about small gains in the amount of leaves, stem and shoots that are produced by a crop but not necessarily in the portion of the crop that can be harvested. And in the long term, it’s going to do more harm to plants than good, Cornish says.
“There’s going to be a tipping point, and that tipping point is different for each crop,” Cornish adds.
Already, rice plants grown in elevated carbon dioxide have been shown to produce more tillers, which include the stems and leaves of the plant, but fewer and smaller grains.
One way to prevent the higher carbon dioxide levels from affecting plant growth and yield is through plant crossbreeding and gene manipulation, Metzger says. Both could lead to the creation of varieties of plants whose growth and nutrient levels will be less affected by the higher amounts of carbon dioxide in the environment.
More research is needed to figure out how a plant produces antioxidants, Metzger says.
“I think it’s important that we put some effort into really understanding how those biochemical pathways are controlled and how we can manipulate them without any harmful effects on the plant.”
Source: Ohio State University's College of Food, Agricultural, and Environmental Sciences