COLUMBUS, Ohio – A new study suggests that rising levels of carbon dioxide in the atmosphere could be a boon for agricultural crops, as this greenhouse gas helps crop plants grow and reproduce more.
But that boon comes with a price, said Peter Curtis, a professor of evolution, ecology and organismal biology at Ohio State University. Greater growth and reproduction may hurt the nutritional value of crops.
"If you're looking for a positive spin on rising CO2 levels, it's that agricultural production in some areas is bound to increase," Curtis said. "Crops have higher yields when more CO2 is available, even if growing conditions aren't perfect.
"But there's a tradeoff between quantity and quality. While crops may be more productive, the resulting produce will be of lower nutritional quality."
Nutritional quality declines because while the plants produce more seeds under higher CO2 levels, the seeds contain less nitrogen.
"The quality of the food produced by the plant decreases, so you've got to eat more of it to get the same benefits," Curtis said. "Nitrogen is a critical component for building protein in animals, and much of the grain grown in the United States is fed to livestock.
"Under the rising CO2 scenario, livestock – and humans – would have to increase their intake of plants to compensate for the loss."
The research appears in the current issue of the journal New Phytologist.
To understand the role that rising CO2 levels may play on plant growth, Curtis and his colleagues conducted a meta-analysis – a technique in which researchers pull together data from a large number of similar studies (159, in this case) and summarize the results. Curtis said that this is the first time that researchers have used the meta-analysis technique to determine the effects of climate change on plant reproduction.
The studies were published between 1983 and 2000. The results included data on crop and wild plant species' reproductive responses to estimated CO2 levels at the end of this century. Scientists expect CO2 levels to nearly double by 2100.
The researchers analyzed eight different ways plants respond to higher CO2 levels: number of flowers; number of fruits; fruit weight; number of seeds; total seed weight; individual seed weight; the amount of nitrogen contained in seeds; and a plant's reproductive allocation, a measurement of a plant's capacity to reproduce.
Plants grown at higher CO2 levels had more flowers (an average of 19 percent more in the species studied); more seeds (16 percent more); greater individual seed weight (four percent more); greater total seed weight (25 percent more) and lower concentration of nitrogen in the seeds (a decrease of 14 percent) than those grown at current levels of atmospheric CO2.
Under higher CO2 levels, crop plants showed a notable increase in reproduction while wild plants did not. On average, crops produced more fruits than did wild species (28 percent higher in crops vs. 4 percent higher in wild plants) as well as seeds (21 percent higher vs. 4 percent higher, respectively).
Individual crops varied in their response to increased CO2 levels. Rice seemed to be the most responsive, as its seed production increased an average of 42 percent. Soybean followed with a 20 percent increase in seed, then wheat (15 percent increase) and, finally, corn (5 percent increase).
While crop plants and wild plants had similar increases in total growth (a 31 percent increase), crops allocated the additional weight to reproduction, while wild plants seem to funnel much of it to tasks other than reproduction, Curtis said.
"Wild plants are constrained by what they can do with increased CO2," he said. "They may use it for survival and defense rather than to boost reproduction. Agricultural crops, on the other hand, are protected from pests and diseases, so they have the luxury of using extra CO2 to enhance reproduction."
Even though seed size increased, the amount of nitrogen in the seeds didn't. Nitrogen levels decreased by an average of 14 percent across all plants except cultivated legumes, such as peas and soybeans.
For example, the total number of seeds in wheat and barley plants increased by 15 percent, but the amount of nitrogen in the seeds declined by 20 percent.
"That's bad news," Curtis said. "Nitrogen is important for building protein in humans and animals. If anything, plant biologists want to boost the levels of nitrogen in crops.
"A growing global population demands more food, but humans would have to eat more of the food to get the same nutritional benefits."
On the flip side, legumes are able to use a rise in CO2 to increase the amount of nitrogen they take in. The result is that these plants maintain their nutritional quality during conditions of high CO2 levels.
"Ecologically speaking, changes in the number of flowers, fruits and seeds and their nutritional quality could have far-reaching consequences," Curtis said. "Changes in the amount of nutrients in seeds could affect reproductive success and seedling survival. Such changes could also have long-term effects on ecosystem functioning."
The study was funded by a grant from the National Science Foundation.
Curtis conducted the meta-analysis with Leanne Jablonski of Ohio State and the Marianist Environmental Education Center in Dayton, Ohio, and Xianzhong Wang, of the department of biology at Indiana University-Purdue University in Indianapolis.
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