Field tests prepared through research at the University of California Davis has found for the first time that elevated levels of carbon dioxide can inhibit plants' processing of nitrate into proteins, indicating that the nutritional quality of food crops is at risk as CO2 levels rise.
Arnold Bloom, lead researcher and professor in the UC Davis Department of Plant Sciences, says there are several explanations for nutritional decline, but his study is the first to demonstrate that elevated carbon dioxide inhibits the conversion of nitrate into protein in a field-grown crop.
The assimilation, or processing, of nitrogen, plays a key role in the plant's growth and productivity, Bloom says. In food crops, it is especially important because plants use nitrogen to produce the proteins that are vital for human nutrition. Wheat, in particular, provides nearly one-fourth of all protein in the global human diet.
Many previous laboratory studies had demonstrated that elevated levels of atmospheric carbon dioxide inhibited nitrate assimilation in the leaves of grain and non-legume plants; however there had been no verification of this relationship in field-grown plants.
Building the study
To observe the response of wheat to different levels of atmospheric carbon dioxide, the researchers examined samples of wheat that had been grown in 1996 and 1997 in the Maricopa Agricultural Center near Phoenix, Ariz.
At that time, carbon dioxide-enriched air was released in the fields, creating an elevated level of atmospheric carbon at the test plots, similar to what is now expected to be present in the next few decades. Control plantings of wheat were also grown in the ambient, untreated level of carbon dioxide.
Leaf material harvested from the various wheat tests plots was immediately placed on ice, and then was oven dried and stored in vacuum-sealed containers to minimize changes over time in various nitrogen compounds.
Reviewing changes via three measures
A fast-forward through more than a decade found Bloom and the current research team able to conduct chemical analyses that were not available at the time the experimental wheat plants were harvested.
In the recent study, the researchers documented that three different measures of nitrate assimilation affirmed that the elevated level of atmospheric carbon dioxide had inhibited nitrate assimilation into protein in the field-grown wheat.
"These field results are consistent with findings from previous laboratory studies, which showed that there are several physiological mechanisms responsible for carbon dioxide's inhibition of nitrate assimilation in leaves," Bloom said.
Bloom noted that other studies also have shown that protein concentrations in the grain of wheat, rice and barley — as well as in potato tubers — decline, on average, by approximately 8% under elevated levels of atmospheric carbon dioxide.
"When this decline is factored into the respective portion of dietary protein that humans derive from these various crops, it becomes clear that the overall amount of protein available for human consumption may drop by about 3% as atmospheric carbon dioxide reaches the levels anticipated to occur during the next few decades," Bloom says.
While nitrogen fertilization could partially compensate for this decline in food quality, Bloom believes it would have negative consequences – including higher costs and increased emissions of the greenhouse gas nitrous oxide.
Previous research has indicated that higher CO2 levels could also favor weed strength.
Findings from the wheat field-test study are online in the journal Nature Climate Change. Funding for the study was provided by the USDA's National Institute of Food and Agriculture.