Rising carbon-dioxide levels will result in the production of less nutritious grains and legumes, according to a University of Illinois Urbana-Champaign study.

For the study, the researchers analyzed several varieties of wheat, rice, field peas, soybeans, maize and sorghum grown in fields with atmospheric carbon dioxide levels predicted in the middle of this century. Currently, the atmospheric CO2 concentrations are nearing 400 parts per million and are anticipated to reach 550 ppm by 2050.

All the crops were exposed to estimated CO2 levels using Free Air Concentration Enrichment (FACE). FACE increases the concentration of CO2 in a particular area.

The researchers found that the nutritional quality - particularly iron, proteins and zinc - of several crops reduced following inflated CO2 levels. The content of zinc and iron decreased considerably in wheat, rice, field peas and soybeans, whereas lower levels of proteins was observed in wheat and rice in response to soaring CO2 levels.

"When we take all of the FACE experiments we've got around the world, we see that an awful lot of our key crops have lower concentrations of zinc and iron in them (at high CO2)," plant biology and Institute for Genomic Biology professor Andrew Leakey and an author on the study, said in a press release. "And zinc and iron deficiency is a big global health problem already for at least 2 billion people."

Leakey said that the quality of the nutrients, however, remained stable in sorghum and maize at increased CO2 levels because they use a type of photosynthesis, C4, that already stores carbon dioxide in their leaves.

"C4 is sort of a fuel-injected photosynthesis that maize and sorghum and millet have," Leaky said. "Our previous work here at Illinois has shown that their photosynthesis rates are not stimulated by being at elevated CO2. They already have high CO2 inside their leaves."

Leakey said that further studies in tropical climates with tropical soils need to be conducted to determine the crops' response to higher atmospheric CO2 levels as food insecurity continues to remain one of the biggest threats in developing regions.

The findings are reported in the journal Nature.