Food, Nutrition & Science

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Wednesday May 28, 2014

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Food Quality and Elevated CO2

In the News

Food quality will suffer as CO2 levels continue to rise, according to a recent report from UC Davis and published in the May issue of Nature Climate Change. The report looked at wheat grown under elevated CO2 concentrations, finding that nitrate assimilation was slower under these conditions.

While carbon dioxide (CO2) is naturally present in the atmosphere as part of the Earth’s carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals), human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution. According to the Environmental Protection Agency (EPA), in 2012, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human activities, like the combustion of fossil fuels for energy and transportation, land-use changes and certain industrial processes.

Previous studies have looked at the relationship of CO2 levels to nitrogen source in grain and non-legume plants, but this was the first study to look at this relationship in field-grown plants. In this study, researchers examined wheat grown in 1996 and 1997 under elevated or ambient atmospheric CO2 concentrations the free-air CO2 enrichment (FACE) experiment at Maricopa, Arizona. The experiment at Maricopa released carbon-dioxide into the air to create elevated levels of CO2 in test plots, mimicking future carbon levels on the Earth’s atmosphere.

Researchers indeed found that elevated CO2 inhibited conversion of leaf nitrate into proteins in field grown wheat. In fact, they documented three different measures of nitrate assimilation affected by CO2 levels. Their findings were consistent with those of previous laboratory studies.

“Plant responses to elevated CO2 are highly variable. We have shown in the laboratory that a large portion of this variation derives from differences in which source of nitrogen – ammonium or nitrate – plants are using because elevated CO2 inhibits the conversion of nitrate into proteins. This is the first study verifying that this (CO2 inhibition of nitrate conversion into proteins) also occurs under field conditions,” says Dr. Arnold Bloom, lead author of the study.

The consequences of the increasing CO2/decreasing nitrogen relationship are worrisome. Elevated CO2 lowers protein concentrations in wheat grain, rice grain, potato tuber and barley by about 8%. Wheat alone provides 21% of protein in the human diet. Researchers expect that protein available for human consumption may diminish by about 3% as atmospheric CO2 continues to climb in the next few decades.

“Rising CO2 levels will decrease food quality. Humans and domestic animals will need to consume about 8% more plant material to meet their nutritional requirements,” says Bloom.

Elevated CO2 inhibits photorespiration, a process that was initially was thought to be wasteful, but is now known to provide the energy to convert nitrate into proteins. Nitrogen is the element from the soil that plants require in greatest amounts and most often limits the productivity of plants. Nitrogen is a major constituent of amino acids and nucleotides, the building blocks of proteins and nucleic acids, respectively. Organisms require proteins and nucleic acids for all their basic functions. Plants receiving sufficient nitrogen will usually exhibit robust plant growth, and adequate nitrogen allows an annual crop like wheat or corn to grown to full maturity with the right balance of nutrients.

To combat these inevitable nutrient losses, Bloom suggests that the food industry look at breeding crops that are more tolerant to using ammonium as a nitrogen source, as elevated CO2 does not inhibit the conversion of ammonium into proteins. We can also breed crops that convert more nitrate into proteins in their roots, as root assimilation is not sensitive to elevated CO2. As for the future, Bloom says they will continue to look at the different ways in which plants absorb and process nutrients under a myriad of conditions.

“We are determining the extent to which different plants use nitrate versus ammonium as nitrogen sources and the extent to which different plants assimilate nitrate into proteins in their roots versus their shoots,” he says.