High levels of carbon dioxide cut nutritional value of crops

Although higher levels of atmospheric carbon dioxide have been heralded as a boon for higher crop yields, the news may not be all good, as higher yields are accompanied by lower nutritional value, research has revealed. The same study has also found that higher carbon dioxide levels could change the composition of wild plant communities.

Researchers from Ohio State University have examined nearly 160 publications of research into the effects of elevated carbon dioxide levels on wild and domesticated plants. The have found that there is a consensus that increased levels of the greenhouse gas increases yields of crop plants (see related story). “But there’s a trade off between quantity and quality,” said Peter Curtis, Professor of Plant Ecology at the University, and one of the authors of the research. Although there is greater productivity, nutritional value is lower, he says.

The study revealed a 19% increase in flowers for domesticated crops, 16% more seeds, a 4% increase in individual seed weight, and a 25% increase in total seed weight. This is accompanied by a 14% decrease in nitrogen concentration in seeds.

Why this is happening is not known for certain. However, there are a number of theories, including the dilution effect, where only a limited amount of nitrogen is available, so an increase in the number of seeds results in a reduced share for each one. It is also possible that plants become more efficient under higher levels of carbon dioxide, reducing plants’ requirement for nitrogen uptake, Curtis told edie.

The response to higher carbon levels varied between crop species. Seed production within rice increased by an average of 42%, compared to a 20% increase with soybean, and a 5% increase with corn.

The exception to the rule is the case of legumes, which still show increased seed production, but is accompanied by a corresponding increase in nitrogen uptake, so that the level of nitrogen stays the same in each seed.

The effect of elevated carbon dioxide is not as clear-cut when it comes to wild plants, however. There has been far less research into effects on wild plants, but those that have been carried out show considerable variation between species. Instead of increasing reproductive output, wild plants tend to use the increased carbon available to them in defence and survival mechanisms, such as toughened leaves and protective chemicals, says Curtis.

So why do domesticated plants behave so differently? “We’ve been breeding them for 10,000 years to put their effort into seeds,” Curtis told edie. They also have less of a requirement to protect themselves as Man has also put much effort into protecting them, he says.

However, it is the variation of effects that most interests Curtis. “For wild species this is a mixed blessing,” he says. There are going to be winners and losers, with the victors being fast growing, aggressive species, which will have an even better competitive advantage over their weaker rivals. In the US these include wild radish, and a pernicious vine-like weed known as kudzu. This has serious implications for the makeup of ecosystems, Curtis explains.

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