CO₂ cuts nutrition

March 17, 2016

Climate change will have an impact on plant growth and the nutritional value of the food we produce.

CO₂ cuts nutrition

Climate change is affecting the Earth, through more frequent and intense weather events, such as heatwaves and rising sea levels, and is predicted to do so for generations to come. Changes brought on by anthropogenic climate change, from activities such as the burning of fossil fuels and deforestation, are impacting natural ecosystems on land and at sea, and across all human settlements.

Increased atmospheric carbon dioxide (CO₂) levels – which have jumped by a third since the Industrial Revolution – will also have an effect on agriculture and the staple plant foods we consume and export, such as wheat.

Stressors on agribusiness, such as prolonged droughts and the spread of new pests and diseases, are exacerbated by climate change and need to be managed to ensure the long-term sustainability of Australia’s food production.

Researchers at the Primary Industries Climate Challenges Centre (PICCC), a collaboration between the University of Melbourne and the Department of Economic Development, Jobs, Transport and Resources in Victoria, are investigating the effects of increased concentrations of CO₂ on grain yield and quality to reveal how a more carbon-enriched atmosphere will affect Australia’s future food security.

CO₂ cuts nutrition
An aerial view of the Australian Grains Free Air CO₂ Enrichment (AGFACE) project, where researchers are investigating the effects of increased concentrations of carbon dioxide on grain yield and quality.

Increasing concentrations of CO₂ in the atmosphere significantly increase water efficiency in plants and stimulate plant growth, a process known as the “fertilisation effect”. This leads to more biomass and a higher crop yield; however, elevated carbon dioxide (eCO₂) could decrease the nutritional content of food.

“Understanding the mechanisms and responses of crops to eCO₂ allows us to focus crop breeding research on the best traits to take advantage of the eCO₂ effect,” says Dr Glenn Fitzgerald, a senior research scientist at the Department of Economic Development, Jobs, Transport and Resources.

According to Fitzgerald, the research being carried out by PICCC, referred to as Australian Grains Free Air CO₂ Enrichment (AGFACE), is also being done in a drier environment than anywhere previously studied.

“The experiments are what we refer to as ‘fully replicated’ – repeated four times and statistically verified for accuracy and precision,” says Fitzgerald. “This allows us to compare our current growing conditions of 400 parts per million (ppm) CO₂ with eCO₂ conditions of 550 ppm – the atmospheric CO₂ concentration level anticipated for 2050.”

The experiments involve injecting CO₂ into the atmosphere around plants via a series of horizontal rings that are raised as the crops grow, and the process is computer-controlled to maintain a CO₂ concentration level of 550 ppm.

CO₂ cuts nutrition
Horizontal rings injecting carbon dioxide into the atmosphere as part of the AGFACE project. Credit: AGFACE

“We’re observing around a 25–30% increase in yields under eCO₂ conditions for wheat, field peas, canola and lentils in Australia,” says Fitzgerald.

Pests and disease

While higher CO₂ levels boost crop yields, there is also a link between eCO₂ and an increase in viruses that affect crop growth.

Scientists at the Department of Economic Development, Jobs, Transport and Resources have been researching the impact of elevated CO₂ levels on plant vector-borne diseases, and they have observed an increase of 30% in the severity of the Barley Yellow Dwarf Virus (BYDV).

CO₂ cuts nutrition
Higher CO₂ levels are linked with an increase in the severity of Barley Yellow Dwarf Virus.

Spread by aphids, BYDV is a common plant virus that affects wheat, barley and oats, and causes yield losses of up to 50%.

“It’s a really underexplored area,” says Dr Jo Luck, director of research, education and training at the Plant Biosecurity Cooperative Research Centre. “We know quite a lot about the effects of drought and increasing temperatures on crops, but we don’t know much about how the increase in temperature and eCO₂ will affect pests and diseases.

“There is a tension between higher yields from eCO₂ and the impacts on growth from pests and diseases. It’s important we consider this in research when we’re looking at food security.”

This increased yield is due to more efficient photosynthesis and because eCO₂ improves the plant’s water-use efficiency.

With atmospheric CO₂ levels rising, less water will be required to produce the same amount of grain. Fitzgerald estimates about a 30% increase in water efficiency for crops grown under eCO₂ conditions.

But nutritional content suffers. “In terms of grain quality, we see a decrease in protein concentration in cereal grains,” says Fitzgerald. The reduction is due to a decrease in the level of nitrogen (N2) in the grain, which occurs because the plant is less efficient at drawing N2 from the soil.

The same reduction in protein concentration is not observed in legumes, however, because of the action of rhizobia – soil bacteria in the roots of legumes that fix N2 and provide an alternative mechanism for making N2 available.

“We are seeing a 1–14% decrease in grain-protein concentration [for eCO₂ levels] and a decrease in bread quality,” says Fitzgerald.

“This is due to the reduction in protein and because changes in the protein composition affect qualities such as elasticity and loaf volume. There is also a decrease of 5–10% in micronutrients such as iron and zinc.”

This micronutrient deficiency, referred to as “hidden hunger”, is a major health concern, particularly in developing countries, according to the International Food Research Policy Institute’s 2014 Global Hunger Index: The challenge of hidden hunger.

There could also be health implications for Australians. As the protein content of grains diminishes, carbohydrate levels increase, leading to food with higher caloric content and less nutritional value, potentially exacerbating the current obesity epidemic.

The corollary from the work being undertaken by Fitzgerald is that in a future CO₂-enriched world, there will be more food but it will be less nutritious. “We see an increase in crop growth on one hand, but a reduction in crop quality on the other,” says Fitzgerald.

Fitzgerald says more research into nitrogen-uptake mechanisms in plants is required in order to develop crops that, when grown in eCO₂ environments, can capitalise on increased plant growth while maintaining N2, and protein, levels.

For now, though, while an eCO₂ atmosphere may be good for plants, it might not be so good for us.

– Carl Williams

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