Rising ozone called a threat to corn varieties

Current levels will decrease yields by as much as 10 percent, which is the same level of threat as drought and flood

While rising carbon dioxide and methane levels in the atmosphere are a concern, little attention has been paid to the problem of rising ozone levels.

In a new study, researchers have discovered that some varieties of the corn family are more susceptible than others to yield losses when exposed to high levels of ozone air pollution.

Researchers from the U.S. Agricultural Research Service, the U.S. Department of Agriculture and scientists with the universities of Illinois and Florida and the University of California, Davis, conducted the study.

Ozone enters plants through leaves but, as a reactive molecule, it can damage biological tissues, impair photosynthetic carbon capture and accelerate the aging of leaves.

“The most sensitive varieties (of corn) show more rapid aging and loss of chlorophyll and photosynthetic capacity in elevated ozone,” said Lisa Ainsworth, research molecular biologist with the U.S. Department of Agriculture. “This general pattern is also true for other crops. The genotypes that show the most yield loss also show more acceleration of senescence (aging).”

Even background ozone levels can damage crops. The research has indicated that current ozone levels will decrease corn yields by as much as 10 percent. That represents the same level of threat as drought, flood, pests or diseases, yet ozone until now has been a relatively under-studied threat.

“Ozone is a short-lived air pollutant meaning that, on average, a molecule of ozone only sticks around in the lower atmosphere for a number of days, while carbon dioxide and methane have atmospheric lifetimes of years (five to 200),” said Ainsworth. “The Intergovernmental Panel on Climate Change separates long-lived and short-lived pollutants and perhaps the long-lived pollutants have received more attention.”

Ainsworth said they are only just beginning to understand why some varieties are more susceptible to ozone.

“It is likely a combination of the capacity for the plant to withstand reactive oxygen species stress and the ability of the plant to restrict ozone entry into the leaf. The additional reactive oxygen species stress commonly leads to faster senescence in sensitive varieties.”

Reactive oxygen species is used to describe a number of reactive molecules and free radicals derived from molecular oxygen. Ainsworth said that they have been more successful finding sensitive lines than ozone-tolerant lines.

To track real time consequences of higher atmospheric ozone levels in an agricultural field, the researchers used the Free Air Concentration Enrichment (FACE) facility at the University of Illinois.

The FACE method is used by ecologists and plant biologists to study the response of plant growth to their exposure to various concentrations of gases. Most studies looking at the effects of greenhouse gases have been done under laboratory conditions but there are many missing factors in ultra-controlled conditions. In an open field, researchers can monitor gas exposure along with weather implications and plant competition. The FACE facility at Illinois has an emissions system that monitors wind direction and speed to dose a field with specific gases.

“The level that we’re fumigating to in this study is a level that is commonly found today in China and India,” Ainsworth said in a news release. “So, it’s not excessively high, even though we’re using a concentration that is 2.5 times the level of background ozone in central Illinois.”

The researchers planted 45 hybrid corn plants that represented all the major varieties including popcorn, broom corn, dent, flint and others. They examined the corn to track variations in their responses to high ozone levels and the outcome showed that some hybrids were more sensitive than others to ozone exposure.

“We found two maize lines whose offspring were more sensitive to ozone pollution, regardless of which other types of corn we bred them with,” said Andrew Leakey, plant biology professor with the University of Illinois, in the news release. “Their genetic deficiencies manifested in different ways when exposed to the high ozone conditions.”

He added that more genetic analysis and more experiments will be needed.

Currently, the research team is mapping the genetic loci associated with ozone response. Ainsworth said that there are some loci that increase the ozone sensitivity and they are following up on those with ongoing experiments. In addition, they are following up on the genetic markers associated with the ozone response as well as doing transcriptome sequencing to identify the genes that are responsive to ozone across diverse inbred lines of corn.

“Ozone is a short-lived pollutant,” said Ainsworth. “It is also a secondary pollutant formed when clouds of hydrocarbon pollution and NOx (nitrogen oxides) react with sunlight. Because it is short-lived, it can be effectively controlled with regulation of emissions. However, we don’t yet have global emissions standards, and until we do, ozone pollution will continue to be problematic. Ozone is a greenhouse gas, so it is also contributing to global climate change, which is another reason to try to limit its production.”

She said floods, droughts, heat waves, new pests and diseases tend to be bigger worries than ozone, but as the climate continues to warm and ozone has greater impact on corn yields, that may change.

The research was published in the journal Global Change Biology.

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