Ozone and the world food crisis

Heat waves, droughts and fuel prices are just a few reasons for the current global food crisis that is making headlines around the world. Research by William Manning of the University of Massachusetts Amherst indicates that rising background levels of ozone in the atmosphere are a likely contributor to the problem, lowering the yield of important food crops, such as wheat and soybeans.

“Plants are much more sensitive to ozone than people, and a slight increase in exposure can have a large impact on their productivity,” says Manning, a professor of plant, soil and insect sciences. “The new ozone standard set by the US EPA in March 2008 is based on protecting human health, and may not be strict enough to protect plants.” Manning served on the Clean Air Science Advisory Committee for the EPA in 1997 when the previous air quality standard for ozone was developed.

According to Manning, emission controls on cars have been successful in reducing short periods of high ozone levels called peaks, but average concentrations of ozone in the atmosphere throughout the year, called the background level, are increasing as polluted air masses from Asia travel to the US and then on to Europe. Background levels are now between 20 and 45 parts per billion in Europe and the United States, and are expected to increase to between 42 and 84 parts per billion by 2100.

Manning was recently part of a team of researchers studying how ozone levels in the Yangtze Delta affect the growth of oilseed rape, a member of the cabbage family that produces one-third of the vegetable oil used in China. By growing the plants in chambers that controlled the ozone environment, the team showed that exposure to elevated ozone reduced the size and weight, or biomass, of the plants by 10 to 20%. Production of seeds and oil was also reduced.

“What was surprising about this research was that plants exposed to ozone levels that peaked in the late afternoon suffered more damage than plants exposed to a steady ozone concentration throughout the day, even though average ozone concentrations were the same for both groups,” says Manning. “This shows that current ozone standards that rely on average concentrations would underestimate crop losses.”

Additional research in the Yangtze Valley, which accounts for nearly half of China’s crop production, showed that wheat was more sensitive to ozone than rice. “Plants vary widely in their sensitivity to ozone, and varieties of the same species can react differently,” says Manning. “Some of the most sensitive plant species are from the legume and cabbage families, which include radishes, broccoli and soybeans.”

Plants can limit ozone damage for short periods of time by reducing the size of pores on their leaves called stomata. This reduces the uptake of ozone, but also carbon dioxide, which is used as the plants make food through the process of photosynthesis. Chronic exposure results in reduced photosynthesis, plant growth and yields. In the long term, leaf injury occurs when the amount of ozone taken in exceeds the leaf’s capacity to provide antioxidants to counter its effects.

This summer, Manning will be investigating the effects of ozone on a variety of plants in the Pioneer Valley of Massachusetts, where ozone levels are often above the EPA standard as pollution from New York City and Washington, DC moves northwards during the day. Plants will be grown in open fields, and some will be treated with a compound that blocks the effects of ozone. If the treated plants are healthier than the untreated group, then ozone will be the cause.