Visible ozone injury in horticultural crops

In Central Europe tropospheric ozone is likely to be the most important gaseous air pollutant in recent years. Elevated concentrations of ozone, as they occur regularly during spring and summer, have the potential to cause visible injury and yield losses in many sensitive crop species.

In lettuce (Lactuca sativa L., top row: unexposed control) leaf chlorosis and brown necroses are visible in older leaves after chronic exposure to ozone (bottom row).

Radish (Raphanus sativus L.) is one of the most sensitive crops towards ozone injury and yield loss. After acute ozone exposure, marked necroses occur, which are easily visible on both, the upper (left) and lower surface (right) of the leaf.

After an acute exposure to realistic concentrations of ozone, the older leaves of spinach (Spinacia oleracea L.) exhibit extended, white-colored necroses.
As it is typical for ozone injury symptoms, these necroses are largely restricted to the interveinal sectors of the leaves.

In onion (Allium cepa L.) numerous small necroses occur after chronic ozone exposure. After acute exposure, these nearly-white necroses may damage large areas of the medium and older leaves.

In medium and old leaves of carrot (Daucus carota L.) the interveinal leaf zones show chlorotic discoloration after chronic ozone exposure.

In a recent study the majority of 12 horticultural crop species exhibited clearly visible ozone injury symptoms. After a 2- to 3-week chronic exposure to ozone concentrations typical for Southern Germany (ca. 135 µg/m3, 8 hours/day), chlorotic discoloration frequently occured, often accompanied by small necrotic lesions. After an acute exposure, mimicking realistic ozone episodes in the same region (ca. 250 µg/m3, 4 hours/day), necrotic damage often became dramatic.

"External quality", i.e. the visible appearance of the produce, is crucial for the marketability of horticultural crops, including vegetables. This quality criterion, however, may be considerably affected by visible leaf injury due to the exposure to tropospheric ozone, as it regularly occurs in Central Europe during the growing season.

For vegetable growers it is important to know that this type of injury cannot be mitigated by common measures of plant protection. When an episode of high ozone is imminent, however, it might be advisable to avoid additional watering of the plants. During shortage of water, plants tend to keep closed their stomates, those little pores in the leaf surface that allow carbon monoxide (as well as air pollutants like ozone) to enter into the leaf. And less ozone uptake means less ozone damage. Therefore, well-timed cultivation measures can help to avoid leaf injury due to ozone at least temporarily.

These are results of a study undertaken jointly with the Federal Agriculture Research Center (FAL, Braunschweig), Institute of Agroecology, and funded by the Ministry of Economy, Transportation, Agriculture and Viticulture of Rheinland-Pfalz (Mainz).

Chlorotic discoloration of the leaves (top row, right) and numerous small necroses, spread over the entire area of the leaf (bottom row, right), are typical ozone injury symptoms in zucchini (Cucurbita pepo L.). For comparison, undamaged leaves (not exposed to ozone) are shown on the left side.