Air Pollutants: Effects of Ozone on Crop Yield and Quality

Air Pollutants: Effects of Ozone on Crop Yield and Quality


Air pollutants have been known for more than one hundred years to affect both the yield and quality of agricultural crops. This is true for a wide range of pollutants including sulfur dioxide, fluorides, and nitrogen-containing pollutants such as nitrogen oxides (NO, NO2) and ammonia (NH3). Today, the main focus is on the effects of the regional occurrence of elevated tropospheric ozone (O3).

In North America and Europe, where emissions of sulfur dioxide and fluorides have declined, the problem of ozone pollution has received the most attention. In contrast, in many developing countries emissions of the number of pollutants are increasing and there are strong indications of adverse effects on crops. However, these effects have been studied to a much lesser extent than in Europe, North America, Japan, and Australia.


Systematic studies of ozone effects on crop yield and quality started around 1950 in California, U.S.A., where the problems caused by ozone and other photochemical oxidants were first discovered. At that time, the studies were mainly of an observational nature, and they were based on visible injury appearing on the plant leaves. 

Controlled experiments in the laboratory began later; however, initially, these yielded largely qualitative data which were not suitable as a basis to estimate the magnitude of potential effects in the field. But, it should be noted here that in certain crops, such as spinach and tobacco, leaf injury is of large economic importance, apart from potential reductions in absolute yield.

In some crops, visible leaf injury expressed as necrotic spots on the leaves, is the most pronounced effect of ozone and other pollutants, while in others a reduction in the leaf life span, the so-called premature senescence, is most important, as it reduces the length of the growth period. The latter occurs in wheat, in which characteristic leaf injury at moderate ozone exposure is generally lacking, while early senescence in response to ozone is pronounced.


With the introduction of the open-top chamber (OTC) as an exposure system, which permits semicontrolled exposure of plants to gaseous pollutants under ecologically realistic conditions in the field, an important step was taken toward a quantitative understanding of the impacts of ozone and other pollutants.

An OTC is a transparent plastic cylinder through which air (ambient air, filtered air, or air enriched with various levels of pollutant gases) is ventilated using a fan. OTCs can be mounted in plots of a field-grown crop and kept in place throughout the growing season. Although the system alters the microclimate of the plants to a certain extent, the ecological realism is much larger than in a closed chamber system in the laboratory.


In the U.S.A., a large experimental program, the NCLAN (National Crop Loss Assessment Network), involving the use of OTCs of crops, was executed during the 1980s. The NCLAN results showed that ambient levels of ozone had the potential to reduce the yield of a number of crops, including soybean, wheat, and alfalfa.

In addition, economic estimates of crop losses for the U.S.A. indicated an annual loss for the farmers in the range of a few billion U.S. dollars. A similar program, the European Open-Top Chamber Programme (EOTC), was organized in Western Europe beginning in the second half of the 1980s until the early 1990s. Negative effects of ozone levels typical of wide areas of Europe were observed in beans and spring wheat.

In some, but not all, experiments with pastures the main effect of ozone was on the species composition, with clover being replaced by grasses. This effect acts to reduce the protein content and thus the fodder quality of the yield. Towards the end of the 1990s a European research program, Changing Climate and Potential Impacts on Potato Yield and Quality (CHIP), studied the effects of ozone on potatoes. It was concluded that current ozone levels in Europe can cause significant effects on potatoes, but that the sensitivity is less than in wheat.

Based on the experiences of both research programs in North America and Europe, it can be concluded that soybean, wheat, tomato, pulses, and watermelon can be classified as highly sensitive to ozone. Barley and certain fruits are rather insensitive, while potato, rapeseed, sugar beet, maize, and rice are intermediate with respect to ozone sensitivity. There exists a certain degree of intraspecific variation in ozone sensitivity, such as in different bean varieties, but a recent European study resulted in a rather uniform ozone response pattern for different cultivars of wheat.


The relationship between grain yield and ozone exposure was most consistent in spring wheat across cultivars and experimental sites, and, considering the pronounced sensitivity observed for this crop, the combined data were used as a basis to derive critical levels for ozone effects under the Convention on Long-Range Transboundary Air Pollution of the United Nations Economic Commission for Europe (UNECE). For this purpose, the ozone exposure index AOT40 (the accumulated exposure over a concentration threshold of 40 nmol mol1 ozone based on hourly averages) was used


An important recent development in the field of crop loss assessments is the consideration of ozone uptake by the plant leaves, in place of a statistical index to characterize the concentration of ozone in the air surrounding the plants, such as the AOT40 exposure index. The key to this method is the quantification of ozone diffusion through the stomata of leaves.

The stomatal conductance of the plant leaves varies with a number of factors, such as soil and air moisture contents, temperature, solar radiation, phenology, and the levels of other pollutants, including carbon dioxide. Stomatal conductance models can now be used to establish relationships between yield loss and ozone uptake. The relationship between relative yield in two Swedish wheat cultivars and the calculated, cumulated uptake of ozone by flag leaves, taking into account an uptake rate threshold of 5 nmol m2 s 1 (CUO5 ). This threshold provided the best correlation between effect and exposure, and it may represent the biochemical defense capacity of the plants. However, it remains to be shown that the two thresholds are directly related to each other.


Much less attention has been paid to the influences of ozone on crop quality as compared to efforts made to understand the relationship with yield. The most discussed quality aspect has been the protein concentration or content, for instance, of wheat grains. The combined results of sixteen open-top chamber experiments performed in four different European countries (Finland, Sweden, Denmark, and Switzerland) were used to show the relationship between grain yield and grain quality, and grain yield and the off-take of grain protein per unit ground area, at different ozone or carbon dioxide levels.

From this, it can be inferred that relative to control (always the open-top chamber treatment with non-filtered air), the grain concentration of protein was higher and yield was lower in elevated ozone, which is the opposite result of situations with elevated carbon dioxide or in chambers where the ozone had been reduced by using charcoal filters. This situation could be attributed to a so-called growth dilution effect by elevated carbon dioxide and air filtration stimulating the carbohydrate accumulation relatively more than the uptake of nitrogen at a given nitrogen fertilizer application rate, while in situations of elevated ozone, carbohydrate accumulation was negatively affected more strongly than protein accumulation.

However, the protein yield per unit of ground area was negatively affected by ozone and enhanced by elevated carbon dioxide, although the latter effect tended to saturate at a level determined by the availability of nitrogen in the soil.


There is strong evidence that current ozone concentrations over large areas in the industrialized world are high enough to cause yield loss in several important agricultural crops, but it remains a challenge to quantify these effects exactly. New approaches based on the uptake of the pollutant by plants, rather than the concentration in the air surrounding the plant, are promising. The protein concentration of the yield tends to increase with increasing ozone in some crops in connection with declining yield. On the other hand, the protein yield per unit of the ground area tends to decrease.

In future research, the understanding of additional quality effects of air pollutant exposure should be given more attention. In the developing world, such as many countries in Asia, there is a great risk for crop losses due to a number of air pollutants for which emission rates are increasing dramatically. A loss in food production in these countries represents a much larger problem than in the industrialized world. More research should be devoted to this problem in the decades to come.

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