During this process of uptake of carbon dioxide plants can also take up pollutants. Once the pollutants enter the leaf, the pollutants may injure leaf tissue and upset the normal metabolic functioning of the plant. Depending on the pollutant, the upper or lower leaf surface of the plant will exhibit symptoms. For these pollutants, reliable air monitoring is necessary to associate plant injury with a pollutant episode. However, a few pollutants, eg. fluoride, are absorbed through the leaf surface and remain within the plant and can be assayed for high concentrations but the occurrence of these pollutants is usually quite low or limited in distribution.
Plants can vary dramatically in sensitivity to pollutants.
Many species of crops and landscape ornamentals are quite resistant while others are extremely sensitive. Additionally, within a specific crop sensitivity can vary widely among cultivars. Presumably, subtle genetic differences are responsible for changes in sensitivity although the basis for these differences are not always well understood. Ozone is a powerful oxidizing chemical which affects many cellular components and biochemical processes. As mentioned previously, ozone is the most important pollutant with respect to plant injury.