Ecophysiological Responses of Plants to Air Pollution

Abstract

In industrialised and heavily populated regions of the world, air pollution has an important influence on vegetation, affecting the production, abundance and distribution of plants. The principal primary pollutants are reactive compounds of sulphur, nitrogen and hydrocarbons emitted during the combustion of fossil fuels in industry, transport and from intensive farming. Increasing in importance is the secondary pollutant ozone. The direct effect of pollutants on plants is strongly influenced by the extent of their uptake into the plant tissues. Pollution also affects soils through increasing soil acidity and altering the balance of soil nutrients affecting roots and plant nutrition. Air pollution affects plants interactions with pests and pathogens and the sensitivity of vegetation to cold and drought stress. Understanding the ecophysiological responses and mechanisms by which air pollutants affect plants helps policymakers to set guidelines for air pollution to protect vegetation, crops and ecosystems.

Key Concepts:

  • Fossil fuel combustion and intensive agriculture are the main sources of air pollutants, which may be transported over long distances to affect plant life on a global scale.

  • In developed nations, sulphur dioxide, smoke and acid rain were the main acute air pollution concerns in the past, but now ozone and reactive nitrogen may pose the greatest threats.

  • Pollutants are deposited to plants as dry deposition of gases and particles and wet deposition in rain and cloudwater.

  • Stomata and cuticle on plant's surfaces provide an important control on uptake by vascular plants.

  • Bryophytes and lichens are vulnerable to air pollution as they lack a protective cuticle.

  • Acid rain, ozone, SO2 and other pollutants can directly injure the physiology of above ground plant tissues such as leaf gas exchange, carbon transport or flowering.

  • Soils are affected by increased acidity and nitrogen deposition which have long term effects on plant nutrient uptake, root system health and species competition.

  • Biodiversity and structure of plant communities appears to be damaged by long term nitrogen deposition.

  • Air pollution increases plant attack from pests and pathogens and increases vulnerability to cold and drought stress.

  • More understanding is needed of damage mechanisms to inform air quality guidelines to protect plants and their ecosystems.

Keywords: air pollutants; wet and dry deposition; acidification; nitrogen; ozone; plant uptake; plant physiology; ecology

Figure 1.

Reduced crown density, a symptom of forest decline, in trees of the Harz mountains in northern Germany. The extent of thinning of the needles in the crown is widely used in forest tree health surveys.

Figure 2.

The resistance analogy used to describe the processes of deposition of air pollutants to vegetation. Symbols: ra=aerodynamic resistance; rb=boundary layer resistance; and rc=canopy resistance. The canopy resistance is made up of resistances offered by the stomata, the leaf surface (cuticle), the surface water and the soil.

Figure 3.

Gas exchange in cotyledon leaves of cabbage measured one day after an experimental acid rain treatment (a 30 min spray of acidic water of pH 3.0 compared with a control spray of pH 5.6). The rate of photosynthesis was not affected by the acid spray treatment. In contrast, as the light was lowered, the stomatal resistance increased markedly in the control plants but showed much less change in the acid‐treated leaves. This showed that the normal closing of stomata in the dark had not occurred in the acid‐treated leaves.

Figure 4.

Proposed mechanism for type I forest decline in Norway spruce trees, as seen in parts of central Europe in the 1980s (after Roberts et al., ). The yellowing of needles is ultimately explained by magnesium deficiency, which results mainly from reduced uptake by the roots and leaching of this nutrient from the soil.

Figure 5.

Relationship between total plant species richness and (above) modelled Nitrogen deposition at 22 heather moorlands sites in northern Britain 2006, and (below) the percentage Nitrogen content of the surface litter layer at each site (Caporn et al., ).

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Caporn, Simon JM(Feb 2013) Ecophysiological Responses of Plants to Air Pollution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003206.pub2]