Soot, like particles in general, may affect vegetation by both physical and chemical processes. Physically, pa rticles may cover the leaf surface and reduce the amount of light available for photosynthesis, or may occlude stomata. Occlusion may lead to increased resistance to gas exchange, or may prevent full stomatal closure, leading to water stress. Increased transpiration is a common response to particle exposure. Infestation by pests and pathogens is likely to be enhanced.
Chemically, soot is likely to adhere strongly to leaf surface waxes because of its organic content, and therefore accumulate on surfaces. As well as carbon and organic matter, soot may contain toxic heavy metals that eventually enter and may accumulate in the soil. The subject of dust deposition has been reviewed by Farmer (1993). Little is known of the direct effects of soot, which is usually associated with pollutant gases (sulphur dioxide, nitrogen oxides) and/or heavy metals.
Air concentrations of soot as 'black smoke' have been measured in the U.K. for many years by local authorities. Air concentrations have declined markedly in recent years, because emissions have fallen by almost 80% in the past 30 years (www.naei.org.uk). Most black smoke now comes from road traffic (diesel engines) and industry. Deposition rates for soot are rarely measured, and exposure is judged by amounts retained on leaves. However, removal of particles from plant surfaces by rain varies very greatly among plant species, from those that are 'self-cleaning' (the Lotus effect) to those that accumulate large quantities (Neinhuis & Barthlott, 1997).
|Critical Load/ Level|
No estimate available