Air pollutants

Air is normally 78% hydrogen and 21% oxygen; the majority of the remaining 1% being provided by water vapor and carbon dioxide. It can also contain pollutants which are classified into 2 groups:
- pollutants emanating from one or a few localized sources and affecting plants located nearby (a factory, a volcano, etc.) (sulfur dioxide, nitrogen oxides, chlorinated compounds, ammonia, ethylene, volatile herbicides like 2,4-D and dust);
- general atmospheric pollutants emanating from a multitude of dispersed sources (car engines, etc.) and affecting the atmosphere over large areas (ozone, peroxyacetyl nitrates = PAN).

The gaseous effluents can remain unchanged in the atmosphere, one speaks then of direct pollutants , or enter into reaction with other normal or abnormal components of the air; we are dealing with secondary pollutants . The latter are dependent on mechanisms such as photochemical reactions, the formation of free radicals, oxidations ... giving rise to new compounds. These reactions are still poorly understood; they are the source of oxidizing (ozone) and photochemical ("smog") compounds.


Among these pollutants, two, in particular, cause damage in plant pathology and on lettuce:

• Ozone

Disrupted functions in plants
- loss of turgor in guard cells;
- modification of membrane permeability;
- reduction of gas exchange;
- acceleration of starch hydrolysis.

Symptoms observed in plants
- small necrotic spots, in depression, varying from white to dark brown, even black and red depending on the species;
- early senescence of tissues, accompanied by chlorosis and necrosis;
- loss of turgor and bleaching of tissues;
- premature fall of leaves and fruits;
- reduction in plant growth and yields.

Ozone is the most damaging air pollutant, the precursors of which are nitrogen oxides and hydrocarbons.

On lettuce, it causes the appearance of a multitude of small lesions more or less dark between the main veins of the blade of old leaves. These changes end up turning yellow and necrosis; the affected leaves therefore chlorinate and become senescent prematurely. This physiological phenomenon is still quite rare. There are differences in sensitivity between varieties.

• Peroxyacetyl nitrate (PAN)

The disturbed functions in plants are characterized by a collapse of the mesophyll cells, the place of which is filled with air.

The symptoms observed in plants correspond to the appearance of a shiny, even metallic appearance, as well as a silvery to bronzed tint ("sylver leaf") on the underside of the leaves.

The PAN (gasoline vapor +/- O3 or NO2) is a photochemical phytotoxic compounds. Damage is observed in very sunny regions where there are strong hydrocarbon emissions linked to road traffic, near large towns and in the presence of fog. It is now observed in northern Europe.

On lettuce, it causes leaf discolorations occurring on a large number of plants; the leaves taking on a metallic bronze to silver tint on the underside of the leaf blade. This modification of the coloring of the limbus is in fact due to the absorption of "PAN" at the level of the stomata. This gas then diffuses to the cells of the mesophyll and destroys them. Air pockets form between the lower epidermis and the palisade cells; they are at the origin of the metallic aspect of the leaves.


The damage appears in some urban and peri-urban production areas, especially after periods of calm, sunny weather, with heavy fog. There are differences in sensitivity between varieties.

Several factors condition the severity of the damage caused by these pollutants, we can point out: their specific toxicity, their concentration, the duration of their emission, the factors acting on their diffusion, the sensitivity of plant species (direct pollutants), their stage of growth, the variety cultivated and the conditions of production.