Plant injury, pigmentation

Howell, R. K. Phenols, ozone, and their involvement in pigmentation and physiology of plant injury, pp. 94-105. In M. Dugger. Ed. Air Pollution Effects on Plant Growth. ACS Symposium Series 3. Washington. D.C. American Chemical Society, 1974. 

Phenols, Ozone, and Their Involvement in Pigmentation and Physiology of Plant Injury... 

It is a common phenomenon for phenolic moieties to accumulate in plant tissues after exposures to stresses. Concentrations of phenols are increased in ozone-treated plant tissues (3, ). Reddish-brown polymerized pigments also develop in a diversity of plants injured by ozone. This response appears to be a general reaction and not a specific result of ozone injury ( ). Similar pigments have been identified in plant tissues during senescence and by curing processes ( ). 

Figure 1-4. Alfalfa leaves exposed to ambient concentrations of ozone a. From a resistant clone and shows no injury or pigment accumulation. b. From a susceptible plant and shows both leaf injury and pigment accumulation.

Acute symptoms of injury from various pollutants in different horticultural and agronomic groups are visible on the affected plant. Symptom expressions produced include chlorosis, necrosis, abscission of plant parts, and effects on pigment systems. Major pollutants which produce these injuries include sulfur dioxide, peroxyacetyl nitrate (PAN), fluorides, chlorides, nitrogen dioxide, ozone, and particulate matter minor pollutants are ethylene, chlorine, ammonia, and hydrogen chloride. Symptoms of acute injury are often used to identify pollutant source and to estimate agricultural damage. 

Enzymatic browning reactions, which have been known for almost 100 years, are complex enzyme catalysed reactions involving the oxidation of phenohc compounds by some oxidoreductases in the presence of air oxygen. Oxidation products are quinones that are transformed by subsequent enzymatic and spontaneous (non-enzymatic) reactions into various coloured pigments. Quinones and other reaction products in plants have microbistatic effects and are able to prevent the spread of microbial and viral infections. Polymeric insoluble reaction products are an effective mechanical barrier against further erosion at the injury site of a plant. 

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