Phenols in Cotton Seedlings Resistant and Susceptible to Alternaria macrospora

In healthy cotton seedlings, stems have a lower phenol content than leaves, but resistant plants have an altogether relatively higher phenol content than susceptible plants. Phenols extracted from infected plants can inhibit the growth of A. macrospora in vitro. In cotton plants infected with A. macrospora, phenols are oxidized by polyphenoloxidase rather than peroxidase and catalase. The main oxidative activity was around the developing necrotic area but activity was detected far from, necrosis as well. Though pre–inoculation mechanical injuries operated the phenol oxidation mechanism in the plant, they neither prevented nor encouraged the increase in disease severity. Isozyme pattern showed that contribution of all participants in the pathological interaction to the oxidative mechanism occurred in the diseased plant. A negative linear correlation was found between polyphenoloxidase activity, phenol accumulation and resistance. This study suggests that the phenol oxidative mechanism, participates in cotton plant resistance to A. macrospora.