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Brown-Pigment Formation in Tobacco Leaves Infected with Alternaria. Harvey W. Spurr, Jr., Research Plant Pathologist, (also Associate Professor, N.C. State University, Raleigh), Southern Region, Agricultural Research Service, U.S. Department of Agriculture, Oxford Tobacco Research Laboratory, Oxford, N.C. 27565; C. E. Main, Associate Professor of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27607. Phytopathology 64:738-745. Accepted for publication 14 December 1973. DOI: 10.1094/Phyto-64-738.

Brown-spot leaf lesions from field-grown susceptible and resistant cultivars were separated mechanically into prehalo, halo, and necrotic tissue zones for comparative studies with adjacent healthy tissue. A gradient increase in total nitrogen, total protein, and oxidase activities was observed from prehalo, to halo, to necrotic tissues. The enzymatic oxidation of phenols was a primary reaction resulting in the formation of pigments. Enzymatically oxidized phenols were shown to complex with extracted proteins to form brown pigments. The results, in toto, support the hypothesis that complexing between certain oxidized phenols, proteins, and perhaps other constituents, results in brown-pigment formation and necrosis. This process seems basic to lesion development. The formation of brown pigments may be a nonspecific process in plants, activated by many types of infections or injuries. A notable difference in oxidase activity was observed between the susceptible and resistant cultivars studied using chlorogenic acid as a substrate. The resistant cultivar had a gradient increase in activity from prehalo to necrotic tissue, whereas all lesion tissue zones of the susceptible cultivar had similarly high activities.

Additional keywords: Alternaria alternata.