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Carbohydrate Changes in Tobacco Systemically Protected Against Blue Mold by Stem Infection with Peronospora tabacina. S. D. Salt, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091; S. Q. Pan, and J. Kuc. Department of Plant Pathology, University of Kentucky, Lexington 40546-0091. Phytopathology 78:733-738. Accepted for publication 10 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-733.

Soluble carbohydrates increased in tobacco leaves and stems that were protected against blue mold by stem inoculation with sporangia of Peronospora tabacina. Increases ranged from 1.1-fold to eightfold by 3 wk after inoculation. Most of the increase was in free glucose; lesser increases in fructose and a variable increase in sucrose were found. Disruption of phloem translocation of photosynthate or ontogenic changes caused by the stem infection did not account for the changes in sugar levels. Starch content was not reduced, and invertase or amylase activities were not elevated, except in necrotic stem tissues where starch decreased and amylase and invertase activities increased. These changes in necrotic stem tissues were insufficient to account for the sugar increases found throughout immunized plants. Crude extracts from infected tissues of inoculated stems liberated significantly greater amounts of soluble carbohydrate from tobacco stem cell wall preparations than did extracts of uninfected tissues. Administration of exogenous sugars to tobacco plants by various means did not consistently affect susceptibility of tobacco to blue mold. Neither glucose nor fructose inhibited germination of P. tabacina sporangia at physiological concentrations, but fructose inhibited at levels approximately sixfold the highest levels detected in tissue.