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Physiology and Biochemistry

Physiological Basis for Tipburn Development in Head Lettuce. I. J. Misaghi, Department of Plant Pathology, University of California, Davis, CA 95616, Present address: Department of Plant Pathology, University of Arizona, Tucson, AZ 85721; R. G. Grogan, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:1744-1753. Accepted for publication 8 July 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1744.

Tipburn was induced consistently on the inner leaves of mature detached heads of lettuce that were held at 30 C for 5 days. Comparisons were made of respiration rates and metabolites of healthy and tipburned lettuce heads. Respiration rate increased with increase in temperature from 5 to 35 C, and substantial increases occurred in the levels of citrate, isocitrate, succinate, fumarate, and all of the soluble amino acids in plants subjected to 30 C as compared with heads kept at 5 C. Increases in the levels of organic and amino acids were detected before symptom development. Concentrations of water-soluble and total calcium in inner and middle leaves of mature heads were less than those in outer leaves. Moreover, concentration of soluble and total calcium in heads of tolerant lettuce cultivars was greater than in susceptible cultivars. Typical tipburn symptoms were induced in mature heads held at 21 C by treatment with potassium salts of fumaric, succinic, and citric acids. In view of the chelating potential of organic acids, particularly citric, we hypothesize that tipburn development is a manifestation of a localized calcium deficiency that results from chelation of calcium by organic acids and other metabolites that are increased in plants during exposure to elevated temperature.