Physiological Basis for Tipburn Development in Head Lettuce

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Abstract
Tipburn was induced consistently on the inner leaves of mature detached heads of lettuce [Lactuca sativa var. capitata ''Calmar'' and ''Calicel''] that were held at 30.degree. 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 5-35.degree. 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.degree. C as compared with heads kept at 5.degree. C. Increases in the levels of organic and amino acids were detected before symptom development. Concentrations of water-soluble and total Ca in inner and middle leaves of mature heads were less than those in outer leaves. Concentration of soluble and total Ca in heads of tolerant lettuce cultivars was greater than in susceptible cultivars. Typical tipburn symptoms were induced in mature heads held at 21.degree. C by treatment with potassium salts of fumaric, succinic, and citric acids. The chelating potential of organic acids, particularly citric, suggest that tipburn development is a manifestation of a localized Ca deficiency that results from chelation of Ca by organic acids and other metabolites that are increased in plants during exposure to elevated temperature.