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Physiology and Biochemistry

Constituents of Orange Juice that Stimulate the Germination of Conidia of Penicillium digitatum. P. du T. Pelser, Postharvest Research Pathologist, Outspan Citrus Centre, Nelspruit, 1200, Republic of South Africa; J. W. Eckert, Professor of Plant Pathology, University of California, Riverside, CA 92502. Phytopathology 67:747-754. Accepted for publication 17 November 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-747.

Conidia of Penicillium digitatum germinated 90% in a diluted dialysate of orange juice, but not in distilled water. Maximum germination was observed in 1% (v/v) orange juice dialysate at pH 3.5 - 8.0, but germ tube development was most vigorous at pH 4.0 - 5.5. The constituents of the dialysate which were adsorbed by an anion exchange resin (anionic fraction) were most active in stimulating germination, whereas the cationic and neutral fractions supported vigorous germ tube development, but did not initiate germination. A mixture of L-ascorbic, citric, and L-malic acids stimulated germination to essentially the same level as a dilution of the anionic fraction which contained the same concentration of these acids. Citric or L-malic acids, alone or combined, did not significantly increase germination. Both L-ascorbic and dehydroascorbic acid induced a higher level of spore germination than isoascorbic acid (D-ascorbic acid). Of the major carbohydrates present in the neutral fraction of orange juice, glucose (2.56 g/ 100 ml) alone supported 50-70% germination, whereas equivalent concentrations of fructose and sucrose were less effective and myo-inositol was inactive. A solution containing 76.8 mg glucose and 29.4 mg citric acid/ 100 ml, concentrations equivalent to 3% orange juice dialysate, resulted in 31% germination and more vigorous germ tube development than that observed in solutions containing higher concentrations of glucose alone. Several other organic acids likewise were stimulatory when combined with glucose, but succinic, fumaric, maleic, and acetic acids were inactive. The combination of glucose and citric acid produced higher germination at pH 4.5 than at pH 6.0. Relatively simple mixtures of nutrients stimulated a high rate of germination. Maximum germination was observed in spores incubated in (i) glucose combined with one of several organic acids, (ii) glucose combined with L-ascorbic acid, and (iii) citric acid combined with L-ascorbic acid. Exogenous nitrogen-containing nutrients were not required for maximum germination, but vigorous germ tube growth was observed only in solutions containing mixtures of amino acids and vitamins.