VIEW ARTICLE

Resistance

Temperature and Gamma Irradiation Effects on Scoparone, a Citrus Phytoalexin Conferring Resistance to Phytophthora citrophthora. U. Afek, Institute for Technology and Storage of Agricultural Products, Department of Fruit and Vegetable Storage, ARO, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel; A. Sztejnberg, The Hebrew University of Jerusalem, Department of Plant Pathology and Microbiology, Faculty of Agriculture, P.O. Box 12, Rehovot 76100, Israel. Phytopathology 83:753-758. Accepted for publication 10 February 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-753.

The effects of gamma irradiation and temperature of incubation on accumulation of the phytoalexin scoparone were investigated in citrus inoculated with the fungus Phytophthora citrophthora. In all the tested citrus species, scoparone concentration in the branches was higher and lesion length was shorter when the incubation temperature after inoculation with P. citrophthora was 28 rather than 20 C. The temperature effect was pronounced, especially in inoculated branches of rough lemon, which after 4 days at 28 C had a scoparone concentration of 290 μg/g fresh weight, compared with 42 μg/g fresh weight at 20 C. The comparable lesion length in this species was 5.2 mm at 28 C compared with 11.0 mm at 20 C. Gamma irradiation affected citrus resistance only at 300–400 Krad. Maximum scoparone concentration in inoculated branches accumulated after irradiation with 400 Krad and reached 970 and 530 μg/g fresh weight in macrophylla and sour orange, respectively, and 100 and 82 μg/g fresh weight in rough lemon and shamouti, respectively. In the fruit, a maximum scoparone concentration of 94.7 μg/g fresh weight accumulated in inoculated grapefruit after irradiation with 400 Krad.