TEMPERATURE AND GAMMA-IRRADIATION EFFECTS ON SCOPARONE, A CITRUS PHYTOALEXIN CONFERRING RESISTANCE TO PHYTOPHTHORA-CITROPHTHORA

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 mug/g fresh weight, compared with 42 mug/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 mug/g fresh weight in macrophylla and sour orange, respectively, and 100 and 82 mug/g fresh weight in rough lemon and shamouti, respectively. In the fruit, a maximum scoparone concentration of 94.7 mu/g fresh weight accumulated in inoculated grapefruit after irradiation with 400 Krad.