PHOTOTOXICITY OF CITRUS-JAMBHIRI TO FUNGI UNDER ENHANCED UV-B RADIATION - ROLE OF FURANOCOUMARINS

Extracts of Citrus jambhiri foliage exposed to and shielded from UV-B radiation were assayed for phytochemical changes and phototoxicity aga inst four fungal pathogens, two of which (Fusarium solani and F. oxysp onum) are causative agents of root rots and two of which (Penicillium italicum and P. digitatum) are associated with fruit rots. Conidial pi gment mutants of these four fungal species were assayed to determine w hether pigments play a role in protecting fungi against plant photosen sitizers. Exposure to 10.2 kJ/day UV-B radiation for 95 days significa ntly reduced phototoxicity of leaf extracts to fungi. Although furanoc oumarin levers were reduced by UV-B, analysis of covariance revealed t hat variation in phototoxicity of the extracts cannot be attributed en tirely to variation in furanocoumarin content; thus, the possibility e xists that nonfuranocoumarin phototoxic constituents, as yet unidentif ied, respond to UV-B exposure and contribute to overall phototoxic def ense of C. jambhiri against pathogens. Root lot fungi were substantial ly more sensitive to furanocoumarin phototoxicity than were fruit rot fungi, a pattern consistent with the amount of light exposure normally experienced by these fungi when associated with phototoxic plants. Al though pigmented strains of air four species displayed greater resista nce to phototoxicity of pure furanocoumarins, no strain differences we re detected in assays of foliar extracts; this finding also suggests t hat nonfuranocoumarin constituents may be involved in the phototoxic d efense of C. jambhiri against pathogens.