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Citral, a monoterpene aldehyde synthesized by several plant genera, has been reported to exhibit antimicrobial activity. For the first time, we report that critral exhibits UV-A (315-400 nm) light enhanced oxygen-dependent toxicity against a series of Escherichia coli strains differing in DNA repair and catalase proficiency. Those E. coli strains carrying a gene leading to catalase deficiency (katF) are particularly sensitized to inactivation by citral and UV-A treatment when compared to catalase proficient strains (katF+). Consistent with these in vivo observations, citral when treated with UV-A in vitro produces H2O2. When tested against Fusarium oxysporum and F. solani, fungal root pathogens of Citrus, enhanced toxicity by citral in the presence of UV-A was demonstrated, while dark toxicity was negligible. When the plasmid pBR322 was treated with citral in the presence of UV-A, a change in conformation from the covalently closed circular to the open circular and, ultimately, the linear form was observed. The change in plasmid conformation corresponded to a reduction in transforming activity. Holding plasmid DNA which had been treated with UV-A light in the presence of citral at 4 degrees C for 22 h in the dark resulted in continued degradation of the DNA and loss of transforming activity. Holding plasmid DNA treated with UV-A or citral alone under identical conditions had no detectable effect on either plasmid conformation or transforming activity.

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Extracts ofCitrus jambhiri foliage exposed to and shielded from UV-B radiation were assayed for phytochemical changes and phototoxicity against four fungal pathogens, two of which (Fusarium solani andF. oxysporum) are causative agents of root rots and two of which (Penicillium italicum andP. digitatum) are associated with fruit rots. Conidial pigment mutants of these four fungal species were assayed to determine whether pigments play a role in protecting fungi against plant photosensitizers. Exposure to 10.2 kJ/ day UV-B radiation for 95 days significantly reduced phototoxicity of leaf extracts to fungi. Although furanocoumarin levels were reduced by UV-B, analysis of covariance revealed that variation in phototoxicity of the extracts cannot be attributed entirely to variation in furanocoumarin content; thus, the possibility exists that nonfuranocoumarin phototoxic constituents, as yet unidentified, respond to UV-B exposure and contribute to overall phototoxic defense ofC. jambhiri against pathogens. Root rot fungi were substantially 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. Although pigmented strains of all four species displayed greater resistance to phototoxicity of pure furanocoumarins, no strain differences were detected in assays of foliar extracts; this finding also suggests that nonfuranocoumarin constituents may be involved in the phototoxic defense ofC. jambhiri against pathogens.

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