ACCUMULATION OF THE PHYTOALEXIN, 6,7-DIMETHOXYCOUMARIN, IN ROOTS AND STEMS OF CITRUS SEEDLINGS FOLLOWING INOCULATION WITH PHYTOPHTHORA-CITROPHTHORA

An antifungal substance, 6,7-dimethoxycoumarin (DMC), was isolated from stems and roots of citrus seedlings infected with P. citrophthora and quantified using a sensitive spectrophotofluorimetric technique. DMC was detected at highest concentrations in infected stem tissue (> 1 μmol g-1 fresh weight). Tissue along the margins of infection sites on stems contained low concentrations (< 0·1 μmol g-1 fresh weight), but uninfected seedlings and uninfected tissue located 0·5 to 10 cm from the margin of lesions on infected plants contained < 1 nmol g-1 fresh weight. DMC was detected at lower concentrations (0·8 μmol g-1 fresh weight) in infected root tips but not in adjacent, apparently healthy tissue. Its concentration in infected tissue was negatively correlated with the extent of invasion of the root tip. DMC was not detected in stem tissue which had been killed prior to inoculation, nor in fungal mycelium grown in culture, nor in cultured callus tissue infected by the fungus. The concentration of DMC in inoculated stem bark and roots was positively correlated with resistance of seedlings to fungal infection. The production of DMC concentrations which inhibit growth of the fungus in the stems and roots was more rapid in the resistant species Poncirus trifoliata × Citrus sinensis (troyer citrange) than in the susceptible species C. aurantium (rough lemon). It is concluded that DMC is a phytoalexin which is produced enzymically by living cells in the plant in response to the invading fungus. © 1990.