Umbelliferone, a phytoalexin associated with resistance of immature Marsh grapefruit to Penicillium digitatum

A low concentration (2 μg^-1 FW) of 7-hydroxycoumarin (umbelliferone) was found in albedo of non-inoculated mature yellow Marsh grapefruit (Citrus paradisi). The concentration of umbelliferone in albedo of immature green grapefruit (2 months before full maturity) was 17 μg g^-1 FW. Four days following inoculation with Penicillium digitatum, the accumulation of umbelliferone in the albedo of mature and immature grapefruit was 28 and 250 μg g^-1 FW, respectively. By that period, the infected area in the yellow fruit was as large as 230 mm², as compared with 8 mm² in the green fruit. The EC₅₀ of umbelliferone to P. digitatum growth in vitro was 95 μg ml^{- 1}. Additionally, umbelliferone inhibited growth of several other pathogenic fungi in vitro. It is suggested that umbelliferone plays a role in the defense mechanisms of immature grapefruit against P. digitatum.