Changes in the content of antibacterial isorhamnetin 3-glucoside and quercetin 3′-glucoside following inoculation of onion ( Allium cepa L. cv. Red Creole) with Pseudomonas cepacia

Inoculation of purple onion ( Allium cepa), cv. Red Creole, bulbs with Pseudomonas cepacia resulted in the death of 80 % of the cells of the pathogen in the tissues by the third day after inoculation and an increase in phenolic concentration for 5 days after inoculation. The increase in phenolic compounds in inoculated tissues was 3–5 times larger than the increase following wounding alone and was preceded by a 25–35% loss in protein content and a 50% reduction in sugar concentration during the first 24 h. Inoculation with the saprophytic species, P. fluorescens or the non-pathogen P. marginalis resulted in changes in phenolic, protein and sugar contents in the tissues similar to those observed for wounding alone. In addition, all the cells of the saprophytic and non-pathogenic species were dead within 3–5 days after inoculation. Two flavonol glycosides, isorhamnetin 3-glucoside and quercetin 3′-glucoside were among the main phenols which accumulated in response to inoculation. Both compounds were more toxic to the pathogenic bacterium than to the non-pathogenic species in agar plate assays; isorhamnetin 3-glucoside being more toxic than quercetin 3′-glucoside at all the concentrations tested. The resistance of fresh onion scales to bacterial infection may be ascribed to the increasing accumulation of the two glycosides following inoculation. However, these compounds are unlikely to play a significant role in pathogenesis since P. cepacia resumes growth in the onion tissues while the glycosides are still present at high concentrations.