Variation in the glucosinolate content of oilseed rape (Brassica napus L.) leaves. II. Response to infection by Alternaria brassicae (Berk.) Sacc.

The glucosinolates are thought to contribute to resistance to pests and diseases in members of the Cruciferae, including oilseed rape (Brassica napus) and they are known to accumulate in Brassica tissues after infestation by various pests. The present study investigated the changes in glucosinolate concentration in leaves of oilseed rape following inoculation with the dark leaf spot pathogen (Alternaria brassicae). Fourth and sixth leaves of the single-low cultivar Bienvenu (low in erucic acid) and the double-low cultivar Cobra (low in erucic acid and glucosinolate) were removed at intervals up to twenty days after inoculation and analysed for glucosinolate content using HPLC. Glucosinolates accumulated in inoculated leaves of both cultivars but the accumulation was greater for cv. Bienvenu, especially in sixth leaves. Among the glucosinolates, aliphatic compounds accumulated rapidly in cv. Bienvenu, but later declined. Indolyl and aromatic glucosinolates accumulated in both cultivars, but at a slower rate than the aliphatic glucosinolates. There were differences in the extent to which individual glucosinolates accumulated after inoculation. Disease symptoms were initially more extensive on cv. Cobra than on cv. Bienvenu but were similar on corresponding leaves of the two cultivars by the end of the experiment. However, sixth leaves had significantly less lesioning than fourth leaves. Glucosinolate accumulation in infected oilseed rape may restrict the spread of existing fungal infection or inhibit subsequent attempted infections, especially in younger leaves.