Secondary metabolites of the leaf surface affected by sulphur fertilisationand perceived by the diamondback moth

Summary. Summary. Oilseed rape, Brassica napus L. (cv Express),plants were grown under three different sulphur regimes:sulphur-free (S₀), normal sulphur (S_n, normal field concentration)and a sulphur-rich (S₊, 2 × concentration of S_n). Weperformed dual choice oviposition assays with the diamondbackmoth, Plutella xylostella, using real plants and,for the first time with this insect, artificial leaves sprayedwith methanolic leaf-surface extracts. The results mirroredthose of a separate study of preferences for whole plants.Females laid more eggs on surrogate leaves that weretreated with S_n extracts than on S₀ plants, while only aslight, not significant, difference was observed betweenextracts of normal and sulphur-rich plants. This showsthat chemical compounds on the leaf surface mediate theoviposition preference and that the female insect canperceive the quality of the host-plants in terms of theirfertilisation status.Since leaf volatiles are known to be oviposition stimulants,we investigated the effects of leaf-surface extracts oninsect olfactory responses using electroantennograms(EAGs). In agreement with the behavioural data, we foundthat extracts of sulphur-treated plants yielded higher EAGamplitudes than the S₀ extracts. Since the leaf content of thevolatiles isothiocyanates is influenced by sulphur nutrition,we analysed the extracts for these compounds. Above thedetection threshold of our GC-MS system, no isothiocyanateswere found. Thus, other compounds present in thesurface extracts must be perceived by the antenna.However, the HPLC analysis revealed 11 differentglucosinolates. Progoitrin (2-Hydroxy-3-butenyl) andgluconapoleiferin (2-Hydroxy-4-pentenyl), which belong tothe hydroxy-alkene class of glucosinolates, were the mostabundant compounds. The total glucosinolate contentsharply increased from S₀ to S_n plants, whereas it was slightlylower in _n versus S₊ plants. Since it is known that glucosinolatescan stimulate oviposition, it seems likely that theincreased content we observed was influencing the insectpreference in this study too.