Asynchrony in the nitrogen and glucosinolate leaf-age profiles of Brassica: is this a defensive strategy against generalist herbivores?

The distribution of nitrogen, chemical defences (glucosinolates) and herbivory was compared with respect to leaf-age in four cultivars of oilseed Brassica. Feeding preferences of woodpigeons Columba palumbus and the slug Deroceras reticulatum, showed that mature leaves were 1.7 times more likely to be damaged than those in younger age classes (allowing for their greater biomass), even though they were less nutritious. Due to a high concentration of glucosinolates (up to 75 (mu)mol per gram of dry leaf weight (g dwt^-1)), the ratio of nitrogen to defence was maximal in young foliage. Thus, despite also having a high total nitrogen content, approximately 1.5 times more nitrogen was locked-up as glucosinolate in the young leaves compared with mature ones. This asynchrony ensures that young foliage is better defended: a feature which may be important in deterring herbivores from feeding on the most valuable parts of the plant. In particular, older leaves may help to absorb limited amounts of herbivory during the overwintering rosette stage. All cultivars exhibited similar patterns of decline in the glucosinolate to nitrogen ratio with age, but there was significant variation in the way plants allocated nitrogen to older leaves, and in the rates of decline exhibited by aliphatic, indolyl and aromatic glucosinolates.