EFFECTS OF PLANT NUTRITION ON THE BALANCE OF INSECT RELEVANT CARDENOLIDES AND GLUCOSINOLATES IN ERYSIMUM CHEIRANTHOIDES

The possible effects of environmental stress on plant chemistry that a re important to herbivorous insects were examined by growing a wild cr ucifer, Erysimum cheiranthoides, under different nutrient regimes. Ovi position by the cabbage butterfly, Pieris rapae, is thought to be affe cted by the balance of glucosinolates (stimulants) and cardenolides (d eterrents) at the surface of leaves. E. cheiranthoides seedlings were provided with three levels of nitrogen and two levels of sulfur for a period of 15 days before analysis of semiochemicals in whole leaf tiss ue and at the surface of the foliage. The ratio of cardenolides to glu cosinolates in the plants at elevated C/N ratios followed the carbon/n utrient balance hypothesis. However, a high nitrogen supply enhanced b iomass production to the extent that concentrations of secondary compo unds were unchanged or reduced. The concentration of glucosinolates (g lucoiberin and glucocheirolin) at the surface was positively related t o whole tissue levels. However, cardenolide (erysimoside and erychrosi de) concentrations, which were highest in leaf tissue of nitrogen-defi cient plants, had the lowest surface levels on foliage of these plants . Possible reasons for differential expression of cardenolides and glu cosinolates in a plant as a result of nutrient deficiency are discusse d.