Spatial Variation of Foliar Chemicals within Brassicaceae Radish Plant, Raphanus sativus L. and Its Effect on Performance of a Generalist Insect, Spodoptera litura Fab.

Abstract

Foliar chemicals are variable within a plant, which can affect plant–herbivore interactions. This study was carried out to quantify concentrations of primary and secondary substances (nitrogen, water, total nonstructural carbohydrates, and sinigrin) of young and old leaves of Brassicaceae plant, Raphanus sativus L. and evaluate performance and survival of a generalist herbivore Spodoptera litura Fab. on them. Forty to fifty-day-old R. sativus plants were used in both foliar chemical analysis and insect performance bioassays. Leaves located on the third to the sixth node from the base of the plant were defined as old leaves and the remaining leaves (from seventh node to the plant apex) of the plant were referred as young leaves in this study. All foliar chemicals except water differed significantly between young and old leaves. Moreover, young leaves were more nutritious but much more chemically defended than old leaves. Performance and survival of S. litura were reduced on young leaves as compared with old leaves. Male and female larval duration only differed significantly on young leaves. Female larval development time was longer than male development time on young leaves, but not on older leaves. In summary, this study supported the optimal defense theory and revealed that defenses in young leaves were stronger against female than male S. litura larvae.