Comparative physiological responses in Chinese cabbage induced by herbivory and fungal infection

Fungal infection of Chinese cabbage leaves by Alternaria brassicae has earlier been shown to have detrimental effects on larval development of the chrysomelid beetle Phaedon cochleariae. Furthermore, adults of this leaf beetle avoid fungus-infected Chinese cabbage leaves for oviposition and feeding. However, herbivory had no impact on fungal growth. In this study, we investigated physiological responses of the host plant to herbivore attack and fungal infection in order to elucidate the mechanisms of the described ecological interactions between the fungus and the herbivore. Changes in primary factors (water, C/N ratio, protein, sucrose) and defense-related plant compounds (glucosinolates, anthocyanins, peroxidase) were measured. Herbivory and fungal infection reduced the sucrose concentration of leaves and increased amounts of indole glucosinolates as well as total anthocyanins. In addition, water content was slightly lower in insect-damaged but not in infected leaves. Higher levels of peroxidase activity resulted exclusively from fungal infection. The C/N ratio and total protein content remained unaffected by either treatment. The implications of the induced plant changes on the herbivore are discussed.