Abstract:
Herbivory in some Nicotiana species is known to induce alkaloid production. This study examined herbivore-induced
defenses in the nornicotine-rich African tobacco N. africana, the only Nicotiana species indigenous to Africa. We tested the
predictions that: 1) N. africana will have high constitutive levels of leaf, flower and nectar alkaloids; 2) leaf herbivory by the
African bollworm Helicoverpa armigera will induce increased alkaloid levels in leaves, flowers and nectar; and 3) increased
alkaloid concentrations in herbivore-damaged plants will negatively affect larval growth. We grew N. africana in large pots
in a greenhouse and exposed flowering plants to densities of one, three and six fourth-instar larvae of H. armigera, for four
days. Leaves, flowers and nectar were analyzed for nicotine, nornicotine and anabasine. The principal leaf alkaloid was
nornicotine (mean: 28 mg/g dry mass) followed by anabasine (4.9 mg/g) and nicotine (0.6 mg/g). Nornicotine was found in
low quantities in the flowers, but no nicotine or anabasine were recorded. The nectar contained none of the alkaloids
measured. Larval growth was reduced when leaves of flowering plants were exposed to six larvae. As predicted by the
optimal defense theory, herbivory had a localized effect and caused an increase in nornicotine concentrations in both
undamaged top leaves of herbivore damaged plants and herbivore damaged leaves exposed to one and three larvae. The
nicotine concentration increased in damaged compared to undamaged middle leaves. The nornicotine concentration was
lower in damaged leaves of plants exposed to six compared to three larvae, suggesting that N. africana rather invests in
new growth as opposed to protecting older leaves under severe attack. The results indicate that the nornicotine-rich N.
africana will be unattractive to herbivores and more so when damaged, but that potential pollinators will be unaffected
because the nectar remains alkaloid-free even after herbivory.
