How to be sweet?: Extrafloral nectar allocation by Gossypium hirsutum fits optimal defense theory predictions

Plants employ nectar for two distinct functions. Floral nectar has traditionally been viewed in the context of pollination. Extrafloral nectar on the other hand, can act as an indirect defense,. allowing the plant to recruit predators and parasitoids. Whereas this makes for a clear-cut categorization, in reality the functions may not be so discrete. Extrafloral nectar may serve a role in pollination, while floral nectar can be utilized by predators and parasitoids and thus can contribute to plant defense. Here we use the optimal defense theory to generate predictions with respect to allocation patterns of defensive nectar. These predictions are then tested using novel data on the production patterns of bracteal and foliar nectar in cotton, Gossypium hirsutum. Bracteal nectar production shows a distinct peak on the day of anthesis, followed by a prolonged secretion during fruit maturation. This suggests that bracteal nectaries function in pollination as well as defense. Constitutive nectar production at the bracteal nectaries exceeds foliar nectar secretion by a factor of between 80 and 110. Whereas nectar production at foliar nectaries is induced following herbivore damage to leaves, bracteal nectar production is not induced in response to fruit damage and even decreases when the fruiting structure is subjected to herbivory. This shows that both an inducible and a constitutive variant of an indirect defense can occur within one plant species. This pattern of nectar allocation fits the optimal defense theory, which predicts high levels of constitutive defenses in valuable tissues such as fruits and induced defenses in less valuable tissues such as leaves. This result lends support to the interpretation that indirect defense has been a selective force in the evolution of bracteal nectaries. Although optimal defense theory has described direct defense allocation solely in terms of probability of attack and value of the plant structure, we argue that herbivore accessibility is an additional factor shaping the allocation of indirect defenses.