When a plant is attacked by an insect herbivore a variety of genetic cascades are triggered. The adaptive role of some of the pathways is well established, e.g. induced synthesis of defense compounds that are metabolically costly in absence of herbivores can help decrease damage when herbivores are present. Other responses are less well understood. Export of resources from attacked tissue might enhance growth in other parts of the plant, or be used for defense compound synthesis. Alternatively these exported resources might be stored for later mobilization after herbivory is relaxed. We constructed a mathematical model to explore adaptive value of resource storage as an induced response to herbivory. First, we simulate plant growth over time as a function of labile carbon allocation among leaf growth, defense, and storage. In addition to the effects of herbivory on leaf area, we model a shift in the balance of resource allocation, away from growth, and toward storage and defense synthesis. We examine how different rates of insect herbivory affect biomass accumulation depending on carbon allocation to induced defense and carbon storage (each ranging from 0 to 20% of available carbon).
Model output shows that induced defense synthesis is beneficial at a range of herbivory levels. In addition, induced carbon storage was beneficial at high damage levels. When nearly all of the leaf area is consumed, allocation of carbon toward storage provides a source of carbon for new leaf growth after the herbivore population has departed. Interestingly, there is an optimal rate of carbon allocation toward storage: if too large a proportion of carbon is allocated away from new growth, the time required for complete consumption of leaf biomass can decrease to the point that insufficient carbon is stored for regrowth after the attack. Our results indicate that under extreme herbivore attack, induced carbon storage can benefit long-term growth, while such storage is likely to be costly at lower levels of herbivore attack. This cost of storage under mild attack might decrease if resource export results in a decrease in food quality from the herbivore perspective, a topic that deserves further consideration.
