Inducible Plant Defenses Against Herbivory - Cost of Induced Defences

Cost of Induced Defences

Central to the concept of induced defences is the cost involved when stimulating such defences in the absence of insect herbivores. After all, in the absence of cost, selection is expected to favour the most defended genotype. Accordingly, individual plants will only do so when there is a need to. The cost of induced defences to a plant can be quantified as the resource-based trade-off between resistance and fitness (allocation cost) or as the reduced fitness resulting from the interactions with other species or the environment (ecological cost).

Allocation cost is related to the channelling of a large quantity fitness-limited resources to from resistance traits in plants. Such resources might not be quickly recycled and thus, are unavailable for fitness-relevant process such as growth and reproduction. For instance, herbivory on the broadleaf dock (Rumex obtusifolius) by the green dock beetle (Gastrophysa viridula) induces an increased activity in cell wall-bound peroxidase. The allocation of resources to this increased activity results in reduced leaf growth and expansion in R. obtusifolius. In the absence of herbivory, inducing such a defence would be ultimately costly to the plant in terms of development.

Ecological cost results from the disruption of the many symbiotic relationships that a plant has with the environment. For example, jasmonic acid can be used to simulate an herbivore attack on plants and thus, induce plant defences. The use of jasmonic acid on tomato (Lycopersicon esculentum) resulted in plants with fewer but larger fruits, longer ripening time, delayed fruit-set, fewer seeds per plant and fewer seeds per unit of fruit weight. All these features play a critical role in attracting seed dispersers. Due to the consequences of induced defences on fruit characteristics, L. esculentum are less able to attract seed dispersers and this ultimately results in a reduced fitness.