Intra-plant regulation of growth and plant-herbivore interactions

Abstract

Plant modularity and sink/source regulation imply that plants consist of parts which may be functionally semiautonomous, but whose functioning is locally and globally regulated by a large number of competing, physiological sinks and sources. As a consequence, plant functions occur at various hierarchical levels of organization. This has potential repercussions for the ecological and evolutionary responses of plants to environmental challenges. For instance, the functional responses of plants to external disturbances, such as herbivory, may be based on shoot- or module-level reactions, and may not occur at the whole-plant level. While modularity and sink/source regulation are well recognized and included in general plant/herbivore hypotheses, their roles as determinants for plant responses have received less attention. We discuss the ways in which plant modularity and sink/source regulation as internal constraints of growth may modify the effects of herbivory. The discussion consists of four steps. We first review factors which at a very general level promote and constrain tree design and function and particularly growth, the latter being an important determinant of plant quality for herbivores. Next, on the basis of modularity and sink/source regulation, we introduce a null hypothesis of plant functioning to predict the responses of growth to an external challenge. Third, we describe the growth characteristics of deciduous birches (Betula) and evergreen pines (Pinus), as examples of the importance of recognizing inter-specific differences in the internal design and function of trees for herbivory studies. Fourth, we discuss the evolutionary implications of our null model. On the basis of the model, it is obvious that many of the spatial and temporal variations in the suitability of trees to herbivores may result directly from attributes of plant design and function per se. This could explain the occurrence of herbivore-induced susceptibility as well as some forms of induced resistance in plants. Accordingly, some tree traits which lead to herbivore avoidance of trees may not require explanations emphasizing selection for defence.