PRODUCTION OF CHEMICAL DEFENSES IN RELATION TO PLANT-GROWTH RATE

The growth-rate hypothesis outlined by Coley et al. is based on a model which predicts that investment of resources in anti-herbivore defenses by plants should increase as intrinsic growth rate decreases. This prediction is based on a model which assumes alpha > beta (alpha = constant relating defense investment to reduction in growth, beta = constant determining defense effectiveness). Under conditions in which beta > alpha, the model predicts either no investment in anti-herbivore defenses for plants with relatively high intrinsic growth rates or a maximum investment in anti-herbivore defenses for plants with relatively low intrinsic growth rates. Conditions in which beta > alpha are realistic for both immobile and mobile defenses. Thus estimates of alpha and beta are needed to test adequately predictions of the growth-rate hypothesis. In the model, alpha is measured in terms of loss of growth. An analysis of defensive compounds in quaking aspen indicates that aspen may shift allocation of resources from one defense to another and not exhibit any loss of growth. Secondary metabolites may also have additional benefits for plants besides reducing herbivory.