The Dilemma of Plants: to Grow or Defend

Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth ( cell division and enlargement ) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: They must grow fast enough to compete, yet maintain the defenses necessary to survive in the presence of pathogens and herbivores. The physiological trade-off between growth and differentiation processes interacts with herbivory and plant-plant competition to manifest itself as a genetic trade-off between growth and defense in the evolution of plant life history strategies. Evolutionary theories of plant defense are reviewed. We also extend a standard growth rate model by separating its ecological and evolutionary components, andformalizing the role of competition in the evolution of plant defense. We conclude with a conceptual model of the evolution of plant defense in which plant physiological trade-offs interact with the abiotic environment, competition and herbivory. theory ofplant defense has not kept pace. The complexity of the subject has hindered the development of a synthetic framework, because any comprehensive treatment of plant defense theory must transcend both the disciplines and the levels of biological organizaINTRODUCTION N RECENT YEARS, empirical studies of plant/herbivore interactions have increased exponentially (Scriber and Ayres, 1988), but progress toward a unifying and organizing The Quarterly Review of Biology, September 1992, Vol. 67, No.3 Copyright @ 1992 by The University of Chicago. All rights reserved. 0033.5770/92/6703-0002$1.00