Abstract: Plants have constructed diverse defense strategies in response to herbivory over time, with chemical defenses based on plant secondary metabolites playing a key regulatory role in plants-herbivores interspecific interactions. The biosynthesis of plant secondary metabolites is a trade-off strategy for plants to cope with herbivores at the cost of reduced resource allocation to growth and reproduction. Recently, numerous studies have been conducted on how plant secondary metabolites affect behavior of herbivores, individual growth and fitness of plants. However, a comprehensive and in-depth elaboration of the chemical defense strategies of plants is still lacking. This study provides a systematic review of the factors influencing the synthesis and release of plant secondary metabolites, the chemical defense strategies and their formation mechanisms in response to herbivory. Individual plant tissues and organs, population and species composition in the community, species and feeding intensity of herbivores, soil resource availability, growing seasons, environmental stresses all can affect the synthesis and release of plant secondary metabolites. Meanwhile, plants respond to herbivory by enhancing the plasticity of chemical defense, regulating the partitioning pattern of photosynthetic products and the trade-offs relationship between growth, reproduction and defense. The growth-differentiation balance hypothesis, plant apparency hypothesis, optimal defense theory, carbon/nutrient balance hypothesis, growth rate hypothesis, plant defense syndromes hypothesis and error management theory are the main hypotheses used to explain the mechanism of plant chemical defense strategies. As human activities (e.g., grazing) increase and climate change intensifies, researches on plant defense strategies in response to large herbivore feeding, environmental stress and global change should be strengthened in the future from a multidisciplinary perspective, which will be helpful for deeper understanding of the defense processes and mechanisms of plants in response to herbivores.

Key words: plant secondary metabolites, chemical defense strategies, cost of defense, growth-defense trade off, herbivores