Plant Tolerance of Herbivory: Fitness Costs and Mechanisms

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9527900 Simms Plants may respond to disease or herbivory either with resistance, which reduces the level of damage, or they may tolerate damage by reducing its effect on reproduction. Little effort has been devoted to understanding how plants tolerate or compensate for damage. Nevertheless, many plant ecologists have maintained that compensating for damage is an important mechanism by which plants respond to herbivory. Recent studies have revealed genetic variation for tolerance within natural plant populations, as well as trade-offs associated with tolerance: when plants do relatively well in the presence of damage, their siblings do relatively poorly in a damage-free environment. This proposal describes two major experiments designed to explore mechanisms underlying this trade-off in the wild turnip mustard, Brassica rapa. To measure tolerance, half sibs will be exposed to carefully controlled levels and timing of damage produced by both insects and artificial means. These manipulations will be performed in various environments to determine what environmental factors influence the ability of genotypes to compensate for damage. Quantitative genetic analyses will be used to test two hypotheses: H1: Tolerance is more costly than resistance because costs of tolerance are associated with major life-history trade-offs; and H2: The cost of tolerance is caused by a trade-off between ability to compensate for damage and competitive ability. Results from this study will significantly improve understanding of how plants respond to herbivores and pathogens. Furthermore, as tolerance and compensatory growth have become recognized as important agronomic traits, this study will provide important insights as to what trade-offs might be involved in breeding these traits into crop varieties.