Grantee Research Project Results
Plant Defense Strategies Against Herbivores: Tolerance and Resistance in Populus tremuloidesEPA Grant Number: U915896
Title: Plant Defense Strategies Against Herbivores: Tolerance and Resistance in Populus tremuloides
Investigators: Stevens, Michael T.
Institution: University of Wisconsin - Madison
EPA Project Officer: Cobbs-Green, Gladys M.
Project Period: January 1, 2001 through January 1, 2004
Project Amount: $92,439
RFA: STAR Graduate Fellowships (2001)
Research Category: Fellowship - Molecular Biology/Genetics , Academic Fellowships , Biology/Life Sciences
The objectives of this research project are to: (1) evaluate tolerance as a defense strategy by examining genetic and environmental variation in, mechanisms for, and physiological costs of tolerance to herbivory in aspen (Populus tremuloides); (2) examine evolutionary tradeoffs between tolerance and resistance; and (3) investigate the genetic variation and heritability of resistance chemicals.Approach:
Evolutionary models of plant defense assume that defenses involve costs. The presumed basis of these costs is the diversion of resources away from growth and reproduction. Allocation costs arise physiologically from internal competition for limiting resources, but evolutionary costs are seen as genetic tradeoffs between traits. Two plant traits that may be defensive are resistance and tolerance. Resistance is any plant trait that reduces the preference or performance of herbivores, and tolerance is the degree to which plant growth and fitness are affected by herbivore damage relative to the undamaged state. Historically, assessments of the costs of defense have focused on resistance factors. More recently, attention has shifted to include tolerance.
Aspen is well suited to the study of resistance and tolerance. Aspen serves as a host to a diverse array of leaf-feeding insects and is favored by outbreak folivores such as the native forest tent caterpillar (Malacosoma disstria) and the large aspen tortrix (Choristoneura conflictana), and increasingly by the introduced gypsy moth (Lymantria dispar). Aspen leaves have resistance chemicals that are effective against a variety of lepidopteran folivores, but are known to be costly in terms of tree growth. The high intrinsic growth rate of aspen and the costs of its chemical defenses make it likely to have evolved tolerance to herbivory. My research project involves a common garden of 12 aspen genotypes grown under 2 levels of soil nutrients and exposed to 2 levels of defoliation (0 percent and 75 percent). We measured tree growth, photosynthesis, reproductive output, levels of resistance chemicals, and whole-plant allocation patterns to determine how aspen responds to damage. The heritability portion of my experiment involves controlled crosses and previously established genetic plantations of known parentage.
My research will extend current knowledge on plant response to herbivores by simultaneously examining the costs of resistance and tolerance while considering several possible mechanisms of tolerance. It also will contribute to the understanding of the quantitative genetics of resistance in aspen. Because most previous researchers in this area have used short-lived herbaceous plants, my research project broadens our knowledge of tolerance to include a long-lived woody plant. This is of particular interest because of its wide distribution and ecological importance throughout North America.Supplemental Keywords:
fellowship, tolerance, resistance, resistance chemicals, aspen, Populus tremuloides, forest tent caterpillar, Malacosoma disstria, large aspen tortix, Choristoneura conflictana, gypsy moth, Lymantria dispar, herbivory, plant defense, plant defense strategies.