Testing optimal defense theory in Solidago altissima

Background/Question/Methods

Optimal defense theory in plants predicts that the most valuable tissues should be more heavily defended, provided that the benefits of such defenses outweigh the costs. To test this theory one needs to know three things: (1) the risk of tissue damage, (2) the value of the tissue, and (3) the cost of tissue defense. All the tissues of S. altissima (tall goldenrod) are known to be attacked by a number of herbivores and pathogens. We estimated the value of 6 tissue types by removing them and then measuring seed and rhizome mass at the end of the season. That is, either the leaves from the bottom half (mature) or the top half (young) of the plant were removed at the beginning (no branching stage), middle (branching stage), or end of the season (flowering stage). The cost of defense was estimated by protecting some plants with fungicide, insecticide, both, or none. Ten different clones were used and foliar herbivores and pathogens were assessed three times during the growing season. Terpenoids and protease inhibitors were extracted from the removed leaves and measured.

Results/Conclusions

We found that young tissue was significantly more valuable than mature tissue and that mid-season removal of young tissue reduced seed mass the most. The clones differed significantly in their susceptibility to herbivores and pathogens. There was also a significant effect of the pesticide treatments on damage and fitness. Because the plants were grown in the same environment and arranged in randomized complete blocks (i.e., essentially a common garden) any differences between the clones can be considered genetic differences. Principle components analysis including several herbivore densities, two pathogen severity measures, rhizome mass, rhizome length, seed mass, shoot mass, root mass, and other variables illustrated significant genetic differences between the clones, which may translate into different levels of constitutive defense and inducibility once the chemical analysis is completed. Rust infection had the strongest effect on fitness in terms of both seed and rhizome mass.