title: Glucosinolates and trichomes track tissue value in two sympatric mustards.
abstract: Glucosinolates, trichomes, nitrogen, and carbon are not distributed uniformly through the canopies of mustards. In this study, we asked whether glucosinolate concentrations and trichome densities in two sympatric mustards, Brassica kaber and B. nigra, are highest in tissues of greatest value to the plant. We also asked whether nitrogen or carbon content is the stronger predictor of tissue value, and what fraction of each resource is incorporated in glucosinolates. To quantify tissue values, we removed three equal-area fractions (lower, middle, and upper) from the canopies of B. kaber and B. nigra in the greenhouse, as well as whole canopies of naturally growing B. nigra in the field, at two times during growth and measured reductions in their performance relative to controls. We also measured trichome density in both experiments, as well as glucosinolate, nitrogen, and carbon concentrations for the equal-area fractions in the greenhouse. We found that upper leaves had the highest glucosinolate concentrations, trichome densities, and tissue values. Furthermore, young plants in the field had higher trichome densities and tissue values than did older plants. Collectively, these data provide strong support for optimal defense theory and are among the first such evidence for glucosinolates and for physical defenses. The positive relationship between trichome density and tissue value was strong even after we accounted for the effects of leaf expansion. While nitrogen and carbon have both received attention as currencies for trade-offs, our data suggest that nitrogen concentration is a significantly better predictor of tissue value in these two mustard species. Interestingly, <1% of the nitrogen or carbon in leaves was incorporated in glucosinolates, which may explain why glucosinolates lack a consistent response to nitrogen fertilization.
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USA. mbtraw@pitt.edu
authors.forename: M Brian
authors.initials: MB
authors.lastname: Traw
authors.forename: Paul
authors.initials: P
authors.lastname: Feeny
mesh_terms: D000818:Animals; D002244:Carbon / Q000032:analysis / Q000378:metabolism; D005247:Feeding Behavior / Q000502:physiology*; D005961:Glucosinolates / Q000032:analysis / Q000378:metabolism*; D009149:Mustard Plant / Q000737:chemistry* / Q000378:metabolism; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism; D018515:Plant Leaves / Q000737:chemistry*; D013045:Species Specificity; D013729:Terpenes / Q000032:analysis / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D005961:Glucosinolates; D013729:Terpenes; D002244:Carbon; D009584:Nitrogen