The effects of plant genetic variation and soil nutrients on secondary chemistry and growth in a shrubby willow, Salix sericea: patterns and constraints on the evolution of resistance traits

Investigators often examine the factors—genetic or environmental—that determine the concentrations of secondary chemicals and growth, but few have examined both simultaneously. We used a factorial genetic design and manipulated nutrient availability to Salix sericea (Salicaceae) in order to quantify: (1) genetic variation, plasticity, and genetic variation in plasticity for growth rate and the concentrations of two phenolic glycosides (salicortin and 2′-cinnamoylsalicortin), and (2) tradeoffs between secondary chemistry and growth rate, and between the two phenolic glycosides. We found genetic variation and genetic variation in plasticity for both chemicals but not for growth rate. Nutrient fertilization enhanced growth and decreased salicortin concentration. More importantly, nutrient environment affected the expression of genetic variation. Heritability was significant only in the medium (both phenolic glycosides) and in the high (2′-cinnamoylsalicortin only) fertilizer treatments, and there was significant genetic variation in plasticity. There was no evidence suggesting that selection for increased chemical concentration would occur at the expense of growth rate. Finally, it appears that selection in favor of one chemical would result in positive correlational selection for the other. Overall, the evolution of these traits might be constrained by a lack of genetic variation in some environments, but not by negative genetic correlations between the different traits.