Phenotypic plasticity of Senecio vulgaris from contrasting habitat types: Growth and pyrrolizidine alkaloid formation

The growth-differentiation balance hypothesis (GDB), which postulates a phy siological trade-off between growth and differentiation (morphological and chemical), has been tested almost exclusively for carbon-based secondary me tabolites. Little attention has been paid to N-based compounds. in this stu dy we aimed to test the predictions of the GDB hypothesis under field condi tions for growth and pyrrolizidine alkaloid (PA) formation in Senecio vulga ris. We conducted a reciprocal transplant experiment at two sites differing widely in their nutrient supply. These included a conventionally managed v ineyard (V) and a strip of local wild flowers between crop fields, which wa s established to promote species diversity in agroecosystems (C). No fertil izer or pesticides are allowed in such ecological compensation areas. In C, we expected lower growth but higher PA formation than in V. Due to differe ntiated selection regimes in the two habitat types with regard to nutrient (nitrogen) availability in the soil, we also expected different N-allocatio n patterns for the genotypes of the two collection sites. Plants of V produ ced more biomass and were taller than the plants of C. The relatively poor nitrogen conditions in C favored an earlier differentiation towards generat ive organs. In plants of C, higher concentrations of PAs were found than in plants of V. There exists a close negative correlation between growth and PA formation, indicating a trade-off. The origin of the plant material had only a little effect on PA formation. The observed phenotypic reaction of P A formation in S. vulgaris in the two habitats fits quite well the predicti ons of GDB theory. It is shown that this general response is overlaid by ph ysiological factors leading to a pattern of PA accumulation, which is not r eadily predictable by nonmechanistic theories.