BACKGROUND AND AIMS: Spotted knapweed (Centaurea stoebe s.l., Asteraceae) is native to Europe, where it occurs as a diploid (2xEU) and tetraploid cytotype (4xEU), but so far only the tetraploid has been reported in the introduced range in North America (4xNA). In previous studies, significant range shifts have been found towards drier climates in 4xEU compared with 2xEU, and in 4xNA when compared with the native range. In addition, 4x plants showed thicker leaves and reduced specific leaf area compared with 2x plants, suggesting higher drought tolerance in 4x plants. It is thus hypothesized that the 4x cytotype might be better pre-adapted to drought than the 2x, and the 4xNA better adapted than the 4xEU due to post-introduction selection. METHODS: Plants of the three geocytotypes (2xEU, 4xEU and 4xNA ), each represented by six populations, were subjected to three water treatments over 6 weeks in a greenhouse experiment. Plasticity and reaction norms of above- and below-ground biomasses and their ratio, survival rate, stomatal conductance and carbon isotope discrimination were analysed using linear and generalized linear mixed effect models. KEY RESULTS AND CONCLUSIONS: Above-ground and total biomasses of European tetraploids were slightly less affected by drought than those of European diploids, and 4xEU plants maintained higher levels of stomatal conductance under moderate drought than 4xNA plants, thus supporting the pre-adaptation but not the post-introduction evolution hypothesis. Plasticity indexes for most of the traits were generally higher in 2xEU and 4xNA than in 4xEU plants, but these differences were not or were only marginally significant. Interestingly, the effect of population origin and its interaction with treatment was more important than the effects of geocytotype and range. Population means for the control treatment showed several significant associations either with latitude or some aspect of climatic data, suggesting evolution of local adaptations, especially within the 2xEU and 4xEU geocytotypes.
