REACTION NORM VARIATION IN A RARE PLANT, SENECIO-INTEGRIFOLIUS (ASTERACEAE)

This study examines the pattern of reaction norm variation and the ext ent to which genetic constraints could prevent the evolution of broade r ecological tolerance in Senecio integrifolius, a rare and declining plant in Sweden. Following the establishment of biparental progenies, we planted seedlings from each full-sib family in two greenhouse envir onments: full daylight and 50 per cent shade. A 2-year demographic stu dy indicated that full-sib genotypes representing a large population a t a site with a fine-scale mosaic habitat (meadow with clumps of shrub s) had a significantly greater overall performance across the environm ents and a lower sensitivity to a drastic reduction in light intensity than genotypes representing a small and patchily distributed populati on at a site without spatial variation (meadow exposed to full sun), c onfirming the supposition of more broadly adapted genotypes in a large population subject to selection across a wider range of micro-habitat s. There was a significant average effect of genotype for all traits b ut we failed to detect within-population variation in phenotypic respo nse (G x E) for direct components of fitness and conclude that there i s little potential for specialization to particular light regimes. Acr oss-environment correlations using full-sib means were small and posit ive and there was no change in the ranking of population means across the two environments, implying a lack of strong trade-offs within the reaction norm. However, the changes in ecological amplitude may not be of sufficient magnitude to permit long-term survival of remaining pop ulations, as implied by the rapid decline of S. integrifolius in Swede n.