Although the concept of genetic constraints plays an important role in our understanding of the evolution of natural populations, there are still few empirical investigations probing the nature and limits of constraints in plant and animal species, aside from some studies inDrosophila. In the work reported here, we use an induced mutation - artificial selection protocol to analyse constraints on character means and phenotypic plasticity to nutrients inArabidopsis thaliana, an annual crucifer. We induced point mutations in a highly inbred line characterized by an extreme phenotype (very fast life cycle, early flowering, reduced leaf production) and little plasticity. We then selected individuals with increased leaf numbers. The goals were to determine if: (i) it is possible to increase leaf production; (ii) this has an effect on reproductive fitness; (iii) a mutation-selection process simultaneously alters the environmental insensitivity of the plant, thereby allowing phenotypic plasticity; and (iv) changes in the target trait affect other characters or their plasticities. The results demonstrate that: (a) mutations do increase leaf number; (b) this yields a much higher reproductive fitness, owing to the extension of the very short life cycle of the base inbred line; (c) there are no changes in plasticity of leaf number or of any other trait, possibly because few loci are involved in the control of plasticity; (d) changes in leaf number are related to alterations in three other traits comprising a strong set of covarying characters inA. thaliana. Two uncorrelated traits are capable of independent evolution from the constrained set. We therefore suggest that environmentally insensitive ecotypes of A.thaliana can quickly evolve to form ecologically specialized, relatively environmentally invariant genotypes.