Shade-Induced Plasticity and Its Ecological Significance in Wild Populations of Arabidopsis thaliana

In laboratory studies of Arabidopsis thaliana, plants shaded by neighboring vegetation (or subject to treatments mimicking shade) flower at a younger developmental stage, and sometimes earlier in time. We examined whether this shade avoidance response varies among and within natural populations of A. thaliana, and whether it corresponds to variable selection regimes at shaded and unshaded field sites, by conducting a two-year reciprocal transplant study and a parallel greenhouse study that manipulated the presence and timing of shade. In the field, shading had limited or inconsistent impacts on survivorship across several phases of the growing season. The date of bolting was earlier at the shadier site compared to the less shady site, but in the greenhouse there was no significant shadeinduced plasticity for this trait. In both studies, we detected directional selection gradients favoring earlier bolting in shade, but gradients favoring earlier bolting were as strong in nonshaded conditions. The number of rosette leaves at bolting (i.e., the developmental stage of flowering) was significantly reduced by shade in both studies. However, there was either no directional selection on this trait, or selection to flower with more rather than fewer leaves. Despite the contrast in habitats, there was limited differentiation between populations for survivorship, reproductive fitness, size-related or flowering time traits, and no differentiation for trait plasticities. Some traits were variable among families within populations. A trade-off between age and developmental stage may limit the response to selection for flowering time, possibly explaining limited local adaptation. The adaptive significance of shade-induced flowering time plasticity remains equivocal. Future studies of the plasticity of flowering time in A. thaliana should investigate the effects of shading regimes together with other environmental variables on sizeand timing-related traits.