Environmental conditions can influence a plantís rate of development and induce morphological responses that may affect fitness. Plant growth form can also constrain or promote developmental and morphological responses, since plants of different growth form differ in the placement and timing of production of vegetative and reproductive modules. In this study, we examine how growth form controls developmental and morphological responses, and fitness across light environments in Brassica rapa and Arabidopsis thaliana. Erect (wild type) and rosette (mutant) genotypes of Brassica and rosette form genotypes (mutants differing in traits related to rosette size) of Arabidopsis were grown in high light (control) and green light filtered (spectral shade) environments. Spectral shading reduced fitness and altered patterns of development in genotypes of both species. Spectral shading induced a shade avoidance response of increased vertical extension, although the strength of this response was not consistent across all genotypes. For example, hypocotyl extension was relatively greater in the rosette form and internode extension was relatively greater in the erect form of Brassica. Rosette genotypes (Brassica) and genotypes with high rosette allocation (Arabidopsis) suffered relatively high fitness reduction in spectral shade treatments. The results of this study demonstrate that growth form can influence morphological and developmental responses that may have important fitness consequences across a range of environments.

Key words: Arabidopsis thaliana, Brassica rapa , development, fitness, morhological response