Closely related plant species from a number of genera produce either erect (orthotropic) or creeping (plagiotropic) shoots. As a result, corresponding (homologous) organs, such as internodes or petioles, are oriented differently in space. They thus fulfil different functions; the elongation of vertically oriented structures shifts leaf blades into higher regions within the canopy, whereas the elongation of horizontally oriented structures does not. As a consequence, different degrees of internode or petiole plasticity may have evolved in erect and stoloniferous species to adjust the architecture of plants in case they are shaded.
This study aims at testing the hypothesis that vertically oriented spacers have higher degrees of shade-induced plasticity in their length than horizontally oriented spacers. It is further hypothesized that a high degree of plasticity in spacer length involves costs in terms of increased biomass investment into elongating organs.
These hypotheses were tested with nine herbaceous species pairs (each consisting of one erect and one closely related stoloniferous species) belonging to six different families of dicots. Vertical spacers had higher degrees of shade-induced plasticity than horizontal spacers. This was true for internodes and petioles of individual species and also for functionally corresponding (i.e. analogous) spacers within species pairs. In addition, plastic increases in spacer length were positively related to increased spacer weight and to increased biomass allocation to spacers under shaded conditions. These results show that spacer plasticity depends primarily on the function of spacers and not on the phylogenetic relations of the species. They also suggest that there are costs of plasticity in terms of biomass investment into elongating organs. Differences in the balance between costs and benefits of plasticity between vertical and horizontal spacers may have led to the evolution of high degrees of plasticity of vertically oriented spacers and low degrees of plasticity of horizontally oriented spacers, as observed in this study.