Plasticity of Internodes and Petioles in Postrate and Erect Potentilla Species

Abstract

1. In phylogenetically related species, internodes of erect and stoloniferous plants are homologous structures, whereas petioles of prostrate plants and internodes of erect plants are structurally analogous, i.e. they have the same ecological function. 2. The hypothesis was tested that analogous spacing organs would show similar degrees of plasticity in response to shading, whereas homologous spacing organs would not. Four closely related Potentilla species representing a range of growth forms from strictly erect to obligatory stoloniferous were studied. 3. Vertical spacers showed significantly higher degrees of plasticity than horizontal spacers in response to shading, confirming the hypothesis that analogous organs show a similar response, whereas homologous spacers differed significantly in their plasticity. 4. Under shaded conditions high degrees of elongation in vertical spacers were accompanied by a significant increase in biomass allocation to these organs, whereas allocation to horizontal spacers tended to be less than under unshaded conditions. This result suggests that significant biomass investments are associated with plastic elongation responses. 5. The results of the study are discussed in the context of specific selection pressures acting on different growth forms which may alter the degree of morphological plasticity of plant organs during evolution.