Lack of differential plasticity to shading of internodes and petioles withgrowth habit in Convolvulus arvensis (Convolvulaceae)

It has been postulated that the plasticity to shading of spacing organs in plants of different growth habit is more likely related to the analogy of o rgans than to their homology. Accordingly, vertical spacers (internodes in erect species, petioles in prostrate species) should be more plastic than h orizontal spacers (petioles in erect species, internodes in prostrate speci es). This hypothesis was tested in the climbing plant Convolvulus arvensis. Given their facultative erect or prostrate habit, depending on support ava ilability, climbing plants may be model species to test the relationship be tween growth habit and spacer plasticity in the absence of the confounding factors that are typical of interspecific comparisons. The phenotypic corre lations among traits were also addressed. Three shading treatments (100%, 2 0%, and 5% of sunlight) and two support conditions (with and without a stak e on which plants could twine) provided the experimental setting. Traits ev aluated included internode length, petiole length, stem thickness, and biom ass as well as area, shape, and specific area of leaves. The hypothesis was not supported. Internodes were more plastic than petioles in supported ("e rect") as well as in nonsupported ("prostrate") plants, thereby supporting homology, and not analogy, of organs as a factor in explaining plasticity p atterns. Most traits were significantly correlated both in supported and no nsupported plants. Internode and petiole length showed a highly significant positive correlation. This is discussed, and trait correlations are consid ered as possible constraints on the expected pattern of differential spacer plasticity to shading.