Lack of Differential Plasticity to Shading of Internodes and Petioles with Growth Habit inConvolvulus arvensis(Convolvulaceae)

Abstract

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 organs than to their homology. Accordingly, vertical spacers (internodes in erect species, petioles in prostrate species) should be more plastic than horizontal spacers (petioles in erect species, internodes in prostrate species). This hypothesis was tested in the climbing plant Convolvulus arvensis. Given their facultative erect or prostrate habit, depending on support availability, climbing plants may be model species to test the relationship between growth habit and spacer plasticity in the absence of the confounding factors that are typical of interspecific comparisons. The phenotypic correlations among traits were also addressed. Three shading treatments (100%, 20%, and 5% of sunlight) and two support conditions (with and without a stake on which plants could twine) provided the experimental setting. Traits evaluated included internode length, petiole length, stem thickness, and biomass as well as area, shape, and specific area of leaves. The hypothesis was not supported. Internodes were more plastic than petioles in supported (“erect”) as well as in nonsupported (“prostrate”) plants, thereby supporting homology, and not analogy, of organs as a factor in explaining plasticity patterns. Most traits were significantly correlated both in supported and nonsupported plants. Internode and petiole length showed a highly significant positive correlation. This is discussed, and trait correlations are considered as possible constraints on the expected pattern of differential spacer plasticity to shading.