Decoupled variation in floral and vegetative traits

In flowering plants with specialized pollination, a tight fit is expected between floral and pollinator morphologies, and variation in flower size is likely to be maladaptive because pollinators remain of more or less fixed size and behaviour. Berg (1959, 1960) hypothesized that the maintenance of the fit between the floral traits responsible for pollen transfer and the pollinators in the face of the phenotypic response of plants to environmental variation was allowed by the decoupling of the phenotypic variation between vegetative and floral traits. However, studies analysing phenotypic variation and covariation between these two types of traits yield inconsistent evidence for the Berg hypothesis. While several studies support the prediction that floral traits are less variable than vegetative traits, others show that floral traits still respond to environmental variation, sometimes markedly. Furthermore, several studies analysing the morphological integration of vegetative and floral traits confirm the decoupling of phenotypic variation between these two types of traits, while other studies observe positive correlations.

A new paper in Annals of Botany analyses patterns of variation and covariation in leaf size and flower size in two populations of Campanula rotundifolia from contrasting environments that were exposed to different temperature treatments. By analysing correlations between flower size and leaf size within and among temperature treatments, the results confirm that the decoupling of the phenotypic variation between vegetative and floral traits can be dependent on the environment.

Decoupled phenotypic variation between floral and vegetative traits: distinguishing between developmental and environmental correlations. Ann Bot (2013) 111 (5): 935-944. doi: 10.1093/aob/mct050
In species with specialized pollination, floral traits are expected to be relatively invariant and decoupled from the phenotypic variation affecting vegetative traits. However, inferring the degree of decoupling between morphological characters from patterns of phenotypic correlations is difficult because phenotypic correlations result from the superimposition of several sources of covariance. In this study it is hypothesized that, in some cases, negative environmental correlations generated by non-congruent reaction norms across traits overshadow positive developmental correlations and generate a decoupling of the phenotypic variation between vegetative and floral traits. To test this hypothesis, Campanula rotundifolia were grown from two distinct populations under two temperature treatments, and patterns of correlation were analysed between leaf size and flower size within and among treatments. Flower size was less sensitive to temperature variation than leaf size. Furthermore, flower size and leaf size showed temperature-induced reaction norms in opposite directions. Flower size decreased with an increasing temperature, while leaf size increased. Consequently, among treatments, correlations between leaf size and flower size were negative or absent, while, within treatments, these correlations were positive or absent in the cold and warm environments, respectively. These results confirm that the decoupling of the phenotypic variation between vegetative and floral traits can be dependent on the environment. They also underline the importance of distinguishing sources of phenotypic covariance when testing hypotheses about phenotypic integration.