Morphological variation of Cosmos bipinnatus (Asteraceae) and its relation to abiotic variables in central Mexico

BACKGROUND: Morphological variability can lead to serious taxonomic problems in species with wide distribution ranges. Although morphological variability is partly due to ontogenetic programming, abiotic variables can also exert a significant effect on micro- and macromorphological characters. In this paper, we studied the morphological variability (43 characters) of Cosmos bipinnatus associated to different vegetation types in central Mexico. We searched for significant correlations between the overall morphology of C. bipinnatus and abiotic variables such as altitude and soil parameters (pH, organic matter content, NH₄, NO₃, PO₄, total N and total P content). We also analyzed the Simplified Relative Distance Plasticity Index (RDPIₛ). RESULTS: Locality had a significant effect in all but three morphological characters measured. Also, 71.43 % of the characters had a significant correlation with at least one abiotic variable. PO₄ content was significantly correlated with paleae characters, while pH had a significant effect in ligule coloration. Discriminant function analysis revealed that C. bipinnatus individuals collected at grasslands and Pinus forests form separate clusters, while individuals collected at scrubs and Quercus forests showed considerable overlap. The RDPIₛ across all sites showed very low levels of plasticity in almost all characters. CONCLUSIONS: Some abiotic variables (altitude, soil NH₄ and PO₄ content, and soil pH) largely contribute to the differential phenotypic expression of C. bipinnatus in central Mexico. However, we found that the number of external phyllaries, the trichome length, and the petiole area can be considered diagnostic traits of C. bipinnatus as they did not show differences within and between collected sites. We hypothesize that the low levels of plasticity found in C. bipinnatus across sites is due to the high tolerance of the species to different environmental conditions.