A Systematic Study of Cardamine Weed Species in United States Container Nurseries

Abstract

Cardamine species present in the United States nursery trade were identified and characterized based on morphological and molecular evidence. The genus Cardamine L., or the bittercresses, contains some of the most prolific weed species in the container nursery industry. Though most bittercress in the U.S. nursery industry was thought to be hairy bittercress (Cardamine hirsuta), we suspected other species to be present due to the global movement of nursery stock. We surveyed 21 nurseries in California, Mississippi, Missouri, New York, North Carolina, and Oregon to collect a representative sample of bittercress species occurring in the U.S. nursery industry. Type material and other herbarium samples were examined for all species encountered in order to identify each accession and determine which characters were most important for identification. From these nursery surveys four Cardamine species were collected: Cardamine corymbosa, C. hirsuta, C. flexuosa and C. oligosperma, each representing 90%, 6%, 2%, and 2% of collections respectively. C. flexuosa was originally described in Europe but also has distribution throughout Asia. European and Asian populations may represent separate taxa based on molecular evidence. Here we treat them as the same taxon. Type material no longer exists for C. flexuosa as it was described in Europe. A neotype is designated here. C. corymbosa, native to New Zealand, is a newly reported species for the United States. C. hirsuta has worldwide distribution and C. oligosperma is native to the United States. Representatives of each species collected, as well as select herbarium specimens, were chosen for a phylogenetic study to examine the relationships between these species. Sequence was generated for the nrDNA Internal Transcribed Spacer region (ITS) and for the single copy nuclear COP1-Interacting Protein 7 region (CIP7). Sequence data was aligned and subjected to maximum parsimony and bayesian inference. When comparing the phylogenies created by ITS and CIP7 datasets, there were areas of congruence and incongruence. The ITS data resolved C. corymbosa as sister to C. flexuosa whereas the CIP7 data resolved C. corymbosa sister to C. hirsuta. The combined alignment resolved the Cardamine accessions with higher bootstrap (BS) support and posterior probability (PP) than either of the separate matrices alone. In the combined matrix C. hirsuta and C. corymbosa resolve as sister taxa. However, no analysis fully resolved the relationships within this closely related species complex.