Morphology and micro-morphology of achenes and their taxonomic implications to Lactuca species (Cichorieae; Asteraceae)

Lactuca L. is the central genus of Lactucinae (Cichorieae; Asteraceae), containing cultivated lettuce and its wild relatives. In this study, we used Scanning Electron Microscope, Plain Stereo Microscope and Automated Digital Microscope to observe, record and discuss the achene characters and surface micro-features ofLactuca (including taxa of ex-Pterocypsela Shih) and Notoseris Shih species. The taxon sampling consisted of fifteen globally distributed Lactuca species and Chinese originated Notoseris species. The results indicated that the morphological and micro-morphological features of achenes were of great importance to identify Lactuca species at the genus and species level. We conclude that two key features, the presence or absence of beak and the arrangement and shape of epidermal cells, can be used to distinguish Lactuca from Notoseris. The shape and margin of the achene body, the beak length and the number of ribs on either side of achene are key features to classify Lactuca species. The ornamentation of epidermal cells can also provide extra evidence to determine closely related Lactuca species. The interspecific relationships among the Lactuca species based on achene features are consistent with the results of previously molecular systematics of these species.