Seed Traits as a Diagnostic Tool for Salicornioideae (Amaranthaceae): Development of a Dichotomous Identification Key

The subfamily Salicornioideae (Amaranthaceae) comprises taxa with a notoriously complex taxonomy due to limited diagnostic characters, high phenotypic plasticity, and frequent hybridization. This study aims to provide a practical tool for supporting the identification of Salicornioideae taxa in Sardinia (Italy), with a particular focus on seed traits as diagnostic taxonomic characters. Mature seeds from seven taxa belonging to the genera Salicornia, Arthrocaulon, and Halocnemum were collected across Sardinia and examined using a digital microscope. Seed traits, including shape, size ratios, colour, surface ornamentation, and the presence and morphology of hairs, were analysed quantitatively and qualitatively. Morphological data were organized into a character matrix and assessed to identify diagnostic features. A Factor Analysis of Mixed Data (FAMD) was used to evaluate the discriminatory power of quantitative and qualitative seed traits and to support the selection of characters for a seed-based dichotomous identification key. The resulting key enables the discrimination of Sardinian annual and perennial taxa, as well as closely related species and subspecies. The results demonstrate that, within the set of taxa examined, seed traits are consistent and easily observable characteristics that can distinguish Salicornioideae taxa at genus, species, and subspecies levels. Improving taxonomic discrimination of Salicornioideae taxa supports floristic, ecological, and conservation studies of halophytic vegetation in Mediterranean regions.