•     Morphological and ecological differences of two forms of Helosciadium repens are known and described in the literature: aquatic and terrestrial. However, their taxonomic status is currently unknown. The question whether they are genotypically adapted to specific environmental conditions or are those differences a result of phenotypic plasticity is addressed in this study.
•       SSR and ISSR data were used to uncover genotypic differences. Data from drought stress experiments (system water content and relative water content of leaves) were used to evaluate the response to water as an environmental factor. The stomatal index of both forms grown under different water treatments was analysed.
•       The principal component analysis of the SSR and ISSR data revealed no clustering that would correspond with ecotypes. The diversity parameters of the SSR data showed no significant differences. Contrary to the expected pattern, the aquatic populations showed a tendency towards heterozygosity while the terrestrial ones showed a bias towards homozygosity. Both forms responded similarly to the changes in water availability, with newly produced leaves after drought stress that were better adapted to repeated drought stress. Stomatal indices were higher in plants from aquatic habitats, but these difference disappeared when the plants were grown in soil.
•       The observed responses indicate that the differences between forms are due to phenotypic plasticity.

SI_Index_H_repens_all.xlsx- Stomatal counts, epidermis cell counts and stomatal index of the all leaflet pair from aquatic and terrestrial plants.

ISSR_Matrix.xlsx- Binary matrix of the ISSR data.

SSR_data_H_repens.xlsx- data matrix of the microsatellites.

SWC_A-T.xlsx- data of the dry stress experiment (potted aquatics)

SWC_T.xlsx- data of the dry stress experiment (terrestrial