Seed Morphotype and Population Origin Shape Germination Responses of Three Salicornia perennans Willd. subsp. perennans Populations Under Different NaCl Concentrations

Successful germination is a critical stage for the establishment and persistence of annual halophytes in Mediterranean saltmarshes, where highly variable salinity creates narrow recruitment windows. Seed heteromorphism is a common feature of annual halophytes and has been proposed to contribute to recruitment under variable environmental conditions. This study investigated how seed morphotype, NaCl concentration and population origin interact to shape germination responses in Salicornia perennans Willd. subsp. perennans. Seeds from three Tuscan coastal populations (Italy) were separated into two naturally occurring morphotypes (large and small) and exposed to four NaCl concentrations (0, 5, 10 and 20 g L−1) in a fully factorial experiment. Seed morphotype emerged as the main determinant of germination performance: large seeds consistently achieved a higher final germination percentage and germination index than small seeds across populations and salinity treatments. Increasing salinity reduced final germination percentage and germination index but had no significant effect on mean germination time or the time required to reach 50% of final germination (T50) among seeds that germinated, which indicates that NaCl acted as a quantitative filter rather than a kinetic brake. Dose–response modeling yielded comparable EC50 estimates among populations for large seeds, where EC50 represents the salinity at which the fitted germination response reaches the midpoint between the lower and upper asymptotes, whereas the maximum germination capacity differed substantially. Unexpectedly, seeds from San Rossore, the site most influenced by freshwater inputs, maintained the highest germination under severe salt stress. Overall, our results identify seed morphotype as the primary determinant of germination success in S. perennans and find that it overrides differences among populations in their response to salinity.