Can germination strategies help to understand regional co‐occurrence of two morphologically conserved congeneric macrophytes?

Abstract

Abstract In variable environments the production of dormant life stages can help organisms to avoid periods that are unsuitable for growth and reproduction, and different species may rely on different cues to break dormancy. However, it is still unclear to what extent variation in germination strategies can help to explain variation in the distributions of related species and contribute to adaptive radiation. Using standardised laboratory germination trials, we demonstrated the existence of both inter‐ and intraspecific variation in germination strategies in two congeneric species of submerged macrophytes that occur in a dynamic wetland system characterised by strong variation in salinity and hydro‐regime. The ditch grass Ruppia maritima L. typically lives in temporary waterbodies, where it assures high seed set through a short life cycle, early flowering and self‐pollination. Its congeneric Ruppia spiralis L. ex Dumortier grows in (semi‐)permanent waterbodies where it can have a perennial life cycle. Populations are presumed to be maintained mainly through clonal growth, and seed production is lower than in R. maritima. We hypothesised that salt and temperature are two important germination cues in these ecosystems that can explain the observed differences in distribution patterns between these two species. We exposed seeds of both species in a full factorial designed experiment to four different salinities (0‰, 7‰, 14‰ and 21‰) and two different temperature regimes (17 and 24°C, simulating spring and summer temperatures, respectively). The experiment was repeated over three consecutive inundation cycles (30 days of inundation, followed by 30 days of drought). During the third cycle, all seeds were inundated with fresh water, as salinity might constraint germination in species that rely on fresh water for germination. We found that the R. maritima seeds had overall low germination success. The freshwater inundation caused rapid germination in seeds that previously were in a high‐salinity treatment. In R. spiralis, which inhabits more permanent habitats of variable salinity, we detected a more complex response, involving two germination peaks. These two contrasting germination strategies observed within R. spiralis could target spring or summertime growing periods respectively. We believe that the observed variation in germination strategies is an important element in ecological differentiation in the morphologically conserved genus Ruppia and may have contributed to past and ongoing speciation.