Anatomical development of roots of native and non-native submerged aquatic macrophytes in different sediment types

• Sediment organic matter (OM) influences the root intercellular spaces of macrophytes.
• Hydrilla verticillata had small increases of the root intercellular spaces in OM.
• Intercellular spaces increased more in the native Egeria than in the exotic Hydrilla.
• Root plasticity of these plants helps explaining their occurrences in River Paraná.

In this study, we evaluated the effects of organic matter in sediment on the aerenchyma development of the roots of Egeria najas (native to Brazil) and the exotic species Hydrilla verticillata. The questions of this study were: (i) can the structural development of roots be influenced by the substrate with different content in organic matter and (ii) do the quantity and perimeter of the cortex intercellular spaces of the roots increase with increases in sediment organic matter? To answer these questions, we conducted an experiment in a greenhouse. We manipulated E. najas and H. verticillata growing in sediment with different proportions of organic matter. The root cortex of E. najas had significantly greater numbers and perimeters of intercellular spaces compared with the exotic species. The interaction between species and sediment was significant for both response variables, which increased faster in E. najas than in H. verticillata with enhancing sediment organic matter. The superior plasticity of E. najas’ roots might be explained by its adaptation to this specific type of sediment in situ for a longer period of time given its native status. On the other hand, the low capacity to increase intercellular spaces in H. verticillata’s roots in organic sediment might be related to the species’ recent introduction to the area and be the main factor that inhibits its expansion into habitats with sediments containing high levels of organic matter. Our results suggest that the species-specific plasticity of roots in these Hydrocharitaceae may help explaining their distribution in the Upper Paraná basin.