Plasticity in carbon acquisition of the heterophyllous Luronium natans: An endangered freshwater species in Europe

Luronium natans (L.) Raf. (Floating Water-plantain) is an endangered amphibious freshwater species endemic to Europe. We examined the plasticity in carbon acquisition and photosynthesis in L. natans to assess if lack of plasticity could contribute to explain the low competitive ability of the species. The plasticity of photosynthesis in submerged leaves towards inorganic carbon availability was examined and the photosynthesis of submerged, floating and aerial leaves was contrasted. L. natans was shown to be plastic in inorganic carbon uptake, as it was able to effectively acclimate to changed concentrations of free-CO2. The photosynthetic apparatus was down-regulated in plants grown at high CO2. Chlorophyll concentration, Rubisco activity and maximum photosynthesis were significantly lower in submerged leaves of plants grown at high CO2 (200 μM free-CO2) compared to plants grown at low CO2 (18 μM free-CO2). Furthermore, bicarbonate utilization was down-regulated in response to high CO2. Carbon acquisition of submerged, floating and aerial leaves of L. natans differed significantly. The aerial leaves were superior in photosynthesising in air and, surprisingly, the floating leaves had the highest rates of photosynthesis in water. The study did not support the hypothesis that the low competitive ability of L. natans is caused by inefficient photosynthesis or a lack of plasticity in photosynthesis. However, the somewhat low photosynthetic performance of the submerged leaves may be a contributing factor.

Research highlights
► Examination of photosynthetic plasticity in Luronium natans.
► Luronium natans was plastic in inorganic carbon uptake.
► Carbon acquisition of submerged, floating and aerial leaves differed significantly.