Seasonal acclimation in the photosynthetic and respiratory temperature responses of three submerged freshwater macrophyte species

Investigations of seasonal temperature acclimation in gas exchange are few and only exist for terrestrial and marine plants. Here we report on results obtained for three freshwater macrophyte species (Callitriche obtusangula, Potamogeton pectinatus and Potamogeton perfoliatus). We collected plants from the field at monthly intervals and measured photosynthetic and respiratory temperature-response curves. Fitted and calculated parameters were derived from the curves and a simple model was used to evaluate the acclimative capacity to seasonal variation in water temperature. For all species, optimum temperatures for gross photosynthesis showed little temporal variation. In addition, the shape of the temperature-response curves at suboptimal temperature was not optimized to temporal differences in water temperature. The only consistent seasonal trend in gas exchange was a gradual decrease in photosynthetic and respiratory capacity over time. Our measurements and model predictions did not point to an acclimative seasonal response in the thermal dependence of oxygen exchange. Hence, we conclude that either other processes constrain the plants' response, or temporal variation in water temperature is less important than seasonal loss of photosynthetic capacity.