Measuring response of photosynthetic rate of Sphagnum palustre L. to water content: Importance of adjustment of the water content to the habitat condition

Sphagnum mosses play an important role in the carbon sink function of northern peatlands. Sphagnum photosynthesis is influenced by hydrological factors such as gravimetric water content (WC) and water table depth. However, there is no unified method for evaluating the photosynthetic rate under field conditions. In our study, Sphagnum palustre capitula collected from four regions in Japan were adjusted to equilibrium WC at three water levels, then changes in net photosynthetic rate (Pn) under saturated light were measured from equilibrium WC to dry condition. Pn was low at both high and low WC, indicating that there is an optimum WC at which Pn is maximised. The equilibrium WC varied with water level, resulting in different Pn values at equilibrium WC. Thus, to accurately estimate photosynthetic rate, the moisture status of the plants should be considered; and to understand the photosynthetic capacity of the habitat, WC should be adjusted to the equilibrium value corresponding to the water level in the habitat. The maximum Pn differed between collected regions, suggesting an influence of Sphagnum morphology or prior habitat conditions; however, the details remain unknown. Further studies on the correlation between Pn and Sphagnum morphology or environmental conditions in the source habitat are needed to understand and predict the potential response to climate change.