Measuring and modelling environmental influences on photosynthetic gas exchange in Sphagnum and Pleurozium

A mechanistic model has been used to examine the environmental regulation of photosynthetic gas exchange in moss. The effects of water content on conductance to CO2 and on photosynthethic capacity during desiccation were calculated from the carbon isotope discrimination data of Williams & Flanagan (1996, Oecologia 108, pp. 38-46) and combined with the biochemical model of Farquhar et al. (1980, Planta 149, pp. 78-90). The model includes a simple light attenuation function that imparts curvature to the light response curve for net assimilation, enabling the use of physiologically realistic values for the biochemical parameters. Measurements of us exchange for Sphagnum and Pleurozium were made in an old black spruce ecosystem over a growing season in order to assign values to parameters in the model. The calculated maximum rates of carboxylation by Rubisco (V(max) were 5, 14 and 6 micromoles m-2 s-1 for Sphagnum during the spring, summer and autumn seasons of 1996, respectively. The increase in V(max) during the summer was consistent with an increased allocation of resources to the photosynthetic apparatus. In contrast, no seasonal variation in V(max) was observed in Pleurozium with average values of 7, 5 and 7 micromole m-2 s-1 during the spring, summer and autumn, respectively.