Effect of changes in water content on photosynthesis, transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum.

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO2 assimilation with variation in water content occurred for both species. The maximum rates of CO2 assimilation for Sphagnum (approx. 7 μmol m-2 s-1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 μmol m-2 s-1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO2 assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, "on-line" 13CO2 and C18O16O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO2 to ambient CO2 partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of 13CO2 discrimination (approx. 15). Discrimination against 13CO2 increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C18C16O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C18O16O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.