EFFECT OF CHANGES IN WATER-CONTENT ON PHOTOSYNTHESIS, TRANSPIRATION AND DISCRIMINATION AGAINST (CO2)-C-13 AND (COO)-O-18-O-16 IN PLEUROZIUMAND SPHAGNUM

Photosynthetic gas exchange characteristics of two common boreal fores t mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry ou t 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 mu mol m(-2) s(-1)) occurred at a water content of approximately 7 (fresh weight/dry weig ht) while for Pleurozium the maximum rate (approx. 2 mu mol m(-2) s(-1 )) occurred at a water content of approximately 6 (fresh weight/dry we ight). Above and below these water contents CO2 assimilation declined. In both species total conductance to water vapour (expressed as a per centage of the maximum rates) remained nearly constant at a water cont ent above 9 (fresh weight/dry weight), but below this level declined i n a strong linear manner. Short-term, ''on-line'' (CO2)-C-13 and (COO) -O-18-O-16 discrimination varied substantially with changes in moss wa ter 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 (CO2)-C-13 discri mination (approx. 15 parts per thousand). Discrimination against (CO2) -C-13 increased continuously with reductions in water content to a max imum of 27 parts per thousand in Sphagnum and 22 parts per thousand in Pleurozium. In a similar manner (CCO)-C-18-O-16 discrimination increa sed from approximately 30 parts per thousand at full hydration in both species to a maximum of 150 parts per thousand in Sphagnum and 90 par ts per thousand in Pleurozium, at low water content. The observed chan ges in (COO)-O-18-O-16 were strongly correlated to predictions of a me chanistic model of discrimination processes. Field measurements of mos s water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low w ater content.