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

Abstract

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 CO assimilation with variation in water content occurred for both species. The maximum rates of CO assimilation for Sphagnum (approx. 7 μmol m s) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 μmol m s) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO 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" CO and COO discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO to ambient CO partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of CO discrimination (approx. 15). Discrimination against CO increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner CCO 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 COO 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.

The role of carbonic anhydrase in photosynthesis and the activity of the carbon‐concentrating‐mechanism in bryophytes of the class Anthocerotae

“…These gas-exchange characteristics are similar to those of organisms which utilize C % photosynthesis, crassulacean acid metabolism (CAM) or a biophysical carbon-concentrating-mechanism (CCM) (Smith & Griffiths, 1996a,b). This is in contrast to the majority of bryophytes, which are characterized by C $ photosynthetic responses (Green & Snelgar, 1982 ;Proctor et al, 1992 ;Green & Lange 1994 ;Rice & Giles 1994 ;Raven et al, 1998) modified by the effects of CO # diffusion limitation (Williams & Flanagan, 1996). The Anthocerotae also show reduced discrimination against "$C during photosynthesis, with fractionation measured in units per mil (=) (∆) values intermediate between those typical of C $ and C % higher plants (Smith & Griffiths, 1996a,b) and uptake of HCO $ − by the Anthocerotae has been demonstrated (Bain & Proctor, 1980).…”

Development and refinement of proxy-climate indicators from peats

“…However, wetness of the Sphagnum itself also directly influences the d 13 C of its molecules. When the Sphagnum plant is more saturated the water film over the photosynthetic cells impedes the incorporation of CO 2 and so the plant becomes less selective against 13 C (Williams and Flannagan, 1996). Because of these competing influences, and the possible influence of other factors like temperature (Skrzypek et al, 2007), d 13 C records derived from peat sequences must be evaluated carefully (Markel et al, 2010), and in the context of other reconstructed parameters.…”

Stable isotopes and organic geochemistry in peat: Tools to investigate past hydrology, temperature and biogeochemistry

“…Precipitation and evaporation cause isotopic fractionation of hydrogen (δD) and oxygen (δ 18 O), so that the isotopic composition of the meteoric water used by peatland plants reflects a combination of precipitation source and peatland hydrology (Daley et al, in press). Stable carbon isotopes (δ 13 C) give important information on carbon pathways, including fractionation during photosynthesis (White et al, 1994;Williams and Flanagan, 1996), and the recycling of organic matter and consumption of CO 2 and methane by microbial activity (Pancost et al, 2000).…”

“…Science Highlights: Peatlands (δ 13 C increases) and vice versa (Williams and Flanagan, 1996). However, the effect of recycled methane on δ 13 C of Sphagnum biomarkers has the opposite relationship with moisture.…”