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.

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 CO 2 and on photosynthetic capacity during desiccation were calculated from the carbon isotope discrimination data of

The role of external carbonic anhydrase in inorganic carbon acquisition and photosynthesis by Chiamydomonas reinhardii at alkaline pH (8.0) was studied. Acetazolamide (50 micromolar) completely inhibited external carbonic anhydrase (CA) activity as determined from isotopic disequilibrium experiments. Under these conditions, photosynthetic rates at low dissolved inorganic carbon (DIC) were far greater than could be maintained by CO2 supplied from the spontaneous dehydration of HC03-thereby showing that C. reinhardii has the ability to utilize exogenous HCO3-. Acetazolamide increased the concentration of DIC required to half-saturate photosynthesis from 38 to 80 micromolar, while it did not affect the maximum photosynthetic rate. External CA activity was also removed from the cell-wall-less mutant (CW-15) by washing. This had no effect on the photosynthetic kinetics of the algae while the addition of acetazolamide to washed cells (CW-15) increased the K?'c from 38 to 80 micromolar. Acetazolamide also caused a buildup of the inorganic carbon pool upon NaHCO3 addition, indicating that this compound partially inhibited internal CA activity. The effects of acetazolamide on the photosynthetic kinetics of C. reinhardii are likely due to the inhibition of internal rather than a consequence of the inhibition of external CA.Further analysis of the isotopic disequilibrium experiments at saturating concentration of DIC provided evidence consistent with active CO2 transport by C. reinhardii. The observation that C. reinhardii has the ability to take up both CO2 and bicarbonate throws into question the role of external CA in the accumulation of DIC in this alga.of carbon for photosynthesis (23). Findenegg (12) (26) then the function of external CA is called into question.Internal CA appears to facilitate the CO2 supply to Rubisco from the internal HCO3 pool (24, 25). Spalding et al. (24,25) showed that a mutant of C. reinhardii lacking internal CA exhibited high rates of photorespiration and accumulated large concentrations of intracellular inorganic carbon. Their conclusion was that an internal CA was required to facilitate CO2 supply to Rubisco from actively transported HCO3 . Proponents of the theory that CO2 is the only species of carbon to cross the plasmalemma suggest that active transport ofbicarbonate occurs at the chloroplast envelope and that the internal CA is located within the chloroplast (21,22).In this report we provide evidence that C. reinhardii has the ability to take up either CO2 or bicarbonate and that the magnitude of each flux is dependent upon the experimental conditions. Our results suggest that both CO2 and HCO3 accumulation are active and occur at the plasmalemma. We also provide evidence that external CA is not necessary for efficient photosynthesis at alkaline pH.

The physiological role of chloroplastic carbonic anhydrase (CA) was examined by antisense suppression of chloroplastic CA (on average 8% of wild type) in Nicotiana tabacum. Photosynthetic gas-exchange characteristics of low-CA and wild-type plants were measured concurrently with short-term, on-line stable isotope discrimination at varying vapor pressure deficit (VPD) and light intensity. Low-CA and wild-type plants were indistinguishable in the responses of assimilation, transpiration, stomatal conductance, and intercellular CO, concentration to changing VPD or light intensity. At saturating light intensity, low-CA plants had lower discrimination against 13C0, than wild-type plants by 1.2 to 1.8%0. Consequently, tissue of the low-CA plants was higher in 13C than the control plants. It was calculated that low-CA plants had chloroplast CO, concentrations 13 to 22 pmol mol-' lower than wild-type plants. Discrimination against C'80'60 in low-CA plants was 20% of that of the wild type, confirming a role of chloroplastic CA in the mechanism of discrimination against C'80'60 (AC'80'60). As VPD increased, stomatal closure caused a reduction in chloroplastic CO, concentration, and since VPD and chloroplastic CO, concentration act in opposing directions on AC'80'60, no effect of VPD was seen on AC'80'60.

ABSTRACIrSynechococcus kopoliensis was grown over a wide range of dissolved inorganic carbon (DIC) concentrations (4-25,000 micromolar) which were obtained by varying culture pH (6.2-9.6)