CO2 and O2 Exchange in Two Mosses, Hypnum cupressiforme and Dicranum scoparium

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Abstract
Photosynthetic CO2 and O2 exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using 18O2 to trace O2 uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18°C, 330 microliters per liter CO2, 21% O2) the net photosynthetic rate was around 40 micromoles CO2 per gram dry weight per hour in H. cupressiforme and 50 micromoles CO2 per gram dry weight per hour in D. scoparium. The CO2 compensation point lay between 45 and 55 microliters per liter CO2 and the enhancement of net photosynthesis by 3% O2versus 21% O2 was 40 to 45%. The ratio of O2 uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO2 showed a high apparent Km (CO2) even in nonsaturating light. On the other hand, O2 uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O2 concentration (saturating level as low as 30% O2), and by 65% by decreasing the CO2 concentration to the compensation point. Although O2 is a competitive inhibitor of CO2 uptake it could not replace CO2 completely as an electron acceptor, and electron flow, expressed as gross O2 production, was inhibited by both high O2 and low CO2 levels. At high CO2, O2 uptake was 70% lower than the maximum at the CO2 compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction.