CO(2) and O(2) Exchange in Two Mosses, Hypnum cupressiforme and Dicranum scoparium.

Photosynthetic CO(2) and O(2) exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using (18)O(2) to trace O(2) uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18 degrees C, 330 microliters per liter CO(2), 21% O(2)) the net photosynthetic rate was around 40 micromoles CO(2) per gram dry weight per hour in H. cupressiforme and 50 micromoles CO(2) per gram dry weight per hour in D. scoparium. The CO(2) compensation point lay between 45 and 55 microliters per liter CO(2) and the enhancement of net photosynthesis by 3% O(2)versus 21% O(2) was 40 to 45%. The ratio of O(2) uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO(2) showed a high apparent K(m) (CO(2)) even in nonsaturating light. On the other hand, O(2) uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O(2) concentration (saturating level as low as 30% O(2)), and by 65% by decreasing the CO(2) concentration to the compensation point. Although O(2) is a competitive inhibitor of CO(2) uptake it could not replace CO(2) completely as an electron acceptor, and electron flow, expressed as gross O(2) production, was inhibited by both high O(2) and low CO(2) levels. At high CO(2), O(2) uptake was 70% lower than the maximum at the CO(2) compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction.