Combined effects of varying water content and CO2 concentration on photosynthesis in Spagnum fuscum

The photosynthesis of Spagnum fuscum (Schimp.) Klinggr. at different water contents and CO₂ concentrations was measured in the laboratory. The optimal water content for photosynthesis near the current atmospheric CO₂ concentration is 600–800% (percentage of dry weight). The decrease in photosynthesis is very steep towards lower water contents and less steep towards higher water contents. The optimal water content range moves higher and becomes wider with increasing CO₂ concentration. At 3000 ppm there is no longer any decrease in photosynthesis with increasing water content. The water content of S. fuscum has a considerable effect on the response of photosynthesis to CO₂ concentration. In a moss saturated with water, photosynthesis increases gradually until 8000 ppm CO₂, but this saturation concentration becomes lower with decreasing water content, being c. 1500 ppm at a water content of 700–800%. An increase in CO₂ concentration over 300 ppm will raise photosynthesis very little in dry moss with a water content of only 300–400%.