An assessment of the relationship between chlorophyll a fluorescence and CO2 gas exchange from field measurements on a moss and lichen

The relationship between CO2 exchange and relative electron-transport rate through photosystem II (ETR, measured using chlorophyll a fluorescence) was determined for a moss and a green algal lichen, photobiont probably Trebouxia sp., in the field in Antarctica. Net photosynthesis (NP) and dark respiration (DR) were measured over temperatures from zero to 25 degrees C and gross photosynthesis (GP) calculated (GP = NP -t DR). The strong response of DR to temperature in these organisms resulted in substantial changes in CO2 exchange rates. The moss Bryum argenteum Hedw. showed a strong, linear relationship between GP and ETR. This was an unexpected result since mosses are C-3 plants and, in higher plants, this group normally has a curvilinear GP versus ETR relationship. It is suggested that suppression of DR in the light might be involved. The lichen, Umbilicaria aprina Nyl., had nonlinear relationships between ETR and GP that were different at each measurement temperature. In some cases the lowest ETR was at the higher CO2 exchange rates. It is suggested that these relationships are the result of strong quenching mechanisms that are inversely proportional to GP. The results support a growing impression that the relationships between ETR and CO2 exchange are complex in these organisms and different from those found fbr higher plants.