LIGHT AND DARK RESPIRATION AND THE EFFECT OF THE LACUNAL SYSTEM ON REFIXATION OF CO2 IN SUBMERGED AQUATIC PLANTS

An open water-flow system was used to measure the effects of O2 concentrations on net photosynthesis and photorespiration (CO2 evolution in the light resulting from glycolate metabolism) in four submerged aquatic plants and to evaluate the effect of the lacunal system on refixation of CO2 in the light. The results showed that photorespiration is present in all species examined: Fontinalis antipyretica L., Elodea canadensis Michx., Lobelia dortmanna L., and Littorella uniflora (L.) Aschers. The ratio of light/dark respiration indicate an efficient refixation of respired CO2 with the lowest efficiencies in Fontinalis and Elodea, which respectively lack and possess few lacunae. The efficiency of refixation caused by the lacunae was investigated by using intact and sectioned leaves of Lobelia and Littorella and a minimum recycling of 50-70% of the CO2 evolution in the light was found. The CO2 compensation points of Elodea and Littorella showed minor increase to increasing O2 concentrations. From spring to autumn, the compensation point of Elodea at saturating O2 doubled from 3 to 6 μM CO2. Minor changes occurred in Littorella with a compensation point about 25 μM. © 1979.