IMMEDIATE RESPONSES OF PHOTOSYNTHESIS AND DARK RESPIRATION OF LATE SUMMER STANDS OF EQUISETUM-FLUVIATILE L TO INCREASING CONCENTRATIONS OF ATMOSPHERIC CO2

Short-term responses of net photosynthesis, apparent dark respiration and gross photosynthesis of Equisetum fluviatile to increasing concent rations of atmospheric CO2 were studied by using transplanted stands o f natural origin. Three transplantations with biomasses of 274, 407, a nd 401 g dry weight m(-2) were established six weeks before the measur ements in late August. Net photosynthesis and apparent dark respiratio n was measured from the change of CO2 concentration inside polycarbona te chambers with diameter of 0.455 m and volume of 0.207 m(3). Altoget her 50 experiments for determination of CO2 influx rates and 24 for ef flux rates were run without any pre-treatment to higher CO2 and each o f them lasted 20-30 min. The response of net photosynthesis of E. fluv iatile to CO2 enrichment was less clear than the response to temperatu re or irradiance. Nevertheless, the stands showed an increase of ca. 2 5 % in net photosynthesis when the CO2 concentration in air was increa sed from ambient to 500-600 ppm. When the CO2 concentration was > 600 ppm the increase was ca. 60 %. A multilinear regression model combinin g solar radiation, temperature and CO2 concentration could only explai n 46.4 % of the variation in the observed rates of net photosynthesis. The apparent dark respiration was positively correlated with temperat ure but inversely related to CO2 concentration. When the CO2 concentra tion was doubled from ambient the stands of E. fluviatile reduced thei r apparent dark respiration by ca. 50 %. Under higher CO2 concentratio n E. fluviatile appeared more effective than in the ambient concentrat ion, as the production lost through respiration decreased. When the co ncentration of atmospheric CO2 was < 500 ppm, 57.5 % of gross producti on was respired whereas above 500 ppm of CO2 the corresponding proport ion was only 34.2 %. As the enrichment with CO2 resulted in decreased respiration rates and it was known from long-term growth and photosynt hesis experiments that neither shoot growth in length in E. fluviatile is stimulated by higher CO2 concentrations nor do the stands show dow n-regulation of photosynthesis after several weeks of CO2 enrichment, it was concluded that the extra carbon fixed was allocated to storage through growth of below-ground biomass.