Einfluß verschiedener CO2-Konzentrationen auf Wachstum und stoffliche Zusammensetzung von Lemna minor L. bei Nitrat- und Ammoniumernährung

Abstract

Summary Lemna minor L. was grown in air with different CO 2 -concentrations with nitrate or ammonium as only nitrogen source under constant conditions (4500 Lux, 25 °C) for several generations. The plants were harvested in the exponential growth phase. After harvest the dry matter per frond and the contents of free sugars, starch, nitrate, ammonium, free amino acids and total reduced nitrogen were determined. The growth rate, the dry matter per frond and the contents of free sugars and starch increased from 100 to 9000 ppm CO 2 on both nitrogen sources. The growth rate increased from 100 to 9000 ppm by about 70% and the dry matter per frond was doubled in this concentration interval. The content of free sugars was also about doubled and the starch content increased to the five fold level from 100 to 9000 ppm CO 2 . On nitrate the growth rate at 9000 ppm was a little lower, but the dry matter per frond was higher on nitrate. In contrast, total reduced nitrogen was mainly affected by the form of nitrogen supplied and CO 2 -concentration had only a small effect. On ammonium medium the total reduced nitrogen per milligram dry matter was about 20% higher than on nitrate. On nitrate medium the internal nitrate content increased with increasing CO 2 -concentrations (doubled from 100 to 9000 ppm). In this concentration range the free ammonium decreased to about 50% of the level at 100 ppm. On ammonium medium the internal ammonium was always about twice the level of nitrate grown plants. The interesting changes in free amino acids were similar for both nitrogen sources: a five fold increase from 100 to 9000 ppm CO 2 . The separation of the free amino acids showed that the major increase occured in the amide content (glutamine and asparagine). Alanine, glutamic acid and aspartic acid also increased. The contents of glycine, serine and histidine showed remarkable decreases from 100 to 9000 ppm CO 2 . For the photorespiration related amino acids glycine and serine the decrease can be explained by changes in relative activities of the RubP-carboxylase/oxygenase in favor of the carboxylase reaction by increasing CO 2 -concentrations. As a consequence, less P-glycolate, the primary substrate for photorespiration, is produced by the oxygenase reaction. Growth rate, dry matter accumulation per frond and free sugars and starch contents were dependent on the CO 2 -concentration, but ammonium and nitrate had only minor effects. Inorganic nitrogenous compounds and free amino acids were affected by both CO 2 - concentration and form of nitrogen supplied. In contrast, total reduced nitrogen per dry matter was affected by the form of nitrogen, but CO 2 -concentration had only a minor effect. High CO 2 -levels did not suppress primary assimilation of nitrogen into amino acids, but amino acid intermediary metabolism and/or the flow of amino acids into macromolecules was limited.