Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985. Metabolism of glycollate by Lemna minor L. grown on nitrate or ammonium as nitrogen source.—J. exp. Bot. 36: 1685-1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutions containing either NH4+ or NO3− as nitrogen source was allowed to assimilate [1-14C]- or [2-14C]glycollate during a 20 min period in darkness or in light. The incorporation of radioactivity into water-soluble metabolites, the insoluble fraction, and into the CO2 released was measured. In addition the extractable activity of phosphoenolpyruvate carboxylase was determined. During the metabolism of [2-14C]glycollate in darkness, as well as in the light, NH4+ grown plants evolved more 14CO2 than NO3− grown plants. Formate was labelled only from [2-14C]glycollate and in NH4+ grown plants it was significantly less labelled in light than in darkness. In NO3− grown plants formate showed similar radioactivity after dark and light labelling. The radioactivity in glycine was little influenced by the nitrogen source. Amounts of radioactivity in serine implied that the further metabolism of serine was reduced in darkness compared with its metabolism in the light under both nitrogen regimes. In illuminated NH4+ plants, serine was labelled through a pathway starting from phosphoglycerate. After [1-14C]glycollate feeding NH4+ grown plants contained markedly more radioactive aspartate and malate than NO3− plants indicating a stimulated phosphoenolpyruvate carboxylation in plants grown on NH4
