Ureide Metabolism in Non-nodulated Phaseolus vulgaris L

The distribution of ureide-N was studied throughout vegetative and reproductive growth of non-nodulated Phaseolus vulgaris L. (bushbean) grown in nitrate nutrient solution. Largest increases in ureide-N per plant were correlated with flowering and early pod formation and with seed filling. Highest amounts of ureides per organ were measured in stems and axillary trifoliates. Highest concentrations (μmol ureide-N g−1 fr. wt.) were measured in young developing organs and stems. Seeds did not accumulate ureides until the ureide content of pods had reached a maximum. Results obtained using the inhibitor of xanthine oxidase, allopurinol, are consistent with the origin of ureides via purine degradation but alternative pathways cannot be discounted. Leaves and stems were shown to have the ability to degrade allantoate via an enzymic proces

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

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

Sulfur dioxide as a sulfur source in duckweeds (Lemna minor L.)

Isotope competition experiments withLemna minor L. indicate that SO2-sulfur enters the sulfur amino acids of the proteins and the sulfoquinovose of the sulfolipids following oxidation to SO42− and subsequent reduction

H2S as sulfur source in Lemna minor L.: II. Direct incorporation into cysteine and inhibition of sulfate assimilation

Bei Lemna minor L. hemmt H2S in subtoxischen Konzentrationen die Assimilation von SO42−. Es wird bei der Bildung von Cystein direkt eingebaut ohne vorangehende Oxidation zu SO42− mit anschliessender Reduktion

Subcellular localization of O-acetylserine sulfhydrylase in spinach leaves

A combination of differential centrifugation and isopycnic sucrose density gradient centrifugation of extracts from spinach leaves (Spinacia oleracea L.) shows that about 20% of the O-Acetylserine sulfhydrylase are associated with chloroplasts. No appreciable amounts of O-Acetylserine sulfhydrylase band with mitochondrial and peroxisomal marker enzymes

Changes in Gas Exchange and in the Activities of Proteolytic Enzymes during Senescence of Wheat Leaves (Triticum aestivum L.)

Gas exchange rates, nitrogen an chlorophyll contents and the activities of aminopeptidase, carboxypeptidase and neutral endopeptidase were measured in the different leaves of field grown wheat (Triticum aestivum L.) during grain development. Like the macroscopic senescence criteria (yellowing, drying) the physiological changes occurred in strong gradients in the whole plant. First the bottom Teaves and finally the flag leaf senesced. Early in senescence net photosynthesis decreased and chlorophyll was degraded, but the leaf remained turgid and dark respiration was only partially reduced. The C02 compensation concentration increased during this period and finally no defined value could be obtained, because no equilibrium existed between carboxylating and decarboxylating processes. Aminopeptidase activity increased during leaf development, readied a maximum and decreased simultaneously with the mobilization of leaf nitrogen. Carboxypeptidase also increased during leaf development and decreased during senescence, but this loss occurred later than that of aminopeptidase. In contrast to the two exopeptidases, the endopeptidase activity increased with the beginning of senescence and peaked during protein mobilization. Maximal activity appeared first in the lowest leaf and then sequentially in the upper leaves. Inside the leaf the activity increase progressed from the tip to the base and finally the leaf sheath mobilized its nitrogen. The endopeptidase appears to be associated with senescence mobilization of proteins. During wheat leaf senescence first net photosynthesis and chlorophyll content decreased and the endopeptidase activity and the CO2 compensation concentration increased. Then dark respiration, aminopeptidase and a little later, also carboxypeptidase activity decreased, while endopeptidase was still increasing. Finally leaf fresh weight decreased, endopeptidase reached its maximum and then it also lost its activity. When the breakdown of the photosynthesis apparatus is progressing, although the cells are still turgid and active, macromolecules can be degraded and the breakdown products can be transferred into translocation compounds and loaded into the phloem. It is known, that in cereals under certain conditions (i.e. water stress, artificial senescence of detached leaves) the pattern of proteolytic enzymes during protein mobilization is not or only slightly changed in the manner mentioned above. A possible mechanism of regulation in the whole plant is the flux of assimilates into the grains affecting their requirement for nitrogen compounds. For a slow export from the leaves to the grains enough amino acids may be produced by regular protein turnover, but for a rapid mobilization from a healthy leaf a more effective mechanism (i. e. increasing endopeptidase activity) may be required

Incorporation of 14CO2 in photosynthetic pigments of Chlorella pyrenoidosa

Tracer kinetic studies of chloroplast pigments of Chlorella pyrenoidosa were carried out in a special steady-state apparatus which allowed the simultaneous recording of oxygen evolution, CO2-fixation and 14CO2-incorporation. A special cylindrical vessel which permits labeling experiments with larger algae suspensions (800 ml) is described.-1. After 2 h of 14CO2-photosynthesis (fixation rate 100–160 μmol CO2/μmol chlorophyllxh) 3.3% of the total 14C-uptake (5.66 mCi) was found in the Chlorella lipid fraction. Total and specific radioactivity were higher in total carotenoids than in the chlorophylls. Chlorophyll a gave a higher labeling degree (2.4%) than chlorophyll b (1.3%).-2. Among the carotenoids α-and β-carotene were labeled after 2 h 14CO2 exposure with the same specific radioactivity and with a particular high labeling degree of c. 19%. The xanthophylls exhibit lower labeling degree (violaxanthin 5.1%, zeaxanthin 1.9%, lutein 1.4%, antheraxanthin 1.3%, and neoxanthin 0.7%).-3. During the 4 h 12CO2-exposure period, which followed the 2 h 14CO2-incorporation time, the specific and total radioactivity of the α-and β-carotene pools decrease with a concomitant increase in the α-ionone-(lutein) and β-ionone xanthophylls (violaxanthin, zeaxanthin, antheraxanthin). The possibility, that the decrease of 14C label in the carotenes may in part be due to a photo-oxidative degradation, is discussed.-4. Calculation of biological half-life-times from the 14C-incorporation kinetics during the first hour of the experiment, when the pigment concentration is almost unchanged, results in times from 30 to 60 min. Half lives are shorter in the precursor pools such as chlorophyll a (30 min), α-carotene (40 min) and β-carotene (50 min) and violaxanthin (60 min) respectively

H2S as a Sulfur Source in Lemna minor L.: Effect on Growth, Sulfur Content and Sulfur Uptake

Lemna minor L. was cultivated with 6 ppm H2S in atmospheric air. Compared to organisms in H2S-free air, it had a smaller but constant specific growth rate for 36 days. It showed a pronounced increase in sulfate concentration. H2S quickly reduced the sulfate uptake. The mechanism of regulation of sulfate uptake is discussed.Bei Lemna minor L. hemmt H2S in subtoxischen Konzentrationen die Assimilation von SO42−. Es wird bei der Bildung von Cystein direkt eingebaut ohne vorangehende Oxidation zu SO42− mit anschliessender Reduktion