The Diurnal Pattern of Nitrate Uptake and Reduction by Spinach (Spinacia oleracea L.)

Spinach plants were grown in bowls of aerated nutrient solutionin a controlled environment chamber for 24 h, and harvestedevery 3·5-5 h to record their growth, nitrate and wateruptake, and plant nitrate concentration. Twelve such experimentsare described, either with a 14/10 h dark/light regime, or continuouslight or darkness. The irradiance was either 110, 320, or 510µmol m^-2 s^-1 (PPFD). All these regimes began at the endof the light period of a 14/10 h dark/light regime (510 µmolm^-2 s^-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almostunabated at about 17 µmol h^-1 through the initial 14-hdark period, and then fell away sharply if the light was notrestored, but increased slightly when it was. With continuouslight at 510 µmol m^-2 s^-1, uptake rate rose steadily forthe first 24 h of light, and then fell sharply for about 6 h.Shoot nitrate concentration increased about three-fold in thedark phase, and declined in the light at a rate which was positivelyrelated to the irradiance. Root nitrate concentration was severaltimes higher than that of the shoot: its diurnal change wassmaller (relative to the mean) than that of the shoot. Nitratereduction occurred to a small extent in the dark, and increasedrapidly as soon as the lights came on, to remain at a roughlyconstant rate (related to the irradiance) throughout the lightphase. Dry matter increase in the light was related to irradiance,but with little increase above 320 µmol m^-2 s^-1. Respiratoryweight loss in the dark was not detectable. Rate of fresh weightincrease was approximately constant throughout light and darkperiods. The results compare quite well with the predictions of a simplesimulation model, based on the pump/leak principle.Copyright1994, 1999 Academic Press Spinacia oleracea, nitrate, uptake, reduction, influx, efflux, diurnal, regulation, model, simulation     

Simulation Model of the Coupling between Nitrification and Denitrification in a Freshwater Sediment

A model was constructed to simulate the results of experiments which investigated nitrification and denitrification in the freshwater sediment of Lake Vilhelmsborg, Denmark (K. Jensen, N. P. Sloth, N. Risgaard-Petersen, S. Rysgaard, and N. P. Revsbech, Appl. Environ. Microbiol. 60:2094-2100, 1994). The model output faithfully represented the profiles of O₂ and NO₃^- and rates of nitrification, denitrification, and O₂ consumption as the O₂ concentration in the overlying water was increased from 10 to 600 μM. The model also accurately predicted the response, to increasing O₂ concentrations, of the integrated (micromoles per square meter per hour) rates of nitrification and denitrification. The simulated rates of denitrification of NO₃^- diffusing from the overlying water (D_w) and of NO₃^- generated by nitrification within the sediment (D_n) corresponded to the experimental rates as the O₂ concentration in the overlying water was altered. The predicted D_w and D_n rates, as NO₃^- concentration in the overlying water was changed, closely resembled those determined experimentally. The model was composed of 41 layers 0.1 mm thick, of which 3 represented the diffusive boundary layer in the water. Large first-order rate constants for nitrification and denitrification were required to completely oxidize all NH₄⁺ diffusing from the lower sediment layers and to remove much of the NO₃^- produced. In addition to the flux of NH₄⁺ from below, the model required a flux of an electron donor, possibly methane. Close coupling between nitrification and denitrification, achieved by allowing denitrification to tolerate some O₂ (~10 μM), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O₂) resulted in too high NO₃^- concentrations and too low rates of denitrification.     

Effect of dietary fiber on microbial activity and microbial gas production in various regions of the gastrointestinal tract of pigs.

The microbial activity, composition of the gas phase, and gas production rates in the gastrointestinal tract of pigs fed either a low- or a high-fiber diet were investigated. Dense populations of culturable anaerobic bacteria, high ATP concentrations, and high adenylate energy charges were found for the last third of the small intestine, indicating that substantial microbial activity takes place in that portion of the gut. The highest microbial activity (highest bacterium counts, highest ATP concentration, high adenylate energy charge, and low pH) was found in the cecum and proximal colon. Greater microbial activity was found in the stomach and all segments of the hindgut in the pigs fed the high-fiber diet than in the pigs fed the low-fiber diet. Considerable amounts of O2 were found in the stomach (around 5%), while the content of O2 in gas samples taken from all other parts of the gastrointestinal tract was < 1%. The highest concentrations and highest production rates for H2 were found in the last third of the small intestine. No methane could be detected in the stomach or the small intestine. The rate of production and concentration of methane in the cecum and the proximal colon were low, followed by a steady increase in the successive segments of the hindgut. A very good correlation between in vivo and in vitro measurements of methane production was found. The amount of CH4 produced by pigs fed the low-fiber diet was 1.4 liters/day per animal. Substantially larger amounts of CH4 were produced by pigs fed the high-fiber diet (12.5 liters/day)(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of 4.5S RNA depletion on Escherichia coli protein synthesis and secretion.

We examined the synthesis of individual proteins following depletion of 4.5S RNA by using a strain deficient in the induction of heat shock proteins. We found that initially the synthesis of all proteins was equally affected, and the peptide elongation rate was reduced by approximately 10%. For up to 1 generation time after the onset of inhibition of total protein synthesis, the processing of secreted proteins was unaffected. After further depletion of 4.5S RNA, accumulation of precursors of secreted proteins was observed under some growth conditions.     

Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove,Phuket Island,Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0–53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added³⁵S-label was recovered in the chromium reducible pools (FeS₂ and S⁰) (41–91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50–100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)_HCl and Fe(III)_HCl, were generally low and Fe(III)_HCl was only present in the upper surface layers (0–5 cm). Maximum concentrations of dissolved Fe²⁺ (35–285 M) occurred just about the depth where dissolved H₂S accumulated. Furthermore Fe²⁺ and H₂S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.Author of correspondence.     

Carbon allocation in developing spruce needles. Enzymes and intermediates of sucrose metabolism

In lyophilized needles of Norway spruce ( Picea abies [L.] Karsten) and starting from bud break, we determined enzyme activities (sucrose phosphate synthase [SPS; EC 2.4,1.14]. sucrose synthase [SS; EC 2.4,1.13]. acid invertase [AI; EC 3.2,1.26]) and intermediates (starch, sucrose, glucose, fructose; fructose 6-phosphate, fructose 2.6-bisphosphate [F26BP]) of carbohydrate metabolism together with needle weight, shoot length, chlorophyll and protein. For up to 110 days after bud break, samples were taken twice a week from about 25-year-old trees under field conditions. At least three periods can be distinguished during needle maturation. During the first period (up to 45 days after bud break) Al showed the highest extractable activity. This coincided with very high levels of F26BP (up to 11 pmol [mg dry weight]⁻¹) and a transient increase of starch in parallel to a decrease of sucrose. The interval between 45 and 70 days after bud break was characterized by high SS activity (ratio of fructose/glucose >1), much decreased levels of F26BP (down to below 1 pmol [mg dry weight]⁻¹), and a pronounced increase in the dry weight/fresh weight ratio. In parallel, starch declined and soluble carbohydrates increased. Finally, needle maturation was characterized by decreasing SS and continuously increasing SPS activities, so that the ratio of SPS/SS increased more than 6-fold. AI. however, did not decline with maturation. Changes in pool sizes of metabolites and enzyme activities (AI. SPS) are consistent with current concepts on sink/source transition. SS is obviously important with regard to the synthesis of structural polysaccharides.     

Myosin in onion (Allium cepa) bulb scale epidermal cells: involvement in dynamics of organelles and endoplasmic reticulum

Studied with the fluorochrome 3,3-dihexyloxacarbocyanine iodide [(DIOC₆(3)], the dynamic system of the endoplasmic reticulum (ER) in epidermal cells of onion bulb scales consists of long, tubular strands moving together with organelles in the deeper cytoplasm, and of a less mobile network composed of tubular and lamellar elements at the cell periphery. Treatment with the sulfhydryl-reagent N-ethylmaleimide (NEM) inhibited organelle and ER movement, and caused the fusion of ER-tubules into flat sheets. Fixed, long, tubular ER strands were formed by lowering the cytosolic pH of NEM-treated cells. Both these observations indicate the involvement of myosin in the dynamics of organelles and ER. Using a monoclonal antibody against murine skeletal muscle myosin (known to cross-react with plant myosin; Tang et al. 1989, J. Cell Sci. 92: 569–574), myosin was identified by immunofluorescence microscopy. Mapping the distribution of myosin, actin filaments, ER, and organelles in different phases of recovery after centrifugation of epidermal cells, co-localization of myosin with ER and organelles but not with actin filaments was observed, supporting the hypothesis that a membrane bound motor protein exists in onion epidermal cells, which translocates organelles and the endoplasmic reticulum along actin filaments.     

Phytohormonal changes in intact shoots of wheat and oilseed rape treated with the acylcyclohexanedione growth retardant prohexadione calcium

The effects of the acylcyclohexanedione-type growth retardant prohexadione calcium on seedling growth and endogenous levels of immunoreactive phytohormone-like substances in shoots of wheat ( Triticum aestivum L. cv. Kanzler) and oilseed rape ( Brassica napus L. ssp. napus cv. Lirajet) were studied. After treatment of seedlings with increasing retardant concentrations in hydroponics, plant height and fresh weight of shoots were reduced by up to 40%. Concomitantly, the amount of immunoreactive gibberellins decreased, on a fresh weight basis, when compared with levels in the shoots of control plants. In contrast, the levels of abscisic acid and dihydrozeatin riboside and isopentenyladenosine-type cytokinins were considerably elevated by the growth retardant. The content of 3-indoleacetic acid decreased slightly. These results suggest that, in addition to its effect on gibberellin content, prohexadione calcium also influences the levels of endogenous abscisic acid and cytokinins.     

Structural analysis and molybdenum-dependent expression of the pAO1-encoded nicotine dehydrogenase genes of Arthrobacter nicotinovorans

The genes of nicotine dehydrogenase (NDH) were identified, cloned and sequenced from the catabolic plasmid pA01 of Arthrobacter nicotinovorans. In immediate proximity to this gene cluster is the beginning of the 6-hydroxy-L-niotine oxidase (6-HLNO) gene. NDH is composed of three subunits (A, B and C) of M_r 30011, 14924 and 87677. It belongs to a family of bacterial hydroxylases with a similar subunit structure; they have molybdopterin dinucleotide, FAD and Fe-S clusters as cofactors. Here the first complete primary structure of a bacterial hydroxylase is provided. Sequence alignments of each of the NDH subunits show similarities to the sequences of eukaryotic xanthine dehydrogenase (XDH) but not to other known molybdenum-containing bacterial enzymes. Based on alignment with XDH it is inferred that the smallest subunit (NDHB) carries an iron-sulphur cluster, that the middle-sized subunit (NDHA) binds FAD, and that the largest NDH subunit (NDHC) corresponds to the molybdopterin-binding domain of XDH. Expression of both the ndh and the 6-hlno genes required the presence of nicotine and molybdenum in the culture medium. Tungsten inhibited enzyme activity but not the synthesis of the enzyme protein. The enzyme was found in A. nicotinovorans cells in a soluble form and in a membrane-associated form. In the presence of tungsten the fraction of membrane-associated NDH increased.