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1.
Additions of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS), result in an increase in NH 3 in seedling leaves of C 3 (wheat [ Triticum aestivum cv. Kolibri] and barley [ Hordeum vulgare var Perth]) and C 4 (corn [ Zea mays W6A × W182E] and sorghum [ Sorghum Vulgare var MK300]) plants. NH 3 accumulation is higher in C 3 (about 17.8 micromoles per gram fresh weight per hour) than in C 4 (about 4.7 micromoles) leaves. Under ideal conditions, when photosynthesis is not yet inhibited by the accumulation of NH 3, the rate of NH 3 accumulation is about 16% of the apparent rate of photosynthesis. A maximum accumulation of NH 3 was elicited by 2.5 millimolar MSX and was essentially independent of the addition of NO 3− during either the growth or experimental period. When O 2 levels in the air were reduced to 2%, MSX resulted in some accumulation of NH 3 (6.0 micromoles per gram fresh weight per hour). At these levels of NH 3, there was no significant inhibition of rates of CO 2 fixation. There was also a minor, but significant, accumulation of NH 3 in corn roots treated with MSX. Inhibitors of photorespiration (isonicotinic hydrazide, 70 millimolar; 2-pyridylhydroxymethanesulfonic acid, 20 millimolar) or transaminase reactions (aminooxyacetate, 1 millimolar) inhibited the accumulation of NH 3 in both C 3 and C 4 leaves. These results support the hypothesis that GS is important in the assimilation of NH 3 in leaves and that the glycine-serine conversion is a major source of that NH 3. 相似文献
2.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
3.
Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH 3, NO 3−, or N 2. Expressed as a function of relative growth rate, steady state cellular P contents and PO 4 assimilation rates did not vary with N-source. However, N-source did alter the maximal PO 4-limited growth rate achieved by the cultures: the NO 3− and N 2 cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH 3 grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO 3−-grown cells were the smallest (627 ± 34 micromoles C · 10 −9 cells), while NH 3-grown cells were largest (900 ± 44 micromoles C · 10 −9 cells) and N 2-fixing cells were intermediate (726 ± 48 micromoles C · 10 −9 cells) in size. In the NO 3−-and N 2-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH 3-grown cells. Heterocysts were absent in NH 3-grown cultures but were present in both the N 2 and NO 3− cultures. In the NO 3−-grown cultures C 2H 2 reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N 2-fixing cultures the acetylene:N 2 ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal. Compared with NH3, the assimilation of NO3− and N2 resulted either in a decrease in cellular C (NO3− and N2 cultures) or in a lower maximal growth rate (N2 culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO3− or N2 compared with NH3. 相似文献
4.
Chlamydomonas reinhardii cells, growing photoautotrophically under air, excreted to the culture medium much higher amounts of NO 2− and NH 4+ under blue than under red light. Under similar conditions, but with NO 2− as the only nitrogen source, the cells consumed NO 2− and excreted NH 4+ at similar rates under blue and red light. In the presence of NO 3− and air with 2% CO 2 (v/v), no excretion of NO 2− and NH 4+ occurred and, moreover, if the bubbling air of the cells that were currently excreting NO 2− and NH 4+ was enriched with 2% CO 2 (v/v), the previously excreted reduced nitrogen ions were rapidly reassimilated. The levels of total nitrate reductase and active nitrate reductase increased several times in the blue-light-irradiated cells growing on NO 3− under air. When tungstate replaced molybdate in the medium (conditions that do not allow the formation of functional nitrate reductase), blue light activated most of the preformed inactive enzyme of the cells. Furthermore, nitrate reductase extracted from the cells in its inactive form was readily activated in vitro by blue light. It appears that under high irradiance (90 w m −2) and low CO 2 tensions, cells growing on NO 3− or NO 2− may not have sufficient carbon skeletons to incorporate all the photogenerated NH 4+. Because these cells should have high levels of reducing power, they might use NO 3− or, in its absence, NO 2− as terminal electron acceptors. The excretion of the products of NO 2− and NH 4+ to the medium may provide a mechanism to control reductant level in the cells. Blue light is suggested as an important regulatory factor of this photorespiratory consumption of NO 3− and possibly of the whole nitrogen metabolism in green algae. 相似文献
5.
We examined nitrate-dependent Fe 2+ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “ Candidatus Brocadia sinica” anaerobically oxidized Fe 2+ and reduced NO 3− to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein −1 min −1, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “ Ca. Brocadia sinica” (10 to 75 nmol NH 4+ mg protein −1 min −1). A 15N tracer experiment demonstrated that coupling of nitrate-dependent Fe 2+ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO 3− by “ Ca. Brocadia sinica.” The activities of nitrate-dependent Fe 2+ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO 3− ± SD of “ Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe 2+ oxidation was further demonstrated by another anammox bacterium, “ Candidatus Scalindua sp.,” whose rates of Fe 2+ oxidation and NO 3− reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein −1 min −1, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe 2+ oxidation and the anammox reaction decreased the molar ratios of consumed NO 2− to consumed NH 4+ (ΔNO 2−/ΔNH 4+) and produced NO 3− to consumed NH 4+ (ΔNO 3−/ΔNH 4+). These reactions are preferable to the application of anammox processes for wastewater treatment. 相似文献
6.
Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH 4+, NO 2−, NO 3−) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO 2 release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH 4+ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O 2 consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO 3− and NO 2− assimilation resulted in a larger stimulation of TCA cycle CO 2 release than did NH 4+, but a much smaller stimulation of mitochondrial O 2 consumption. NH 4+ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO 3− and NO 2− assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH 4+, NO 3− and NO 2− assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO 3− and NO 2− assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O 2 consumption during NO 3− and NO 2− assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO 3− and NO 2− reduction. 相似文献
7.
The role of NO 3− and NO 2− in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley ( Hordeum vulgare L.) seedlings was investigated. Barley leaves contained 6 to 8 micromoles NO 2−/gram fresh weight × hour of endogenous NiR activity when grown in N-free solutions. Supply of both NO 2− and NO 3− induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO 2− and NO 3− over a 24 hour period). In NO 3−-supplied leaves, NiR induction occurred at an ambient NO 3− concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO 2− until the ambient NO 2− concentration was 0.5 millimolar. Nitrate accumulated in NO 2−-fed leaves. The amount of NO 3− accumulating in NO 2−-fed leaves induced similar levels of NiR as did equivalent amounts of NO 3− accumulating in NO 3−-fed leaves. Induction of NiR in NO 2−-fed leaves was not seen until NO 3− was detectable (30 nanomoles/gram fresh weight) in the leaves. The internal concentrations of NO 3−, irrespective of N source, were highly correlated with the levels of NiR induced. When the reduction of NO 3− to NO 2− was inhibited by WO 42−, the induction of NiR was inhibited only partially. The results indicate that in barley leaves NiR is induced by NO 3− directly, i.e. without being reduced to NO 2−, and that absorbed NO 2− induces the enzyme activity indirectly after being oxidized to NO 3− within the leaf. 相似文献
8.
K + content and concentration within the apoplast of mesophyll tissue of pea ( Pisum sativum L., cv Argenteum) leaflets were determined using an elution procedure. Following removal of the epidermis, a 1 centimeter (inside diameter) glass cylinder was attached to the exposed mesophyll tissue and filled with 5 millimolar CaCl 2 solution (1°C). From time-course curves of cumulative K + diffusion from the tissue, the amount of K + of extracellular origin was estimated. Apoplastic K + contents for leaves from plants cultured in nutrient solution containing 2 or 10 millimolar K + were found to range from 1 to 4.5 micromoles per gram fresh weight, comprising less than 3% of the total K + content within the lamina tissue. Assuming an apoplastic solution volume of 0.04 to 0.1 milliliters per gram fresh weight and a Donnan cation exchange capacity of 2.63 micromoles per gram fresh weight (experimentally determined), the K + concentration within apoplastic solution was estimated at 2.4 to 11.8 millimolar. Net movement of Rb + label from the extracellular compartment within mesophyll tissue into the symplast was demonstrated by pulse-chase experiments. It was concluded that the mesophyll apoplast in pea has a relatively low capacitance as an ion reservoir. Apoplastic K + content was found to be highly sensitive to changes in xylem solution concentration. 相似文献
9.
Nitrogen-14 and nitrogen-15 nuclear magnetic resonance (NMR) spectra were recorded for freshly dissected buds of Picea glauca and for buds grown for 3, 6 and 9 weeks on shoot-forming medium. Resonances for Glu (and other αNH 2 groups), Pro, Ala, and the side chain groups in Gln, Arg, Orn, and γ-aminobutyric acid could be detected in in vivo15N NMR spectra. Peaks for α-amino groups, Pro, NO 3− and NH 4+ could also be identified in 14N NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that 15N-labeled NH 4+ and NO 3− are first incorporated into the amide group of Gln and then in the αNH 2 pool. Subsequently, it also emerges in Ala and Arg. These data suggest that the glutamine synthetase/ glutamate synthase pathway functions under these conditions. The assimilation of NH 4+ is much faster than that of NO 3−. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH 4+ rather than NO 3−. If NH 4+ is omitted from the medium, no NO 3− is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH 4+ and NO 3− are both required for the induction of nitrate- and nitrite reductase. 相似文献
10.
Macroalgae has bloomed in the brackish lake of Shenzhen Bay, China continuously from 2010 to 2014. Gracilaria tenuistipitata was identified as the causative macroalgal species. The aim of this study was to explore the outbreak mechanism of G. tenuistipitata, by studying the effects of salinity and nitrogen sources on growth, and the different nitrogen sources uptake characteristic. Our experimental design was based on environmental conditions observed in the bloom areas, and these main factors were simulated in the laboratory. Results showed that salinity 12 to 20 ‰ was suitable for G. tenuistipitata growth. When the nitrogen sources'' (NH 4
+, NO 3
−) concentrations reached 40 µM or above, the growth rate of G. tenuistipitata was significantly higher. Algal biomass was higher (approximately 1.4 times) when cultured with NH 4
+ than that with NO 3
− addition. Coincidentally, macroalgal bloom formed during times of moderate salinity (∼12 ‰) and high nitrogen conditions. The NH 4
+ and NO 3
− uptake characteristic was studied to understand the potential mechanism of G. tenuistipitata bloom. NH 4
+ uptake was best described by a linear, rate-unsaturated response, with the slope decreasing with time intervals. In contrast, NO 3
− uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model, with kinetic parameters Vmax = 37.2 µM g −1 DM h −1 and Ks = 61.5 µM. Further, based on the isotope 15N tracer method, we found that 15N from NH 4
+ accumulated faster and reached an atom% twice than that of 15N from NO 3
−, suggesting when both NH 4
+ and NO 3
− were available, NH 4
+ was assimilated more rapidly. The results of the present study indicate that in the estuarine environment, the combination of moderate salinity with high ammonium may stimulate bloom formation. 相似文献
11.
An experiment was conducted to determine the extent that NO 3− taken up in the dark was assimilated and utilized differently by plants than NO 3− taken up in the light. Vegetative, nonnodulated soybean plants ( Glycine max L. Merrill, `Ransom') were exposed to 15NO 3− throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing 14NO 3−, with plants sampled subsequently at each light/dark transition over 3 days. The rates of 15NO 3− absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of 15NO 3− reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed 15NO 3− in roots and decreased efficiency of reduction of 15NO 3− in the shoot. The rate of incorporation of 15N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant 15NO 3− reduction in darkness. A large portion of the 15NO3− retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of 15NO3− acquired during the light period. Taking into account reduction of NO3− from all endogenous pools, it was apparent that plant reduction in a given light period (~13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO3− (8.42 micromoles per hour) during that period. The primary source of substrate for NO3− reduction in the dark was exogenous NO3− being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO3− in the light occurred in the root system. 相似文献
12.
Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH 4
+-N) and nitrate N (NO 3
−-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH 4
+-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH 4
+-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH 4
+-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg −1 adsorbed 2.3 mg NH 4
+-N g −1 in solutions with 50 mg NH 4
+ L −1). Compared with NH 4
+-N, none of NO 3
−-N was adsorbed to biochars at different NO 3
− concentrations. Instead, some NO 3
−-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH 4
+-N (or NH 3) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO 3
−-N pollution. 相似文献
13.
The objective of this study was to improve the growth of in vitro shoot cultures of Brugmansia × candida ‘Creamsickle’. Several mineral nutrient experiments were conducted to determine the effect of NH 4+, NO 3−, K +, FeSO 4/EDTA, ZnSO 4, MnSO 4, and CuSO 4 on quality, leaf width and length, size and weight of shoot mass, and shoot number. The experiment to determine the levels of NH 4+, NO 3−, and K +, was conducted as a 2-component NH 4+: K + mixture crossed by [NO 3−] and resulted in an experimental design free of ion confounding and capable of separating the effects of proportion and concentration. The results of the NH 4+-K +-NO 3− experiment revealed a region in the design space where growth was significantly improved; the region generally had lower total nitrogen and lower NH 4+:K + ratios than MS medium. The experiments to determine the appropriate levels of Fe, Zn, Mn, and Cu were conducted at six log levels ranging from 0 to 1 mM. Of the four metal salts tested, MnSO 4 had the least effect on in vitro shoot growth and its concentration was reduced from 0.1 mM (MS level) to 0.001 mM. CuSO 4 had large effects on in vitro shoot growth and was increased from 0.0001 mM to 0.001 mM. A 2-level factorial of NH 4+-K +-NO 3−, FeSO 4/EDTA, and ZnSO 4 was conducted and several formulations identified for their improvements of quality and growth. In addition to the changes to MnSO 4 and CuSO 4, these formulations were characterized by lower levels of NH 4+, K +, NO 3− and Zn, and higher levels of FeSO 4/EDTA. Overall, several nutrient formulations were identified as superior to MS medium for growth of in vitro shoot cultures of B. ‘Creamsickle’. 相似文献
14.
Chlamydomonas reinhardii cells, after a period of dark anaerobic adaptation, evolve H 2 not only in the dark but also in the light. Our results show that high irradiances impair prolonged H 2 evolution, while under low irradiances or darkness H 2 evolution proceeds for more than 50 hours. NO 3− and NO 2− suppress H 2 evolution both in the dark or under low irradiance. Apparently the cells prefer these oxidized nitrogen sources to protons as electron acceptors, since both NO 3− and NO 2− become reduced to NH 4+, which is excreted to the culture medium in high amounts. H 2 evolution started once these oxidized anions were largely depleted from the medium. Moreover, H 2 evolution was consistently associated with NH 4+ excretion even if NH 4+ was already present in high amounts in the medium. This observation indicates that the cells utilize not only their carbohydrate but also their protein reserves as sources of reducing power for H 2 evolution. This conclusion was supported by the observation that when nitrogen-starved cells were made anaerobic in a nitrogen-free medium, they not only evolved H 2 at very high rates but excreted concomitantly NH 4+ up to concentrations in the millimolar range. 相似文献
15.
Dissimilatory reduction of NO 2− to N 2O and NH 4+ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2O production by this mechanism. In batch cultures with defined media, NO 2− reduction to NH 4+ was favored by high glucose and low NO 3− concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3− concentrations. With succinate as the energy source, little or no NO 2− was reduced to NH 4+ but N 2O was produced. Resting cell suspensions reduced NO 2− simultaneously to N 2O and free extracellular NH 4+. Chloramphenicol prevented the induction of N 2O-producing activity. The Km for NO 2− reduction to N 2O was estimated to be 0.9 mM NO 2−, yet the apparent Km for overall NO 2− reduction was considerably lower, no greater than 0.04 mM NO 2−. Activities for N 2O and NH 4+ production increased markedly after depletion of NO 3− from the media. Amendment with NO 3− inhibited N 2O and NH 4+ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4+ but not of N 2O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2O at rates equal to the wild type. These observations suggest that N 2O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2− to NH 4+. 相似文献
16.
Uptake of NO 3− by nonnodulated soybean plants ( Glycine max L. Merr. cv Ransom) growing in flowing hydroponic culture at 22 and 14°C root temperatures was measured daily during a 31-day growth period. Ion chromatography was used to determine removal of NO 3− from solution during each 24-hour period. At both root-zone temperatures, rate of NO 3− uptake per plant oscillated with a periodicity of 3 to 5 days. The rate of NO 3− uptake per plant was consistently lower at 14°C than 22°C. The lower rate of NO 3− uptake at 14°C during the initial 5 to 10 days was caused by reduced uptake rates per gram root dry weight, but with time uptake rates per gram root became equal at 14 and 22°C. Thereafter, the continued reduction in rate of NO 3− uptake per plant at 14°C was attributable to slower root growth. 相似文献
17.
Nitrate reductase (NR) in maize ( Zea mays cv W64A × W182E) roots has been stabilized in vitro by the addition of chymostatin to extraction buffer. Contrary to previous observations, levels of NR were higher in the mature root than in root tip sections when chymostatin was included in the extraction buffer. Two forms of NR were identified, an NADH monospecific NR found mainly in the 1cm root tip and an NAD(P)H bispecific NR found predominantly in mature regions of the root. During the first 10 days of seedling growth, NR activity in the root ranged from 50 to 80% of the activities found in the leaf (a maximum of 2.4 micromoles NO −2 produced per hour per gram fresh weight was measured at 4 days). 相似文献
18.
Short-term changes in pyridine nucleotides and other key metabolites were measured during the onset of NO 3− or NH 4+ assimilation in the dark by the N-limited green alga Selenastrum minutum. When NH 4+ was added to N-limited cells, the NADH/NAD ratio rose immediately and the NADPH/NADP ratio followed more slowly. An immediate decrease in glutamate and 2-oxoglutarate indicates an increased flux through the glutamine synthase/glutamate oxoglutarate aminotransferase. Pyruvate kinase and phospho enolpyruvate carboxylase are rapidly activated to supply carbon skeletons to the tricarboxylic acid cycle for amino acid synthesis. In contrast, NO 3− addition caused an immediate decrease in the NADPH/NADP ratio that was accompanied by an increase in 6-phosphogluconate and decrease in the glucose-6-phosphate/6-phosphogluconate ratio. These changes show increased glucose-6-phosphate dehydrogenase activity, indicating that the oxidative pentose phosphate pathway supplies some reductant for NO 3− assimilation in the dark. A lag of 30 to 60 seconds in the increase of the NADH/NAD ratio during NO 3− assimilation correlates with a slow activation of pyruvate kinase and phospho enolpyruvate carboxylase. Together, these results indicate that during NH 4+ assimilation, the demand for ATP and carbon skeletons to synthesize amino acid signals activation of respiratory carbon flow. In contrast, during NO 3− assimilation, the initial demand on carbon respiration is for reductant and there is a lag before tricarboxylic acid cycle carbon flow is activated in response to the carbon demands of amino acid synthesis. 相似文献
19.
Net photosynthetic rates of Spirodela polyrrhiza turions, at low O 2 levels, were 6.2 and 38.8 micromoles O 2 per gram fresh weight per hour at 1 millimolar HCO 3− and CO 2 saturation, respectively, and much lower in a regular low-pH growth solution. Air equilibration O 2 concentrations decreased rates considerably, except at CO 2 saturation. The surfacing rate of turions in various inorganic carbon surroundings correlated positively with their photosynthetic rates, but were the same at high and low O 2 levels. The relevance of these findings in relation to environmental conditions conductive to germination of autotrophically growing turions is discussed. 相似文献
20.
Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO 3− or NH 4+, the solutions being maintained at pH 5.5. In NO 3−-fed plants excess nutrient anion over cation uptake was equivalent to net OH − efflux, and the total charge from NO 3− and SO 42− reduction equated to the sum of organic anion accumulation plus net OH − efflux. In NH 4+-fed plants a large H + efflux was recorded in close agreement with excess cation over anion uptake. This H + efflux equated to the sum of net cation (NH 4+ minus SO 42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO 3− that is taken up and reduced in NO 3−-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH 4+-fed plants absorbed NH 4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO 3−-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients. 相似文献
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