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1.
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
2.
Ricinus communis was used to test the Ben Zioni-Dijkshoorn hypothesis that NO 3 uptake by roots can be regulated by NO 3 assimilation in the shoot. The rate of the anion charge from assimilated NO 3− (and SO 42−) was followed in its distribution between organic acid anion accumulation and HCO 3− efflux into the nutrient solution. In plants adequately supplied with NO 3−, HCO 3− efflux accounted for between 56 and 63% of the anion charge. When the plants were subjected to a low NO 3 regime HCO 3− excretion accounted for only 23% of the charge. A comparison of mature plants growing for a 10-day period at the two levels of NO 3 nutrition revealed that the uptake of NO 3− at the higher level was increased 3-fold, whereas K uptake was unaltered. To trace ion movement within the plant, the ionic constituents of xylem and phloem sap were determined. In xylem sap these constituents were found to be predominantly K +, Ca 2+, and NO 3−, whereas in the phloem sap they were mainly K + and organic acid anions. Results have been obtained which may be interpreted as providing direct evidence of NO 3 uptake by roots regulated by NO 3 reduction in the tops, the process being facilitated by the recirculation of K + in the plant. 相似文献
3.
Ricinus communis L. was used to test the Dijkshoorn-Ben Zioni hypothesis that NO 3− uptake by roots is regulated by NO 3− assimilation in the shoot. The fate of the electronegative charge arising from total assimilated NO 3− (and SO 42−) was followed in its distribution between organic anion accumulation and HCO 3− excretion into the nutrient solution. In plants adequately supplied with NO 3−, HCO 3− excretion accounted for about 47% of the anion charge, reflecting an excess nutrient anion over cation uptake. In vivo nitrate reductase assays revealed that the roots represented the site of about 44% of the total NO 3− reduction in the plants. To trace vascular transport of ionic and nitrogenous constituents within the plant, the composition of both xylem and phloem saps was thoroughly investigated. Detailed dry tissue and sap analyses revealed that only between 19 and 24% of the HCO 3− excretion could be accounted for from oxidative decarboxylation of shoot-borne organic anions produced in the NO 3− reduction process. The results obtained in this investigation may be interpreted as providing direct evidence for a minor importance of phloem transport of cation-organate for the regulation of intracellular pH and electroneutrality, thus practically eliminating the necessity for the Dijkshoorn-Ben Zioni recycling process. 相似文献
4.
The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H + or OH – into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations (NH 4
++Ca 2++ Mg 2++K ++Na +) and anions (NO 3
–+Cl –+SO 4
2–+H 2PO 4
–) and charge balance is maintained by excretion of an amount of H + or OH – which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H + and in particular OH – ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.The contemporary view is that primary active extrusion of H +, catalyzed by a membrane-located ATPase, is the major driving force for secondary transport of cations and anions across the plasma membrane. However, the fact that net OH – extrusion often occurs (since excess anion absorption commonly takes place) implies there is a yet-to-be characterized OH – ion efflux mechanism at the plasma membrane that is associated with anion uptake. There is, therefore, a need for future studies of the uptake mechanisms and stoichiometry of anion uptake; particularly that of NO 3
– which is often the predominant anion absorbed. Another related phenonenon which requires detailed study in terms of cation-anion balance is localized rhizosphere acidification which can occur in response to deficiencies of Fe and P. 相似文献
5.
The effect of nitrogen form (NH 4-N, NH 4-N + NO 3−, NO 3−) on nitrate reductase activity in roots and shoots of maize ( Zea mays L. cv INRA 508) seedlings was studied. Nitrate reductase activity in leaves was consistent with the well known fact that NO 3− increases, and NH 4+ and amide-N decrease, nitrate reductase activity. Nitrate reductase activity in the roots, however, could not be explained by the root content of NO 3−, NH 4-N, and amide-N. In roots, nitrate reductase activity in vitro was correlated with the rate of nitrate reduction in vivo. Inasmuch as nitrate reduction results in the production of OH − and stimulates the synthesis of organic anions, it was postulated that nitrate reductase activity of roots is stimulated by the released OH − or by the synthesized organic anions rather than by nitrate itself. Addition of HCO 3− to nutrient solution of maize seedlings resulted in a significant increase of the nitrate reductase activity in the roots. As HCO 3−, like OH −, increases pH and promotes the synthesis of organic anions, this provides circumstantial evidence that alkaline conditions and/or organic anions have a more direct impact on nitrate reductase activity than do NO 3−, NH 4-N, and amide-N. 相似文献
7.
The influence of NO 3− uptake and reduction on ionic balance in barley seedlings ( Hordeum vulgare, cv. Compana) was studied. KNO 3 and KCl treatment solutions were used for comparison of cation and anion uptake. The rate of Cl − uptake was more rapid than the rate of NO 3− uptake during the first 2 to 4 hours of treatment. There was an acceleration in rate of NO 3− uptake after 4 hours resulting in a sustained rate of NO 3− uptake which exceeded the rate of Cl − uptake. The initial (2 to 4 hours) rate of K + uptake appeared to be independent of the rate of anion uptake. After 4 hours the rate of K + uptake was greater with the KNO 3 treatment than with the KCl treatment, and the solution pH, cell sap pH, and organic acid levels with KNO 3 increased, relative to those with the KCl treatment. When absorption experiments were conducted in darkness, K + uptake from KNO 3 did not exceed K + uptake from KCl. We suggest that the greater uptake and accumulation of K + in NO 3−-treated plants resulted from ( a) a more rapid, sustained uptake and transport of NO 3− providing a mobile counteranion for K + transport, and ( b) the synthesis of organic acids in response to NO 3− reduction increasing the capacity for K + accumulation by providing a source of nondiffusible organic anions. 相似文献
8.
An experiment was conducted to test the hypothesis that, when nitrogenase and nitrate reductase both contribute to the nitrogen nutrition of a nodulated legume, nitrogenase activity is inversely proportional to the rate of accumulation of organic nitrogen derived from the reduction of nitrate. Trifolium subterraneum L. plants, inoculated with Rhizobium trifolii and sown as small swards, were allowed to establish a closed canopy and steady rates of growth, dinitrogen fixation, and nitrogen accumulation. Swards were then supplied with nutrient solutions of 0, 0.5, 1.0, or 2.5 m m NO 3− with a 29.69% enrichment of 15N and allowed to grow for a further 33 days. Harvests were made to measure dry weight, nitrogen accumulation, 15N accumulation, NO 3− content and nitrogenase activity by acetylene reduction assay. Since the 15N of the plant organic matter could have been derived only from the NO 3− of the nutrient solution, its rate of accumulation provided a measure of the rate of NO 3− reduction. It was found that as this rate increased in response to external NO 3− concentration the rate of nitrogenase activity decreased proportionately. It is concluded that the reduction of nitrate and the reduction of dinitrogen act in a complementary manner to supply a plant with organic nitrogen for growth. 相似文献
9.
An investigation was carried out to study the cation-anion balance in different tissues of tomato plants supplied with nitrate, urea, or ammonium nitrogen in water culture. Irrespective of the form of nutrition, a very close balance was found in the tissues investigated (leaves, petioles, stems, and roots) between total cations (Ca, Mg, K and Na), and total anions (NO3−, H2PO4−, SO4−−, Cl−) total non-volatile organic acids, oxalate, and uronic acids. In comparison with the tissues of the nitrate fed plants, the corresponding ammonium tissues contained lower concentrations of inorganic cations, and organic acids and a correspondingly higher proportion of inorganic anions. Tissues from the urea plants were intermediate between the other 2 treatments. These results were independent of concentration or dilution effects, caused by growth. In all tissues approximately equivalent amounts of diffusible cations (Ca++, Mg++, K+ and Na+), and diffusible anions (No3−, SO4−−, H2PO4−, Cl−) and non-volatile organic acids were found. An almost 1:1 ratio occurred between the levels of bound calcium and magnesium, and oxalate and uronic acids. This points to the fact that in the tomato plant the indiffusible anions are mainly oxalate and pectate. Approximately equivalent values were found for the alkalinity of the ash, and organic anions (total organic acids including oxalate, and uronic acids). The influence of nitrate, urea, and ammonium nitrogen nutrition on the cation-anion balance and the organic acid content of the plant has been considered and the effects of these different nitrogen forms on both the pH of the plant and the nutrient medium and its consequences discussed. 相似文献
10.
We examined nitrate assimilation and root gas fluxes in a wild-type barley ( Hordeum vulgare L. cv Steptoe), a mutant ( nar1a) deficient in NADH nitrate reductase, and a mutant ( nar1a; nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO 3− induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a; nar7w mutant. These results indicate that, under NH 4+ nutrition, 14% of root carbon catabolism is coupled to NH 4+ absorption and assimilation and that, under NO 3− nutrition, 5% of root carbon catabolism is coupled to NO 3− absorption, 15% to NO 3− assimilation, and 3% to NH 4+ assimilation. The additional energy requirements of NO 3− assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH 4+ and NO 3− absorption and assimilation constitute a significant portion of root respiration. 相似文献
11.
Chloride or nitrate decreased a pH gradient (measured as [ 14C]methylamine accumulation) in tonoplast-enriched vesicles. The ΔpH decrease was dependent on the anion concentration. These effects are independent of the anion-sensitive H +-ATPase of the tonoplast, since the pH gradient (acid inside) was imposed artificially using a pH jump or a K + gradient and nigericin. 4,4′-Diisothiocyano-2,2′-stilbene disulfonic acid partially prevented the decrease in pH gradient induced by Cl −. Two possible models to account for this anion-dependent decrease of ΔpH are: (a) H + loss is accompanied by Cl − or NO 3− efflux from the vesicles via H +/anion symport systems on the tonoplast and (b) H + loss is accompanied by Cl − or NO 3− uptake into the vesicles via H +/anion antiport systems. Depending on the requirements and conditions of the cell, these two systems would serve to either mobilize Cl − and NO 3− stored in the vacuole for use in the cytoplasm or to drive anions into the vacuole. Chloride or nitrate also decreased a pH gradient in fractions containing plasma membrane and Golgi, implying that these membranes may have similar H +-coupled anion transport systems. 相似文献
12.
Soybeans ( Glycine max L. Merr., cv Kingsoy) were grown on media containing NO 3− or urea. The enrichments of shoots in K +, NO 3−, and total reduced N (N r), relative to that in Ca 2+, were compared to the ratios K +/Ca 2+,NO 3−/Ca 2+, and N r/Ca 2+ in the xylem saps, to estimate the cycling of K +, and N r. The net production of carboxylates (R −) was estimated from the difference between the sums of the main cations and inorganic anions. The estimate for shoots was compared to the theoretical production of R − associated with NO 3− assimilation in these organs, and the difference was attributed to export of R − to roots. The net exchange rates of H + and OH − between the medium and roots were monitored. The shoots were the site of more than 90% of total NO 3− reduction, and N r was cycling through the plants at a high rate. Alkalinization of the medium by NO 3−-fed plants was interrupted by stem girdling, and not restored by glucose addition to the medium. It was concluded that the majority of the base excreted in NO 3− medium originated from R − produced in the shoots, and transported to the roots together with K +. As expected, cycling of K + and reduced N was favoured by NO 3− nutrition as compared to urea nutrition. 相似文献
13.
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
14.
Glutamine is the first major organic product of assimilation of 13NH 4+ by tobacco ( Nicotiana tabacum L. cv. Xanthi) cells cultured on nitrate, urea, or ammonium succinate as the sole source of nitrogen, and of 13NO 3− by tobacco cells cultured on nitrate. The percentage of organic 13N in glutamate, and subsequently, alanine, increases with increasing periods of assimilation. 13NO 3−, used for the first time in a study of assimilation of nitrogen, was purified by new preparative techniques. During pulse-chase experiments, there is a decrease in the percentage of 13N in glutamine, and a concomitant increase in the percentage of 13N in glutamate and alanine. Methionine sulfoximine inhibits the incorporation of 13N from 13NH 4+ into glutamine more extensively than it inhibits the incorporation of 13N into glutamate, with cells grown on any of the three sources of nitrogen. Azaserine inhibits glutamate synthesis extensively when 13NH 4+ is fed to cells cultured on nitrate. These results indicate that the major route for assimilation of 13NH 4+ is the glutamine synthetase-glutamate synthase pathway, and that glutamate dehydrogenase also plays a role, but a minor one. Methionine sulfoximine inhibits the incorporation of 13N from 13NO 3− into glutamate more strongly than it inhibits the incorporation of 13N into glutamine, suggesting that the assimilation of 13NH 4+ derived from 13NO 3− may be mediated solely by the glutamine synthetase-glutamate synthase pathway. 相似文献
15.
Evidence is presented that chlorate is an extremely good analog for nitrate during nitrate uptake by intact barley ( Hordeum vulgare cv. Fergus) roots. The depletion of ClO 3− or NO 3− from uptake media over 2 to 6 hours by seedlings was found to be dependent on combined NO 3− plus ClO 3− concentrations, and total anion uptake was equivalent at different NO 3−/ClO 3− ratios. After loading barley seedlings with 36ClO 3− for 6 hours, kinetic parameters were derived from the analysis of efflux of [ 36Cl] chlorate into unlabeled solution. On the basis of this analysis, the half times for exchange for the cytoplasmic and vacuolar phases were 17 minutes and 20 hours, respectively. 相似文献
16.
Unlike many plants reported in the literature, lupins do notexcrete OH - in amounts equivalent to the net excess of inorganicanion uptake over inorganic cation uptake. To investigate themechanisms involved in the maintenance of charge balance, nutrientuptake and organic anion accumulation of lupins and peas suppliedwith a range of NO -3 concentrations, were compared. Lupins absorbed less NO -3 than peas on a dry weight basis, whichlargely accounted for the smaller excess of anion uptake overcation uptake in lupins than in peas at the same NO -3 supply.When anion uptake exceeded cation uptake, peas excreted an equivalentcharge of OH -, whereas lupins excreted much smaller amountsof OH - than the excess of anion over cation uptake. It was calculatedthat lupins excreted significant amounts of organic anions whenanion uptake exceeded cation uptake, whereas organic anion excretionfrom peas was negligible, regardless of their NO -3 supply andcation-anion balance. In this study, organic anion excretion was measured from lupinroots grown in near-sterile conditions while supplied with NO -3at 0, 500 and 2000 µM. Although complete sterility wasnot achieved, there was close agreement between the organicanion excreted and the excess anion over cation uptake. Copyright1994, 1999 Academic Press Lupinus angustifolius L., Pisum sativum L., organic acid, nutrient uptake 相似文献
18.
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
19.
The comparative induction of nitrate reductase (NR) by ambient NO 3− and NO 2− as a function of influx, reduction (as NR was induced) and accumulation in detached leaves of 8-day-old barley ( Hordeum valgare L.) seedlings was determined. The dynamic interaction of NO 3− influx, reduction and accumulation on NR induction was shown. The activity of NR, as it was induced, influenced its further induction by affecting the internal concentration of NO 3−. As the ambient concentration of NO 3− increased, the relative influences imposed by influx and reduction on NO 3− accumulation changed with influx becoming a more predominant regulant. Significant levels of NO 3− accumulated in NO 2−-fed leaves. When the leaves were supplied cycloheximide or tungstate along with NO 2−, about 60% more NO 3− accumulated in the leaves than in the absence of the inhibitors. In NO 3−-supplied leaves NR induction was observed at an ambient concentration of as low as 0.02 m m. No NR induction occurred in leaves supplied with NO 2− until the ambient NO 2− concentration was 0.5 m m. In fact, NR induction from NO 2− solutions was not seen until NO 3− was detected in the leaves. The amount of NO 3− accumulating in NO 2−-fed leaves induced similar levels of NR as did equivalent amounts of NO 3− accumulating from NO 3−-fed leaves. In all cases the internal concentration of NO 3−, but not NO 2−, was highly correlated with the amount of NR induced. The evidence indicated that NO 3− was a more likely inducer of NR than was NO 2−. 相似文献
20.
The influence of nitrogen stress on net nitrate uptake resulting from concomitant 15NO 3− influx and 14NO 3− efflux was examined in two 12-day-old inbred lines of maize. Plants grown on 14NO 3− were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate containing 98.7 atom% 15N. The nitrate concentration of the roots declined from approximately 100 to 5 micromolar per gram fresh weight during deprivation, and 14NO 3− efflux was linearly related to root nitrate concentration. Influx of 15NO 3− was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in 15NO 3− influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied. 相似文献
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