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1.
The effect of NaCl and Na 2SO 4 salinity on NO 3− assimilation in young barley ( Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO 3− transporter was affected very little; the major effect of the salts was on its activity. Both Cl − and SO 42− salts severely inhibited uptake of NO 3−. When compared on the basis of osmolality of the uptake solutions, Cl − salts were more inhibitory (15-30%) than SO 42− salts. At equal concentrations, SO 42− salts inhibited NO 3− uptake 30 to 40% more than did Cl − salts. The absolute concentrations of each ion seemed more important as inhibitors of NO 3− uptake than did the osmolality of the uptake solutions. Both K + and Na + salts inhibited NO 3− uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity. Unlike NO3− uptake, NO3− reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO3− in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO3− reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO3− transporter, rather than that of the NO3− reduction system, may be a critical factor to plant survival during salt stress. 相似文献
2.
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
3.
At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na +. The rate of photosynthesis greatly exceeded the CO 2 supply rate and indicated that HCO 3− was taken up by a Na +-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na + and exceeded the CO 2 supply rate by 8 to 45 times. The apparent photosynthetic affinity ( K½) for DIC was high (6-40 micromolar) and was not markedly affected by Na + concentration, whereas with air-grown cells K½ (DIC) decreased by more than an order of magnitude in the presence of Na +. Lithium, which inhibited Na +-dependent HCO 3− uptake in air-grown cells, had little effect on Na +-independent HCO 3− uptake by standing culture cells. A component of total HCO 3− uptake in standing culture cells was also Na +-dependent with a K½ (Na +) of 4.8 millimolar and was inhibited by lithium. Analysis of 14C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na +-independent CO 2 transport system. The conversion from Na +-independent to Na +-dependent HCO 3− uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO 3− for subsequent photosynthetic fixation. 相似文献
4.
Susceptible corn roots exposed to the host-selective toxin of Helminthosporium carbonum took up and retained more NO 3−, Na +, Cl −, 3-o-methylglucose, and leucine than did control roots. Stimulatory effects on uptake were more pronounced with freshly cut roots than with roots that were washed and aged. Solutes were accumulated against a concentration gradient, and toxin-treated tissues developed a steeper gradient than did control tissues. Toxin affected both the low and high affinity uptake systems for Na + and Cl −. Toxin did not affect uptake of Na 2−, K +, Ca 2+, phosphate ion (H 2PO 4− and HPO 4−), SO 4−, and glutamic acid. No toxin-induced leakage of any solute tested was detected within 5 to 6 hr after initial exposure to toxin. The data suggest that toxin from H. carbonum does not cause the general plasma membrane derangement caused by other host-selective toxins. Instead, H. carbonum toxin may cause specific changes in characteristics of the plasmalemma, which result in increased uptake of certain solutes. 相似文献
5.
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
6.
Week-old wheat seedlings absorbed at least 40% NO 3− from NaNO 3 when preloaded with K + than when preloaded with Na + or Ca 2+. Cultures of Triticum vulgare L. cv. Arthur were grown for 5 days on 0.2 m m CaSO 4, pretreated for 48 hours with either 1 m m CaSO 4, K 2SO 4, or Na 2SO 4, and then transferred to 1 m m NaNO 3. All solutions contained 0.2 m m CaSO 4. Shoots of K +-preloaded plants accumulated three times more NO 3− than shoots of the other two treatments. Initially, the K +-preloaded plants contained 10-fold more malate than either Na +- or Ca 2+-preloaded seedlings. During the 48-hour treatment with NaNO 3, malate in both roots and shoots of the K +-preloaded seedlings decreased. Seedlings preloaded with K + reduced 25% more NO 3− than those preloaded with either Na + or Ca 2+. These experiments indicate that K + enhanced NO 3− uptake and reduction even though the absorption of K + and NO 3− were separated in time. Xylem exudate of K +-pretreated plants contained roughly equivalent concentrations of K + and NO 3−, but exudate from Na + and Ca 2+-pretreated plants contained two to four times more NO 3− than K +. Therefore K + is not an obligatory counterion for NO 3− transport in xylem. 相似文献
7.
The influence of the allelopathic compound ferulic acid (FA) on nitrogen uptake from solutions containing both NO 3− and NH 4+ was examined in 8-day-old nitrogen-depleted corn ( Zea mays L.) seedlings. Concurrent effects on uptake of Cl − and K + also were assessed. The presence of 250 micromolar FA inhibited the initial (0-1 hours) rate of NO 3− uptake and also prevented development of the NO 3−-inducible accelerated rate. The pattern of recovery when FA was removed was interpreted as indicating a rapid relief of FA-restricted NO 3− uptake activity, followed by a reinitiation of the induction of that activity. No inhibition of NO 3− reduction was detected. Ammonium uptake was less sensitive than NO 3− uptake to inhibition by FA. An inhibition of Cl − uptake occurred as induction of the NO 3− transport system developed in the absence of FA. Alterations of Cl − uptake in the presence of FA were, therefore, a result of a beneficial effect, because NO 3− uptake was restricted, and a direct inhibitory effect. The presence of FA increased the initial net K + loss from the roots during exposure to the low K, ammonium nitrate uptake solution and delayed the recovery to positive net uptake, but it did not alter the general pattern of the response. The implications of the observations are discussed for growth of plants under natural conditions and cultural practices that foster periodic accumulation of allelopathic substances. 相似文献
8.
The effect of exogenous NH 4+ on NO 3− uptake and in vivo NO 3− reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO 3− uptake. Pretreatment with NH 4Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO 3− uptake. When NH4+ was given at the start of NO3− nutrition, the time pattern of NO3− uptake was the same as in plants receiving no NH4+. After 6 hours, however, the NO3− uptake rate (NUR) and root NRA were inhibited by NH4+ to a maximum of 45% and 60%, respectively. The response of the NUR of NO3−-induced plants depended on the NH4Cl concentration. Below 1 millimolar NH4+, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH4+ at 2 millimolar and above caused a rapid and transient stimulation of NO3− uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3−-induced plants, root NRA responded less and more slowly to NH4+ than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH4+ inhibition of the NUR of NO3−-induced plants. We conclude that repression of the NUR by NH4+ depends on NH4+ assimilation. The repression by NH4+ was least at the lowest and highest NH4+ levels tested (0.04 and 25 millimolar). 相似文献
9.
The effect of NO 3
− on intracellular pH (pH i) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO 3
− induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO 3
−- induced pH change was not due to modulation of Na +/H + exchange, since it was also observed in Na +/H + antiport-deficient mutants. Though NO 3
− is known to inhibit vacuolar-type (V) H +-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A 1. NO 3
−/HCO 3
− exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO 3
−, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B.S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448– 11454). Accumulation of intracellular NO 3
−, measured by the Greiss method after reduction to NO 2
−, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO 3
− with H + (or the equivalent counter-transport of NO 3
− for OH −). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO 3
− and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO 3
− and was accentuated by acidification of the extracellular space. The putative NO 3
−-H + cotransport was inhibited markedly by ethacrynic acid and by α-cyano-4-hydroxycinnamate, but only marginally by 4,4′-diisothiocyanostilbene-2,2′ disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO 3
−-induced pH changes in mammalian cells may be related, though not identical, to the NO 3
−-H + cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO 3
− with H + the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral. 相似文献
10.
Net uptakes of K + and NO 3− were monitored simultaneously and continuously for two barley ( Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K + and NO 3− uptake in the absence of NH 4+ or Cl −. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH 4+, K +, Cl −, or NO 3− affected the cultivars very differently: (a) the presence of NH 4+ as NH 4Cl stimulated net NO 3− uptake in Prato barley but inhibited net NO 3− uptake in Olli barley; (b) Cl − inhibited net NO 3− uptake in Prato but had little effect in Olli; and (c) NH 4+ as (NH 4) 2SO 4 inhibited net K + uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl − initially inhibited net K + uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH 4+ and Cl − uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption. 相似文献
11.
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
12.
We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO 3
−), ammonium (NH 4
+), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L −1 for NO 3
− and NH 4
+, 0.5–50 μmol N L −1 for urea, 0.5–100 μmol N L −1 for glu and cyanate, and 0.5–200 μmol N L −1 for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μ m, 1.51±0.06 d −1 and 1.50±0.05 d −1, respectively). However, cultures grown on cyanate, NO 3
−, and NH 4
+ had lower half saturation constants (K μ, 0.28–0.51 μmol N L −1). N uptake kinetics were measured in NO 3
−-deplete and -replete batch cultures of P. donghaiense. In NO 3
−-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO 3
−, NH 4
+, urea and algal amino acids; uptake was saturated at or below 50 μmol N L −1. In NO 3
−-replete batch cultures, NH 4
+, urea, and algal amino acid uptake kinetics were similar to those measured in NO 3
−-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth. 相似文献
13.
Tomato plants ( Lycopersicon esculentum L. var. Ailsa Craig) were grown in water culture in nutrient solution in a series of 10 increasing levels of nitrate nutrition. Using whole plant data derived from analytical and yield data of individual plant parts, the fate of anion charge arising from increased NO 3 assimilation was followed in its distribution between organic anion accumulation in the plant and OH − efflux into the nutrient solution as calculated by excess anion over cation uptake. With increasing NO 3 nutrition the bulk of the anion charge appeared as organic anion accumulation in the plants. OH − efflux at a maximum accounted for only 20% of the anion charge shift. The major organic anion accumulated in response to nitrate assimilation was malate. The increase in organic anion accumulation was paralleled by an increase in cation concentration (K +, Ca 2+, Mg 2+, Na +). Total inorganic anion levels (NO 3−, SO 42−, H 2PO 4−, Cl −) were relatively constant. The effect of increasing NO 3 nutrition in stimulating organic anion accumulation was much more pronounced in the tops than in the roots. 相似文献
14.
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
15.
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
16.
Potassium ions at low concentrations stimulate cytokinin-dependent betacyanin synthesis in Amaranthus tricolor seedlings more than other alkali metal ions when tested as the chloride salts. The sequence of relative stimulation is K + > Rb + > (Na + = Li +). Calcium and Mg 2+ ions are inhibitory at concentrations > 1 millimolar when tested as chlorides. Anions also have an effect on the degree of alkali metal stimulation in the order PO 43− > NO 3− > Cl −. The high activity of phosphate may be partly due to its chelating effect on inhibitory Ca 2+ ions, or to effects on K + uptake. A mixture of Na + and K + in the presence of phosphate is more effective than either cation alone. This result may be due either to effects on tyrosine transport or on the potassium uptake system. Phytochrome-dependent betacyanin synthesis shows the same stimulation by Na + plus K +. The effect of a number of inhibitors of transport systems on betacyanin accumulation is reported. The possible role of the ionic environment of cells in their metabolic regulation is discussed, particularly in relation to cytokinin action. 相似文献
17.
Assimilation of NO 3− and NH 4+ by perennial ryegrass ( Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH 4+, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO 3− had been reduced, whereas 97% of the NH 4+ had been assimilated. During the early stages (0 to 8 hours) of NO 3− uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO 3−, NH 4+ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO 3− uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate. 相似文献
18.
Transcellular Cl − movement across acinar cells is the rate-limiting step for salivary gland fluid secretion. Basolateral Nkcc1 Na +-K +-2Cl − cotransporters play a critical role in fluid secretion by promoting the intracellular accumulation of Cl − above its equilibrium potential. However, salivation is only partially abolished in the absence of Nkcc1 cotransporter activity, suggesting that another Cl − uptake pathway concentrates Cl − ions in acinar cells. To identify alternative molecular mechanisms, we studied mice lacking Ae2 and Ae4 Cl −/HCO 3− exchangers. We found that salivation stimulated by muscarinic and β-adrenergic receptor agonists was normal in the submandibular glands of Ae2−/− mice. In contrast, saliva secretion was reduced by 35% in Ae4−/− mice. The decrease in salivation was not related to loss of Na +-K +-2Cl − cotransporter or Na +/H + exchanger activity in Ae4−/− mice but correlated with reduced Cl − uptake during β-adrenergic receptor activation of cAMP signaling. Direct measurements of Cl −/HCO 3− exchanger activity revealed that HCO 3−-dependent Cl − uptake was reduced in the acinar cells of Ae2−/− and Ae4−/− mice. Moreover, Cl −/HCO 3− exchanger activity was nearly abolished in double Ae4/Ae2 knock-out mice, suggesting that most of the Cl −/HCO 3− exchanger activity in submandibular acinar cells depends on Ae2 and Ae4 expression. In conclusion, both Ae2 and Ae4 anion exchangers are functionally expressed in submandibular acinar cells; however, only Ae4 expression appears to be important for cAMP-dependent regulation of fluid secretion. 相似文献
19.
The product of pxcA (formerly known as cotA) is involved in light-induced Na +-dependent proton extrusion. In the presence of 2,5-dimethyl- p-benzoquinone, net proton extrusion by Synechocystis sp. strain PCC6803 ceased after 1 min of illumination and a postillumination influx of protons was observed, suggesting that the PxcA-dependent, light-dependent proton extrusion equilibrates with a light-independent influx of protons. A photosystem I (PS I) deletion mutant extruded a large number of protons in the light. Thus, PS II-dependent electron transfer and proton translocation are major factors in light-driven proton extrusion, presumably mediated by ATP synthesis. Inhibition of CO 2 fixation by glyceraldehyde in a cytochrome c oxidase (COX) deletion mutant strongly inhibited the proton extrusion. Leakage of PS II-generated electrons to oxygen via COX appears to be required for proton extrusion when CO 2 fixation is inhibited. At pH 8.0, NO 3− uptake activity was very low in the pxcA mutant at low [Na +] (~100 μM). At pH 6.5, the pxcA strain did not take up CO 2 or NO 3− at low [Na +] and showed very low CO 2 uptake activity even at 15 mM Na +. A possible role of PxcA-dependent proton exchange in charge and pH homeostasis during uptake of CO 2, HCO 3−, and NO 3− is discussed. 相似文献
20.
The effect of the exogenous and endogenous NO 3− concentration on net uptake, influx, and efflux of NO 3− and on nitrate reductase activity (NRA) in roots was studied in Phaseolus vulgaris L. cv. Witte Krombek. After exposure to NO 3−, an apparent induction period of about 6 hours occurred regardless of the exogenous NO 3− level. A double reciprocal plot of the net uptake rate of induced plants versus exogenous NO 3− concentration yielded four distinct phases, each with simple Michaelis-Menten kinetics, and separated by sharp breaks at about 45, 80, and 480 micromoles per cubic decimeter. Influx was estimated as the accumulation of 15N after 1 hour exposure to 15NO3−. The isotherms for influx and net uptake were similar and corresponded to those for alkali cations and Cl−. Efflux of NO3− was a constant proportion of net uptake during initial NO3− supply and increased with exogenous NO3− concentration. No efflux occurred to a NO3−-free medium. The net uptake rate was negatively correlated with the NO3− content of roots. Nitrate efflux, but not influx, was influenced by endogenous NO3−. Variations between experiments, e.g. in NO3− status, affected the values of Km and Vmax in the various concentration phases. The concentrations at which phase transitions occurred, however, were constant both for influx and net uptake. The findings corroborate the contention that separate sites are responsible for uptake and transitions between phases. Beyond 100 micromoles per cubic decimeter, root NRA was not affected by exogenous NO3− indicating that NO3− uptake was not coupled to root NRA, at least not at high concentrations. 相似文献
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