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1.
A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants ( Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar NO 3− (low N or high N conditions, respectively), were provided with 200 micromolar NO 3− or ClO 3− during the uptake period. Initial rates of NO 3− uptake were several times higher in low N plants than in high N plants. Within 10 min, uptake in the former plants declined to a new steady rate which was sustained for the remainder of the experiment. No such time-dependent changes were evident in the high N plants. Rates and patterns of net chlorate uptake exhibited almost identical dependence upon previous nitrate provision. NO 3− ( 36ClO 3−) influx, by contrast, appeared to be independent of NO 3− pretreatment prior to influx determination. Nitrate efflux, estimated by several different methods, was strongly correlated with internal nitrate concentration of the roots. 相似文献
2.
Effects of NO 2−, ClO 3−, and ClO 2− on the induction of nitrate transport and nitrate reductase activity (NRA) as well as their effects on NO 3− influx into roots of intact barley ( Hordeum vulgare cv Klondike) seedlings were investigated. A 24-h pretreatment with 0.1 mol m −3 NO 2− fully induced NO 3− transport but failed to induce NRA. Similar pretreatments with ClO 3− and ClO 2− induced neither NO 3− transport nor NRA. Net ClO 3− uptake was induced by NO 3− but not by ClO 3− itself, indicating that NO 3− and ClO 3− transport occur via the NO 3− carrier. At the uptake step, NO 2− and ClO 2− strongly inhibited NO 3− influx; the former exhibited classical competitive kinetics, whereas the latter exhibited complex mixed-type kinetics. ClO 3− proved to be a weak inhibitor of NO 3− influx ( Ki = 16 mol m −3) in a noncompetitive manner. The implications of these findings are discussed in the context of the suitability of these NO 3− analogs as screening agents for the isolation of mutants defective in NO 3− transport. 相似文献
3.
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
4.
The nature of the injury and recovery of nitrate uptake (net uptake) from NaCl stress in young barley ( Hordeum vulgare L, var CM 72) seedlings was investigated. Nitrate uptake was inhibited rapidly by NaCl, within 1 minute after exposure to 200 millimolar NaCl. The duration of exposure to saline conditions determined the time of recovery of NO 3− uptake from NaCl stress. Recovery was dependent on the presence of NO 3− and was inhibited by cycloheximide, 6-methylpurine, and cerulenin, respective inhibitors of protein, RNA, and sterol/fatty acid synthesis. These inhibitors also prevented the induction of the NO 3− uptake system in uninduced seedlings. Uninduced seedlings exhibited endogenous NO 3− transport activity that appeared to be constitutive. This constitutive activity was also inhibited by NaCl. Recovery of constitutive NO 3− uptake did not require the presence of NO 3−. 相似文献
5.
In vivo NO 3− reduction in roots and shoots of intact barley ( Hordeum vulgare L. var Numar) seedlings was estimated in light and darkness. Seedlings were placed in darkness for 24 hours to make them carbohydrate-deficient. During darkness, the leaves lost 75% of their soluble carbohydrates, whereas the roots lost only 15%. Detached leaves from these plants reduced only 7% of the NO 3− absorbed in darkness. By contrast, detached roots from the seedlings reduced the same proportion of absorbed NO 3−, as did roots from normal light-grown plants. The rate of NO 3− reduction in the roots accounted for that found in the intact dark-treated carbohydrate-deficient seedlings. The rates of NO 3− reduction in roots of intact plants were the same for approximately 12 hours, both in light and darkness, after which the NO 3− reduction rate in roots of plants placed in darkness slowly declined. In the dark, approximately 40% of the NO 3− reduction occurred in the roots, whereas in light only 20% of the total NO 3− reduction occurred in roots. A lesser proportion was reduced in roots because the leaves reduced more nitrate in light than in darkness. 相似文献
6.
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
7.
Nitrate and NO 2− transport by roots of 8-day-old uninduced and induced intact barley ( Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO 3− and NO 2− was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO 3− and NO 2−. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO 3− and NO 2− uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO 3− and NO 2− uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO 3− uptake ranged from 34 to 36 micromolar and of NO 2− uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO 3− and NO 2− inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO 3− was a more effective inhibitor than NO 2−. Nitrate and NO 2− transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited. 相似文献
8.
Nitrate uptake of Na + -deficient Amaranthus tricolor L. cv Tricolor seedlings from complete culture solution was stimulated by about 210% within 5 hours by application of 0.5 millimolar NaCl. From a Na + -preloading experiment, intracellular Na + was shown to be responsible for the stimulation of NO 3− uptake. The results suggest a possible role of Na + in NO 3− uptake in C 4 plants. 相似文献
9.
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
10.
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
11.
An experiment was conducted to investigate the relative changes in NO 3− assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants ( Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar 15NO 3− for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of 15NO 3− decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous 15NO 3− assimilation processes, which were distinctly different than those in 15NO 3− uptake. During the extended dark period, translocation of absorbed 15N out of the root to the shoot varied rhythmically. The adjustments were independent of 15NO 3− uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of 15NO 3− always was limited more than uptake. The assimilation of 15N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. The plants clearly retained a capacity for 15NO 3− reduction and synthesis of insoluble reduced- 15N even when 15NO 3− uptake was severely restricted and minimal carbohydrate reserves remained in the tissue. 相似文献
12.
Membrane associated nitrate reductase (NR) was detected in plasma membrane (PM) fractions isolated by aqueous two-phase partitioning from barley ( Hordeum vulgare L. var CM 72) roots. The PM associated NR was not removed by washing vesicles with 500 millimolar NaCl and 1 millimolar EDTA and represented up to 4% of the total root NR activity. PM associated NR was stimulated up to 20-fold by Triton X-100 whereas soluble NR was only increased 1.7-fold. The latency was a function of the solubilization of NR from the membrane. NR, solubilized from the PM fraction by Triton X-100 was inactivated by antiserum to Chlorella sorokiniana NR. Anti-NR immunoglobulin G fragments purified from the anti-NR serum inhibited NO 3− uptake by more than 90% but had no effect on NO 2− uptake. The inhibitory effect was only partially reversible; uptake recovered to 50% of the control after thorough rinsing of roots. Preimmune serum immunoglobulin G fragments inhibited NO 3− uptake 36% but the effect was completely reversible by rinsing. Intact NR antiserum had no effect on NO 3− uptake. The results present the possibility that NO 3− uptake and NO 3− reduction in the PM of barley roots may be related. 相似文献
13.
In soybean ( Glycine max L. Merr. cv Kingsoy), NO 3− assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO 3− uptake by roots. The net NO 3− uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO 3− uptake and C excretion rates by roots. These results suggest that NO 3− uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO 3− uptake, and excretion of HCO 3− ions was probably released by malate decarboxylation. NO 3− uptake rate increased when the supply of NO 3− to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO 42−. We conclude that in situ, NO 3− reduction rate in the shoot may control NO 3− uptake rate in the roots via the translocation rate of malate in the phloem. 相似文献
15.
Using 13NO 3−, effects of various NO 3− pretreatments upon NO 3− influx were studied in intact roots of barley ( Hordeum vulgare L. cv Klondike). Prior exposure of roots to NO 3− increased NO 3− influx and net NO 3− uptake. This `induction' of NO 3− uptake was dependent both on time and external NO 3− concentration ([NO 3−]). During induction influx was positively correlated with root [NO 3−]. In the postinduction period, however, NO 3− influx declined as root [NO 3−] increased. It is suggested that induction and negative feedback regulation are independent processes: Induction appears to depend upon some critical cytoplasmic [NO 3−]; removal of external NO 3− caused a reduction of 13NO 3− influx even though mean root [NO 3−] remained high. It is proposed that cytoplasmic [NO 3−] is depleted rapidly under these conditions resulting in `deinduction' of the NO 3− transport system. Beyond 50 micromoles per gram [NO 3−], 13NO 3− influx was negatively correlated with root [NO 3−]. However, it is unclear whether root [NO 3−] per se or some product(s) of NO 3− assimilation are responsible for the negative feedback effects. 相似文献
16.
Short-term (10 minutes) measurements of plasmalemma NO 3− influx ( oc) into roots of intact barley plants were obtained using 13NO 3−. In plants grown for 4 days at various NO 3− levels (0.1, 0.2, 0.5 millimolar), oc was found to be independent of the level of NO 3− pretreatment. Similarly, pretreatment with Cl − had no effect upon plasmalemma 13NO 3− influx. Plants grown in the complete absence of 13NO 3− (in CaSO 4 solutions) subsequently revealed influx values which were more than 50% lower than for plants grown in NO 3−. Based upon the documented effects of NO 3− or Cl − pretreatments on net uptake of NO 3−, these observations suggest that negative feedback from vacuolar NO 3− and/or Cl − acts at the tonoplast but not at the plasmalemma. When included in the influx medium, 0.5 millimolar Cl − was without effect upon 13NO 3− influx, but NH 4+ caused approximately 50% reduction of influx at this concentration. 相似文献
17.
The oxidation of NH 4+ by Nitrosomonas europaea was insensitive to 10 mM NaClO 3 (sodium chlorate) but was strongly inhibited by NaClO 2 (sodium chlorite; Ki, 2 μM). The oxidation of NO 2− by Nitrobacter winogradskyi was inhibited by both ClO 3− and ClO 2− ( Ki for ClO 2−, 100 μM). N. winogradskyi reduced ClO 3− to ClO 2− under both aerobic and anaerobic conditions, and as much as 0.25 mM ClO 2− was detected in the culture filtrate. In mixed N. europaea-N. winogradskyi cell suspensions, the oxidation of both NH 4+ and NO 2− was inhibited in the presence of 10 mM ClO 3− after a 2-h lag period, despite the fact that, under these conditions, ClO 2− was not detected in the filtrate. The data are consistent with the hypothesis that, in mixed culture, NH 4+ oxidation is inhibited by ClO 2− produced by reduction of ClO 3− by the NO 2− oxidizer. The use of ClO 3− inhibition of NO 2− oxidation in assays of nitrification by mixed populations necessitates cautious interpretation unless it can be shown that the oxidation of NH 4+ is not affected. 相似文献
18.
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
19.
The effects of CO 2-limited photosynthesis on 15NO 3− uptake and reduction by maize ( Zea mays, DeKalb XL-45) seedlings were examined in relation to concurrent effects of CO 2 stress on carbohydrate levels and in vitro nitrate reductase activities. During a 10-hour period in CO 2-depleted air (30 microliters of CO 2/ per liter), cumulative 15NO 3− uptake and reduction were restricted 22 and 82%, respectively, relative to control seedlings exposed to ambient air containing 450 microliters of CO 2 per liter. The comparable values for roots of decapitated maize seedlings, the shoots of which had previously been subjected to CO 2 stress, were 30 and 42%. The results demonstrate that reduction of entering nitrate by roots as well as shoots was regulated by concurrent photosynthesis. Although in vitro nitrate reductase activity of both tissues declined by 60% during a 10-hour period of CO 2 stress, the remaining activity was greatly in excess of that required to catalyze the measured rate of 15NO 3− reduction. Root respiration and soluble carbohydrate levels in root tissue were also decreased by CO 2 stress. Collectively, the results indicate that nitrate uptake and reduction were regulated by the supply of energy and carbon skeletons required to support these processes, rather than by the potential enzymatic capacity to catalyze nitrate reduction, as measured by in vitro nitrate reductase activity. 相似文献
20.
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
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