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1.
Nitrate reduction was studied as a function of carbohydrate concentration in detached primary leaves of barley ( Hordeum vulgare L. cv Numar) seedlings under aerobic conditions in light and darkness. Seedlings were grown either in continuous light for 8 days or under a regimen of 16-hour light and 8-hour dark for 8 to 15 days. Leaves of 8-day-old seedlings grown in continuous light accumulated 4 times more carbohydrates than leaves of plants grown under a light and dark regimen. When detached leaves from these seedlings were supplied with NO 3− in darkness, those with the higher levels of carbohydrates reduced a greater proportion of the NO 3− that was taken up. In darkness, added glucose increased the percentage of NO 3− reduced up to 2.6-fold depending on the endogenous carbohydrate status of the leaves. Both NO 3− reduction and carbohydrate content of the leaves increased with age. Fructose and sucrose also increased NO 3− reduction in darkness to the same extent as glucose. Krebs cycle intermediates, citrate and succinate, did not increase NO 3− reduction, whereas malate slightly stimulated it in darkness. In light, 73 to 90% of the NO3− taken up was reduced by the detached leaves; therefore, an exogenous supply of glucose had little additional effect on NO3− reduction. The results indicate that in darkness the rate of NO3− reduction in primary leaves of barley depends upon the availability of carbohydrates. 相似文献
2.
The effects of several photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO 3− and NO 2− assimilation were studied using detached barley ( Hordeum vulgare L. cv Numar) leaves in which only endogenous NO 3− or NO 2− were available for reduction. Uncouplers of oxidative phosphorylation greatly increased NO 3− reduction in both light and darkness, while photosynthetic inhibitors did not. The NO2− concentration in the control leaves was very low in both light and darkness; 98% or more of the NO2− formed from NO3− was further assimilated in control leaves. More NO2− accumulated in the leaves in light and darkness in the presence of photosynthetic inhibitors. Of this NO2−, 94% or more was further assimilated. It appears that metabolites, either external or internal to the chloroplast, capable of reducing NADP (which, in turn, could reduce ferredoxin via NADP reductase) might support NO2− reduction in darkness and light when photosynthetic electron flow is inhibited by photosynthetic inhibitors. Nitrite assimilation was much more sensitive to uncouplers in darkness than in light: in darkness, 74% or more of NO2− formed from NO3− was further assimilated, whereas in light, 95% or more of the NO2− was further assimilated. 相似文献
3.
An experiment was conducted to investigate the reduction of endogenous NO 3−, which had been taken up by plants in darkness, during the course of the subsequent light period. Vegetative, nonnodulated soybean plants ( Glycine max [L]. Merrill, `Ransom') were exposed to 1.0 millimolar 15NO 3− for 12 hours in darkness and then returned to a solution containing 1.0 millimolar 14NO 3− for the 12 hours `chase' period in the light. Another set of plants was exposed to 15NO 3− during the light period to allow a direct comparison of contributions of substrate from the endogenous and exogenous sources. At the end of the 15NO 3− exposure in the dark, 70% of the absorbed 15NO 3− remained unreduced, and 83% of this unreduced NO 3− was retained in roots. The pool of endogenous 15NO 3− in roots was depleted at a steady rate during the initial 9 hours of light and was utilized almost exclusively in the formation of insoluble reduced-N in leaves. Unlabeled endogenous NO 3−, which had accumulated in the root prior to the previous dark period, also was depleted in the light. When exogenous 15NO 3− was supplied during the light period, the rate of assimilation progressively increased, reflecting an increased rate of uptake and decreased accumulation of NO 3− in the root tissue. The dark-absorbed endogenous NO 3− in the root was the primary source of substrate for whole-plant NO 3− reduction in the first 6 hours of the light period, and exogenous NO 3− was the primary source of substrate thereafter. It is concluded that retention of NO 3− in roots in darkness and its release in the following light period is an important whole-plant regulatory mechanism which serves to coordinate delivery of substrate with the maximal potential for NO 3− assimilation in photosynthetic tissues. 相似文献
4.
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
5.
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
6.
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
7.
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
8.
Soybean ( Glycine max [L.] Merr.) seeds were imbibed and germinated with or without NO 3−, tungstate, and norflurazon (San 9789). Norflurazon is a herbicide which causes photobleaching of chlorophyll by inhibiting carotenoid synthesis and which impairs normal chloroplast development. After 3 days in the dark, seedlings were placed in white light to induce extractable nitrate reductase activity. The induction of maximal nitrate reductase activity in greening cotyledons did not require NO 3− and was not inhibited by tungstate. Induction of nitrate reductase activity in norflurazon-treated cotyledons had an absolute requirement for NO 3− and was completely inhibited by tungstate. Nitrate was not detected in seeds or seedlings which had not been treated with NO 3−. The optimum pH for cotyledon nitrate reductase activity from norflurazon-treated seedlings was at pH 7.5, and near that for root nitrate reductase activity, whereas the optimum pH for nitrate reductase activity from greening cotyledons was pH 6.5. Induction of root nitrate reductase activity was also inhibited by tungstate and was dependent on the presence of NO 3−, further indicating that the isoform of nitrate reductase induced in norflurazon-treated cotyledons is the same or similar to that found in roots. Nitrate reductases with and without a NO 3− requirement for light induction appear to be present in developing leaves. In vivo kinetics (light induction and dark decay rates) and in vitro kinetics (Arrhenius energies of activation and NADH:NADPH specificities) of nitrate reductases with and without a NO 3− requirement for induction were quite different. Km values for NO 3− were identical for both nitrate reductases. 相似文献
9.
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
10.
The objectives of this study were to select and initially characterize mutants of soybean ( Glycine max L. Merr. cv Williams) with decreased ability to reduce nitrate. Selection involved a chlorate screen of approximately 12,000 seedlings (progeny of mutagenized seed) and subsequent analyses for low nitrate reductase (LNR) activity. Three lines, designated LNR-2, LNR-3, and LNR-4, were selected by this procedure. In growth chamber studies, the fully expanded first trifoliolate leaf from NO3−-grown LNR-2, LNR-3, and LNR-4 plants had approximately 50% of the wild-type NR activity. Leaves from urea-grown LNR-2, LNR-3, and LNR-4 plants had no NR activity while leaves from comparable wild-type plants had considerable activity; the latter activity does not require the presence of NO3− in the nutrient solution for induction and on this basis is tentatively considered as a constitutive enzyme. Summation of constitutive (urea-grown wild-type plants) and inducible (NO3−-grown LNR-2, LNR-3, or LNR-4 plants) leaf NR activities approximated activity in leaves of NO3−-grown wild-type plants. Root NR activities were comparable in wild-type and mutant plants grown on NO3−, and roots of both plant types lacked constitutive NR activity when grown on urea. In both growth chamber- and field-grown plants, oxides of nitrogen [NO(x)] were evolved from young leaves of wild-type plants, but not from leaves of LNR-2 plants, during in vivo NR assays. Analysis of leaves from different canopy locations showed that constitutive NR activity was confined to the youngest three fully expanded leaves of the wild-type plant and, therefore, on a total plant canopy basis, the NR activity of LNR-2 plants was approximately 75% that of wild-type plants. It is concluded that: (a) the NR activity in leaves of NO3−-grown wild-type plants includes both constitutive and inducible activity; (b) the missing NR activity in LNR-2, LNR-3, and LNR-4 leaves is the constitutive component; and (c) the constitutive NR activity is associated with NO(x) evolution and occurs only in physiologically young leaves. 相似文献
11.
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
12.
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
13.
It is unclear if the relative content of NO 3− and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO 3− in the root. Experiments were conducted to determine whether NO 3− and reduced N in xylem exudate of vegetative, nonnodulated soybean plants ( Glycine max [L.] Merr., `Ransom') originated from exogenous recently absorbed 15NO 3− or from endogenous 14N pools. Plants either were decapitated and exposed to 15NO 3− solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to 15NO 3− in the dark and in the light. Considerable amounts of 14NO 3− and reduced 14N were transported into the xylem, but almost all of the 15N was present as 15NO 3−. Dissimilar changes in transport of 14NO 3−, reduced 14N and 15NO 3− during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of 15N transport into the xylem of decapitated plants was only 21 to 36% of the 15N delivered to the shoot of intact plants. Based on the proportion of total 15N which was found as reduced 15N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed 15NO 3− was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO 3− and total reduced N in the xylem exudate. 相似文献
14.
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.
Corn seedlings ( Zea mays cv W64A × W182E) were grown hydroponically, in the presence or absence of NO 3−, with or without light and with NH 4Cl as the only N source. In agreement with earlier results nitrate reductase (NR) activity was found only in plants treated with both light and NO 3−. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by transfer of the proteins to nitrocellulose paper and reaction with antibodies prepared against a pure NR showed that crude extracts prepared from light-grown plants had a polypeptide of approximately 116 kilodaltons (the subunit size for NR) when NO 3− was present in the growth medium. Crude extracts from plants grown in the dark did not have the 116 kilodalton polypeptide, although smaller polypeptides, which reacted with NR-immunoglobulin G, were sometimes found at the gel front. When seedlings were grown on Kimpack paper or well washed sand, NR activity was again found only when the seedlings were exposed to light and NO 3−. Under these conditions, however, a protein of about 116 kilodaltons, which reacted with the NR antibody was present in light-grown plants whether NO 3− was added to the system or not. The NR antibody cross-reacting protein was also seen in hydroponically grown plants when NH 4Cl − was the only added form of nitrogen. These results indicate that the induction of an inactive NR-protein precursor in corn is mediated either by extremely low levels of NO 3− or by some other unidentified factor, and that higher levels of NO 3− are necessary for converting the inactive NR cross-reacting protein to a form of the enzyme capable of reducing NO 3− to NO 2−. 相似文献
16.
The effects of N source (6 m m nitrogen as NO 3− or urea) and tungstate (0, 100, 200, 300, and 400 μ m Na 2 WO 4) on nitrate metabolism, nodulation, and growth of soybean ( Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO 3− content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO 3−-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO 3−-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 μ m tungstate levels, acetylene reduction activity of nodulated roots of NO 3−-grown plants exceeded the activity of comparable urea-grown plants. 相似文献
17.
Potential competition between CO 2 and NO 2− photoassimilation for photogenerated reductant ( e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO 2 and/or NO 2− to produce the highest demand for reductant by CO 2 and/or NO 2− assimilatory processes (enzymes). Even in the presence of CO 2 fixation, NO 2− reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO 2 assimilation. Regardless of the magnitude of the incident light energy, CO 2 assimilation did not repress NO 2− reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO 2 as well as with inorganic nitrogen photoassimilation. 相似文献
18.
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
19.
The patterns of molybdenum (MoO 42−) absorption and transport were investigated in intact bean ( Phaseolus vulgaris L.) and rice ( Oryza sativa L. cv. I.R.8) plants. The mobility of MoO 42− absorbed by roots and by leaves was compared with that of a freely mobile element, Rb +. Although MoO 42− absorption by bean roots was nearly as high as that of Rb +, its transport to the shoot was considerably less. When MoO 42− was fed to one of the primary leaves, most of it was transported to the stem and root. Evidence obtained here showed that MoO 42− was mobile. Experiments with intact rice seedlings revealed large differences in the absorption and transport of MoO 42− between the plants grown in CaSO 4 and those in Hoagland solution. Molybdate uptake by excised rice roots was suggested to be an active process since it was greatly inhibited by a metabolic inhibitor. The presence of Mn 2+, Zn 2+, Cu 2+, CI −, or SO 42− in the absorption medium reduced MoO 42− uptake which was markedly enhanced by the presence of Fe 2+. 相似文献
20.
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
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