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1.
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
2.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
3.
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
4.
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
5.
Young bean plants ( Phaseolus vulgaris L. var Saxa) were fed with 3.5 or 10 millimolar N in either the form of NO 3− or NH 4+, after being grown on N-free nutrient solution for 8 days. The pH of the nutrient solutions was either 6 or 4. The cell sap pH and the extractable activities of phosphoenolpyruvate carboxylase and of pyruvate kinase from roots and primary leaves were measured over several days. The extractable activity of phosphoenolpyruvate carboxylase (based on soluble protein) from primary leaves increased with NO3− nutrition, whereas with NH4+ nutrition and on N-free nutrient solution the activity remained at a low level. Phosphoenopyruvate carboxylase activity from the roots of NH4+-fed plants at pH 4 was finally somewhat higher than from the roots of plants grown on NO3− at the same pH. There was no difference in activity from the root between the N treatments when pH in the nutrient solutions was 6. The extractable activity of pyruvate kinase from roots and primary leaves seemed not to be influenced by the N nutrition of the plants. The results are discussed in relation to the physiological function of both enzymes with special regard to the postulated functions of phosphoenolpyruvate carboxylase in C3 plants as an anaplerotic enzyme and as part of a cellular pH stat. 相似文献
6.
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.
To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH 4+ and NO 3? uptake from mixed sources, NH 4+ and NO 3? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH 4+-fed axis than in the NO 3?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH 4+-fed axis can be accounted for by the retransport of CH 2O equivalents from the root back to the shoot with amino acids produced by NH 4+ assimilation. Uptake of NH 4+ or NO 3? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH 4+ and NO 3? were supplied to separate halves of the split-root system, the cumulative NH 4+ to NO 3? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH 4+ to NO 3? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH 4+ and NO 3? uptake by roots. For whole roots supplied with both NH 4+ and NO 3?, the restriction in uptake of NO 3? may involve a stimulation of NO 3? efflux rather than an inhibition of NO 3? influx. While only the net uptake of NH 4+ and NO 3? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH 4+, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH 4+ was supplied with NO 3? to whole-root systems or separately to an axis of the split-root system. In the case of NO 3?, however, the number of net efflux events increased from 8 when NO 3? was supplied to a separate axis of the split-root system to between 19 and 24 when NO 3? was supplied with NH 4+ to whole-root systems. 相似文献
8.
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
9.
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
10.
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
11.
Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate
reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms
is limited. We fertilized 1-year-old, half-sib black walnut ( Juglans nigra L.) seedlings with ammonium (NH 4
+) [as (NH 4) 2SO 4], nitrate (NO 3
−) (as NaNO 3), or a mixed N source (NH 4NO 3) at 0, 800, or 1,600 mg N plant −1 season −1. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed.
Specific leaf NRA was higher in NO 3
−-fed and NH 4NO 3-fed plants compared to observed responses in NH 4
+-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring
greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO 3
−-fed than in NH 4
+-fed plants. Exogenous NO 3
− was assimilated in roots or stored, so no difference was observed in NO 3
− levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over
NO 3
− or NH 4
+ alone, suggesting NH 4NO 3 is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source
and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms. 相似文献
12.
Alfalfa ( Medicago sativa L.) N-sufficient plants were fed 1·5 mM N in the form of NO 3−, NH 4+ or NO 3− in conjunction with NH 4+, or were N-deprived for 2 weeks. The specific activity of phosphoenolpyruvate carboxylase (PEPC) from the non-nodulated roots of N-sufficient plants was increased in comparison with that of N-deprived plants. The PEPC value was highest with NO 3− nutrition, lowest with NH 4+ and intermediate in plants that were fed mixed salts. The protein was more abundant in NO 3−-fed plants than in either NH 4+- or N mixed-fed plants. Nitrogen starvation decreased the level of PEPC mRNA, and nitrate was the N form that most stimulated PEPC gene expression. The malate content was significantly lower in NO 3−-deprived than in NO 3−-sufficient plants. Root malate accumulation was high in NO 3−-fed plants, but decreased significantly in plants that were fed with NH 4+. The effect of malate on the desalted enzyme was also investigated. Root PEPC was not very sensitive to malate and PEPC activity was inhibited only by very high concentrations of malate. Asparagine and glutamine enhanced PEPC activity markedly in NO 3−-fed plants, but failed to affect plants that were either treated with other N types or N starved. Glutamate and citrate inhibited PEPC activity only at optimal pH. N-nutrition also influenced root nitrate and ammonium accumulation. Nitrate accumulated in the roots of NO 3−- and (NO 3− + NH 4+)-fed plants, but was undetectable in those administered NH 4+. Both the nitrate and the ammonium contents were significantly reduced in NO 3−- and (NO 3− + NH 4+)-starved plants. Root accumulation of free amino acids was strongly influenced by the type of N administered. It was highest in NH 4+-fed plants and the most abundant amides were asparagine and glutamine. It was concluded that root PEPC from alfalfa plants is N regulated and that nitrate exerts a strong influence on the PEPC enzyme by enhancing both PEPC gene expression and activity. 相似文献
13.
Net fluxes of NH 4+ and NO 3− into roots of 7-day-old barley ( Hordeum vulgare L. cv Prato) seedlings varied both with position along the root axis and with time. These variations were not consistent between replicate plants; different roots showed unique temporal and spatial patterns of uptake. Axial scans of NH 4+ and NO 3− net fluxes were conducted along the apical 7 centimeters of seminal roots of intact barley seedlings in solution culture using ion-selective microelectrodes in the unstirred layer immediately external to the root surface. Theoretically derived relationships between uptake and concentration gradients, combined with experimental observations of the conditions existing in our experimental system, permitted evaluation of the contribution of bulk water flow to ion movement in the unstirred layer, as well as a measure of the spatial resolution of the microelectrode flux estimation technique. Finally, a method was adopted to assess the accuracy of this technique. 相似文献
15.
In the atmosphere, ammonia (NH 3) is the third most abundant N species which, due to various natural and anthropogenic sources, can locally reach high concentrations. The acquisition of atmospheric NH 3 by plant shoots will lead to two opposing effects on acid-base balance. Absorption and dissolution of NH 3 will cause an alkalinisation, while the assimilation of NH 3 results in an acidification. Different rates of these processes would lead to an acid-base imbalance with consequences for the ionic balance of the plant. As there is only a limited capacity for biochemical disposal of excess H + in shoots, pH regulation may involve a pattern of (in)organic ion flow between shoots and roots followed by H +/OH ? extrusion into the media via roots. The acquisition of NH 3 as additional N source should lead to a reduction in the ratio of mol H +/OH ? gained per mol N assimilated. We have recently investigated the NH 3 acquisition by Lolium perenne L. cv. Centurion and studied the effects of gas phase NH 3 on growth, acid-base balance and water-use efficiency. The experiments, therefore, included the application of a range of 14NH 3 to the shoots and of 15N as NO 3?, NH 4+ or NH 4NO 3 to the roots. After a summary of the main conclusions from those experiments, we discuss the implications of the use of atmospheric NH 3 for the mineral composition of the plants. Over the range of NH 3 supplied, plants from all treatments could utilize gas-phase NH 3. Plants receiving NO 3? via their roots had a higher capacity to use gaseous NH 3 than those growing with NH 4+. NH 3 assimilation in shoots reduced both the acid load with NH 4+ nutrition and the alkaline load with NO 3? supply to the roots. The most significant effect of fumigation on the ion balance was an increase in K + within all treatments, and this effect was highest in the NH 4+-fed plants. The results of the experiments support predictions of a combination of neutralizing biochemical reactions as well as transport of organic anion salts between shoots and roots as possible acid-base regulation mechanisms of the whole plant. 相似文献
16.
At root temperature below 14 C the absorption of 15N from NH 4+ greatly exceeded that from NO 2− by tillers of Lolium multiflorum and Lolium perenne under conditions where pH, external concentration, plant N status, and pretreatment temperature were varied. There was a marked increase in the temperature sensitivity of NO 3− transport below 14 C, irrespective of the temperature at which plants were grown previously. A marked increase in the temperature sensitivity was also seen for NH 4+ transport, but this occurred at the lower temperature of 10 C. Pretreatment of roots at 8 C lowered this still further to 5 C. Above and below these transition temperatures the Q 10 values for NO 3− and NH 4+ transport were similar. Thus, the increased absorption of NH 4+ relative to NO 3− at low temperatures seems to be related primarily to the difference in transition temperatures. 相似文献
17.
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
18.
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
19.
The carbon and nitrogen partitioning characteristics of wheat ( Triticum aestivum L.) and maize ( Zea mays L.) grown hydroponically at a constant pH on either 4 m M or 12 m M NO 3
- or NH 4
+ nutrition were investigated using either 14C or 15N techniques. Greater allocation of 14C to amino-N fractions occurred at the expense of allocation of 14C to carbohydrate fractions in NH 4
+-compared to NO 3
--fed plants. The [ 14C]carbohydrate:[ 14C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 m M NO 3
--compared to 12 m M NH 4
+-fed wheat. In both 4 m M and 12 m M N-fed maize the [ 14C]carbohydrate:[ 14C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO 3
--compared to NH 4
+-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO 3
--and the other in NH 4
+-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of 14N and 15N within separate sets of plants confirmed that NH 4
--fed plants accumulated more amino-N compounds than NO 3
--fed plants. Wheat roots supplied with 15NH 4
+ for 8 h were found to accumulate 15NH 4
+ (8.5 g 15N g -1 h -1) whereas in maize roots very little 15NH 4
+ accumulated (1.5 g 15N g -1 h -1)It is proposed that the observed accumulation of 15NH 4
+ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH 4
+, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C 4 maize plants ensure greater carbon availability to the root than in the C 3 wheat plants. These differences in carbon and nitrogen partitioning between NO 3
--and NH 4
+-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO 3
- and NH 4
+ nutrition. 相似文献
20.
Comparative studies on the effect of nitrogen (N) form on iron (Fe) uptake and distribution in maize ( Zea mays L. cv Yellow 417) were carried out through three related experiments with different pretreatments. Experiment 1: plants were precultured in nutrient solution with 1.0×10 –4 M FeEDTA for 6 d and then exposed to NO 3–N or NH 4–N solution with 1.0×10 –4 M FeEDTA or without for 7 d. Experiment 2: plants were precultured with 59FeEDTA for 6 d and were then transferred to the solution with different N forms, and 0 and 1.0×10 –4 M FeEDTA for 8 d. Experiment 3: half of roots were supplied with 59FeEDTA for 5 d and then cut off, with further culturing in treatment concentrations for 7 d. In comparison to the NH 4-fed plants, young leaves of the NO 3-fed plants showed severe chlorosis under Fe deficiency. Nitrate supply caused Fe accumulation in roots, while NH 4–N supply resulted in a higher Fe concentration in young leaves and a lower Fe concentration in roots. HCl-extractable (active) Fe was a good indicator reflecting Fe nutrition status in maize plants. Compared with NO 3-fed plants, a higher proportion of 59Fe was observed in young leaves of the Fe-deficient plants fed with NH 4–N. Ammonium supply greatly improved 59Fe retranslocation from primary leaves and stem to young leaves. Under Fe deficiency, about 25% of Fe in primary leaves of the NH 4-fed plants was mobilized and retranslocated to young leaves. Exogenous Fe supply decreased the efficiency of such 59Fe retranslocation. The results suggest that Fe can be remobilized from old to young tissues in maize plants but the remobilization depends on the form of N supply as well as supply of exogenous Fe. 相似文献
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