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1.
Background: The complementary use of different forms of soil nitrogen (N) might lead to a higher productivity of mixed forests than monocultures, but convincing evidence for temperate mixed forests is scarce. Aims: We searched for species differences in N uptake rates and the preference for NH 4+, NO 3? or glycine among five temperate broad?leaved tree species ( Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Tilia cordata) in a mature mixed stand. Methods: 15N tracer was added to the soil and its accumulation in fine root biomass was analysed after 10 min, 1 h and 1 d. Results: The estimated root uptake rates of the species were in the range of 5–46 µg N g ?1 root h ?1 for NH 4+, 6–86 µg N g ?1 h ?1 for NO 3? and 4–29 µg N g ?1 h ?1 for glycine during the first hour after tracer application. Carpinus, Tilia and Acer tended to prefer NH 4+ over NO 3?, while Fraxinus showed equal preference for both N forms and Fagus seemed to prefer NO 3?. Conclusions: The five co-existing tree species differed in uptake rates and partly in their N form preference, but complementarity in the use of different N forms seems to be of minor importance in this forest because tree species appear to be rather flexible in their N form use. 相似文献
2.
Three-year-old Scots pine ( Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH 4/NO 3 uptake ratio and proton efflux. N was supplied in three NH 4-N/NO 3-N ratios, 3:1, 1:1 and 1:3, either as 15NH 4+ 14NO 3 or as 14NH 4+ 15NO 3. Total N and 15N acquisition of different plant parts were measured. Averaged over the whole tree, the NH 4/NO 3 uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively.
The excess cation-over-anion uptake value (C a-A a) appeared to be linearly related to the natural logarithm of the NH 4/NO 3 uptake ratio. Further, this uptake ratio was related to the NH 4/NO 3 ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern
as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause
soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers. 相似文献
3.
Background and aimsPlants differ in their ability to use different nitrogen (N) chemical forms, these differences can be related to their ecology and drive community structure. The capacity to uptake intact organic N has been observed in plants of several ecosystems. However, soil organic N uptake by Mediterranean plants is unknown despite organic N being abundant in Mediterranean ecosystems. We compare the uptake of different N forms in two widespread coexisting Mediterranean forest trees with contrasting ecophysiological characteristics: Quercus ilex and Pinus halepensis.
MethodsTo estimate root uptake rate of each N form we used equimolar solutions (1 mM N) of 15NO3
?, 15NH4
+ and 15N-13C glycine. ResultsNH4
+ and glycine were taken up at a similar rate, but faster than NO3
? in both species. Intact dual labeled glycine was found in both species, demonstrating that both species can absorb intact organic N. ConclusionsDespite their ecological differences, both species had similar preference for N forms suggesting no fundamental niche complementarity for N uptake. The higher preference for NH4
+ and glycine over NO3
? possibly reflects adaptation to the differing proportions of N forms in Mediterranean soils. 相似文献
4.
The effect of NO 2 fumigation on root N uptake and metabolism was investigated in 3-month-old spruce ( Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO 2 to the plant N budget was measured during a 48 h fumigation with 100mm 3m ?3 NO 2. Plants were pre-treated with various nutrient solutions containing NO 2 and NH 4+, NO 3? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH 4+ in the solution for 6d led to an increased capacity for NO 3? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO 3? uptake. In fumigated plants, NO 2 uptake accounted for 20–40% of NO 3? uptake. Root NO 3? uptake in plants supplied with NH 4+plus NO 3? solutions was decreased by NO 2 fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO 2 and NH 4+ and were fumigated or not with 100mm 3m ?3 NO 2 for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and 15N labelling. Root NH 4+ uptake was approximately three times higher than NO 3? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO 3? nor NH 4+ accumulated in the shoot, showing that all the absorbed NO 2 was assimilated. Root NO 3? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO 2-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed. 相似文献
5.
Studies that quantify plant δ 15N often assume that fractionation during nitrogen uptake and intra-plant variation in δ 15N are minimal. We tested both assumptions by growing tomato ( Lycopersicon esculetum Mill. cv. T-5) at NH 4+ or NO ?3 concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ 15N was not significantly different from source δ 15 N for plants grown on either nitrogen form. No intra-plant variation in δ 15N was observed for plants grown with NH +4. In contrast. δ 15N of leaves was as much as 5.8% greater than that of roots for plants grown with NO ?3. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH +4 is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO ?3 assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ 15N of NO ?3 to become enriched relative to organic nitrogen; the δ 15N of NO ?3 was 11.1 and 12.9% greater than the δ 15N of organic nitrogen in leaves and roots, respectively. Leaf δ 15N may therefore be greater than that of roots because the NO ?3 available for assimilation in leaves originates from a NO ?3 pool that was previously exposed to nitrate assimilation in the root. 相似文献
6.
The kinetics of NH 4
+ and NO 3
− uptake in young Douglas fir trees ( Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N
species, the V max of NH 4
+ uptake was higher than that of NO 3
− uptake. The K m of NH 4
+ uptake and K m of NO 3
− uptake differed not significantly. When both NH 4
+ and NO 3
− were present, the V max for NH 4
+ uptake became slightly higher, and the K m for NH 4
+ uptake remained in the same order. Under these conditions the NO 3
− uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μ M total N). This inhibition by NH 4
+ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap} 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.
Tomato growth was examined in solution culture under constant pH and low levels of NH 4+ or NO 3?. There were five nitrogen treatments: 20 mmoles m ?3 NH 4+, 50 mmoles m ?3 NO 3?, 100 mmoles m ?3 NH 4+ 200 mmoles m ?3 NO 3?, and 20 mmoles m ?3 NH 4++ 50 mmoles m ?3 NO 3?. The lower concentrations (20 mmoles m ?3 NH 4+ and 50 mmoles m ?3 NO 3?) were near the apparent Km for net NH 4+ and NO 3? uptake; the higher concentrations (100 mmoles m ?3 NH 4+ and 200 mmoles m ?3 NO 3?) were near levels at which the net uptake of NH 4+ or NO 3? saturate. Although organic nitrogen contents for the higher NO 3? and the NH 4++ NO 3? treatments were 22.2–30.3% greater than those for the lower NO 3? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH 4+ or NO 3? concentrations are held constant over the root surface at near the apparent Km concentration. Relative growth rates for the two NH 4+ treatments were much higher than have been previously reported for tomatoes growing with NH 4+ as the sole nitrogen source. Initial growth rates under NH 4+ nutrition did not differ significantly ( P≥ 0.05) from those under NO 3? or under combined NH 4++ NO 3?. Growth rates slowed after 10–15 d for the NH 4+ treatments, whereas they remained more constant for the NO 3? and mixed NH 4++ NO 3? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH 4+ nutrition may have resulted from a reduction in Ca 2+, K +, and/or Mg 2+ absorption. 相似文献
9.
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
10.
The complex interplay between photosynthesis and the uptake of nitrogen was investigated in samples from five lakes of different size and trophic state. When enriched with 15NH 4+, the photosynthetic rate was often reduced for 4–5 h in samples believed to be nitrogen deficient. This implies that energy was reallocated from photosynthesis to the uptake and assimilation of N. Stimulation in C uptake at low levels of NH 4+ enrichment was followed by a progressive decline with further NH 4+ enrichment. On other occasions when ambient NH 4+ was undetectable, nutrient regeneration by zooplankton supplied a significant fraction of the required nitrogen. At these times and when the plankton had sufficient available N, there usually was no change in photosynthetic rate with either NH 4+ or NO 3?enrichment. Typically, little NO 3? was taken up and no photosynthetic response was observed. On two occasions, however, the uptake of NO 3? was significant due to high NO 3? and low NH 4+ levels early in the season. At one of these times there was a reduction in photosynthesis with NO 3? enrichment. A further complication was observed when photosynthesis decreased with NH 4+ enrichment but increased with NO 3? enrichment despite negligible NO 3? uptake. These observations illustrate that the complex metabolism of these two nitrogen sources is not fully understood. At optimum light intensity, C:N uptake ratios, even under NH 4+ enrichment, are only sufficient to maintain the cellular C:N ratio unless much of the fixed C is respired or excreted. Three observations suggest that photosynthesis and N uptake are not coupled, (i) Photoinhibition of C uptake, but not N uptake was observed when low light adapted populations are exposed to high light conditions, (ii) The light intensity for maximum N uptake was slightly less than that for carbon. (iii) Dark N uptake was always near 50% of the maximum rate in the light whereas the C uptake was near 2% of P opt. Certainly, there is an interconnection because dark C uptake was enhanced by NH 4+ enrichment. 相似文献
11.
In short-term water culture experiments with different 15N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH 4
+ at a higher rate than NO 3
–. Maximum NO 3
– uptake by the whole plant occurred at 120 mg L –1 NO 3
–-N, whereas NH 4
+ absorption was saturated at 240 mg L –1 NH 4
+-N. 15NH 4
+ accumulated in roots and to a lesser degree in both leaves and stems. However, 15NO 3
– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH 4
+ (15–60 mg L –1 N) to nutrient solutions containing 120 mg L –1 N as 15N labeled nitrate reduced 15NO 3
– uptake. Maximum inhibition of 15NO 3
– uptake was about 55% at 2.14 m M NH 4
+ (30 mg L –1 NH 4
+-N) and it did not increase any further at higher NH 4
+ proportions.In a long-term experiment, the effects of concentration and source of added N (NO 3
– or NH 4
+) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH 4
+ versus NO 3
– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO 3
–-N:NH 4
+-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L –1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO 3
– or NH 4
+ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO 3
–-N/NH 4
+-N ratio. With increasing proportions of NH 4
+ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH 4
+, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO 3
–-N:NH 4
+-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH 4
+ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO 3
–:NH 4
+ ratio. 相似文献
12.
Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH 4+ and NO 3?) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO 3? > NH 4+ > chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletion period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within 10 days. Upon N resupply, the RON pool increased 3 × faster than either the inorganic or protein pools, suggesting that protein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for both NH 4+ and NO 3? varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO 3? showed an initial lag phase, but the initial uptake of NH 4+ was enhanced and was present only when the intracellular NH 4+ pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool size and maximum assimilation uptake rates for both NH 4+ and NO 3? suggested a feedback control on assimilation uptake by the buildup and depletion of organic compounds. Enhanced uptake and the accumulation of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demand in C. linum. 相似文献
13.
The influence of various nitrogen (N) and sulphur (S) forms on the uptake of manganese (Mn) in young spring barley ( Hordeum vulgare L cv Golf) plants was examined in both a hydroponic system and in a soil-based system. The soil was a typical Danish Mn-deficient soil viz. a sandy loam soil developed on old marine sediments. Plants growing in solution culture with NO 3– as the only N source had a higher Mn uptake than plants receiving mixtures of NO 3– and NH 4+. These findings were opposite to the results obtained in the soil-based experiments, where plants fertilized with NO 3– as the only N source accumulated much less Mn than plants fertilized with NH 4+. Combining the results of these experiments confirmed that NH 4+ acted as a powerful antagonist to Mn 2+ during uptake but that this antagonistic effect was more than compensated for by the influence of NH 4+ in reducing plant-unavailable Mn(IV) to plant-available Mn(II) in the soil. Furthermore the soil experiments showed that fertilizers containing sulphur in the form of reduced S (thiosulphate) had a strong mobilizing effect on Mn, and enabled the plants to accumulate large amounts of Mn in the biomass compared with oxidized S (sulphate). Thus, fertilization with thiosulphate may be very effective in alleviating Mn-deficiency in soils developed on old marine sediments where Mn availability is limiting plant growth. 相似文献
14.
NH 4+ and NO 3? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO 3?up take followed saturable kinetics (K 2=4.9 μg-at N t ?1, V max= 2.85 μg- at N, g(wet) ?1. h ?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO 3? uptake was reduced one-half in the presence of NH 4+, but presence of NO 3? had no effect on NH 4+ uptake. Darkness reduced both NO 3? and NH 4+ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO 3? uptake followed saturable kinetices: K 2=13.1 μg-at N. l ?1. V max=3.05 μg-at N. g(wet) ?1. h ?1.NH 4+ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K 2=5.3 μg-at N.1–1, V max= 2.38 μg-at N G (wet) ?1.h ?1: at higher concentration uptake increased lincarly with concentrations. NO 3?and NH 4+ were taken up simulataneously: presence of one form did not affect uptake of the other. 相似文献
15.
Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH 4 +) and/or nitrate (NO 3 ?) to improve yields. However, little is known about net NH 4 +/NO 3 ? fluxes and their relation with H + fluxes in poplar roots. In this study, net NH 4 +/NO 3 ? fluxes in association with H + fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH 4 + and NO 3 ? fluxes was found along root tips of P. popularis. The maximal net uptake of NH 4 + and NO 3 ? occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH 4 + uptake was induced by ca. 48 % with provision of NO 3 ? together, but net NO 3 ? uptake was inhibited by ca. 39 % with the presence of NH 4 + in poplar roots. Furthermore, inactivation of plasma membrane (PM) H +-ATPases by orthovanadate markedly inhibited net NH 4 +/NO 3 ? uptake and even led to net NH 4 + release with NO 3 ? co-provision. Linear correlations were observed between net NH 4 +/NO 3 ? and H + fluxes in poplar roots except that no correlation was found between net NH 4 + and H + fluxes in roots exposed to NH 4Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH 4 +/NO 3 ? uptake and that net NH 4 +/NO 3 ? fluxes and the interaction of net fluxes of both ions are tightly associated with H + fluxes in poplar roots. 相似文献
16.
Rates of NH 4+ and NO 3? uptake were determined by accumulation of 15N in plant tissue and by disappearance of nutrient from the medium. Agreement between rates calculated by the two methods was good, averaging 82.7% (SD = 15.8%) and 91.2% (SD = 13.7%) for NH 4+ and NO 3? uptake, respectively. An average of 93.4 and 96.0% of added 15NH 4+ and 15NO 3? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH 4+ may contribute to discrepancies between NH 4+ uptake rates calculated by 15N accumulation and disappearance of NH 4+ from the medium. The influence of tissue composition on uptake of NH 4+, NO 3? and PO 43- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH 4+ uptake, V max was 188 μmol NH 4+. g dry wt ?1. h ?1 and K s ranged from 9.3 to 13.4 μM, but there was no correlation between kinetic parameters and tissue nitrogen content. For NO 3?, both kinetic parameters were higher for plants with low tissue nitrogen than for plants with high tissue nitrogen. Maximum rates were 169 and 75.4 μmol NO 3?. g dry wt ?1. h ?1, and K s was 13.3 and 2.31 μM for low and high tissue nitrogen plants, respectively. Estimates of uptake in the field suggested that NH 4+ accounted for 65% and NO 3? for up to 35% of total nitrogen uptake during the summer. Nutrient uptake rates of field-collected plants also indicated that E. prolifera in Yaquina Bay, Oregon was not likely to have been nitrogen-limited, but may have been phosphorus-limited. 相似文献
17.
Kinetic parameters for NH 4+ and NO 3? uptake were measured in intact roots of Lolium perenne and actively N 2-fixing Trifolium repens. Simultaneously, net H + fluxes between the roots and the root medium were recorded, as were the net photosynthetic rate and transpiration of the leaves. A Michaelis–Menten-type high-affinity system operated in the concentration range up to about 500 mmol m ?3 NO 3? or NH 4+. In L. perenne, the Vmax of this system was 9–11 and 13–14 μmol g ?1 root FW h ?1 for NO 3? and NH 4+, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g ?1 root FW h ?1. The Km for NH 4+ uptake was much lower in L. perenne than in T. repens ( c. 40 compared with 170 mmol m ?3), while Km values for NO 3? absorption were roughly similar (around 130 mmol m ?3) in the two species. There were no indications of a significant efflux component in the net uptake of the two ions. The translocation rate to the shoots of nitrogen derived from absorbed NO 3?-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH 4+. Trifolium repens had higher rates of transpiration and net photosynthesis than L. perenne. Measurements of net H + fluxes between roots and nutrient solution showed that L. perenne absorbing NO 3? had a net uptake of H +, while L. perenne with access to NH 4+ and T. repens, with access to NO 3? or NH 4+, in all cases acidified the nutrient solution. Within the individual combinations of plant species and inorganic N form, the net H + fluxes varied only a little with external N concentration and, hence, with the absorption rate of inorganic N. Based on assessment of the net H + fluxes in T. repens, nitrogen absorption rate via N 2 fixation was similar to that of inorganic N and was not down-regulated by exposure to inorganic N for 2 h. It is concluded that L. perenne will have a competitive advantage over T. repens with respect to inorganic N acquisition. 相似文献
18.
An in vitro system was established for the characterisation of inorganic nitrogen uptake by sugarcane plantlets of variety NCo376. After multiplication and rooting, plantlets (0.27–0.3 g fresh mass) were placed on N-free medium for 4 days, and then supplied with 2–20 mM N as NO 3 ?-N only, NH 4 +-N only or NO 3 ?-N + NH 4 +-N (as 1:1). With few exceptions, on all the tested N media, the in vitro plants always had a higher V max for NH 4 +-N (28.69–66.51 μmol g ?1 h ?1) than for NO 3 ?-N uptake (10.24–30.19 μmol g ?1 h ?1) and the K m indicated a higher affinity for NO 3 ?-N (0.02–7.38 mM) than for NH 4 +-N (0.06–9.15 mM). When N was applied as 4 and 20 mM to varieties N12, N19 and N36, the interaction between variety, N form and concentration resulted in differences in the V max and K m. The high N-use efficient varieties (N12 and N19), as determined in previous pot and field trials, behaved similarly under all tested conditions and displayed a lower V max and K m than the low N-use efficient ones (NCo376 and N36). Based on this finding, it was suggested that the N-use efficient designation (from pot and field trials) may not be ascribed solely to N uptake. Assessment of the relative preference index (RPI) for NO 3 ?-N and NH 4 +-N uptake revealed that, at present, the RPI has no application in sugarcane due to its preferential uptake of NH 4 +-N. 相似文献
19.
The competitive ability for N uptake by four intertidal seaweeds, Stictosiphonia arbuscula (Harvey) King et Puttock, Apophlaea lyallii Hook. f. et Harvey, Scytothamnus australis Hook. f. et Harvey, and Xiphophora gladiata (Labillardière) Montagne ex Harvey, from New Zealand is described by the uptake kinetics for NO 3?, NH 4+, and urea. This is the first study to report uptake kinetics for N uptake by a range of southern hemisphere intertidal seaweeds in relation to season and zonation. Species growing at the highest shore positions had higher NO 3? and urea uptake at both high and low concentrations and had unsaturable NH 4+ uptake in both summer and winter. Although there was evidence of some feedback inhibition of Vmax for NO 3? uptake by Stictosiphonia arbuscula growing at the lower vertical limits of its range, rates were high compared with species growing lower on the shore. Our results highlight the superior competitive ability for N uptake of certain high intertidal seaweeds, and consistent with our previous findings we can conclude that intertidal seaweeds in southeast New Zealand are adapted to maximizing N acquisition in a potentially N‐limiting environment. 相似文献
20.
The effect of external inorganic nitrogen and K + content on K + uptake from low-K + solutions and plasma membrane (PM) H +-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4 mM K + and inorganic nitrogen as NO 3-, NO 3-/NH 4+ or NH 4+ and then starved of K + for 24, 48 and 72 h. NH 4+ in full nutrient solution significantly affected the uptake efficiency and accumulation of K +, and this effect was less pronounced at the high K + concentration. In contrast, the translocation rate of K + to the shoot was not altered. Depletion assays showed that plants grown with NH 4+ more efficiently depleted the external K + and reached higher initial rates of low-K + uptake than plants grown with NO 3-. One possible influence of K + content of shoot, but not of roots, on K + uptake was evidenced. Enhanced K +-uptake capacity was correlated with the induction of H + extrusion by PM H +-ATPase. In plants grown in high K + solutions, the increase in the active H + gradient was associated with an increase of the PM H +-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2 mM K +, only the initial rate of H +-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H +-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO 3--grown plants. The results suggest that K + homeostasis in NH 4+-grown sorghum plants may be regulated by a high capacity for K + uptake, which is dependent upon the H +-pumping activity of PM H +-ATPase. 相似文献
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