A comparative kinetic analysis of nitrate and ammonium influx in two early-successional tree species of temperate and boreal forest ecosystems

Root NO₃ ^? and NH₄ ⁺ influx systems of two early‐successional species of temperate (trembling aspen: Populus tremuloides Michx.) and boreal (lodgepole pine: Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forest ecosystems were characterized. NO₃ ^? and NH₄ ⁺ influxes were biphasic, consisting of saturable high‐affinity (HATS) and constitutive non‐saturable low‐affinity transport systems (LATS) that were evident at low and relatively high N concentrations, respectively. NO₃ ^? influx via HATS was inducible (IHATS); nitrate pre‐treatment resulted in 8–10‐fold increases in the V_max for influx in both species. By contrast, HATS for NH₄ ⁺ were entirely constitutive. In both species, V_max values for NH₄ ⁺ influx were higher than those for NO₃ ^? uptake; the differences were larger in pine (6‐fold) than aspen (1·8‐fold). In aspen, the K_m for NH₄ ⁺ influx by HATS was approximately 3‐fold higher than for IHATS NO₃ ^? influx, while in pine the K_m for IHATS NO₃ ^? influx was approximately 3‐fold higher than for NH₄ ⁺ influx. The aspen IHATS for NO₃ ^? influx appeared to be more efficient than that of pine (V_max values for aspen being approximately 10‐fold higher and K_m values being approximately 13‐fold lower than for pine). By contrast, only small differences in values for the NH₄ ⁺ HATS were evident between the two species. The kinetic parameters observed here probably result from adaptations to the N availabilities in their respective natural habitats; these may contribute to the distribution and niche separation of these species.     

Dynamics of nitrogen remobilization in defoliated Phleum pratense and Festuca pratensis under short and long photoperiods

The impact of photoperiod on the rate and magnitude of N remobilization relative to uptake of inorganic N during the recovery of shoot growth after a severe defoliation was compared over 18 days in two temperate grass species, timothy (Phleum pratense L. cv. Bodin) and meadow fescue (Festuca pratensis Huds. cv. Salten). Plants were grown in flowing solution culture with N supplied as 20 mM NH₄NO₃ and pre-treated by extending the 11 h photosynthetically significant light period either by 1 h (short-day or SD plants) or 7 h (long-day or LD plants) of very low light intensity, during the 10 days prior to defoliation. Following a single severe defoliation, ¹⁵N-labelled NH₄⁺ or NH₄⁺+ NO₃^? was supplied over a 20-day recovery period under the same SD and LD conditions. Changes in the relative contributions of remobilized N and newly acquired mineral N to shoot regrowth were assessed by sequential harvests. Both absolute and relative rates of N remobilization from root and stubble fractions were higher in LD than SD plants of both species, with the enhancement more acute but of shorter duration in timothy than fescue. Remobilized N was the predominant source of N for shoot regrowth in all treatments between days 0 and 8 after cutting; on average more so for fescue than timothy, because the presence of NO₃^? reduced the proportional contribution of remobilized N to the regrowth of timothy but not of fescue. Net uptake of mineral N began to recover between days 4 and 6 after cutting, with NO₃^? uptake restarting 1 or 2 days earlier than NH₄⁺ uptake, even when NH₄⁺ was the only form of N supply. LD timothy plants supplied solely with NH₄⁺ were slowest to resume uptake of mineral N. Supplying NO₃^? in addition to NH₄⁺ after defoliation promoted shoot regrowth rate but not remobilization of N. Rates of regrowth (shoot dry weight production per plant) were not correlated with rates of N remobilization from stubble either over the short-term (days 0–8) or longer term (days 0–18), interpreted as evidence against a causal dependence of regrowth rate on N remobilization under these conditions. The results are discussed in relation to hypotheses for source/sink-driven rates of N remobilization and their interactions with mineral N uptake following defoliation.     

Salt-induced NO<Subscript>3</Subscript><Superscript>-</Superscript> uptake inhibition in cowpea roots is dependent on the ionic composition of the salt and its osmotic effect

Salinity remarkably inhibits NO₃ ^- uptake but the mechanisms are not well understood. This study was addressed to elucidate the role of ionic and osmotic components of salinity on NO₃ ^- influx and efflux employing classic kinetics involving a low affinity transport system (LATS) and a high affinity transport system (HATS). In the presence of KCl, NaCl, and Na₂SO₄ at 100 mM concentrations, in both LATS and HATS, Michaelis constant (K_m) was similar for the three salts and maximum rate (V_max) decreased as follows: KCl > NaCl > Na₂SO₄, compared to control indicating a non-competitive interaction with NO₃ ^-. Unexpectedly, iso-osmotic solutions (osmotic potential Ψ_π = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased K_m in both the LATS and the HATS, but V_max did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, K_m and V_max were higher than under the salt treatments. The salts increased slightly NO₃ ^- efflux in the following order KCl > NaCl > Na₂SO₄. In contrast, mannitol strongly stimulated and the PEG inhibited NO₃ ^- efflux. The obtained data reveal that salinity effects were not dependent on the anion type (Cl^- versus SO₄ ^2-) indicating a non-competitive inhibition mechanism between Cl^- and NO₃ ^-. In contrast, the cation types (K⁺ versus Na⁺) had a pronounced effect. The osmotic component is important to net NO₃ ^- uptake affecting remarkably the influx in both LATS and HATS components of cowpea roots.     

Effect of nitrogen (NH4 NO3) supply on absorption of ammonium and nitrate by conventional and hybrid rice during reproductive growth

Although NH₄ ⁺ has generally been accepted as the preferred N source for fertilising rice, some workers have concluded tha NO₃ ^- is as effective as NH₄ ⁺. The present glasshouse study exmined the relative uptake of NH₄ ⁺ and NO₃ ^- from solution and cultures containing 5–120 mg N/L supplied as NH₄NO₃ by a hybrid rice (India) and a conventional rice cultivar (Japonica). At all levels of N supply, the hybrid rice had higher leaf area and higher rates of uptake of total N than the conventional cultivar. Net photosynthesis rates were similar for both cultivars at the highest rates of N supply, but were lower at 5–40 mg N/L for the hybrid cultivar than for the conventional cultivar. At all levels of N supply, the conventional rice cultivar absorbed more NH₄ ⁺ than NO₃ ^-. In contrast, the hybrid rice absorbed more NH₄ ⁺ than NO₃ ^- at the low levels of N supply (5–40 mg N/L), but more NO₃ ^- than NH₄ ⁺ at the high levels of at 80 and 120 mg N/L. It is concluded that the uptake of N by rice is under genetic control and also dependent on levels of N supply. Thus the appropriate form of N fertiliser for rice may depend on cultivar and rates of N supply.     

Studies of the Uptake of Nitrate in Barley : IV. Electrophysiology

Transmembrane electrical potential differences (Δψ) of epidermal and cortical cells were measured in intact roots of barley (Hordeum vulgare L. cv Klondike). The effects of exogenous NO₃⁻ on Δψ (in the concentration range from 100 micromolar to 20 millimolar) were investigated to probe the mechanisms of nitrate uptake by the high-affinity (HATS) and low-affinity (LATS) transport systems for NO₃⁻ uptake. Both transport systems caused depolarization of Δψ, demonstrating that the LATS (like the HATS) for NO₃⁻ uptake is probably mediated by an electrogenic cation (H⁺?) cotransport system. Membrane depolarization by the HATS was “inducible” by NO₃⁻, and saturable with respect to exogenous [NO₃⁻]. By contrast, depolarization by the LATS was constitutive, and first-order in response to external [NO₃⁻]. H⁺ fluxes, measured in 200 micromolar and in 5 millimolar Ca(NO₃)₂ solutions, failed to alkalinize external media as anticipated for a 2 H⁺:1 NO₃⁻ symport. However, switching from K₂SO₄ solutions (which were strongly acidifying) to KNO₃ solutions at the same K⁺ concentration caused marked reductions in H⁺ efflux. These observations are consistent with NO₃⁻ uptake by the HATS and the LATS via 2 H⁺:1 NO₃⁻ symports. These observations establish that the HATS for nitrate uptake by barley roots is essentially similar to those reported for Lemna and Zea mays by earlier workers. There are, nevertheless, distinct differences between barley and corn in their quantitative responses to external NO₃⁻.     

PHOTOSYNTHETIC RESPONSE OF LAKE PLANKTON TO COMBINED NITROGEN ENRICHMENT1

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 ¹⁵NH₄⁺, 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 NH₄⁺ enrichment was followed by a progressive decline with further NH₄⁺ enrichment. On other occasions when ambient NH₄⁺ 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 NH₄⁺ or NO₃^?enrichment. Typically, little NO₃^? was taken up and no photosynthetic response was observed. On two occasions, however, the uptake of NO₃^? was significant due to high NO₃^? and low NH₄⁺ levels early in the season. At one of these times there was a reduction in photosynthesis with NO₃^? enrichment. A further complication was observed when photosynthesis decreased with NH₄⁺ enrichment but increased with NO₃^? enrichment despite negligible NO₃^? 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 NH₄⁺ 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 NH₄⁺ enrichment.     

UPTAKE OF INORGANIC NITROGEN BY CODIUM FRAGILE SUBSP. TOMENTOSOIDES (CHLOROPHYTA)1

The uptake of nitrate, nitrite and ammonium by Codium fragile subsp. tomentosoides (van Goor) Silva was measured at different combinations of temperature (6–30 C) and irradiance (0–140 μEin.m-^2. s^-1). Uptake of all three forms of N was greater at 12–24 C than at 6 and 30 C. Although uptake was stimulated by light, saturation occurred at relatively low irradiance (7–28 μEin m^-2 s^-1, depending on the N source and temperature). The Michaelis-Menten uptake constants (V_max K)varied with temperature. V_max was greatest at intermediate temperatures and K was lowest at lower temperatures. The V_maxfor NH₄⁺ was higher and the K, for NH₄⁺was lower than those for NO₃^-_- and NO₂^-_-. Codium was capable of simultaneously taking up all three forms of inorganic N although the presence of NH₄⁺ reduced the uptake of both NO₃^-_- and NO₂^-_-. The results of this study indicate that part of the ecological success of Codium in a N-limited environment may be due to its N uptake capabilities.     

Uptake and Assimilation of NO(3) and NH(4) by Nitrogen-Deficient Perennial Ryegrass Turf

Assimilation of NO₃⁻ and NH₄⁺ 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 NH₄⁺, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO₃⁻ had been reduced, whereas 97% of the NH₄⁺ had been assimilated. During the early stages (0 to 8 hours) of NO₃⁻ 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 NO₃⁻, NH₄⁺ 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 NO₃⁻ uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Ammonium uptake in Lemna gibba G 1, related membrane potential changes, and inhibition of anion uptake

In N-starved (?N) fronds of Lemna gibba L. G 1, NH₄⁺ uptake rates were several-fold those of NO₃^?-supplied (+N) fronds. NO₃^?, uptake in +N-plants was slow and not inhibited by addition of NH₄⁺. However, in ?N-plants with higher NO₃^? and still higher NH₄⁺ uptake rates, addition of NH₄⁺ immediately reduced the NO₃^? uptake rates to about one third until the NH₄⁺ was consumed. The membrane potential (E_m) decreased immediately upon addition of NH₄⁺ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where E_m remained at the level of the K⁺ diffusion potential (E_D) until NH₄⁺ was removed. In N-starved plants NH₄⁺ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH₄⁺ similarly to NO₃^? uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO₃^? and H₂PO ₄^? into the negatively charged cells of Lemna is mediated by anion/H⁺ cotransport, but NH₄⁺ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NH₄⁺ uniport, the linear component diffusion of NH₄⁺ or NH₃. Inhibition of anion/H⁺ cotransport by high NH₄⁺ influx rates may be due to loss of the proton-driving force, Δμ?H⁺, across the plasmalemma. Reversible inhibition by NH₄⁺ of the H⁺ extrusion pump may contribute to the finding that Δμ?H⁺ cannot be reconstituted in the presence of higher NH₄⁺ concentrations.     

Uptake and inhibition kinetics of nitrogen in Microcystis aeruginosa: Results from cultures and field assemblages collected in the San Francisco Bay Delta,CA

The nitrogen (N) uptake kinetic parameters for Microcystis field assemblages collected from the San Francisco Bay Delta (Delta) in 2012 and non-toxic and toxic laboratory culture strains of M. aeruginosa were assessed. The ¹⁵N tracer technique was used to investigate uptake of ammonium (NH₄⁺), nitrate (NO₃⁻), urea and glutamic acid over short-term incubations (0.5–1 h), and to study inhibition of NO₃⁻, NH₄⁺ and urea uptake by NH₄⁺, NO₃⁻ and NH₄⁺, respectively. This study demonstrates that Delta Microcystis can utilize different forms of inorganic and organic N, with the greatest capacity for NH₄⁺ uptake and the least for glutamic acid uptake, although N uptake did not always follow the classic Michaelis–Menten hyperbolic relationship at substrate concentrations up to 67 μmol N L⁻¹. Current ambient N concentrations in the Delta may be at sub-saturating levels for N uptake, indicating that if N loading (especially NH₄⁺) were to increase, Delta Microcystis assemblages have the potential for increased N uptake rates. Delta Microcystis had the highest specific affinity, α, for NH₄⁺ and the lowest for NO₃⁻. In culture, N uptake by non-toxic and toxic M. aeruginosa strains was much higher than from the field, but followed similar N utilization trends to those in the field. Neither strain showed severe inhibition of NO₃⁻ uptake by NH₄⁺ or inhibition of NH₄⁺ uptake on NO₃⁻, but both strains showed some inhibition of urea uptake by NH₄⁺.     

SHORT TERM UPTAKE OF NUTRIENTS BY ENTEROMORPHA PROLIFERA (CHLOROPHYCEAE)1

Rates of NH₄⁺ and NO₃^? 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 NH₄⁺ and NO₃^? uptake, respectively. An average of 93.4 and 96.0% of added ¹⁵NH₄⁺ and ¹⁵NO₃^? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH₄⁺ may contribute to discrepancies between NH₄⁺ uptake rates calculated by ¹⁵N accumulation and disappearance of NH₄⁺ from the medium. The influence of tissue composition on uptake of NH₄⁺, NO₃^? and PO₄^3- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH₄⁺ uptake, V_max was 188 μmol NH₄^+. 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 NO₃^?, 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 NO₃^?. 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 NH₄⁺ accounted for 65% and NO₃^? 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.     

Effects of pH,phosphate and ammonia on the rate of uptake of nitrate and ammonia by freshwater phytoplankton

The effect of phosphate (PO₄ ⁺³) and pH in regulating nitrate (NO₃) and ammonia (NH₃ ⁺) uptake by phytoplankton was investigated in two Oklahoma lakes using ¹⁵N tracers. Addition of PO₄ ⁺³ above ambient concentrations had a negligible effect on the rate of uptake of NO₃ ^– or NH₃ ⁺. Manipulation of pH of lake water had little effect on uptake of either NO₃ ^– or NH₃ ⁺. A correlation analysis suggested that NO₃ ^– is not used by phytoplankton when NH₃ ⁺ concentrations exceed about 210 µg NH₃ ⁺-N(1)^–1.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH₄⁺ and NO₃^? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO₃^?up take followed saturable kinetics (K₂=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. NO₃^? uptake was reduced one-half in the presence of NH₄⁺, but presence of NO₃^? had no effect on NH₄⁺ uptake. Darkness reduced both NO₃^? and NH₄⁺ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO₃^? uptake followed saturable kinetices: K₂=13.1 μg-at N. l^?1. V_max=3.05 μg-at N. g(wet)^?1. h^?1.NH₄⁺ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K₂=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. NO₃^?and NH₄⁺ were taken up simulataneously: presence of one form did not affect uptake of the other.     

Effects of gypsum on the uptake,assimilation and cycling of 15N-labelled ammonium and nitrate-N by perennial ryegrass

Solution culture studies have shown that plant uptake of NH₄ ⁺ and NO₃ ^- can be improved by increasing the concentration of Ca²⁺ in the root environment: the same may be true for grass grown in soil culture. An experiment was set up to see whether gypsum (CaSO₄ 2H₂O) increased the rate at which perennial ryegrass absorbed ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^- from soil.The results demonstrated that gypsum increases the rates of uptake of both NH₄ ⁺ and NO₃ ^- by perennial ryegrass. However because there was little potential for mineral-N loss from the experimental system, either by gaseous emission or by N immobilization, long term improvements in fertilizer efficiency were not observed. Nitrogen cycling from shoots to roots commenced once net uptake of N into plants had ceased. Labelled N transferred thus to roots underwent isotopic exchange with unlabelled soil N. It was suggested that this exchange of N might constitute an energy drain from the plant, if plant organic N was exchanged for soil inorganic N. The fact that the exchange occurred at all cast doubt on the suitability of the ¹⁵N-isotope dilution technique for assessing fertilizer efficiency in medium to long term experiments. There was evidence that the extra NO₃ ^--N taken up by plants on the all-nitrate treatments as a result of gypsum application, was reduced in root tissue rather than in shoots, but to the detriment of subsequent root growth and N uptake.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

Absorption of nitrate and ammonium ions by Lolium perenne from flowing solution cultures at low root temperatures

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49 d to low root temperature (5°C) prior to treatment at root temperatures of 3, 5, 7 and 9°C for 41 d with common air temperature of 20/15°C day/night and solution pH 5·0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH₄ or NO3 at 10 mmol m^?3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5°C had very high dry matter contents as reflected by the fresh weight: freeze-dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9°C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NHJ but at any given temperature, the [N] in plants grown with NHJ was significantly greater than in those grown with NO₃. The specific absorption rate (SAR) of NH⁺₄ was greater at all temperatures than SAR-NO₃. In plants grown with NH⁺, 3–5% of the total N was recovered as NH⁺₄, whereas in those grown with NO^?₃ the unassimilated NO^?₃ rose sharply between 7 and 9°C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH⁺₄ lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO^?₃-grown plants. Plants grown with NH⁺₄ had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in Concentration to those in NO^?₃ grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH⁺₄ or NO^?₃. The additional residual N (protein) in NH⁺₄ grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ 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 NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ 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.