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.     

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.     

METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE-LIMITING STEP FOR UPTAKE1

Non-linear time courses of ammonium (NH₄⁺) depletion from the medium and internal accumulation of soluble nitrogen (N) in macroalgae imply that the rate-limiting step for ammonium uptake changes over time. We tested this hypothesis by measuring the time course of N accumulation in N-limited Ulva rigida C. Agardh. Total uptake was measured as removal of NH₄⁺ from medium. Rates for the component processes (transport of NH₄⁺ across the membrane = R_v assimilation of tissure NH₄⁺ into soluble N compounds = R_a, assimilation of tissue NH₄⁺ into soluble N compounds = R_a and incorporation of soluble N compounds into macromolecules = R₁) were determined by measuring the rate of labelling of the major tissue N pools after the addition of ¹⁵N-ammonium. The results indicate that nitrogen-specific rates (mass N taken up / mass N present / unit time) are ranked in the order of R_t < R_a < R₁ Absolute uptake rates (μmol N. mg dry wt^?1. h^?1) showed a different relationship. Membrane transport appears to be inhibited when NH₄⁺ accumulates in the tissue. Maximum uptake rates occur when assimilation of NH₄⁺ into soluble N compounds begins. Assimilation of NH₄⁺ into soluble N compounds was initially faster than incorporation of soluble N compounds into macromolecules. Implications of rate limitations caused by differences in maximal rates and maximal pool sizes are discussed.     

CHANGES IN INTRACELLULAR NITROGEN POOLS AND FEEDBACK CONTROLS ON NITROGEN UPTAKE IN CHAETOMORPHA LINUM (CHLOROPHYTA)1

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH₄⁺ and NO₃^?) 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 > NO₃^? > NH₄⁺ > 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 NH₄⁺ and NO₃^? varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO₃^? showed an initial lag phase, but the initial uptake of NH₄⁺ was enhanced and was present only when the intracellular NH₄⁺ 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 NH₄⁺ and NO₃^? 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.     

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.     

CHARACTERIZATION OF NITROGEN UPTAKE BY NATURAL POPULATIONS OF AUREOCOCCUS ANOPHAGEFFERENS (CHRYSOPHYCEAE) AS A FUNCTION OF INCUBATION DURATION,SUBSTRATE CONCENTRATION,LIGHT, AND TEMPERATURE1

Nitrogen uptake studies were conducted during an aestival “brown tide” bloom in Shinnecock Bay, Long Island, New York. The same station was sampled in late July and mid-August 1995 when Aureococcus anophagefferens composed >90% and 30–40% of the total cell density, respectively. Experiments were designed to examine the effect of incubation duration on the uptake kinetics, and the effect of light and temperature dependencies of NH₄⁺, urea, and NO₃^? uptake. Maximum specific uptake rates (V'_max) decreased in the order NH₄⁺, urea, NO₃^? and were nonlinear with time for NH₄⁺ and urea, both of which exhibited an exponential decline between 1 and 10 min and then did nut significantly change for 60 min. Nitrogen uptake kinetic experiments exhibited a typical hyperbolic response for urea and NO₃^?. Half-saturation constants. (K_s) were calculated to he 0.03 and 0.12 μmol · L^?1 for urea and NO₃^?; respectively, but could not be calculated for NH₄⁺ under these experimental conditions. Nutrient uptake rate versus, irradiance (NI) experiments showed that maximum uptake rates occurred at ≤% of incident irradiance on both sampling dates and that values of V′_max-cell (NH₄⁺) were on average 30% greater than V′_max-cell (urea). A7°–9°C temperature decrease in incubation temperature between the two NI experiments in August resulted in a 30% decrease in V′_max-cell(NH₄⁺), no change in V′_max-cell(urea), and a 3–4-fold decrease in calculated K_lt values for both NH₄⁺ and urea. The results from these experiments demonstrate that A. anophagefferens has a higher affinity for NH₄⁺ and urea than for NO₃^? and that this particular species is adapted to use these substrates at low irradiances and concentrations. The data presented in this study are also consistent with the hypothesis that A. anophagefferens may be an oceanic clone that was displaced by an anomalous oceanographic event.     

UPTAKE OF NITRATE,AMMONIUM, AND UREA BY NITROGEN-STARVED CULTURES OF MICROMONAS PUSILLA (PRASINOPHYCEAE): TRANSIENT RESPONSES1

Nitrogen uptake rates were measured as a function of time following saturating additions (15 μMg-at N·^?1) of ¹⁵N-labelid ammonium, urea, and nitrate to N-starved cultures of the picoflagellate Micromonas pusilla Butcher. Uptake rates were estimated from both the accumulation of ¹⁵N into the cells and the disappearance of nitrogen from the medium. Transient elevated (surge) uptake rates of NH₄⁺ and urea were observed after enrichment. During the first 5 min the initial urea and NH₄⁺ uptake rates were 2- and 4-fold greater than the maximum growth rate (μM_max)observed prior to No₃^? depletion in the cultures. The elevated urea uptake rates declined quickly to a relatively constant value, whereas the initial rates of NH₄⁺ uptake declined rapidly but were followed by a subsequent increase prior to remaining roughly constant. Nitrate was not taken up as readily by N-starved M. pusilla as the reduced N forms. Although NO₃⁺ uptake commenced immediately after enrichment (i.e. no lag period) the N-Specific rate over the next 6 h averaged half the μM_max observed during NO₃^? replete conditions.     

The role of N-remobilisation and the uptake of NH4+ and NO3- by Lolium perenne L. in laminae growth following defoliation under field conditions

Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results in a large decline in N uptake (60%) and/or N₂ fixation (80%). The source of N used for initial shoot growth following defoliation relies mainly on mobilisation of N reserves from tissues remaining after defoliation. To date, most studies investigating N-mobilisation have been conducted, with isolated plants grown in controlled conditions. The objectives of this study were for Lolium perenne L., grown in a dense canopy in field conditions, to determine: 1) the contribution of N-mobilisation, NH₄ ⁺ uptake and NO₃ ^- uptake to growing shoots after defoliation, and 2) the contribution of the high (HATS) and low (LATS) affinity transport systems to the total plant uptake of NH₄ ⁺ and NO₃ ^-. During the first seven days following defoliation, decreases in biomass and N-content of roots (34% and 47%, respectively) and to a lesser extent stubble (18% and 43%, respectively) were observed, concomitant with mobilisation of N to shoots. The proportion and origin of N used by shoots (derived from reserves or uptake) was similar to data reported for isolated plants. Both HATS and LATS contributed to the total root uptake of NH₄ ⁺ and NO₃ ^-. The V_max of both the NH₄ ⁺ and NO₃ ^- HATS increased as a function of time after defoliation, and both HATS systems were saturated by substrate concentrations in the soil at all times. The capacity of the LATS was reduced as soil NO₃ ^- and NH₄ ⁺ concentrations decreased following defoliation. Data from ¹⁵N uptake by field-grown plants, and uptake rates of NH₄ ⁺ and NO₃ ^- estimated by excised root bioassays, were significantly correlated, though uptake was over-estimated by the later method. The results are discussed in terms of putative mechanisms for regulating N uptake following severe defoliation. This revised version was published online in June 2006 with corrections to the Cover Date.     

KINETICS OF NITRATE,AMMONIUM, AND UREA UPTAKE BY FOUR INTERTIDAL SEAWEEDS FROM NEW ZEALAND1

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 NO₃^?, NH₄⁺, 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 NO₃^? and urea uptake at both high and low concentrations and had unsaturable NH₄⁺ uptake in both summer and winter. Although there was evidence of some feedback inhibition of V_max for NO₃^? 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.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

15N MEASUREMENTS OF AMMONIUM AND NITRATE UPTAKE BY ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA): COMPARISON OF NET NUTRIENT DISAPPEARANCE,RELEASE OF AMMONIUM AND NITRATE,AND 15N ACCUMULATION IN ALGAL TISSUE 1

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by ¹⁵N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH₄ ⁺ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw ^{? 1}·h ^{? 1} in U. fenestrata and 32 μmol N·g dw ^{? 1}·h ^{? 1} in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH₄ ⁺ release rate of 11 μmol N·g dw ^{? 1}·h ^{? 1}. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH₄ ⁺ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH₄ ⁺ disappearance and ¹⁵N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH₄ ⁺ concentrations, provided that isotope dilution is taken into account. On average, 96% of added ¹⁵NH₄ ⁺ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO₃ ^? was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO₃ ^? pools) or physiological/metabolic changes required before the uptake of NO₃ ^? . During the subsequent days, NO₃ ^? uptake rates were similar for the two macroalgae but much lower than NH₄ ⁺ uptake rates (1.97–3.19 μmol N·g dw ^{? 1}·h ^{? 1}). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added ¹⁵NO₃ ^? was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH₄ ⁺ uptake experiments conducted with N‐rich macroalgae and that determination of ¹⁵N atom % enrichment of the dissolved NH₄ ⁺ is recommended to avoid poor isotope recovery and underestimation of uptake rates.     

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.     

NITROGEN UPTAKE BY FUCUS SPIRALIS (PHAEOPHYCEAE)1,2

Ammoniun, nitrate and nitrite update by Fucus spiralis L. from the Massachusetts coast was examined. Uptake of all appeared to follow saturation type nutrient uptake kinetics, with uptake often restricted at ambient nutrient concentrations. Although only relatively large difference in K₈ values could be easily distinguished, K₈ values for NO_3? and NH₄₊ were generally similar and low compared with NO_2?. There was also some suggestion that K₈ was reduced at lower temperatures. At 15 C. V_max for light and dark uptake for both NH₄+ and NO_3?, and light uptake of N0_2? were similar, suggesting comparable potential use at higher concentrations. Ammonium and NO_3?uptake decreased at lower temperatures giving Q_ro values of 1.8 and 1.6, respectively, between 5 and 15°C. Nitrate and NH₄+ were taken up together and high levels of NH₄+ did not inhibit NO_3? uptake. Light did not affect uptake of either but did stimulate NO_2? uptake. Ammonium and NO_3? uptake were highest in apical frond and whole young plants, and lowest in slower growing, older frond and stipe. On a relative basis. NO_3?, NH₄₊ and NO_2? were estimated to have contributed ca. 59, 39 and 2% respectively, to the yearly N uptake by apical frond. During winter, NO_3? would provide ca. twice the N to F. spiralis as would, NH₄+. From summer to early fall, when NO_3? levels are lower, NO_3? and NH₄+ would be used in comparable amounts.     

THE RELATIVE IMPORTANCE OF WATER MOTION ON NITROGEN UPTAKE BY THE SUBTIDAL MACROALGA ADAMSIELLA CHAUVINII (RHODOPHYTA) IN WINTER AND SUMMER1

The influence of seawater velocity (1.5–12 cm · s^?1) on inorganic nitrogen (N) uptake by the soft‐sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO₃^? uptake in either winter or summer, indicating that NO₃^?‐uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH₄⁺ uptake, suggesting that mass‐transfer limitation of NH₄⁺ is likely during summer for natural populations. Uptake kinetics (V_max, K_s) were similar among three populations of A. chauvinii at sites with different mean flow speeds; however, uptake rates of NO₃^? and NH₄⁺ were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake.     

AMMONIUM,NITRATE, PHOSPHATE,AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

The dependence of substrate saturated uptake of ¹⁵NH₄⁺, ¹⁵NO₃^?, ³²PO₄^3?, and ¹⁴CO₂ on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO₄^3? uptake was not dependent upon PPFD at any time of the year, whereas NH₄⁺, NO₃^?, and CO₂ uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH₄⁺, NO₃^? and CO₂ uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH₄⁺, NO₃^? and PO₄^3? were incorporated in the dark at measurable rates most of the year, whereas dark CO₂ uptake was always near 0 relative to light uptake. CO₂ and NO₃^? uptake were more strongly influenced by PPFD than was NH₄^3? uptake. The PPFD dependence of PO₄^3?, NH₄⁺, NO₃^? and CO₂ uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.     

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.     

Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings

The effect of NO₂ 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 NO₂ to the plant N budget was measured during a 48 h fumigation with 100mm³m^?3 NO₂. Plants were pre-treated with various nutrient solutions containing NO₂ and NH₄⁺, NO₃^? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH₄⁺ in the solution for 6d led to an increased capacity for NO₃^? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO₃^? uptake. In fumigated plants, NO₂ uptake accounted for 20–40% of NO₃^? uptake. Root NO₃^? uptake in plants supplied with NH₄⁺plus NO₃^? solutions was decreased by NO₂ fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO₂ and NH₄⁺ and were fumigated or not with 100mm³m^?3 NO₂ 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 NH₄⁺ uptake was approximately three times higher than NO₃^? 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 NO₃^? nor NH₄⁺ accumulated in the shoot, showing that all the absorbed NO₂ was assimilated. Root NO₃^? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO₂-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed.     

Differential responses of root uptake kinetics of NH4+ and NO3− to enriched atmospheric CO2 concentration in field-grown loblolly pine

The nitrogen requirement of plants is predominantly supplied by NH₄⁺ and/or NO₃^? from the soil solution, but the energetic cost of uptake and assimilation is generally higher for NO₃^? than for NH₄⁺. We found that CO₂ enrichment of the atmosphere enhanced the root uptake capacity for NO₃^?, but not for NH₄⁺, in field-grown loblolly pine saplings. Increased preference for NO₃^? at the elevated CO₂ concentration was accompanied by increased carbohydrate levels in roots. The results have important implications for the potential consequences of global climate change on plant-and ecosystem-level processes in many temperate forest ecosystems.     

Relationships among nitrogen accumulation, nitrogen assimilation and plant growth in chrysanthemums

Relationships among growth, N accumulation and assimilation were investigated in Chrysanthemum morifolium Ramat cv. Fiesta in experiments testing the effects of varying levels of NO^–3₃supply and of increasing NH⁺₄ added to a constant level of NO^–3₃ Flowing solution culture systems were used to provide NO^?₃at concentrations of 0.03 to 5.0 mol m^–3 and NH⁺₄ levels from 0.05 to 0.3 mmol m^–3 added to 0.1 mol m^–3NO^?₃. Rates of growth, N absorption, accumulation, distribution and utilization were estimated by regression analysis of data obtained from sequential plant harvests, and rates of NO^?₃ and NH^?₄ net uptake were estimated from solution depletion. A sustained ambient NO^?₃ concentration of 0.03 mol m^–3 was evidently adequate to support growth, since relative growth rates were not affected by increasing NO^?₃ supply from 0.03 to 1.0 mol m^–3, nor from 0.25 to 5.0 mol m^–3, in separate experiments. Shoot growth rates were stimulated by NH₄ added to NO^?₃ one experiment, but not when the experiment was repeated under ambient conditions less favorable to growth. Relative accumulation rates for total N increased with increasing NO^?₃ and with NH⁺₄added to NO^?₃ A constant proportion of NO^?₃ taken up was reduced when NO^?₃ alone was supplied. Both the proportion of total N taken up as NO^?₃ and the proportion of NO^?₃ reduced decreased with increasing NH⁺₃ added to NO^?₃ NH⁺₄ uptake apparently must exceed a threshold of about 30% of the total uptake to inhibit NO^?₃ uptake. Utilization of N in chrysanthemum was apparently limited by redistribution since relative accumulation rates for total N were equal to or greater than relative growth rates, in contrast to results reported for several other species. Results of this study and other information support the postulate that NH⁺₄ added to NO^?₃might stimulate growth by increasing transport of reduced N from roots to shoots, thus increasing the supply of reduced N available to support growth of shoot meristems.     

Kinetics of nitrate and ammonium absorption and accompanying H+ fluxes in roots of Lolium perenne L. and N2-fixing Trifolium repens L.

Kinetic parameters for NH₄⁺ and NO₃^? uptake were measured in intact roots of Lolium perenne and actively N₂-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 NO₃^? or NH₄⁺. In L. perenne, the V_max of this system was 9–11 and 13–14 μmol g^?1 root FW h^?1 for NO₃^? and NH₄⁺, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g^?1 root FW h^?1. The K_m for NH₄⁺ uptake was much lower in L. perenne than in T. repens (c. 40 compared with 170 mmol m^?3), while K_m values for NO₃^? 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 NO₃^?-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH₄⁺. 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 NO₃^? had a net uptake of H⁺, while L. perenne with access to NH₄⁺ and T. repens, with access to NO₃^? or NH₄⁺, 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 N₂ 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.