Inorganic nitrogen uptake kinetics of sugarcane (Saccharum spp.) varieties under in vitro conditions with varying N supply

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 NO₃ ^?-N only, NH₄ ⁺-N only or NO₃ ^?-N + NH₄ ⁺-N (as 1:1). With few exceptions, on all the tested N media, the in vitro plants always had a higher V_max for NH₄ ⁺-N (28.69–66.51 μmol g^?1 h^?1) than for NO₃ ^?-N uptake (10.24–30.19 μmol g^?1 h^?1) and the K_m indicated a higher affinity for NO₃ ^?-N (0.02–7.38 mM) than for NH₄ ⁺-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 NO₃ ^?-N and NH₄ ⁺-N uptake revealed that, at present, the RPI has no application in sugarcane due to its preferential uptake of NH₄ ⁺-N.     

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.     

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.     

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.     

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.     

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.     

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 kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ 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 NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

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.     

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.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.     

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.     

EXPERIMENTAL EVIDENCE SUPPORTS THE USE OF δ15N CONTENT OF THE OPPORTUNISTIC GREEN MACROALGA ENTEROMORPHA INTESTINALIS (CHLOROPHYTA) TO DETERMINE NITROGEN SOURCES TO ESTUARIES1

One assumption underlying the use of stable nitrogen isotopes (δ¹⁵N) for determining nitrogen (N) sources is that the δ¹⁵N of primary producers reflects N sources in a predictable manner. To test this assumption, we conducted two experiments. First, we varied δ¹⁵N with constant concentration of NO₃^? or NH₄⁺ to determine whether either nutrient is preferentially selected by the macroalga Enteromorpha intestinalis (L. Link) and if isotopic ratio affected selectivity. Tissue δ¹⁵N increased with δ¹⁵N supplied for both NO₃^? and NH₄⁺ but sequestering of ¹⁵NH₄⁺ was more rapid than for ¹⁵NO₃^?; in addition, some evidence suggested that high relative abundance of ¹⁵N may have decreased NO₃^? uptake. Second, we held δ¹⁵N constant and varied concentrations of either NO₃^? or NH₄⁺ to determine whether fractionation is concentration dependent. Uptake of N was described by a Michaelis‐Menten equation for both NO₃^? and NH₄⁺, with higher V_max and K_1/2 for NH₄⁺ than for NO₃^?. There was no relationship between N concentration and tissue δ¹⁵N for either NO₃^? or NH₄⁺; therefore, no selection for ¹⁴N over ¹⁵N occurred. This study demonstrated that accumulation of ¹⁵N in macroalgal tissue was predictable over a range of water δ¹⁵N values and N concentrations, suggesting that E. intestinalis may be used to assess the availability of N sources to estuarine and coastal communities. However, caution must be used when interpreting tissue δ¹⁵N depending on the primary inorganic form of N available.     

Temporal and spatial variability in nitrogen uptake kinetics during harmful dinoflagellate blooms in the East China Sea

Temporal and spatial variability in the kinetic parameters of uptake of nitrate (NO₃⁻), ammonium (NH₄⁺), urea, and glycine was measured during dinoflagellate blooms in Changjiang River estuary and East China Sea coast, 2005. Karenia mikimotoi was the dominant species in the early stage of the blooms and was succeeded by Prorocentrum donghaiense. The uptake of nitrogen (N) was determined using ¹⁵N tracer techniques. The results of comparison kinetic parameters with ambient nutrients confirmed that different N forms were preferentially taken up during different stages of the bloom. NO₃⁻ (V_max 0.044 h⁻¹; K_s 60.8 μM-N) was an important N source before it was depleted. NH₄⁺ (V_max 0.049 h⁻¹; K_s 2.15 μM-N) was generally the preferred N. Between the 2 organic N sources, urea was more preferred when K. mikimotoi dominated the bloom (V_max 0.020 h⁻¹; K_s 1.35 μM-N) and glycine, considered as a dominant amino acid, was more preferred when P. donghaiense dominated the bloom (V_max 0.025 h⁻¹; K_s 1.76 μM-N). The change of N uptake preference by the bloom-forming algae was also related to the variation in ambient N concentrations.     

Changes in NO3− and K+ uptake by four species in flowing solution culture in response to increased irradiance

Net rates of NO₃^? and K⁺ uptake were compared for oilseed rape (Brassica napus L. cv. Jet neuf), perennial ryegrass (Lolium perenne L. cv. S23), Italian ryegrass (Lolium multiflorum Lam. cv. Augusta) and wheat (Triticum aestivum L. cv. Fen-man) in flowing solution culture during a 4-day sequence of low-low-high-high natural irradiance. Concentrations of NO₃^? (10 μM) and K⁺ (2.5 μM) in solutions were maintained automatically and hourly variation in net uptake of these ions was measured. During the 2 days of low irradiance (<1 MJ m^?2 day^?1) the uptake rates of both ions by all species were low at <1 mmol NO₃^?, m^?2 h^?1 and <0.4 mmol K⁺ m^?2 h^?1. Uptake increased in each species during the first day of high irradiance (7.90 MJ m^?2 day^?1) to >4 mmol NO₃^? m^?2 h^?1 and >1.4 mmol K⁺ m^?1 h^?1. These higher rates were maintained throughout the following night. The lag-time between maximum irradiance and the onset of the highest acceleration in uptake was greater for NO₃^? (5–8 h) than for K⁺ (≤1 h) in rape, wheat and Italian ryegrass. Uptake of NO₃^?, by perennial ryegrass showed an almost constant acceleration for 18 h following maximum irradiance. In all species the measured maximum inflows (uptake rate per unit root length) of both ions were greater than theoretical maximum potential inflows to a non-competing infinite-sink root in soil, by factors of 7 and 36, respectively, for NO₃^? and K⁺, averaged over all species.     

NUTRITIONAL STUDIES OF TWO RED ALGAE. II. KINETICS OF AMMONIUM AND NITRATE UPTAKE1, 2

Similar NH₄₊ and NO₃^?.uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO₃^? was taken up in a rate-sturating fashion described by the Michaelis-Menten equation. NH₄₊ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH₄₊]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH₄₊ uptake at a given concentration than plants not N-Iimited. Only plants with high N content exhibited diel changes inNH₄₊ uptake rates, and showed transient rates of NH₄₊ accumulation which did not greatly exceed the capacity to incorporate N in steady-state growth. NH₄₊ was preferred over NO_3?even in plants preconditioned on NO_3?as the sole N. source, NO_3? uptake was suppressed at 5μM [NH₄₊], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH₄₊uptake than via NO_3?uptake. Changing capacity for NH₄₊ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N-.satiated plants but a large accumulation was possible for N-depleted plants.     

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.     

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.     

Nutrient uptake and alkaline phosphatase (ec 3:1:3:1) activity of emiliania huxleyi (PRYMNESIOPHYCEAE) during growth under n and p limitation in continuous cultures

Emiliania huxleyi (strain L) expressed an exceptional P assimilation capability. Under P limitation, the minimum cell P content was 2.6 fmol P·cell^?1, and cell N remained constant at all growth rates at 100 fmol N·cell^?1. Both, calcification of cells and the induction of the phosphate uptake system were inversely correlated with growth rate. The highest (cellular P based) maximum phosphate uptake rate (V_max^P) was 1400 times (i.e. 8.9 h^?1) higher than the actual uptake rate. The affinity of the P‐uptake system (dV/dS) was 19.8 L·μmol^?1·h^?1 at μ = 0.14 d^?1. This is the highest value ever reported for a phytoplankton species. V_max and dV/dS for phosphate uptake were 48% and 15% lower in the dark than in the light at the lowest growth rates. The half‐saturation constant for growth was 1.1 nM. The coefficient for luxury phosphate uptake (Q_maxt/Q_min) was 31. Under P limitation, E. huxleyi expressed two different types of alkaline phosphatase (APase) enzyme kinetics. One type was synthesized constitutively and possessed a V_max and half‐saturation constant of 43 fmol MFP·cell^?1·h^?1 and 1.9 μM, respectively. The other, inducible type of APase expressed its highest activity at the lowest growth rates, with a V_max and half‐saturation constant of 190 fmol MFP·cell^?1·h^?1 and 12.2 μM, respectively. Both APase systems were located in a lipid membrane close to the cell wall. Under N‐limiting growth conditions, the minimum N quotum was 43 fmol N·cell^?1. The highest value for the cell N‐specific maximum nitrate uptake rate (V_max^N) was 0.075 h^?1; for the affinity of nitrate uptake, 0.37 L·μmol^?1·h^?1. The uptake rate of nitrate in the dark was 70% lower than in the light. N‐limited cells were smaller than P‐limited cells and contained 50% less organic and inorganic carbon. In comparison with other algae, E. huxleyi is a poor competitor for nitrate under N limitation. As a consequence of its high affinity for inorganic phosphate, and the presence of two different types of APase in terms of kinetics, E. huxleyi is expected to perform well in P‐controlled ecosystems.     

Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ClO(3): II. Regulation of NO(3) Efflux by NH(4)

The influence of NH₄⁺, in the external medium, on fluxes of NO₃⁻ and K⁺ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH₄⁺ was without effect on NO₃⁻ (³⁶ClO₃⁻) influx whereas inhibition of net uptake appeared to be a function of previous NO₃⁻ provision. Plants grown at 10 micromolar NO₃⁻ were sensitive to external NH₄⁺ when uptake was measured in 100 micromolar NO₃⁻. By contrast, NO₃⁻ uptake (from 100 micromolar NO₃⁻) by plants previously grown at this concentration was not reduced by NH₄⁺ treatment. Plants pretreated for 2 days with 5 millimolar NO₃⁻ showed net efflux of NO₃⁻ when roots were transferred to 100 micromolar NO₃⁻. This efflux was stimulated in the presence of NH₄⁺. NH₄⁺ also stimulated NO₃⁻ efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO₃⁻ occur via effects upon efflux. By contrast to the situation for NO₃⁻, net K⁺ uptake and influx of ³⁶Rb⁺-labeled K⁺ was inhibited by NH₄⁺ regardless of the nutrient history of the plants. Inhibition of net K⁺ uptake reached its maximum value within 2 minutes of NH₄⁺ addition. It is concluded that the latter ion exerts a direct effect upon K⁺ influx.