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.     

The nitrogen balance of Raphanus sativus x raphanistrum plants. II. Growth, nitrogen redistribution and photosynthesis under NO3− deprivation

Abstract. Wild radish plants deprived of, and continuously supplied with solution NO^?₃ for 7 d following 3 weeks growth at high NO^?₃ supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH₂-N and NO^?₃-N) among individual organs. Initial levels of NO^?₃-N accounted for 25% of total plant N. Following termination of NO^?₃ supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH₂-N concentration declined by about 25% relative to NO^?₃-supplied plants, and endogenous NO^?₃-N content was reduced to nearly zero. Older leaves lost NO^?₃ and NH₂-N, and roots and young leaves gained NH₂-N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight-based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH₂-N concentration, and the relationship was not significantly different for leaves of NO₃^?-supplied compared to NO^?₃-deprived plants. Simulations of the time course of NO^?₃ depletion for plants of various NH₂-N and NO^?₃ compositions and relative growth rates indicated that environmental conditions may influence the importance of NO^?₃ accumulation as a buffer against fluctuations in the N supply to demand ratio.     

Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O₂ m^?2 h^?1), ammonium (NH₄ ⁺), nitrate (NO₃ ^?), dissolved organic nitrogen, dinitrogen gas (N₂), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH₄ ⁺), NO₃ ^? and N₂ fluxes (annual means +14.6, +15.9 and 44.6 μmol N m^?2 h^?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O₂ m^?2 h^?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH₄ ⁺ flux which may have masked reductions in NH₄ ⁺ flux associated with BMA assimilation during the light. Peaks in net N₂ fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO₃ ^? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O₂ m^?2 h^?1.     

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.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

PHYSIOLOGICAL ACCLIMATION OF MARINE PHYTOPLANKTON TO DIFFERENT NITROGEN SOURCES1

We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH₄⁺, NO₃^?, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO₃^? and NH₄⁺ grown cells had similar growth rates and C and X quotas. Therefore, NO₃^? grown cells used up to 48% more energy than NH₄⁺ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a^?1, we suggest that NO₃^?, grown cells do not compensate for the greater energy requirements of NO₃^? reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO₃^?, compared to NH₄⁺ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh₄⁺ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO₃^?, or urea.     

Nitrate photo-assimilation in tomato leaves under short-term exposure to elevated carbon dioxide and low oxygen

The role of photorespiration in the foliar assimilation of nitrate (NO₃^–) and carbon dioxide (CO₂) was investigated by measuring net CO₂ assimilation, net oxygen (O₂) evolution, and chlorophyll fluorescence in tomato leaves (Lycopersicon esculentum). The plants were grown under ambient CO₂ with ammonium nitrate (NH₄NO₃) as the nitrogen source, and then exposed to a CO₂ concentration of either 360 or 700 µmol mol^?1, an O₂ concentration of 21 or 2%, and either NO₃^– or NH₄⁺ as the sole nitrogen source. The elevated CO₂ concentration stimulated net CO₂ assimilation under 21% O₂ for both nitrogen treatments, but not under 2% O₂. Under ambient CO₂ and O₂ conditions (i.e. 360 µmol mol^?1 CO₂, 21% O₂), plants that received NO₃^– had 11–13% higher rates of net O₂ evolution and electron transport rate (estimated from chlorophyll fluorescence) than plants that received NH₄⁺. Differences in net O₂ evolution and electron transport rate due to the nitrogen source were not observed at the elevated CO₂ concentration for the 21% O₂ treatment or at either CO₂ level for the 2% O₂ treatment. The assimilatory quotient (AQ) from gas exchange, the ratio of net CO₂ assimilation to net O₂ evolution, indicated more NO₃^– assimilation under ambient CO₂ and O₂ conditions than under the other treatments. When the AQ was derived from gross O₂ evolution rates estimated from chlorophyll fluorescence, no differences could be detected between the nitrogen treatments. The results suggest that short‐term exposure to elevated atmospheric CO₂ decreases NO₃^– assimilation in tomato, and that photorespiration may help to support NO₃^– assimilation.     

Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH₄⁺ and NO₃^? uptake from mixed sources, NH₄⁺ and NO₃^? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH₄⁺-fed axis than in the NO₃^?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH₄⁺-fed axis can be accounted for by the retransport of CH₂O equivalents from the root back to the shoot with amino acids produced by NH₄⁺ assimilation. Uptake of NH₄⁺ or NO₃^? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH₄⁺ and NO₃^? were supplied to separate halves of the split-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH₄⁺ and NO₃^? uptake by roots. For whole roots supplied with both NH₄⁺ and NO₃^?, the restriction in uptake of NO₃^? may involve a stimulation of NO₃^? efflux rather than an inhibition of NO₃^? influx. While only the net uptake of NH₄⁺ and NO₃^? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH₄⁺, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH₄⁺ was supplied with NO₃^? to whole-root systems or separately to an axis of the split-root system. In the case of NO₃^?, however, the number of net efflux events increased from 8 when NO₃^? was supplied to a separate axis of the split-root system to between 19 and 24 when NO₃^? was supplied with NH₄⁺ to whole-root systems.     

Ammonium influence on nitrogen assimilating enzymes and protein accumulation in suspension cultures of Paul's Scarlet rose

The influence of NH₄⁺ on protein accumulation was examined by growing suspension cultures of Rosa cv. Paul's Scarlet on two defined media. Both contained 1920 μmol of NO₃^? but only one contained 72.8 μmol of NH₄⁺. At the conclusion of a 14-day growth period, cultures grown with NH₄⁺ possessed twice as much protein as cultures grown without NH₄⁺. The influence of NH₄⁺ did not appear to be a substrate effect, since the amount of NH₄⁺ provided accounted for only 10% of the nitrogen recovered in protein. The provision of NH₄⁺ in the starting medium increased the activity (μmol substrate. h^?1· g^?1 fr wt) of glutamate dehydrogenase and glutamate synthase, and reduced the activity of glutamine synthetase. A comparison of the total activity per culture for each of these enzymes with the rate of nitrogen incorporation into protein showed that the enzymatic potential of glutamine synthetase and glutamate dehydrogenase greatly exceeded the actual in vivo rate of nitrogen assimilation through the respective pathways. Thus it was concluded that the availability of either of these enzymes does not limit nitrogen assimilation in rose cells and the fluctuations in their level brought about by NH₄⁺ was of no physiological importance. The activity of glutamate synthase per culture approximated the rate of nitrogen incorporation into protein during early stages of growth, and for that reason may have limited nitrogen assimilation or caused a diversion of nitrogen through the alternative pathway to glutamate catalyzed by glutamate dehydrogenase.     

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.     

Modeling the C Economy of Anabaena flos-aquae: Estimates of Establishment, Maintenance, and Active Costs Associated with Growth on NH(3), NO(3), and N(2)

Steady state cultures of Anabaena flos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH₃, NO₃⁻, or N₂ gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (<0.03 per hour) in the NO₃⁻ and N₂ cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic O₂ evolution with growth rate was greatest when N requirements were met by NO₃⁻ and least when met by NH₃. These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH₃ cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO₃⁻-and N₂-grown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with NO₃⁻ and N₂ assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for NO₃⁻ assimilation were the active costs, while in N₂-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with NO₃⁻ assimilation, N₂ fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N₂ assimilation.     

Demonstration of Both a Photosynthetic and a Nonphotosynthetic CO(2) Requirement for NH(4) Assimilation in the Green Alga Selenastrum minutum

Nitrogen-limited and nitrogen-sufficient cell cultures of Selenastrum minutum (Naeg.) Collins (Chlorophyta) were used to investigate the dependence of NH₄⁺ assimilation on exogenous CO₂. N-sufficient cells were only able to assimilate NH₄⁺ maximally in the presence of CO₂ and light. Inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron also inhibited NH₄⁺ assimilation. These results indicate that NH₄⁺ assimilation by N-sufficient cells exhibited a strict requirement for photosynthetic CO₂ fixation. N-limited cells assimilated NH₄⁺ both in the dark and in the light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, indicating that photosynthetic CO₂ fixation was not required for NH₄⁺ assimilation. Using CO₂ removal techniques reported previously in the literature, we were unable to demonstrate CO₂-dependent NH₄⁺ assimilation in N-limited cells. However, employing more stringent CO₂ removal techniques we were able to show a CO₂ dependence of NH₄⁺ assimilation in both the light and dark, which was independent of photosynthesis. The results indicate two independent CO₂ requirements for NH₄⁺ assimilation. The first is as a substrate for photosynthetic CO₂ fixation, whereas the second is a nonphoto-synthetic requirement, presumably as a substrate for the anaplerotic reaction catalyzed by phosphoenolpyruvate carboxylase.     

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.     

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.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum

Nitrate-limited chemostat cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) were used to determine the effects of nitrogen addition on photosynthesis, dark respiration, and dark carbon fixation. Addition of NO₃⁻ or NH₄⁺ induced a transient suppression of photosynthetic carbon fixation (70 and 40% respectively). Intracellular ribulose bisphosphate levels decreased during suppression and recovered in parallel with photosynthesis. Photosynthetic oxygen evolution was decreased by N-pulsing under saturating light (650 microeinsteins per square meter per second). Under subsaturating light intensities (<165 microeinsteins per square meter per second) NH₄⁺ addition resulted in O₂ consumption in the light which was alleviated by the presence of the tricarboxylic acid cycle inhibitor fluoroacetate. Addition of NO₃⁻ or NH₄⁺ resulted in a large stimulation of dark respiration (67 and 129%, respectively) and dark carbon fixation (360 and 2080%, respectively). The duration of N-induced perturbations was dependent on the concentration of added N. Inhibition of glutamine 2-oxoglutarate aminotransferase by azaserine alleviated all these effects. It is proposed that suppression of photosynthetic carbon fixation in response to N pulsing was the result of a competition for metabolites between the Calvin cycle and nitrogen assimilation. Carbon skeletons required for nitrogen assimilation would be derived from tricarboxylic acid cycle intermediates. To maintain tricarboxylic acid cycle activity triose phosphates would be exported from the chloroplast. This would decrease the rate of ribulose bisphosphate regeneration and consequently decrease net photosynthetic carbon accumulation. Stoichiometric calculations indicate that the Calvin cycle is one source of triose phosphates for N assimilation; however, during transient N resupply the major demand for triose phosphates must be met by starch or sucrose breakdown. The effects of N-pulsing on O₂ evolution, dark respiration, and dark C-fixation are shown to be consistent with this model.     

Mitochondrial Respiration Can Support NO(3) and NO(2) Reduction during Photosynthesis : Interactions between Photosynthesis, Respiration, and N Assimilation in the N-Limited Green Alga Selenastrum minutum

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH₄⁺, NO₂⁻, NO₃⁻) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO₂ release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH₄⁺ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O₂ consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO₃⁻ and NO₂⁻ assimilation resulted in a larger stimulation of TCA cycle CO₂ release than did NH₄⁺, but a much smaller stimulation of mitochondrial O₂ consumption. NH₄⁺ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO₃⁻ and NO₂⁻ assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH₄⁺, NO₃⁻ and NO₂⁻ assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO₃⁻ and NO₂⁻ assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O₂ consumption during NO₃⁻ and NO₂⁻ assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO₃⁻ and NO₂⁻ reduction.     

High NH4+ efflux from roots of the common alpine grass,Festuca nigrescens,at field-relevant concentrations restricts net uptake

Alpine meadows of high ecological value could be severely endangered by anthropogenic N enrichment, modifying the relationships between species and the environment. While a constraint exerted by N availability on alpine plant development has been demonstrated by some fertilization experiments, in others no effect was observed. Basically, the problem is that mineral N absorption has not been characterized in alpine plants. In growth chamber experiments, we investigated the component fluxes of ¹⁵NO₃^? and ¹⁵NH₄⁺ uptake in a tussock grass (Festuca nigrescens) very common and representative of the dominant plant growth form in European alpine meadows. Rates of influx supported data already published for low elevation herbaceous species. These rates were up to ten times higher for NH₄⁺ than for NO₃^? but rates of net uptake were similar for both ions demonstrating the occurrence of elevated NH₄⁺ efflux (80% of primary influx). An increase in external N in the range of field-relevant concentrations did not substantially enhance net uptake. Thus, the alpine plant which is assumed to be adapted to relatively high soil NH₄⁺ responded like an NH₄⁺-sensitive species: as if it was unable to use the incoming nitrogen. It is suggested that the ability of this typical alpine grass to respond to increasing N availability due to global changes is limited.     

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.     

Net fluxes of ammonium and nitrate in association with H+ fluxes in fine roots of Populus popularis

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH₄ ⁺) and/or nitrate (NO₃ ^?) to improve yields. However, little is known about net NH₄ ⁺/NO₃ ^? fluxes and their relation with H⁺ fluxes in poplar roots. In this study, net NH₄ ⁺/NO₃ ^? fluxes in association with H⁺ fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH₄ ⁺ and NO₃ ^? fluxes was found along root tips of P. popularis. The maximal net uptake of NH₄ ⁺ and NO₃ ^? occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH₄ ⁺ uptake was induced by ca. 48 % with provision of NO₃ ^? together, but net NO₃ ^? uptake was inhibited by ca. 39 % with the presence of NH₄ ⁺ in poplar roots. Furthermore, inactivation of plasma membrane (PM) H⁺-ATPases by orthovanadate markedly inhibited net NH₄ ⁺/NO₃ ^? uptake and even led to net NH₄ ⁺ release with NO₃ ^? co-provision. Linear correlations were observed between net NH₄ ⁺/NO₃ ^? and H⁺ fluxes in poplar roots except that no correlation was found between net NH₄ ⁺ and H⁺ fluxes in roots exposed to NH₄Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH₄ ⁺/NO₃ ^? uptake and that net NH₄ ⁺/NO₃ ^? fluxes and the interaction of net fluxes of both ions are tightly associated with H⁺ fluxes in poplar roots.