NITROGENOUS NUTRITION OF ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN CULTURES AND IN THAU LAGOON,SOUTHERN FRANCE1

Alexandrium catenella (Whedon et Kofoid) Balech was isolated from Thau lagoon (northern Mediterranean) and its growth and uptake characteristics measured for nitrate, ammonium, and urea. Although affinity constants did not indicate a preference for ammonium over nitrate, there was a strong inhibition of nitrate uptake by ammonium when both nitrogen (N) sources were present. Nitrogen budgets during growth in cultures revealed major imbalances between decreases in dissolved N and increases in particulate N, indicating excretion of dissolved organic N during the early part of the growth phase and uptake during the later part. A quasi‐unialgal bloom in November 2001 (4×10⁶ cells·L^?1) allowed measurements of uptake of nitrate, nitrite, ammonium, and urea; net and gross growth rate of A. catenella; and grazing rates on this organism. The affinity constants indicate that it is not a strong competitor for the N nutrients tested when these are in low concentrations (<10 μgat N·L^?1), compared with other members of the phytoplankton community. Indirect evidence from cultures indicate that dissolved organic N compounds could be important in triggering those blooms. Finally, the strongly unbalanced growth observed in the field indicates that A. catenella exhibits a storage rather than a growth response to a nutrient pulse and is adapted to low frequency events such as the passage of frontal disturbances. The disappearance of A. catenella was due to grazing that balanced growth at the peak of the bloom.     

Effect of inorganic N on the population,in vitro colonization and morphology of Acetobacter diazotrophicus (syn. Gluconacetobacter diazotrophicus)

We investigated whether Acetobacter diazotrophicus (syn.Gluconacetobacter diazotrophicus) could be recovered only from sugarcane plants either with low or no application of fertiliser N. We report here the enrichment and enumeration of A. diazotrophicus from high N-fertilised samples where high heterotrophic populations reduce the numbers of A. diazotrophicus ultimately diminshing its isolation frequency as reported earlier. The growth medium of micropropagated sugarcane seedlings of the varieties Co 8021, Co 86249, Co 86010, Co 86032, and Co 87025 was amended with potassium nitrate, ammonium nitrate, ammonium chloride and urea. The colonisation and AR activity of A. diazotrophicus were affected in the presence of high levels (25 mM) of ammonium chloride and ammonium nitrate but remained unaffected in low levels of N (i.e 1/10th of MS liquid medium) and with high levels of potassium nitrate (25 mM) and urea (500 ppm). A. diazotrophicus was detected in the inoculated plants both at low and high levels of N based on the amplification of a specific 16S rRNA gene fragment using PCR based method targeting a stretch of 445 bp with primers AC and DI. High levels of N in the growth medium induced morphological changes on A. diazotrophicus cells resulting in long pleomorphic cells. The percentage of pleomorphic cells was in the decending order from NH₄NO₃, NH₄Cl, KNO₃, and urea. These changes were more prominent in ammonium chloride and ammonium nitrate than potassium nitrate, urea and N free medium. The morphological changes and the increased heterotrophic populations may play a role on the survival ofA. diazotrophicus in high N-fertilised samples/environments.     

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.     

Effect of trickle fertigation with three forms of nitrogen on soil pH,levels of extractable nutrients below the emitter and plant growth

The effects of application of nitrogen as calcium nitrate, urea or ammonium sulphate at two rates through the trickle irrigation system on pH and nutrient status of the wetted volume of soil below the emitters and on growth and nutrition of courgette (zucchini) plants (Cucurbita pepo L.) was investigated. Soil acidification, caused by nitrification, occurred to a large extent in the volume of soil immediately below the emitters in the urea and ammonium sulphate treatments. Acidification was greater at the high rate of N addition and more pronounced with ammonium sulphate than urea. A significant amount of applied urea appeared to move through the soil as urea and consequently, at the same rate of N addition, levels of ammonium were lower directly below the emitter and those of nitrate were higher further away from the emitters for the urea than ammonium sulphate treatments. Soil acidification below the emitters resulted in significant decreases in levels of exchangeable Ca, Mg and K and increases in levels of exchangeable Al, EDTA-extractable Fe, Mn, Zn and Cu and bicarbonate-extractable P. Vegetative growth and harvestable yields of courgettes were increased by both irrigation and nitrogen applications. Vegetative growth was generally greater at the low rate of N addition than at the high one and generally followed the order calcium nitrate > urea > ammonium sulphate. However, fruit yields followed the order urea > ammonium sulphate > calcium nitrate and were larger at the high rate of N for urea and ammonium sulphate treatments and unaffected by rate for the calcium nitrate treatments. It is suggested that with fertigation, the form of applied N can have significant physiological effects of plant growth and yields because N may be applied into the root zone on numerous occasions during the growing season.     

Nutrient uptake by tupelo gum and bald cypress from saturated or unsaturated soil

Summary Seedlings of tupelo gum (Nyssa aquatica L.) and bald cypress (Taxodium distichum L. Rich.) were grown in pots containing a sphagnum moss-peat soil mix. Plants approximately 20 to 25 cm tall were subjected to three moisture treatments, saturated-aerated, saturated, and unsaturated soil; and three nitrogen fertilization treatments, control (no N added), urea (a reduced N source), and nitrate (an oxidized N source).Data include dry weights (g/culture) of leaves, stems, and roots; concentrations (percentage of dry weight) and contents (mg/culture) of N, P, K, Ca, and Mg in leaves, stems, and roots. Total dry weight was greater for plants grown in saturated-aerated soil than in either saturated or unsaturated soil. Differences in nutrient absorption and distribution between the plants and among the water treatments were principally the result of growth differences produced by the water treatments. Element contents and often the concentrations of P, K, Ca, or Mg were highest in both species when grown on the saturated-aerated soil and lowest when grown on unsaturated soil. The low levels of N in plants grown on saturated soils were probably the result of denitrification, as shown by the greater content of N in plants grown on soil fertilized with urea as opposed to nitrate. Thus, urea would appear to be a better N source than nitrate for fertilization in swamp forests. Growth of, and nutrient uptake by cypress was restricted less than that of tupelo when the plants were grown on saturated as compared to saturated-aerated soil. Thus, cypress appeared more tolerant than tupelo to the anaerobic root environment found in saturated soil.     

Productivity and Growth of a Natural Population of the Smallest Free-Living Eukaryote under Nitrogen Deficiency and Sufficiency

The influence of dissolved inorganic nitrogen (DIN) enrichments on cell-normalized carbon uptake rate, chlorophyll a content, and apparent cell size of a picoeukaryote (<1 m) (Ostreococcus tauri, the smallest eukaryotic cell) from a natural summer phytoplanktonic assemblage (<200 m) in a northern Mediterranean Lagoon (Thau Lagoon) was studied in 20-L enclosures in June 1995. The natural planktonic community was incubated in situ for 24 h with initial ammonium and nitrate enrichments and compared to a control without enrichment. O. tauri cell-normalized productivity was estimated from the combination of flow cytometric (FCM) enumeration and 2-h (radioactive) carbonate incorporation measured on post-incubation size fractions (<1m). No difference between the effects of the two DIN sources of enrichment on the studied biological parameters was measured during this experiment. Growth of natural O. tauri was perturbed by the low DIN availability in the control with drastic changes in cell productivity, chlorophyll content, and cell cycle (from the variations in apparent cell size) as compared to the DIN sufficiency conditions. On the other hand, a very high specific growth rate for natural O. tauri, up to 8 day^–1 under DIN enrichments, has been estimated from production and abundance data obtained during this experiment. This supports values measured in culture and suggests that the yearly high contribution of picophytoplankton to the total primary production in Thau Lagoon is likely to be due to their high growth rate rather than the previously suggested lack of grazing pressure.     

Seasonal growth pattern of an Icelandic Laminaria population (section Simplices,Laminariaceae, Phaeophyta) containing solid- and hollow-stiped plants

Lamina elongation and content of mannitol, laminaran and nitrate were measured during one year in Laminaria saccharina sensu lato from Iceland. The population contained both solid- and hollow-stiped plants. Growth rate was at its minimum from October to December, and started to increase in mid-winter, slightly earlier at 3 m than at 5 m. The increase in growth rate coincided with a strong reduction in stored carbohydrates and an increase in nitrate content of the laminae, indicating that stored mannitol and laminaran provided extra energy for increased lamina growth and/or for nitrate uptake. The results showed that stored mannitol was utilised before laminaran. The growth rate was at its maximum from April to June, and was reduced from June to July. The ambient nitrate concentration at the locality was low from May to August. The nitrate content of the lamina tissue in relation to dry weight was high during spring but was reduced to low values by July, indicating that nitrate levels limited growth during summer. However, high nitrate concentration of the sea-water and high levels of storage carbohydrates in the plants during autumn indicate that the low growth rate at this time cannot be attributed to lack of nitrate or energy in the form of stored carbon. The Laminaria population in Iceland that was examined showed morphological similarity with L. longicruris populations in Canada (hollow stipe), while the growth pattern corresponds with European L. saccharina populations.     

Population responses of Lake Michigan phytoplankton to nitrogen and phosphorus enrichment

A natural phytoplankton assemblage from Grand Traverse Bay, Lake Michigan, was treated with factorial enrichments of nitrate and phosphorus, with maintained nutrient concentrations ranging from 5 to 60 μg total soluble phosphorus liter⁻¹ and 0.225 to 1.12 mg nitrate-nitrogen liter⁻¹.One container was spiked with added vitamins, a chelator, and trace metals. The assemblage response was monitored at the species level. Significant differences in growth rates as a function of nutrient enrichment were detected at both the division and the species levels. Growth rates associated with the various levels of enrichment are reported for several diatom taxa. Many of the diatom taxa exhibited highly significant (P < 0.01) increases in growth rate after phosphorus enrichment, with the largest effects occurring between 5 and 15 μg total soluble phosphorus liter⁻¹. Significant (P < 0.05) N effects were also observed, and the nature of these effects was found to be taxon-specific. Taxa also showed significant changes in percent composition, due both to time and to nutrient enrichment, indicating a substantial heterogeneity in response at the species level. Experimentally induced population changes were qualitatively similar to those observed in regions of the Great Lakes which have undergone anthropogenic eutrophication. Contribution No. 222 of the Great Lakes Research Division, University of Michigan. Work was supported by funds from the Michigan Sea Grant Program and the Environmental Protection Agency. Contribution No. 222 of the Great Lakes Research Division, University of Michigan. Work was supported by funds from the Michigan Sea Grant Program and the Environmental Protection Agency.     

BIOMASS AND DYNAMICS OF GROWTH OF ULVA SPECIES IN PALMONES RIVER ESTUARY1

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kütz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m^?2 in some spots, corresponding to a thallus area index of nearly 17 m²Ulva·m^?2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self-shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.     

Elevated pCO2 Affects C and N Metabolism in Wild Type and Transgenic Tobacco Exhibiting Altered C/N Balance in Metabolite Analysis

Abstract: Diurnal changes in starch, sugar and amino acid concentrations in source leaves, sink leaves and roots of tobacco plants were determined. In addition to wild type tobacco, transformed plants deficient in root nitrate reductase and exhibiting decreased rates of growth were employed. Further, the growth rates of tobacco plants were modulated by exposure to elevated pCO₂. From the diurnal alterations in metabolite concentrations, the daily turnover of starch and amino N was estimated in order to: (i) elucidate whether turnover rates can be related to growth rates, and (ii) identify individual amino compounds with the potential to indicate nitrogen fluxes and the C/N status of plants. Elevated pCO₂ increased growth rates and daily turnover of starch in both wild type and transformed plants, indicating enhanced rates of photosynthesis. In wild type plants, elevated pCO₂ increased the turnover of amino N, notably glutamine and alanine, in mature source leaves, indicating enhanced nitrate reduction. By contrast, amino N turnover in source leaves of transformed plants was not affected by elevated pCO₂, although nitrate reduction was presumably enhanced. Apparently, export of amino N was increased from the source leaves of transformed plants. This assumption was supported by a significantly increased turnover of amino N in young sink leaves compared to mature source leaves, indicating a preference for acropetal amino N allocation and import into the young leaves of the transformed plants. Further, elevated pCO₂ increased the allocation of leaf‐derived amino N to the roots of transformed plants. This led to increased levels of amino compounds during the entire day, notably glutamate, but did not affect root growth of the transformed plants. The suitability of individual amino compounds as markers for major N fluxes, such as nitrate reduction, photorespiration, and amino N export and import is discussed.     

Nitrate assimilatory properties of barley grown under long-term N limitation: Effects of local nitrate supply in split-root cultures

The effect of nitrate availability on characteristics of the nitrate assimilatory system was investigated in N-limited barley (Hordeum valgare L. cv. Golf), grown with the seminal root system split into initially equal-sized halves. The cultures were continuously supplied with nitrate-N at a relative addition rate (RA) of 0.09 day^?1, which resulted in relative growth rates (RG) that were ca 85% of those observed under surplus nitrate nutrition. The total N addition was divided between the subroots in ratios of 100:0, 80:20, 70:30, 60:40, and 50:50. For comparison, standard cultures were grown at RAs ranging from 0.03 to 0.18 day^?1. Initially, biomass and N partitioning to the subroots responded strongly and proportionally to the nitrate distribution ratio. After 12-14 days no further effect was observed. The V_max for net nitrate uptake and in vitro nitrate reductase (NR) activity were measured in acclimated plants, i.e., after > 14 days under a certain nitrate regime. In subroots fed from 20 to 100% of the total N addition, V_max for net nitrate uptake increased slightly, whereas NR activity was unaffected. Uptake and NR activities were insignificant in the 0%-subroot. Uneven nitrate supply to individual subroots had negligible effect on the whole-plant ability for nitrate uptake, and the relative V_max (unit N taken up per unit N in whole plant tissue and time) remained about 7-fold in excess of the demand set by growth. Balancing nitrate concentrations (the resulting external nitrate concentrations at a certain RA) generally ranged between 2 and 10 μM at growth-limiting RA, both when predicted from uptake kinetics and when actually measured. When comparing split root and standard cultures when acclimated, it appears that uptake and NR activities in roots respond more strongly to over-all nitrate availability than to nitrate availability to individual subroots.     

THE EFFECTS OF LIGHT AND NITROGEN ON GROWTH,PIGMENT CONTENT,AND BIOCHEMICAL COMPOSITION OF GRACILARIA FOLIIFERA V. ANGUSTISSIMA (GIGARTINALES,RHODOPHYTA)1

The combined effects of light intensity and nitrogen (NO₃^?) on growth rate, pigment content, and biochemical composition of Gracilaria foliifera v. angustissima (Harvey) Taylor was investigated using outdoor continuous cultures. Growth of Gracilaria increased linearly with increasing light to 0.43 doublings d^?1 at high light levels (383 ly d^?1 of in situ light), suggesting that light may often limit growth of this plant in nature. Chlorophyll a and phycoerythrin contents were inversely proportional to light level and growth rate. However, pigment content did not affect the growth capacity of Gracilaria. There was no increase in growth or pigment content with increasing additions of nitrogen. The low nitrogen treatment was unenriched seawater that had higher NO₃^? levels than most coastal waters (influent = 8.61 μM; residual = 0.94 μM). When growing near its maximum rate under high light intensities, Gracilaria had a significantly (P < 0.001) lower phycoerythrin: chlorophyll a ratio (phyco: Chl a) than did Gracilaria growing more slowly under lower light (Phyco:Chl a of 2.8 ± 0.2 vs. 3.8 ± 0.3). Faster growing plants also had C:N ratios above 10, indicating N- limitation. In addition to harvesting light the phycobiliproteins of Gracilaria may store nitrogen. Growth rates of Gracilaria correlated negatively with ash (r =–0.85) and positively with the carbon: phycoerythrin ratio (r = 0.85), suggesting that these two indices can be used to estimate growth in the field.     

SHORT‐TERM TEMPORAL VARIABILITY OF AMMONIUM AND UREA UPTAKE BY ALEXANDRIUM CATENELLA (DINOPHYTA) IN CULTURES1

In batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the ¹⁵N tracer technique. Ammonium uptake could be described by Michaelis–Menten kinetics along a substrate gradient of 0.1–10 μgat N · L^?1 for the four strains, while two different patterns were observed for urea uptake with Michaelis–Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (V_max; for saturable kinetics) and in the initial slope α. For ammonium, V_max increased by a factor of 2–10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2–18. Temporal variations of half‐saturation constants (K_s) for both nutrients did not show a clear trend. Increases in V_max and α showed an acclimation of the cells’ uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growth.     

Effect of nitrogenous resource on growth,biochemical composition and ultrastructure of Isochrysis galbana (Isochrysidales,Haptophyta)

The nitrogenous resource used to promote algal growth has cost implications for mass culture processes. The present study therefore aimed to determine the effect of different nitrogenous resources (nitrate, ammonium and urea) on various performance parameters (growth, final cell yield, pigmentation, lipid yield and cellular and sub‐cellular characteristics) in Isochrysis galbana. Growth rate was unaffected by nitrogenous resource, but the final cellular yield in the nitrate and urea treatments far exceeded that evident in the ammonium treatments. The reduced cell yield in ammonium treatments and the earlier onset of the stationary phase was brought about by nitrogen‐starvation due to an increase in pH and resultant ammonia volatilization. This starvation initiated an early onset of lipid accumulation, chlorophyll depletion and an increase in the carotenoid to chlorophyll ratio relative to the other nitrogen (N) source treatments. Hence, in spite of being potentially the preferred source of N by algae (due to its reduced state), ammonium‐nitrogen is undesirable for mass culture. The performance parameters of Isochrysis grown in urea (an organic N source) and nitrate (an inorganic N source) were similar, but lipid accrued earlier in cells grown in medium supplemented with urea. This is advantageous for lipid acquisition for the production of biodiesel since it would reduce the duration of photobioreactor runs. Urea is easily available and considerably cheaper than all the other N sources tested and is thus recommended as the nitrogenous resource for large‐scale culture of I. galbana for biodiesel production.     

Diagnosis of nitrogen deficiency and toxicity of Eucalyptus globulus seedlings by foliar analysis

The relationship between shoot growth and foliar nitrogen (N) in E. globulus seedlings was studied in the glasshouse to determine standard values for N deficiency and toxicity diagnosis. Seedlings were grown for 9 weeks in yellow sand, at 10 rates of N, applied as ammonium sulphate, calcium nitrate or ammonium nitrate. Shoot dry weight (DW) increased linearly with N rate for all forms of N in the deficiency range. Seedlings continued to respond to higher rates of ammonium and ammonium nitrate than to nitrate. Maximum shoot DW for nitrate fed plants and ammonium nitrate fed plants were 51% and 84% respectively of ammonium fed plants. Total N concentration in the youngest fully expanded leaf (YFEL) ranged from 1.0% to 3.3% in deficient and adequate plants. The critical N concentration for deficiency diagnosis (corresponding to 90% maximum yield) in the YFEL, determined from these growth response curves averaged over all N forms, was 2.6% N. For ammonium nitrate fed plants, total N concentration in the YFEL for the severely deficient, deficient, adequate, and toxic ranges were <1.4%, 1.4–2.5%, 2.6–3.5%, > 4.3%. High total N concentrations were associated with growth depression and toxicity symptoms, which differed with N form. For nitrate fed plants, a total N concentration above 3.3% in the YFEL was associated with severe growth depression, and leaf tip necrosis. The adequate concentration range for ammonium nitrate was similar to values found on a field trial with 7 month old E. globulus trees grown on an exforest site.     

STEADY-STATE LUXURY CONSUMPTION AND THE CONCEPT OF OPTIMUM NUTRIENT RATIOS: A STUDY WITH PHOSPHATE AND NITRATE LIMITED SELENASTRUM MINUTUM (CHLOROPHYTA)1

Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non-limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in k_Q·Q_max^?1*, was similar in magnitude to previously reported values but k_Q·Q_max^?1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (k_Qi·K_Qj^?1) is inappropriate for dealing with a species growing under steady-state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed. The capacity for luxury consumption differed between nutrients and was growth rate dependent. At low growth rates, the coefficient of luxury consumption (R_sat) for P was ca. four times that for N. The set of all possible relationships between N and P cell quota under these conditions was reported and these values were then used to establish the cellular N:P niche boundaries for S. minutum. Cell quotas of non-limiting nutrients were not described by the Droop equation. Analysis showed that as the cellular N:P ratio deviates from the optimum ratio, the ability of the Droop equation to describe the relationship between growth rate and non-limiting cell quotas decreases. When non-limiting nutrient cell quotas are saturated, the Droop equation appears to be invalid. Previously reported patterns of non-limiting nutrient utilization are summarized in support of this conclusion. The physiological and ecological consequences of luxury consumption and growth rate dependent optimum ratios are considered.     

EFFECTS OF EXTERNAL INORGANIC NITROGEN CONCENTRATION ON METABOLISM,GROWTH AND ACTIVITIES OF KEY CARBON AND NITROGEN ASSIMILATORY ENZYMES OF LAMINARIA SACCHARINA (PHAEOPHYCEAE) IN CULTURE1

The growth rate of Laminaria saccharina (L.) Lamour. is dependent on inorganic nitrogen in culture. Growth rates were saturated between 5 and 10 μmol · L^?1 nitrate. The activities of ribulose-1,5 bisphosphate carboxylase, phosphoenolpyruvate carboxykinase, mannitol-1-phosphate dehydrogenase, nitrate reductase and glutamine synthetase also varied with the concentration of inorganic nitrogen in the medium. All enzyme activities were lowest at 2.5 μmol · L^?1 nitrate (the lowest concentration used) increasing to a maximum activity between 10 and 30 μmol · L^?1 nitrate. Most enzyme activities followed a hyperbolic curve resembling those described by the Michaelis-Menten equation, with different half-saturation constants.     

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.     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Does nitrogen availability control rates of litter decomposition in forests?

The effects of increased exogenous N availability on rates of litter decomposition were assessed in several field fertilization trials. In a jack pine (Pinus banksiana Lamb.) forest, needle litter decomposed at the same rate in control plots and in plots fertilized with urea and ammonium nitrate (1350 kg N ha^-1) with or without P and K. Mixed needle litter of western hemlock (Tsuga heterophylla (Raf.) Sarg.), western red cedar (Thuja plicata Donn) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) incubated in plots recently amended with sewage sludge (500 kg N ha^-1) lost less weight during 3 years than did litter in control plots. Forest floor material also decomposed more slowly in plots amended with sewage sludge. Paper birch (Betula papyrifera Marsh.) leaf litter placed on sewage sludge (1000 kg N ha^-1), pulp sludge, or sewage-pulp sludge mixtures decomposed at the same rate as leaf litter in control plots. These experiments demonstrate little effect of exogenous N availability on rates of litter decomposition.The influence of endogenous N availability on rates of litter decomposition was examined in a microcosm experiment. Lodgepole pine (Pinus contorta var. latifolia Engelm.) needle litter collected from N-fertilized trees (525 kg N ha^-1 in ammonium nitrate) were 5 times richer in N than needles from control trees (1.56% N versus 0.33% N in control trees), but decomposed at the same rate. Green needles from fertilized trees contained twice as much N as needles from control trees (1.91% N versus 0.88% N), but decomposed at the same rate. These experiments suggest that N availability alone, either exogenous or endogenous, does not control rates of litter decomposition. Increased N availability, through fertilization or deposition, in the absence of changes in vegetation composition, will not alter rates of litter decomposition in forests.