Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH₄ ⁺) [as (NH₄)₂SO₄], nitrate (NO₃ ⁻) (as NaNO₃), or a mixed N source (NH₄NO₃) at 0, 800, or 1,600 mg N plant⁻¹ season⁻¹. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO₃ ⁻-fed and NH₄NO₃-fed plants compared to observed responses in NH₄ ⁺-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO₃ ⁻-fed than in NH₄ ⁺-fed plants. Exogenous NO₃ ⁻ was assimilated in roots or stored, so no difference was observed in NO₃ ⁻ levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO₃ ⁻ or NH₄ ⁺ alone, suggesting NH₄NO₃ is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.     

Micronutrients and nitrogen metabolism

A sand-culture experiment was conducted to study the influence of a deficiency of and an excess of micronutrients on the uptake and assimilation of NH ₄ ⁺ and NO ₃ ⁻ ions by maize. By studying the fate of¹⁵N supplied as¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃, it was demonstrated that in maize plants NH₄−N was absorbed in preference to NO ₃ ⁻ −N. The uptake and distribution of N originating from both NH ₄ ⁺ and NO ₃ ⁻ was considerably modified by deficiency of, or an excess of, micronutrients in the growth medium. The translocation of NH ₄ ⁺ −N from roots to shoots was relatively less than that of NO ₃ ⁻ −N. Deficiency as well as excessive amounts of micronutrients, in the growth medium, substantially reduced the translocation of absorbed N into protein. This effect was more pronounced in the case of N supplied as NO ₃ ⁻ . Amino-N was the predominant non-protein fraction in which N from both NH ₄ ⁺ and NO ₃ ⁻ tended to accumulate. The next important non-protein fractions were NO ₃ ⁻ −N when N was supplied as NO ₃ ⁻ and amide-N when NH ₄ ⁺ was the source. The relative accumulation of¹⁵N into different protein fractions was also a function of imposed micronutrient levels.     

Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NH ₄ ⁺ vs. NO ₃ ⁻ nutrition (50 or 100 mg kg⁻¹ soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NH ₄ ⁺ as the predominant N source for plants grown in NH ₄ ⁺ -treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH ₄ ⁺ than under NO ₃ ⁻ nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH ₄ ⁺ than under NO ₃ ^– supplied at 150 mg N kg⁻¹ soil; whereas at both N levels applied, NH ₄ ⁺ compared to NO ₃ ⁻ nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH ₄ ⁺ vs. NO ₃ ⁻ nutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NH ₄ ⁺ of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH ₄ ⁺ than under NO ₃ ⁻ nutrition.     

Nitrate and ammonium accumulation in bermudagrass in relation to nitrogen fertilization and season

Seasonal changes in nitrate and ammonium concentrations were studied inCynodon dactylon (L.) Pers. plants grown for one year in the field in a Mediterranean area. Plants cultivated in a sandy loam soil were fertilized with nitrate-N or ammonium-N at two application rates (250 and 1000 kg N ha⁻¹ year⁻¹) and compared to controls with no added N. Plots were harvested every three weeks from May to November. Shoots were separated into leaves and stems and analyses carried out in both fractions. Nitrogen applications generally led to elevated nitrate concentrations both in leaves and stems at all sampling dates but had little influence on the ammonium concentrations of the tissues. Higher nitrate and ammonium concentrations were found in stems than in leaves, although no levels higher than 0.22% NO ₃ ⁻ −N and 0.10% NH ₄ ⁺ −N were detected in either fraction. Nitrate tended to accumulate mostly in autumn and spring whereas low accumulations were found in summer. Ammonium showed both in leaves and stems a progressive but limited accumulation throughout the period with a peak in October, followed by a strong decrease in November.     

Accumulation of inorganic and organic osmolytes and their role in osmotic adjustment in NaCl-stressed vetiver grass seedlings

The accumulation of inorganic and organic osmolytes and their role in osmotic adjustment were investigated in roots and leaves of vetiver grass (Vetiveria zizanioides) seedlings stressed with 100, 200, and 300 mM NaCl for 9 days. The results showed that, although the contents of inorganic (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻, SO₄²⁻ and H₂PO₃⁻)) and organic (soluble sugar, organic acids, and free amino acids) osmolytes all increased with NaCl concentration, the contribution of inorganic ions (mainly Na⁺, K⁺, and Cl⁻) to osmotic adjustment was higher (71.50–80.56% of total) than that of organic solutes (19.43–28.50%). The contribution of inorganic ions increased and that of organic solutes decreased in roots with the enhanced NaCl concentration, whereas the case in leaves was opposite. On the other hand, the osmotic adjustment was only effective for vetiver grass seedlings under moderate saline stress (less than 200 mM NaCl).     

The effects of low,regulated supplies of nitrate and ammonium nitrogen on the growth and composition of perennial ryegrass

Summary Perennial ryegrass was grown in flowing solution culture with nitrogen supplied in amounts that increased exponentially,i.e. in parallel with the rate of increase in growth. Nitrogen was supplied as either NO ₃ ⁻ or NH ₄ ⁺ , and the amounts to be added were calculated on the basis of extrapolated values for dry weights obtained from fitted curves. There were two rates of addition for each form of N aimed at providing adequate (5.0 per cent) and less than adequate (2.75 per cent) contents in the plants in each case. Measured plant weights and N concentrations were in close agreement with predicted values over a four week experimental period. There was no effect of N-form at high N, and these plants produced 46 per cent more dry matter than the plants at low N. Only minor differences in overall growth occurred with NO ₃ ⁻ or NH ₄ ⁺ plants at low N, but the NH ₄ ⁺ plants had a greater shoot:root ratio. The absorption rate (m mol Ng⁻ root d⁻¹) for NH ₄ ⁺ -N was therefore greater than for NO ₃ ⁻ -N. The cation/anion composition of the plants was affected in a predicable way, and to a greater or lesser extent at high or low N, respectively, in NO ₃ ⁻ or NH ₄ ⁺ plants. The major changes in cation composition came through effects on potassium absorption. Plants with low NO ₃ ⁻ appeared to be under greater N stress than those with low NH ₄ ⁺ because of the lower shoot:root ratio and the greater C∶N ratio in the shoots.     

Nitrate reductase activity of two leafy vegetables as affected by nickel and different nitrogen forms

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO₃ ⁻–N, NH₄ ⁺–N, NH₄NO₃). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO₃ ⁻–N) or mixed (NH₄NO₃) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO₃ ⁻–N or NH₄NO₃. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO₃ ⁻-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH₄ ⁺–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH₄ ⁺–N the enzyme activity in roots was even higher than in those supplied with NO₃ ⁻–N.     

The influence of elevated rhizosphere dissolved inorganic carbon concentrations on respiratory O2 and CO2 flux in tomato roots

Respiratory CO2 and O2 flux were measured in hydroponically grown Lycopersicon esculentum (L.) Mill. cv. F144 plants at either low (O mol mol^-1) or elevated CO2 concentrations (>2000 mol mol^-1) supplied to the roots. In NO3^- fed plants the consumption of O2 and the engagement of the alternative pathway were increased by elevated dissolved inorganic carbon (DIC = CO2 + HCO3^-) concentrations. This was ascribed to the influence of organic acids on the TCA cycle and electron transport pathways. Inhibition of O2 consumption by elevated DIC in NH4^--fed plants may be due to the reduction requirements of anaplerotic carbon entering the TCA cycle or the removal of carbon from the TCA cycle for amino acid synthesis. In both NO3^- and NH4⁺-fed plants elevated DIC inhibited CO2 release due to high rates of DIC incorporation by phosphoenolpyruvate carboxylase (PEPc) activity. Transient net CO2 consumption due to the inhibition of respiration by salicylhydroxamic acid and KCN, together with high respiratory quotients after the addition of inhibitors of carbonic anhydrase (CA) activity, were also ascribed to high rates of DIC incorporation at elevated DIC concentrations. Ethoxyzolamide, an inhibitor of CA activity, inhibited both DI¹⁴C incorporation into organic products and NO3^- uptake by 81% and 40%, respectively. This, together with a 32% increase in DI¹⁴C accumulation and inhibition of NO3^- uptake upon inhibition of anion transport by diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) may indicate the exchange of HCO3^- for NO3^- across the root plasmalemma. It was concluded that dark incorporation of HCO3^- by PEPc increased at elevated rhizosphere DIC concentrations and that the products of DIC incorporation may stimulate respiratory electron transport. Additional reducing energy and carbon skeletons from the tricarboxylic acid (TCA) cycle would therefore be available for respiration and the reduction and incorporation of NO3^- into amino acids.Key words: Tomato, PEPc, respiration, carbon dioxide nitrate.      

Nitrate or ammonium nutrition in french bean

Summary Bean Plants were grown in a greenhouse in sand irrigated with nutrient solutions containing either 2 mM NO ₃ ^– or 2 mM NH ₄ ⁺ . After 45 days fresh weight of NH ₄ ⁺ plants was half that of NO ₃ ^– plants. Cation concentration in NH ₄ ⁺ plants was 30% less than in NO ₃ ^– plants. Amino acids (SER, ASN, GLN) accummulated 3 to 10 times more in NH ₄ ⁺ plants. The concentration of organic acids (malic, malonic, citric) was 10 to 30 times higher in NO ₃ ^– plants. The ATP-costings for the synthesis of amino acids and organic acids in NH ₄ ⁺ plants was half that of NO ₃ ^– ones: therefore it could not account for the reduction of growth in the ammonium-fed plants.     

Evidence for a periodic excretion of nitrogen by roots of grass-legume associations

Diurnal variation in ion content of the solution bathing roots of two plants growing together in sand culture was analysed for three pairs of grass-legume species (Lolium multiflorum andTrifolium pratense; Zea mays andGlycine hispida; Avena sativa andVicia sativa) and their monospecific controls. Biomass and nitrogen content of plants were determined. Ion concentration (NO ₃ ⁻ , NO ₂ ⁻ , NH ₄ ⁺ , and K⁺) and pH of root solutions were measured for Lolium-Trifolium plant pairs and controls at 6 hours intervals over 36 h, starting at 8 am within a circadian cycle. Root solutions were regularly depleted in NO ₃ ⁻ by the grasses (Lolium-Lolium control) throughout the cycle. For associations involving the legume (Lolium-Trifolium and Trifolium-Trifolium), NO ₃ ⁻ depletion was followed by NO ₃ ⁻ enrichment at night, from late afternoon to early morning; the enrichment was more marked for the Lolium-Trifolium association. Solutions which did not contain NO ₂ ⁻ ions, were enriched by trace amounts of NH ₄ ⁺ ions, largely depleted in K⁺ and alkalanized for all associations throughout the cycle. Repeating the experiment with the three pairs of species at the vegetative phase of development confirmed the previous results: NO ₃ ⁻ enrichment during the night for associations with legumes. When the experiment was repeated with older plants which had almost completed their flowering stage, depletion only was observed and no NO ₃ ⁻ enrichment. These data suggest that NO ₃ ⁻ enrichment results from N excretion from active nodulated roots of the legume, accounting for the increase in both biomass and nitrogen content of the companion grass in grass-legume association. The quantitative importance and periodicity of nitrogen excretion as well as the origin of nitrate enrichment are discussed.     

Salicylic acid-induced adaptive response to copper stress in sunflower (Helianthus annuus L.)

The ameliorative effect of salicylic acid (SA: 0.5 mM) on sunflower (Helianthus annuus L.) under Cu stress (5 mg l⁻¹) was studied. Excess Cu reduced the fresh and dry weights of different organs (roots, stems and leaves) and photosynthetic pigments (chlorophyll a, b and carotenoids) in four-week-old plants. There was a considerable increase in Chl a/b ratio and lipid peroxidation in both the roots and leaves of plants under excess Cu. Soluble sugars and free amino acids in the roots also decreased under Cu stress. However, soluble sugars in the leaves, free amino acids in the stems and leaves, and proline content in all plant organs increased in response to Cu toxicity. Salicylic acid (SA) significantly reduced the Chl a/b ratio and the level of lipid peroxidation in Cu-stressed plants. Under excess Cu, a higher accumulation of soluble sugars, soluble proteins and free amino acids including proline occurred in plants treated with 0.5 mM SA. Exogenous application of SA appeared to induce an adaptive response to Cu toxicity including the accumulation of organic solutes leading to protective reactions to the photosynthetic pigments and a reduction in membrane damage in sunflower.     

Glutamine synthetase oligomers and isoforms in sugarbeet (Beta vulgaris L.)

The -amino-N compounds that accumulate in the thickening storage root of sugarbeet (Beta vulgaris L.) were synthesized in the leaves (NO ₃ ^– nutrition) and also in the lateral roots (NH ₄ ⁺ nutrition). Ammonium stimulated glutamine synthetase (GS, EC 6.3.1.2) activity, especially in the lateral roots. With non-denaturing polyacrylamide-gel isoelectric focussing, simultaneously active charge-isomers of GS were separated in both leaves and roots. The leaf isoforms were active in an octameric and also in a tetrameric form. In the root only octameric isoforms were found. The tetramer was more active than the octamer in the leaf blade and vice versa in the leaf stem. Only the tetramer needed -mercaptoethanol for activity stabilization in vitro. A reactivation, however, of an inactive tetramer by the addition of thiol/thioredoxin was not possible. The same isoforms of GS were separated in different organs of sugarbeet but with different patterns of relative activity. The activity pattern depended also on the N-source of the plant. With increasing age of the plant the number of active GS isoforms declined in both leaves and roots although the in-vitro activity remained unchanged (NO ₃ ^– -fed plants) or even increased (NH ₄ ⁺ -fed plants).Abbreviations GS glutamine synthetase (E.C. 6.3.1.2.) - IEF isoelectric focussing - PAGE polyacrylamide gel electrophoresis This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO₃⁻ or NH₄⁺, the solutions being maintained at pH 5.5. In NO₃⁻-fed plants excess nutrient anion over cation uptake was equivalent to net OH⁻ efflux, and the total charge from NO₃⁻ and SO₄²⁻ reduction equated to the sum of organic anion accumulation plus net OH⁻ efflux. In NH₄⁺-fed plants a large H⁺ efflux was recorded in close agreement with excess cation over anion uptake. This H⁺ efflux equated to the sum of net cation (NH₄⁺ minus SO₄²⁻) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO₃⁻ that is taken up and reduced in NO₃⁻-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH₄⁺-fed plants absorbed NH₄⁺ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO₃⁻-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.     

Effects of Al on nitrogen (NH 4 + and NO 3 − ) uptake,nitrate reductase activity and proton release in two sorghum cultivars differing in Al tolerance

After growth for 17 to 36 days on nutrient solutions with NH₄NO₃ as nitrogen source (pH 4.2) dry matter of sorghum genotype SC0283 was much less affected by Al (1.5 and 3.0 ppm) than that of genotype NB9040. In the absence of Al both cultivars released protons into the nutrient solution as a result of an excess of cationic nutrients taken up. When Al was present, this proton efflux per unit dry weight increased drastically, especially with the sensitive genotype NB9040. Chemical analysis of plant material and continuous analyses of NO ₃ ⁻ and NH ₄ ⁺ in the nutrient solution indicated, that the Al-induced shift in H⁺-balance of both genotypes could almost completely be attributed to a decreased NO ₃ ⁻ /NH ₄ ⁺ uptake ratio. In vivo nitrate reductase activity (NRA) was reduced in the shoot of NB9040 and to a lesser degree in SC0283. Al-induced decrease in NRA was accompanied by similar percentual decreases in NO ₃ ⁻ tissue concentrations. Therefore this decrease is interpreted as being indirect,i.e., the consequence of the reduced NO ₃ ⁻ uptake of the plants. A direct repression of NRA by Al seems also unlikely because nitrate reductase activity of the roots (where cellular Al-concentrations should be higher than in shoots) was not affected in Al-treated plants of either genotype.     

Nutrient utilization in liquid/membrane system for watermelon micropropagation

Watermelon [Citrullus lanatus (Thunberg) Matsumura and Nakai] proliferating shoot meristems from established shoot cultures were inoculated on modified Murashige and Skoog salts medium supplemented with 10 μM 6-benzyladenine (BA) for shoot proliferation and on similar medium supplemented with 1 μM BA and 10 μM gibberellic acid (GA₃) for shoot elongation. Agar-solidified medium and microporous polypropylene membrane rafts in liquid medium were used to support the tissues. Growth over culture time of proliferating and elongating tissues in liquid and agar-solidified media were compared. Nutrient depletion in liquid medium was monitored and quantified using ion selective electrodes. Tissue fresh weights in both proliferation and shoot elongation media were greater in liquid than in agar-solidified medium. Relative dry matter content, however, was greater in agar-solidified than in liquid medium. More shoots elongated in agar-solidified than in liquid medium. The numbers of buds or unelongated shoot meristems, however, were comparable for both the liquid and agar-solidified medium. Proliferating and elongating tissues in liquid medium used Ca⁺⁺ and K⁺ minimally. NO ₃ ⁻ was utilized but not depleted by proliferating tissues. NH ₄ ⁺ , however, was depleted. Most of the NH ₄ ⁺ was utilized by the proliferating tissues within 21 days of culture when growth rate was greatest. At 35 days, residual Ca⁺⁺, K⁺, NO ₃ ⁻ , and NH ₄ ⁺ in proliferation medium were 81.0%, 67.8%, 55.7%, and 1.2% of initial levels, respectively. NO ₃ ⁻ and NH ₄ ⁺ in shoot elongation medium were depleted. The greatest NO ₃ ⁻ and NH ₄ ⁺ utilization was observed during the first 14 days of culture when the largest growth rate was obtained. The residual Ca⁺⁺, K⁺, NO ₃ ⁻ , and NH ₄ ⁺ in shoot elongation medium at 38 days were 63.5%, 37.9%, 21.2%, and 24.3% of initial concentrations, respectively. At the end of experiment, 72.3% and 42.8% of initial sugars were still remaining in the shoot proliferation and shoot elongation medium, respectively. Technical Contribution No. 3236 of the South Carolina Agricultural Experiment Station.     

Utilization of exogenously supplied 14C-saccharose into primary metabolites and alkaloid production in Catharanthus roseus

Partitioning of exogenously supplied U-¹⁴C-saccharose into primary metabolic pool as sugars, amino acids, and organic acids was analyzed and simultaneous utilization for production of alkaloid by leaf, stem, and root in twigs and rooted plants of Catharanthus roseus grown in hydroponic culture medium was determined. Twigs revealed comparable distribution of total ¹⁴C label in leaf and stem. Stems contained significantly higher ¹⁴C label in sugar fraction and in alkaloids [47 kBq kg⁻¹(DM)] than leaf. In rooted plants, label in ¹⁴C in metabolic fractions in root such as ethanol-soluble, ethanol-insoluble, and chloroform-soluble fractions and in components such as sugars, amino acids, and organic acids were significantly higher than in stems and leaves. This was related with significantly higher content of ¹⁴C in alkaloids in stems and leaves. ¹⁴C contents in sugars, amino acids, and organic acids increased from leaf to stem and roots. Roots are the major accumulators of metabolites accompanied by higher biosynthetic utilization for alkaloid accumulation.     

Nitrogen-sodium concentrated complex fertilisers (CCF’s)versus nitrogen-sodium blends: Effects on ammonium and nitrate uptake by perennial ryegrass plants replete with potassium

There is ample experimental evidence that, Na, if supplied in separate fertiliser granules or crystals to N, i.e., in blended fertiliser form, can improve both the yield and the recovery of fertiliser N by grassland swards in situations of limited K supply, but not in situations of K abundance. There is some evidence, though, that in K-replete situations, Na, if supplied in the same fertiliser granule as N, i.e. in concentrated complex fertiliser (CCF) form, also improves dry matter production and N recovery by swards whilst lowering the risk of grass tetany in grazing animals. However, the mechanism for the latter effect of Na on N uptake has never been elucidated, nor has it been clarified whether Na stimulates NH ₄ ⁺ and NO ₃ ⁻ uptake by plants or simply NO ₃ ⁻ uptake alone. The aim of the present study was to see if supplying Na in the same fertiliser pellets (NNa-CCF) as NH₄NO₃ (differentially labelled with¹⁵N), or in separate pellets (NNa-blend), had any effect on the recovery of¹⁵N-labelled NH ₄ ⁺ and NO ₃ ⁻ -N by perennial ryegrass plants growing in a glasshouse under K-replete conditions. The results of the experiment confirmed that using an NNa-CCF was more beneficial to shoot production than using an NNa-blend. However, the differential in shoot production occurred without any corresponding difference in total N (i.e. NH ₄ ⁺ plus NO ₃ ⁻ -N) recovery in shoot tissue. Instead, Na, in the CCF appears to have stimulated NO ₃ ⁻ uptake at the expense of NH ₄ ⁺ absorption, thereby altering the balance between NH ₄ ⁺ and NO ₃ ⁻ -nutrition in favour of NO ₃ ⁻ -nutrition, and stimulating shoot production as a consequence. It was concluded that if grassland is already well supplied with K it would be more beneficial in terms of sward production to apply a Na and N-containing CCF than a blend of separate Na and N-containing granules or crystals.     

Effects of aluminium and mineral nutrition on growth and chemical composition of hydroponically grown seedlings of five different forest tree species

Forest die-back and impaired tree vitality have frequently been ascribed to Al-toxicity and Al-induced nutritional disorders due to increased acidification of forest soils. Therefore, in this experiment effects of Al were studied on growth and nutrient uptake with seedlings of five different forest tree species. During growth in culture solutions with and without Al all five species proved to be very Al-tolerant, despite high accumulation of Al in roots. In the coniferous evergreens Douglas-fir and Scots pine shoot as well as root Al concentrations were significantly higher than in the deciduous broad-leaved species oak and birch. Larch showed intermediate Al levels. In none of the five species did Al reduce nutrient concentrations or the Ca/Al ratio to values below the critical level. Besides differences in Al accumulation, coniferous and broad-leaved species also differed with respect to uptake and assimilation of nitrogen. Due to extra NH ₄ ⁺ uptake, oak and birch showed a much higher N uptake and higher NH ₄ ⁺ preference than the coniferous species. Especially with oak this high NH ₄ ⁺ preference in combination with a low specific root surface area resulted in a high root proton efflux density. In comparison to both broad-leaved trees and Scots pine the NO ₃ ⁻ reduction capacity of larch and Douglas-fir was extremely low. This may have important consequences for both species if grown in NO ₃ ⁻ -rich soils.