Effect of primary leaves on 59Fe uptake by roots and 59Fe distribution in the shoot of iron sufficient and iron deficient bean (Phaseolus vulgaris L.) plants

A study has been made on the effect of primary leaves on iron (Fe) distribution in the shoot. Bean (Phaseolus vulgaris L.) seedlings were precultured in nutrient solution with 8×10^-5 M FeEDTA for 4 days, and then grown further with either 8×10^-5 M FeEDTA (+Fe) or without Fe supply (-Fe) for another 5 days. Thereafter, both +Fe and -Fe plants were treated in three different ways: undisturbed; one primary leaf removed; or one primary leaf shaded, starting two hours before supply ⁵⁹FeEDTA to the roots. The +Fe plants were supplied with 8×10^-5 M ⁵⁹FeEDTA, and the -Fe plants with only 1×10^-6 M ⁵⁹FeEDTA. After 1 to 8 hour uptake periods, plants were harvested and ⁵⁹Fe in different organs was determined. Removal or shading of one primary leaf did not affect ⁵⁹Fe uptake by roots and ⁵⁹Fe translocation to the shoot in +Fe plants. In the -Fe plants, however, removal of one primary leaf decreased ⁵⁹Fe uptake by roots, whereas shading of one primary leaf had no effect on ⁵⁹Fe uptake but slightly enhanced ⁵⁹Fe translocation from roots to the shoot. The quantity of ⁵⁹Fe in primary leaves was positively correlated with quantity of ⁵⁹Fe in the stem in the -Fepplants, but not in the +Fe plants. In both, the +Fe and -Fe plants, the quantity of ⁵⁹Fe in the shoot apex was positively correlated with ⁵⁹Fe in primary leaves. The results suggest that irrespective of the Fe nutritional status of plants, the source of Fe for the shoot apex is Fe retranslocated from primary leaves.     

Combination Effect of NaCl Salinity and Nitrogen Form on Mineral Composition of Sunflower Plants

The effect of two N-forms (NH₄ ⁺ and NO₃ ^–) and NaCl on pattern of accumulation of some essential inorganic nutrients was examined in sunflower (Helianthus annuus L.) cv. Hisun 33. Eight-day-old plants of were subjected for 21 d to Hoagland's nutrient solution containing 8 mM N as NH₄ ⁺ or NO₃ ^·, and salinized with and addition of NaCl to the growth medium had no significant effect on total leaf N. However, root N of NH₄-supplied plants decreased significantly with increase in NaCl concentration, whereas that of NO₃-supplied plants remained unaffected. There was no significant effect of NaCl on leaf or root P, but the NO₃-supplied plants had significa concentration of leaf P than that of NH₄-supplied plants at varying salt treatments. Salinity of the rooting med did not show any significant effect on Na⁺ concentrations of leaves or roots of plants subjected to two differen N. NH₄-treated plants generally had greater concentrations of Cl^– in leaves and roots and lower K⁺ content in leaves than NO₃-supplied plants. Ca²⁺ concentrations of leaves and roots and Mg²⁺ concentrations of leaves decreased in NH₄-supplied plants due to NaCl, but they remained unaffected in NO₃-treated plants.     

Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

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.     

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.     

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.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] as affected by nitrogen source

A study was conducted to elucidate the effect of N form, either NH₄ ⁺ or NO₃ ^–, on growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] grown under 10 mM or 100 mM NaCl in hydroponics. Shoot biomass was not affected by N form, whereas NH₄ ⁺ compared to NO₃ ^– nutrition caused an almost 4-fold reduction in the root biomass at both salinity levels. Under NH₄ ⁺ nutrition, salinity had no effect on the biomass yield, whereas under NO₃ ^– nutrition, increasing salinity from 10 mM to 100 mM caused 23% and 36% reduction in the root and shoot biomass, respectively. The reduced root growth under NH₄ ⁺ nutrition was not attributable to impaired shoot to root C allocation since N form did not affect the overall root sugar concentration and the starch concentration was even higher under NH₄ ⁺ compared to NO₃ ^– nutrition. The low NH₄ ⁺ (2 mM) and generally higher amino-N concentrations in NH₄ ⁺- compared to NO₃ ^–-fed plants indicated that the grass was able to effectively detoxify NH₄ ⁺. Salinity had no effect on Ca²⁺ and Mg²⁺ levels, whereas their concentration in shoots was lower under NH₄ ⁺ compared to NO₃ ^– nutrition (over 66% reduction in Ca²⁺; over 20% reduction in Mg²⁺), but without showing deficiency symptoms. Ammonium compared to NO₃ ^– nutrition did not inhibit K⁺ uptake, and the K⁺-Na⁺ selectivity either remained unaffected or it was higher under NH₄ ⁺ than under NO₃ ^– nutrition. Results suggested that while NH₄ ⁺ versus NO₃ ^– nutrition substantially reduced root growth, and also strongly modified anion concentrations and to a minor extent concentrations of divalent cations in shoots, it did not influence salt tolerance of kallar grass.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Effect of Nitrate and Ammonium Nutrition of Nonnodulated Phaseolus vulgaris L. on Phosphoenolpyruvate Carboxylase and Pyruvate Kinase Activity

Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with 3.5 or 10 millimolar N in either the form of NO₃⁻ or NH₄⁺, after being grown on N-free nutrient solution for 8 days. The pH of the nutrient solutions was either 6 or 4. The cell sap pH and the extractable activities of phosphoenolpyruvate carboxylase and of pyruvate kinase from roots and primary leaves were measured over several days.

The extractable activity of phosphoenolpyruvate carboxylase (based on soluble protein) from primary leaves increased with NO₃⁻ nutrition, whereas with NH₄⁺ nutrition and on N-free nutrient solution the activity remained at a low level. Phosphoenopyruvate carboxylase activity from the roots of NH₄⁺-fed plants at pH 4 was finally somewhat higher than from the roots of plants grown on NO₃⁻ at the same pH. There was no difference in activity from the root between the N treatments when pH in the nutrient solutions was 6. The extractable activity of pyruvate kinase from roots and primary leaves seemed not to be influenced by the N nutrition of the plants.

The results are discussed in relation to the physiological function of both enzymes with special regard to the postulated functions of phosphoenolpyruvate carboxylase in C3 plants as an anaplerotic enzyme and as part of a cellular pH stat.

     

Maize and pigweed response to nitrogen supply and form

As nitrogen management practices change to achieve economic and environmental goals, effects on weed-crop competition must be examined. Two greenhouse experiments investigated the influence of N amount and form on growth of maize and redroot pigweed (Amaranthus retroflexus L.). In Experiment 1, maize and pigweed were grown together in a replacement series (maize: pigweed ratios of 0:4, 1:3, 2:2, 3:1, 4:0) under three NH₄NO₃-N supplies (0, 110, and 220 mg N kg^-1 soil). Maize was planted into established pigweed and plants were harvested 24 days after maize germination. Pigweed responded more to supplemental N than maize and accumulated 2.5 times as much N in shoots at the high N supply. Competition effects were not significant. Maize and pigweed were grown separately in Experiment 2 and supplied 220 mg N kg^-1 as either Ca(NO₃)₂ or (NH₄)₂SO₄ plus a nitrification inhibitor (enhanced ammonium supply, EAS). In maize, EAS treatment did not affect shoot growth and reduced root growth 25% relative to the NO₃-N treatment. In pigweed, shoot and root growth were restricted 23 and 86% by EAS treatment, respectively. Total plant N accumulation under EAS treatment was higher in maize, less in pigweed. Under EAS treatment, pigweed leaves were crinkled and chlorotic; leaf disks extracted in 70% ethanol, pH 3, contained less malate and oxalate but more NH₄ ⁺ compared to the NO₃-N treatment. Maize leaf disk malate levels were generally higher compared to pigweed but were less due to EAS treatment. Ammonium level in maize leaf disks was unaffected by N form and EAS treatment increased oxalate levels. Final bulk soil pH was generally lower in pots where pigweed were grown and tended to be lower due to EAS. Leaf disk malate levels and soil pH were positively associated. Results indicate that pigweed is more likely to compete with maize when high levels of NO₃-N are provided. Enhancing the proportion of N supplied as NH₄ ⁺ should restrict the growth of NH₄ ⁺-sensitive pigweed.     

Importance of the nitrogen source in the grass species Brachiaria brizantha responses to sulfur limitation

Background and aims

Plant responses to S supply are highly dependent on N nutrition. We investigated the effect of S status on metabolic, nutritional, and production variables in Brachiaria brizantha treated with different N forms. Additionally, ¹⁵N and ³⁴S root influx were determined in plants under short- and long-term S deprivation.

Methods

Plants were submitted to soil fertilization treatments consisted of combinations of N forms [without N, ammonium (NH₄ ⁺), nitrate (NO₃ ^?) or NH₄ ⁺+NO₃ ^?] at S rates (0, 15, 30, or 45 mg dm^?3). N and S influx capacity was determined in hydroponically-grown plants.

Results

Shoot production due to S supply increased 53, 145 and 196 % with NH₄ ⁺, NH₄ ⁺+NO₃ ^? and NO₃ ^? treatments, respectively. No or low S impaired protein synthesis and led to high accumulation of N-NO₃ ^? and asparagine in NO₃ ^?-fed plants, both alone and with NH₄ ⁺. Proline accumulation was observed in NH₄ ⁺-fed plants. Short- and long-term S deprivation did not promote considerable changes in ¹⁵N influx. ³⁴S absorption decreased depending on the N form provided: NH₄ ⁺+NO₃ ^? > only NH₄ ⁺ > only NO₃ ^? > low N.

Conclusions

Including both NH₄ ⁺ and NO₃ ^? forms in fertilizer increases N and S intake potential and thereby enhances plant growth, nutritional value and production.     

Growth responses of the perennial legume Sesbania sesban to NH4 and NO3 nutrition and effects on root nodulation

Preference for NH₄⁺ or NO₃⁻ nutrition by the perennial legume Sesbania sesban (L.) Merr. was assessed by supplying plants with NH₄⁺ and NO₃⁻ alone or mixed at equal concentrations (0.5 mM) in hydroponic culture. In addition, growth responses of S. sesban to NH₄⁺ and NO₃⁻ nutrition and the effects on root nodulation and nutrient and mineral composition of the plant tissues were evaluated in a hydroponic setup at a range of external concentration of NH₄⁺ and NO₃⁻ (0, 0.1, 0.2, 0.5, 2 and 5 mM). Seedlings of S. sesban grew equally well when supplied with either NH₄⁺ or NO₃⁻ alone or mixed and had high relative growth rates (RGRs) ranging between 0.19 and 0.21 d⁻¹. When larger plants of S. sesban were supplied with NH₄⁺ or NO₃⁻ alone, the RGRs and shoot elongation rates were not affected by the external concentration of inorganic N. At external N concentrations up to 0.5 mM nodulation occurred and contributed to the N nutrition through fixation of gaseous N₂ from the atmosphere. For both NH₄⁺ and NO₃⁻-fed plants the N concentration in the plant tissues, particularly water-extractable NO₃⁻, increased at high supply concentrations, and concentrations of mineral cations generally decreased. It is concluded that S. sesban can grow without an external inorganic N supply by fixing atmospheric N₂ gas via root nodules. Also, S. sesban grows well on both NH₄⁺ and NO₃⁻ as the external N source and the plant can tolerate relatively high concentrations of NH₄⁺. This wide ecological amplitude concerning N nutrition makes S. sesban very useful as a N₂-fixing fallow crop in N deficient areas and also a candidate species for use in constructed wetland systems for the treatment of NH₄⁺ rich waters.     

Metabolic regulation and gene expression of root phosphoenolpyruvate carboxylase by different nitrogen sources

Alfalfa (Medicago sativa L.) N-sufficient plants were fed 1·5 mM N in the form of NO₃⁻, NH₄⁺ or NO₃⁻ in conjunction with NH₄⁺, or were N-deprived for 2 weeks. The specific activity of phosphoenolpyruvate carboxylase (PEPC) from the non-nodulated roots of N-sufficient plants was increased in comparison with that of N-deprived plants. The PEPC value was highest with NO₃⁻ nutrition, lowest with NH₄⁺ and intermediate in plants that were fed mixed salts. The protein was more abundant in NO₃⁻-fed plants than in either NH₄⁺- or N mixed-fed plants. Nitrogen starvation decreased the level of PEPC mRNA, and nitrate was the N form that most stimulated PEPC gene expression. The malate content was significantly lower in NO₃⁻-deprived than in NO₃⁻-sufficient plants. Root malate accumulation was high in NO₃⁻-fed plants, but decreased significantly in plants that were fed with NH₄⁺. The effect of malate on the desalted enzyme was also investigated. Root PEPC was not very sensitive to malate and PEPC activity was inhibited only by very high concentrations of malate. Asparagine and glutamine enhanced PEPC activity markedly in NO₃⁻-fed plants, but failed to affect plants that were either treated with other N types or N starved. Glutamate and citrate inhibited PEPC activity only at optimal pH. N-nutrition also influenced root nitrate and ammonium accumulation. Nitrate accumulated in the roots of NO₃⁻- and (NO₃⁻ + NH₄⁺)-fed plants, but was undetectable in those administered NH₄⁺. Both the nitrate and the ammonium contents were significantly reduced in NO₃⁻- and (NO₃⁻ + NH₄⁺)-starved plants. Root accumulation of free amino acids was strongly influenced by the type of N administered. It was highest in NH₄⁺-fed plants and the most abundant amides were asparagine and glutamine. It was concluded that root PEPC from alfalfa plants is N regulated and that nitrate exerts a strong influence on the PEPC enzyme by enhancing both PEPC gene expression and activity.     

Effects of supplied nitrogen form on growth and water uptake of French bean (Phaseolus vulgaris L.) plants

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO^– ₃) supply (NN), sole ammonium (NH⁺ ₄) supply (AA) and spatially separated supply of NO^– ₃ and NH⁺ ₄ (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO^– ₃ revealed a 170% higher DM than the root half exposed to NH⁺ ₄. N form affected stable carbon-isotope discrimination () of leaf tissue. In leaves of plants which were supplied with NH⁺ ₄ (AA; NA) was significantly more negative (–29.4, –29.6) than in NN treatment (–28.2). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative of root tissue under NH⁺ ₄ supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO^– ₃ was 104% higher than in those receiving NH⁺ ₄. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.     

Effects of N source on proton excretion,ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4

Summary Non-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO₃; NO₃/NH₄; and NH₄) at a total N level of 4 meq.l^–1, and growth was ccmpared with nodulated plants at zero N (N₂ fixation). Of the combined-N sources, 100 per cent NH₄ resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO₃.During the first 3 weeks, the yield of the N₂-fixing plants was as high as that of the NH₄ plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N₂-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO₃ was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH₄ or N₂ was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N₂ fixation amounted toc. 0.5 meq H⁺.mmol^–1 N_org, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith⁴³ for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO₃/NH₄, NH₄ and N₂ were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH₄ plants ceased growing at a pH of 2.6.