The influence of phosphate nutrition on H ion efflux from the roots of young rape plants

Changes in pH around the roots of young rape plants were studied using a nutrient film technique which allowed either part or all of the root system to be subjected to specific nutrient treatments. The rapidity and direction of change of pH was assessed by embedding absorbing roots in a thin layer of agar containing bromocresol purple. Measurements were also made with a pH microelectrode placed next to the roots. Phosphate-fed plants were deprived of phosphate when 14 days old. Patterns of pH changes round the deprived roots were the same as with phosphate-fed plants until the plants had been deprived of P for three days, when H ion efflux started in the terminal portions of the roots. The lengths of root producing acid and amounts of H ion both increased as the plants became more P deficient. Both P fed and P deprived roots produced HCO₃ ions but the net amount of HCO₃ ion produced by the P deficient roots fell as did nitrate uptake rates. Cation-anion balances measured at the end of the experiment showed that uptake of all anions and K decreased in the P deprived plants but uptake of Ca and Mg were little altered. This resulted in a smaller ratio of anions to cations absorbed which was reflected in the reduced HCO₃ ion efflux.     

Effects of a localized supply of nitrate on NO3 - uptake rate and growth of roots in Lolium multiflorum Lam.

Roots of higher plants are usually exposed to varying spatial and temporal changes in concentrations of soil mineral nitrogen. A split root system was used to see how Lolium multiflorum Lam. roots adapt to such variations to cope with their N requirements. Plants were grown in hydroponic culture with their root system split in two spatially separated compartments allowing them to be fed with or without KNO₃. Net NO₃ ^- uptake, ¹⁵NO₃ ^- influx and root growth were studied in relation to time. Within less than 24 h following deprivation of KNO₃ to half the roots, the influx in NO₃ ^- fed roots was observed to increase (about 200% of the influx measured in plant uniformly NO₃ ^- supplied control plant) thereby compensating the whole plant for the lack of uptake by the N deprived roots. Due to the large NO₃ ^- concentrations in the roots, the NO₃ ^- efflux was also increased so that the net uptake rate increased only slightly (35% maximum) compared with the values obtained for control plants uniformly supplied with NO₃ ^-. This increase in net NO₃ ^- uptake rate was not sufficient to compensate the deficit in N uptake rate of the NO₃ ^- deprived split root in the short term. Over a longer period (>1 wk), root growth of the part of the root system locally supplied with NO₃ ^- was stimulated. An increase in root growth was mainly responsable for the greater uptake of nitrate in Lolium multiflorum so that it was able to fully compensate the deficit in N uptake rate of the NO₃ ^- deprived split root.     

Nitrate uptake and nitrite release by tomato roots in response to anoxia

Excised root systems of tomato plants (early fruiting stage, 2nd flush) were subjected to a gradual transition from normoxia to anoxia by seating the hydroponic root medium while aeration was stopped. Oxygen level in the medium and respiration rate decreased and reached very low values after 12 h of treatment, indicating that the tissues were anoxic thereafter. Nitrate loss from the nutrient solution was strongly stimulated by anoxia (after 26 h) concomitantly with a release of nitrite starting only after 16 h of treatment. This effect was not observed in the absence of roots or in the presence of tungstate, but occurred with whole plants or with sterile in vitro cultured root tissues. These results indicate that biochemical processes in the root involve nitrate reductase. NR activity assayed in tomato roots increased during anoxia. This phenomenon appeared in intact plants and in root tissues of detopped plants. The stimulating effect of oxygen deprivation on nitrate uptake was specific; anoxia simultaneously entailed a release of orthophosphate, sulfate, and potassium by the roots. Anoxia enhanced nitrate reduction by root tissues, and nitrite ions were released into xylem sap and into medium culture. In terms of the overall balance, the amount of nitrite recovered represented only half of the amount of nitrate utilized. Nitrite reduction into nitric oxide and perhaps into nitrogen gas could account for this discrepancy. These results appear to be the first report of an increase in nitrate uptake by plant roots under anoxia of tomato at the early fruiting stage, and the rates of nitrite release in nutrient medium by the asphyxiated roots are the fastest yet reported.     

Fate of nitrate during initial nitrate utilization by nitrogen-depleted dwarf bean

The fate of nitrate and nitrogen-15 was followed during the apparent induction phase (6h) for nitrate uptake by N-depleted dwarf bean (Phaseolus vulgaris L. ev. Witte Krombek). Experiments were done with intact plants and with detached root systems. Qualitatively and quantitatively, xylem exudation from detached roots was a bad estimate of the export of NO^?₃ or NO^?₃-¹⁵N from roots of intact plants. In vivo nitrate reductase activity (NRA) agreed well with in situ reduction, calculated as the difference between uptake and accumulation in whole plants, provided NRA was assayed with merely endogenous nitrate as substrate (‘actual’ NRA). The majority (75%) of the entering nitrate remained unmetabolized. Both nitrate reduction and nitrate accumulation occurred predominantly in the root system. Some (< 25%) of the root-reduced nitrate-N was translocated to the shoot. Nitrate uptake occurred against the concentration gradient between medium and root cells, and probably against the gradient of the electro-chemical potential of nitrate. Part of the energy expended for NO^?₃ absorption came from the tops, since decapitation and ringing at the stem base restricted nitrate uptake.     

Effects of nitrogen dioxide and nitrate nutrition on nodulation, nitrogenase activity, growth, and nitrogen content of bean

The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO₂ (0-0.5 parts per million) on nodulation and in vivo acetylene reduction activity of the roots and on growth and nitrate and Kjeldahl N concentration in shoots was studied in bean (Phaseolus vulgaris L. cv Kinghorn Wax) plants. Exposing 8-day old seedlings for 6 hours each day, for 15 days, to 0.02 to 0.5 parts per million NO₂ decreased total nodule weight at 0 and 1 millimolar nitrate, and nitrogenase (acetylene reduction) activity at all concentrations of nitrate. The pollutant had little effect on root fresh or dry weights. Shoot growth was inhibited by NO₂. The NO₂ exposure increased nitrate concentration in roots only at 20 millimolar nutrient nitrate. Exposure to NO₂ markedly increased Kjeldahl N concentration in roots but generally decreased that in shoots. The experiments demonstrated that nutrient N level and NO₂ concentration act jointly in affecting nodulation and N fixing capability, plant growth and composition, and root/shoot relationships of bean plants.     

Decrease in nitrate uptake and increase in proton release in zinc deficient cotton,sunflower and buckwheat plants

The effect of varied Zn supply on the pH of the nutrient solution and uptake of cations and anions was studied in cotton (Gossypium hirsutum L.), sunflower (Helianthus annuus L.) and buckwheat (Fagopyrum esculentum Moench) plants grown under controlled environmental conditions in nutrient solutions with nitrate as source of nitrogen. With the appearance of visual Zn deficiency symtoms, the pH of the nutrient solutions decreased from 6 to about 5 whereas the pH increased to about 7 when the plants were adequately supplied with Zn. In Zn deficient plants the pH decrease was associated with a shift in the cation-anion uptake ratio in favour of cation uptake. Of the major ions, uptake of Ca²⁺ and K⁺ was either not affected or only slightly lowered whereas NO₃ ^- uptake was drastically decreased in Zn deficient plants. Although the Zn nutritional status of plants hardly affected the NO₃ ^- concentrations in the plants, the leakage of NO₃ ^- from roots of Zn deficient plants into a diluted CaCl₂ solution was nearly 10 times higher than that of plants adequately supplied with Zn. In contrast to Zn deficiency, Mn deficiency in cotton plants neither affected NO₃ ^- uptake nor the pH of the nutrient solution.The results indicate that, probably as a consequence of the role of Zn in plasma membrane integrity and nitrogen metabolism, when Zn is deficient in dicotyledonous species net uptake of NO₃ ^- is particularly depressed which in turn results in an increase in cation-anion uptake ratio and a corresponding decrease in external pH. The ecological relevance of this rhizosphere acidification is discussed.     

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.     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. IV. Flow and metabolism of inorganic nitrogen and malate depending on nitrogen nutrition and salt treatment

Ricinus plants were supplied with nutrient solutions containingdifferent N-sources or different nitrate concentrations andwere also exposed to mild salinity. Between 41 and 51 d aftersowing, the ratio of inorganic to total nitrogen in xylem andphloem saps, the content of inorganic nitrogen and malate intissues, and nitrate reductase activities were determined. Theflows of nitrate, ammonium, and malate between root and shootwere modelled to identify the site(s) of inorganic nitrogenassimilation and to show the possible role of malate in a pH-statmechanism. Only in the xylem of nitrate-fed plants did inorganicnitrogen, in the form of nitrate, play a role as the transportsolute. The nitrate percentage of total nitrogen in the xylemsap generally increased in parallel with the external nitrateconcentration. The contribution of the shoot to nitrate reductionincreased with higher nitrate supply. Under salt treatment relativelymore nitrate was reduced in the root as compared with non-treatedplants. Ammonium was almost totally assimilated in the root,with only a minor recycling via the phloem. Nitrate reductaseactivities measured in vitro roughly matched, or were somewhatlower than, calculated rates of nitrate reduction. From therates of nitrate reduction (OH -production) and rates of malatesynthesis (2H⁺-production) it was calculated that malate accumulationcontributed 76, 45, or 39% to the pH-stat system during nitratereduction in plants fed with 0.2, 1.0 or 4.0 mM nitrate, malateflow in the phloem played no role. In tissues of ammonium-fedplants no malate accumulation was found and malate flows inxylem and phloem were also relative low. Key words: Ammonium, Ricinus communis, phloem, xylem, transport, nitrate, nitrate reductase, nitrogen assimilation, malate     

Ion currents and the nitrogen status of roots of Hordeum vulgare and non-nodulated Trifolium repens

Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH⁺₄ or NO^-₃, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H⁺ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H⁺ extrusion while assimilation of NO^-₃, results in net OH^-₃ efflux. Growth on NH⁺₄, as compared to growth on NO^-₃, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.     

Labile anion binding by roots of two plum clones

The anion-exchange capacities (AEC) of roots of intact plants of two plum clones [Marianna 2624 (Prunus cerasifera × P. munsoniana) and Myrobalan 3-J (P. cerasifera)] were assessed with an anionic dye, eosin Y. The positively charged eosin-specific adsorption sites were metabolically dependent and also affected by nutrient status. Myrobalan 3-J roots adsorbed twice as much exchangeable NO₃ as did Marianna 2624. Since we previously showed that net nitrate uptake by Myrobalan 3-J persisted at half the ambient nitrate concentration as that characteristic of Marianna 2624, the data provide circumstantial support for a functional role of labile anion binding in active uptake at dilute concentrations of ambient nitrate.     

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.     

Effect of Phloem-Translocated Malate on NO(3) Uptake by Roots of Intact Soybean Plants

In soybean (Glycine max L. Merr. cv Kingsoy), NO₃⁻ assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO₃⁻ uptake by roots. The net NO₃⁻ uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO₃⁻ uptake and C excretion rates by roots. These results suggest that NO₃⁻ uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO₃⁻ uptake, and excretion of HCO₃⁻ ions was probably released by malate decarboxylation. NO₃⁻ uptake rate increased when the supply of NO₃⁻ to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO₄²⁻. We conclude that in situ, NO₃⁻ reduction rate in the shoot may control NO₃⁻ uptake rate in the roots via the translocation rate of malate in the phloem.     

Phosphorus deficiency does not enhance proton release by roots of soybean [Glycine max (L.) Murr.]

Little work has been done on root exudation in soybean under P deficiency. This study examined the effect of P supply on release of protons and carboxylates by roots of soybean (Glycine max Heinong 35), and to correlate the release with excess uptake of cations over anions. Plants were either reliant on N₂ fixation or supplied with nitrate and were grown in nutrient solution with 1–50 μM P for 7 weeks. Release of protons and carboxylates from roots, and concentrations of Ca, Mg, K, Na, P, S, Cl and N in plants were measured weekly from week 4. Unlike in many other species, P deficiency decreased proton release per unit root biomass in N₂-fixing plants and increased release of hydroxyl ions in nitrate-fed soybean. While P deficiency generally decreased uptake of K, Ca, Mg, S, Cl and P, it increased nitrate uptake per unit root biomass. Irrespective of P supply, amounts of protons released correlated well with excess uptake of cations over anions by the roots. Phosphorus deficiency increased release of carboxylates but the amounts released were small. The results suggest that soybean displays strategies of P acquisition through decreasing proton release which favors P mobilization in acid soils, and increasing root-to-shoot ratio and specific root length.     

The effect of chloride on nitrate reductase level,on anaerobic nitrite production,and on nitrate content in excisedPisum sativum L. roots

The effect was studied of chloride ions, added in the form of different salts, on nitrate reductase (NR) level in excised pea roots, on anaerobic nitrite production in an assay medium lacking both nitrate and n-propanol, on nitrate content in the roots, and on in vivo NR activity determined in an assay medium containing 5% n-propanol. The presence of Cl in nitrate containing nutrient solutions resulted in lower NR levels, however counterions supplied together with Cl tended to modify slightly this general trend. The negative effect of Cl ions was also apparent, when Cl ions were applied before nitrate ions. Anaerobic nitrite production in the medium lacking both nitrate and n-propanol was not influenced by chloride ions. Nitrate content in the roots was reduced in the presence of chloride both at 3 mM and 15 mM NO₃ in nutrient solutions; however, at 16 mM NO₃, nitrate content in the roots exoeeded even in the presence of 15 mM Cl nitrate content in those root segments which were cultivated in a nutrient solution with 6 mM nitrate, which is the concentration at which NR reaches the level of saturation in excised pea roots. The results obtained suggest that a special induction nitrate pool exists in plant cells besides the storage and metabolic nitrate pools.     

Diurnal modulation of phosphoenolpyruvate carboxylation in pea leaves and roots as related to tissue malate concentrations and to the nitrogen source

Phosphoenolpyruvate (PEP) carboxylation was measured as dark ¹⁴CO₂ fixation in leaves and roots (in vivo) or as PEP carboxylase (PEPCase) activity in desalted leaf and roof extracts (in vitro) from Pisum sativum L. cv. Kleine Rheinländerin. Its relation to the malate content and to the nitrogen source (nitrate or ammonium) was investigated. In tissue from nitrate-grown plants, PEP carboxylation varied diurnally, showing an increase upon illumination and a decrease upon darkening. Diurnal variations in roots were much lower than in leaves. Fixation rates in leaves remained constantly low in continuous darkness or high in continuous light. Dark CO₂ fixation of leaf slices also decreased when leaves were preilluminated for 1 h in CO₂-free air, suggesting that the modulation of dark CO2 fixation was related to assimilate availability in leaves and roots. Phosphoenolpyruvate carboxylase activity was also measured in vitro. However, no difference in maximum enzyme activity was found in extracts from illuminated or darkened leaves, and the response to substrate and effectors (PEP, malate, glucose-6-phosphate, pH) was also identical. The serine/threonine protein kinase inhibitors K252b, H7 and staurosporine, and the protein phosphatase 2A inhibitors okadaic acid and cantharidin, fed through the leaf petiole, did not have the effects on dark CO₂ fixation predicted by a regulatory system in which PEPCase is modulated via reversible protein phosphorylation. Therefore, it is suggested that the diurnal modulation of PEP carboxylation in vivo in leaves and roots of pea is not caused by protein phosphorylation, but rather by direct allosteric effects. Upon transfer of plants to ammonium-N or to an N-free nutrient solution, mean daily malate levels in leaves decreased drastically within 4–5 d. At that time, the diurnal oscillations of PEP carboxylation in vivo disappeared and rates remained at the high light-level. The coincidence of the two events suggests that PEPCase was de-regulated because malate levels became very low. The drastic decrease of leaf malate contents upon transfer of plants from nitrate to ammonium nutrition was apparently not caused by increased amino acid or protein synthesis, but probably by higher decarboxylation rates.Abbreviations CAM crassulacean acid metabolism - PEP Phosphoenolpyruvate - PEPCase phosphoenolpyruvate carboxylase - PP protein phosphatase - PK protein kinase This work was supported by the Deutsche Forschungsgemeinschaft. B. Baur was a recipient of a doctoral grant, and L. Leport recipient of a post-doctoral grant of the DFG. The skilled technical assistance of Eva Wirth and Maria Lesch is gratefully acknowledged.     

The effect of calcium deficiency on nitrate absorption and assimilation in pumpkin plants

The absorption of nitrate and the activity of nitrate reductase were much lower in Ca-deficient plants ofCururbita pepo L., cv. ‘Kveta’ than in normal plants grown in complete nutrient solution for a period of 8 days. After the addition of nitrate to the nutrient medium, nitrate reductase activity in the roots of NO₃-deficient plants sharply rose during the first 6 h and then remained constant during the following 6 h; the content of endogenous NO₃ ^? rose slowly and continuously. These processes were depressed in (Ca, NO₃)-deficient plants independently of the addition of Ca²⁺ to the medium in the variant with NO₃ ^?. Thus it seems that the whole nitrogen metabolism,i.e. both NO₃ ^? absorption and the synthesis of nitrate reductase, is impaired in Ca-deficient plants.     

Nitrate Uptake by Roots as Regulated by Nitrate Reduction Products of the Shoot

A mechanism is proposed by which secondary products of nitrate reduction in the shoot control the uptake of nitrate by the roots. KNO₃ enters the roots and is translocated to the shoot where nitrate is reduced and, at the same time, malate is produced. The reduction of nitrate is stoichiometric to the synthesis of malate (1). Part of the K-malate moves down to the root system in which malate is oxidized, yielding KHCO₃ which exchanges for KNO₃. Nitrate reduction in the shoot promotes the synthesis of malate which, after its translocation to the root, allows the preferential uptake of nitrate. Thus, plants reducing large amounts of nitrate may take up the anion without a superfluous accumulation of the cation. Furthermore, the utilization of nitrate by the shoot regulates its uptake by the root.     

Ionic balance,proton efflux,nitrate reductase activity and growth ofHippophaë rhamnoides L. ssp.rhamnoides as influenced by combined-N nutrition or N2 fixation

Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N₂ fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH ₄ ⁺ , a mixture of NH ₄ ⁺ and NO ₃ ^? producing the highest yields. Growth was severely reduced when solely dependent on N₂ fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N₂-fixing plants. Thein vivo nitrate reductase activity (NRA) of NO ₃ ^? dependent plants was almost entirely located in the roots. However, when grown with a combination of NO ₃ ^? and NH ₄ ⁺ , root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH ₄ ⁺ , NO ₃ ^? contributing only for approximately 20 per cent to the total N assimilation. Specific rates of N acquisition and ion uptake were generally highest in NO ₃ ^? +NH ₄ ⁺ plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH ₄ ⁺ plants than in the NO ₃ ^? plants. Measured extrusions of H⁺ or OH^? (HCO ₃ ^? ) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N₂ fixation amounted to 0.45–0.55 meq H⁺. (mmol N_org)^?1. Without acidity control and in the presence of NH ₄ ⁺ , specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N₂-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance. The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites.     

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.