Influence of nitrate on uptake of ammonium by nitrogen-depleted soybean: is the effect located in roots or shoots?

In non-nodulated soybean [Glycine max (L.) Merrill cv. Ransom]plants that were subjected to 15 d of nitrogen deprivation inflowing hydroponic culture, concentrations of nitrogen declinedto 1.0 and 1.4mmol Ng^–1 dry weight in shoots and roots,respectively, and the concentration of soluble amino acids (determinedas primary amines) declined to 40µmol g^–1 dry weightin both shoots and roots. In one experiment, nitrogen was resuppliedfor 10 d to one set of nitrogen-depleted plants as 1.0 mol m^–3NH₄⁺ to the whole root system, to a second set as 0.5 mol m^–3NH₄⁺ plus 0.5 mol m^–3 NO₃^– to the whole root system,and to a third set as 1.0 mol m^–3 NH₄⁺ to one-half ofa split-root system and 1.0 mol m^–3 NO₃^– to theother half. In a second experiment, 1.0 mol m^–3 of nitrogenwas resupplied for 4 d to whole root systems in NH₄⁺ : NO₃^–ratios of 1:0, 9:1, and 1:1. Nutrient solutions were maintainedat pH 6.0. When NH₄⁺ was resupplied in combination with NO₃^– to thewhole root system in Experiment I, cumulative uptake of NH₄⁺for the 10 d of resupply was about twice as great as when NH₄⁺was resupplied alone. Also, about twice as much NH₄⁺ as NO₃^–was taken up when both ions were resupplied to the whole rootsystem. When NH₄⁺ and NO₃^– were resupplied to separatehalves of a split-root system, however, cumulative uptake ofNH₄⁺ was about half that of NO₃^–. The uptake of NH₄⁺,which is inhibited in nitrogen-depleted plants, thus is facilitatedby the presence of exogenous NO₃^–, and the stimulatingeffect of NO₃^– on uptake of NH₄⁺ appears to be confinedto processes within root tissues. In Experiment II, resupplyof nitrogen as both NH₄⁺ and NO₃^– in a ratio of either1:1 or 9:1 enhanced the uptake of NH₄⁺. The enhancement of NH₄⁺uptake was 1.8-fold greater when the NH₄⁺: NO₃^–-resupplyratio was 1:1 than when it was 9:1; however, only 1.3 timesas much NO₃^– was taken up by plants resupplied with the1 :1 exogenous ratio. The effect of NO₃^– on enhancementof uptake of NH₄⁺ apparently involves more than net uptake ofNO₃^– itself and perhaps entails an effect of NO₃^–uptake on maintenance of K⁺ availability within the plant. Theconcentration of K⁺ in plants declined slightly during nitrogendeprivation and continued to decline following resupply of nitrogen.The greatest decline in K⁺ concentration occurred when nitrogenwas resupplied as NH₄⁺ alone. It is proposed that decreasedavailability of K⁺ within the NH₄⁺-resup-plied plants inhibitedNH₄⁺ uptake through restricted transfer of amino acids fromthe root symplasm into the xylem. Key words: Ammonium, Glycine max, nitrate, nitrogen-nutrition, nitrogen stress, split-root cultures     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjectedto 15 d of nitrogen deprivation were resupplied for 10 d with1.0 mol m^–3 nitrogen provided as NO₃^–, NH₄⁺, orNH₄⁺+NO₃^– in flowing hydroponic culture. Plants in a fourthhydroponic system received 1.0 mol m^–3 NO₃^– duringboth stress and resupply periods. Concentrations of solublecarbohydrates and organic acids in roots increased 210 and 370%,respectively, during stress. For the first day of resupply,however, specific uptake rates of nitrogen, determined by ionchromatography as depletion from solution, were lower for stressedthan for non-stressed plants by 43% for NO₃^- resupply, by 32%for NH₄⁺ + NO₃^– resupply, and 86% for NH₄⁺ resupply. Whenspecific uptake of nitrogen for stressed plants recovered torates for non-stressed plants at 6 to 8 d after nitrogen resupply,carbohydrates and organic acids in their roots had declinedto concentrations lower than those of non-stressed plants. Recoveryof nitrogen uptake capacity of roots thus does not appear tobe regulated simply by the content of soluble carbon compoundswithin roots. Solution concentrations of NH₄⁺ and NO₃^– were monitoredat 62.5 min intervals during the first 3 d of resupply. Intermittent‘hourly’ intervals of net influx and net effluxoccurred. Rates of uptake during influx intervals were greaterfor the NH₄⁺ -resupplied than for the NO₃^– -resuppliedplants. For NH₄⁺ -resupplied plants, however, the hourly intervalsof efflux were more numerous than for NO₃^– -resuppliedplants. It thus is possible that, instead of repressing NH₄⁺influx, increased accumulation of amino acids and NH₄⁺ in NH₄⁺-resupplled plants inhibited net uptake by stimulation of effluxof NH₄⁺ absorbed in excess of availability of carbon skeletonsfor assimilation. Entry of NH₄⁺ into root cytoplasm appearedto be less restricted than translocation of amino acids fromthe cytoplasm into the xylem. Key words: Ammonium, nitrate, nitrogen-nutrition, nitrogen-stress, soybean     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: II. K +/Na+ SELECTIVITY OF ROOT TISSUES9

This paper reports the effects of low O₂ concentration (0–01,0–055, and 0.115mol m^–3) in nutrient solutions onK⁺/Na⁺ selectivity of growing and mature root tissues of 6-to 8-d-old, intact, wheat (Triticum aestivum cv. Gamenya) seedlings. Increases in anaerobic catabolism and decreases in O₂ uptake,K⁺ uptake and K⁺/Na⁺ selectivity were all more pronounced and/oroccurred at higher external O₂ concentrations in the apex (0–2mm) than in the expanding tissues (2–4 mm); these growingtissues were, in turn, more affected than the expanded tissuesof the roots (4–12 mm). Selectivity for K⁺ over Na⁺ in roots and shoots was particularlysensitive to O₂ deficiency. For example, in apical tissues (0–2mm) K ⁺ /Na⁺ selectivity was already reduced at 0.115 mol m^–3O₂, yet at this O₂ concentration there was no effect on eithergrowth or (K⁺/Na⁺) uptake. Upon transfer from 0.01 to 0.26 mol m^–3 O₂, a detailedstudy of the 12 mm root tips showed that 70% of these tips regainedhigh (K⁺ + Na⁺) concentrations and K⁺/^Na+ ratios. In contrast,there was no recovery in the remaining 30% of the 12 mm roottips. Net K⁺ transport to the shoots during the period afterre-aeration was negative for the population as a whole. Theseverity of these effects supports the view that the root tipsand the stele were more susceptible to O2 deficiency than wasthe cortex of the fully-developed root tissues. Key words: Hypoxia, K⁺/Na⁺ selectivity, expanded and expanding tissues     

Ontogenetic Changes in the Chemical Composition of Ricinus communis Grown with NO-3 or NH+4 as N Source

Ricinus communis L. var. Gibsonii was grown in Long Ashton nutrientmedium with either 12mol m^–3 NO^–₃ or 8.0 mol m^–3NH⁺₄ as N source. Two plants from each N treatment were harvestedtwice a week and analysed for C, N, P, S, NO^–₃, SO^2–₄Cl^–K⁺Na⁺, Ca²⁺ Mg²⁺ and ash alkalinity. Statistical analysis of thedata showed that the effect of age and N source was differentfor the chemical variables analysed. Thus [Na⁺] was unaffectedby age or N source, and for both N sources [Mg²⁺] started atthe same level and decreased at the same rate as the plantsmatured. With NH⁺₄ as N source, [SO^2–₄] was higher thanwith NO^–₃, but did not alter with age. The concentrations,in mmol g^–1 dry wt, of C, organic N, K⁺ and Ca²⁺ weredifferent for the two N sources, but the levels of these variablesaltered with age in the same way for both N sources; i.e. therewas no age x N interaction. In the case of P, NO^–₃, Cl^– and COO^–, however,age-related variations were different for the two N sources.It is concluded, inter alia, that [Na⁺] is determined by external[Na⁺] alone, and that K⁺, Ca²⁺ and Cl^– are the inorganicions actively involved in charge balance during ion uptake bythe roots. Key words: Ontogeny, Chemical composition, Plant nutrition     

Ammonium Nutrition in Ricinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps

Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinuscommunis: its effect on plantgrowth and the chemical compositionof the whole plant, xylem and phloem saps.—J. exp. Bot.37: 1599–1610. The growth and chemical composition of Ricinus communis cultivatedhydroponically on 12 mol m ^{– 3} NO₃^–-N were comparedwith plants raised on a range of NH₄⁺-N concentrations. At NH₄⁺-Nconcentrations between 0·5 and 4·0 mol m^–3,fresh- and dry-weight yields of 62-d-old plants were not significantlydifferent from those of the NO₃^–-N controls. Growth wasreduced at 0·2 mol m^–3 NH₄⁺-N and was associatedwith increased root. shoot and C: organic N ratios, suggestingthat the plants were N-limited. At 8·0 mol m^–3NH₄⁺-N, growth was greatly restricted and the plants exhibitedsymptoms of severe ‘NH₄⁺ toxicity’. Plants growingon NH₄⁺-N showed marked acidification of the rooting medium,this effect being greatest on media supporting the highest growthrates. Shoot carboxylate content per unit dry weight was lower at mostNH₄⁺-N concentrations than in the NO₃^–-N controls, althoughit increased at the lowest NH₄⁺-N levels. Root carboxylate contentwas comparable on the two N sources, but also increased substantiallyat the lowest NH₄⁺-N levels. N source had little effect on inorganic-cationcontent at the whole-plant level, while NO₃^– and carboxylatewere replaced by Cl as the dominant anion in the NH₄⁺-N plants.This was reflected in the ionic composition of the xylem andleaf-cell saps, the latter containing about 100 mol m^–3Cl^– in plants on 8·0 mol m^–3 NH₄⁺. Xylem-saporganic-N concentration increased more than threefold with NH₄⁺-N(with glutamine being the dominant compound irrespective ofN source) while in leaf-cell sap it increased more than 12-foldon NH₄⁺-N media (with arginine becoming the dominant species).In the phloem, N source had little or no effect on inorganic-cation,sucrose or organic-N concentrations or sap pH, but sap fromNH₄⁺-N plants contained high levels of Cl^– and serine. Collectively, the results suggested that the toxic effects ofhigh NH₄⁺ concentrations were not the result of medium acidification,reduced inorganic-cation or carboxylate levels, or restrictedcarbohydrate availability, as is commonly supposed. Rather,NH₄⁺ toxicity in R. communis is probably the result of changesin protein N turnover and impairment of the photorespiratoryN cycle. Key words: Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, Phloem, amino acids, carboxylate     

Nitrogen utilization by plant species from acid heathland soils: II. Growth and shoot/root partitioning of NO3- assimilation at constant low pH and varying NO3-/NH4+ ratio

The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m^–3N, varying the N0₃^–/NH₄⁺ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N0₃^–/NH₄⁺ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH₄⁺ by NO₃^– tended to enhance the plant's growth ratewhen compared to NH₄⁺ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO₃^–increasing from 0 to 100%. In the presence of NH₄⁺, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH₄⁺. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO₃^–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO₃^– [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO₃^– <0.20), D. flexuosashowed a strong preference for NO₃^– at low external nitrate(the relative uptake ratio for N0₃^– reaching a value of2.0 at 10% NO₃^–). The ecological significance of thisprominent high preference for NO₃^– at low NO₃^–/NH₄⁺ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N0₃^–/NH₄⁺ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate     

Effect of Abscisic Acid on the K+ Efflux and Membrane Potential of Nicotiana tabacum L. Leaf Cells

K⁺ efflux from tobacco (Nicotiana tabacum L, cv. Samsun NN)leaf discs into the external medium was increased and the membranepotential (Em) changed in the positive direction with a changein pH from 8.0 to 4.0. E_m was affected by the external concentrationof KCl, greatly decreasing with a change in concentration from1 mM to 100 mM. The equilibrium potential of the membrane forK⁺ (E_k) was decreased in a Nernst fashion with increasing externalconcentrations of KCl. E_k is more positive than E_m above ca.50 µM KCl. Most of the experiments were carried out underconditions in which the difference between the electrochemicalpotential for K⁺ on the inside to the outside of the cell (µ_kis positive. Thus, K⁺ may passively flow to the outside of thecells accompanied by the depolarization of the membrane. Abscisic acid (ABA) increased the K⁺ efflux under conditionsof passive transport. K⁺ efflux was accelerated with an increasingconcentration of ABA, being maximal at 10^–4 M–10^–3M. This acceleration was due to the enhancement of the potassiummotive force (µ_k/F) which is the force causing the netpassive transport of K⁺. The membrane potential was decreasedfrom –205 mV to –170 mV by 2 x 10^–4 M ABAwithin 10 min. The depolarization was not transient, being lostfor at least 3 hr. These results show that ABA accelerated passive K⁺ efflux, whichaccompanied depolarization of the membrane. (Received June 22, 1981; Accepted August 24, 1981)     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

Nutrient Solution pH Control using Dipolar Buffers in Studies of Trifolium repens L. Nitrogen Nutrition

In studies of Trifolium repens nitrogen nutrition, the controlof nutrient solution pH using dipolar buffers, was evaluatedin tube culture under sterile conditions. Five buffers; MES,ADA, ACES, BES and MOPS with pK₂s (20 °C) of 6.15, 6.60,6.90, 7.15 and 7.20 respectively, at a concentration of 2.0mol m^–3, were provided to inoculated Trifolium repensgrowing in nutrient solution containing 7.13 mol m^–3 nitrogenas (NH₄)₂SO₄. Initial pH of each solution was adjusted to theappropriate buffer pK₂ Two buffers, ADA and ACES completelyinhibited plant growth. The remaining buffers had little effectin limiting pH change, although plant dry matter was higherand nodule numbers lower in the presence of these buffers. MESand MOPS were supplied to nutrient solutions with and without7.13 mol m^–3 (NH₄)₂SO₄, at concentrations ranging from0–12 mol m^–3. MES at 9 mol m^–3 and 12 molm^–3 reduced growth of plants reliant on the symbiosisfor providing nitrogen. The provision of MES to plants providedwith NH₄⁺ significantly increased plant yield and reduced nodulenumber at all concentrations. MOPS did not affect plant yieldor nodule number. The use of dipolar buffers in legume nitrogennutrition studies is considered in terms of buffering capacity,and the side effects on plant growth and symbiotic development. Key words: Ammonium, Dipolar buffer, Nitrogen nutrition, pH control, Symbiosis, Trifolium repens     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

Inhibition of Nitrate Assimilation in Roots in the Presence of Ammonium: The Moderating Influence of Potassium

Experiments were conducted to investigate the effect of concentrationof NH₄⁺ in nutrient solution on root assimilation of NO₃^–and to determine whether the NH₄⁺NO₃^– interaction wasmodified in the presence of K⁺. Dark-grown, detopped corn seedlings(cv. Pioneer 3369A) were exposed for 8 h to 0.15 mM Ca(NO₃)₂and varying concentrations of (NH₄)₂SO₄ in the absence or presenceof 0.15 mM K₂SO₄. The accelerated phase of NO₃^– uptakeappeared most sensitive to restriction by additions of 0.15mM (NH₄)₂SO₄. In the absence of K⁺, the restriction increasedonly slightly even when solution (NH₄)₂SO₄, was increased from0.15 mM to 12.5 mM which was accompanied by an increase of NH₄⁺in the tissue from about 7.0 to 35 µmol g^–1 fr.wt. of root. Increasing concentrations of solution NH₄⁺ progressivelyinhibited net K+ uptake. At the highest solution NH₄⁺ concentrations,there was an initial net efflux of K⁺ and no net influx occurredduring the treatment period. The severity of the NH₄)SO₄ restrictionof NO₃^– uptake was moderated considerably in the presenceof K⁺ as long as a net influx of K⁺ occurred. However, net influxof K⁺ was not associated with alteration of NH₄⁺ uptake, assimilation,or accumulation in the root tissue. The lack of correlationbetween the severity of restriction of NO₃^– uptake andendogenous NHJ suggested the restriction resulted from an effectexerted by exogenous NH₄⁺ which tended to saturate at lowersolution NHJ concentrations or by inhibitory factors generatedduring assimilation of NH₄⁺. Several mechanisms were postulatedto account for the moderating influence of K⁺. In all experiments,root NO₃^– reduction was restricted by the presence ofambient NH₄⁺. The quantitative decreases in reduction tendedto be less than decreases in NO₃^– uptake and therefore,could result from inhibition solely of uptake with subsequentlimitation in availability of substrate for the reduction process,but the possibility of a direct effect on reduction could notbe excluded.     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Efficiency of K+ Utilization by Barley Varieties: Activation of Pyruvate Kinase

Barley (Hordeum vulgare L.) varieties differed in their raponseto [K⁺]₀, in terms of their utilization efficiencies (UE = freshweight. concentration of [K⁺]₁^–1). At low [K⁺]₀, Compana,an efficient-non-responder demonstrated superior utilizationof absorbed K⁺. On the other hand, at high [K⁺]₀, Fergus (anefficient responder) and BT 334 (an inefficient responder) hadhigher UE values for K⁺ than Compana which performed poorlyat this [K⁺]₀. Kinetic parameters for K⁺ activation of the enzyme pyruvatekinase from 12 barley varieties, representing a range of UEvalues, were determined. Varieties showed substantial differencesin their V_max values (P<0·01). Compana, an efficientvariety, had the highest V_max (31 µmol g^–1 freshwt. h^–1) which was about 50% higher than that of Mingo,an inefficient variety. By contrast, K_m values for the enzymeswere not significantly different among varieties The mean valuesfor all varieties (3·9±0·15 mol m^–3K⁺) is far below the estimated cytoplasmic [K⁺] (100-200 molm^–3). It is, therefore, unlikely that differences in theutilization of K⁺ by these varieties can be explained on thebasis of differential requirements for (K⁺) activation of theseenzymes. Alternative possibilities for differences in the utilizationof K⁺ are discussed. Key words: K⁺ utilization efficiency, Pyruvate kinase, Barley varieties     

Potassium Transport in Enteromorpha intestinalis (L.) Link: II. EFFECTS OF MEDIUM COMPOSITION AND METABOLIC INHIBITORS

In springwater (25.5 mol m^–3 Cl^–, 20.4 mol m^–3Na⁺, 0.14 mol m^–3 K⁺) Enteromorpha intestinalis couldnot survive for more than a few weeks unless provided with 0.5mol m^–3 K⁺ in the medium or alternatively exposed to seawaterfor 1 day per week. Maintenance of a cytoplasmic K⁺ level ofabout 200 mol m^–3 is critical for the maintenance of normalmetabolic activity. Net gains of intracellular K⁺ occurred whenthe plants were transferred from low-salinity to seawater; converselylarge net losses occurred when plants were transferred fromseawater to springwater. These two processes were not simplythe reverse of one another; net gain of K⁺ involved a largeincrease in the tracer flux both into and out of the cell butnet loss of K⁺ virtually halted the tracer flux into the cell.Any injury incurred by rapid salinity changes was short-lived;plants were rapidly able to adjust intracellular [K₁.K⁺). K⁺(orto some extent Rb⁺) was found to be necessary in the effluxmedium for ⁴²K⁺ exchange to occur. The osmotic concentrationof the medium was also important but extracellular Na⁺ and Cl^–concentrationswere not critical. K⁺ influx and efflux in both springwaterand seawater were largely independent of light and were sensitivein varying degrees to a range of common metabolic inhibitorsand uncouplers. The results are best explained by the presenceof an active K⁺ influx, generated by an ATP-dependent K⁺ pumpat the plasmalemma. Key words: Enteromorpha, Potassium transport, Salinity changes, Uncouplers, Inhibitors     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake

The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H⁺ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The K_m values of thethree affinity sites were 1.4 x 10^–3 M, 1.3 x 10^–4M, 4.3 x 10^–5 M; the maximum velocity values were 3.3x 10^–8, 4.5 x 10^–9, 1.8 x 10_–9 mol/10 min/4x 10⁶ cells. (Received April 18, 1981; Accepted August 25, 1981)     

Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

Imsande, J. 1986. Inhibition of nodule development in soybeanby nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max(L.) Merr.] is inhibited by continuous growth in the presenceof 4· mol m^–3 KNO₃ The presence of 4·0 molm^–3 ‘starter nitrate’ for 3-6 d during noduledevelopment, however, subsequently stimulates nodule dry weightaccumulation and nitrogenase activity. These stimulations occureven though 4· mol m^–3 nitrate temporarily delaysnodule development, i.e. the late steps of nodule developmentare reversibly inhibited by a short-term exposure to 4·0mol m^–3 nitrate. On the other hand, treatment with 4·0mol m^–3 nitrate in excess of 14 d significantly reducesnodule dry weight Thus, extended growth in the presence of 4·0mol m^–3 KNO₃ seems to block both early and late stepsof nodule development. Nodulation of hydroponically grown soybeansis also inhibited by continuous growth in the presence of 2·0mol m^–3 (NH₄)₂SO₄ This inhibition is not caused by acidityof the growth medium. On the other hand, nodule development6 d after inoculation with Rhizoblum japonicum is not delayedby a 7-d exposure to 2·0 mol m^–3 (NH₄)₂SO₄ butis partially inhibited by a prolonged exposure to (NH₄)₂SO₄Because repression of nodulation by 4·0 mol m^–3KNO₃ is more severe than that by 2·0 mol m^–3 (NH₄)₂SO₄and because ammonium taken up by the soybean plant is not activelyoxidized to nitrate, it is suggested that there are at leasttwo mechanisms by which nitrate utilization represses noduleformation in soybean. Key words: Glycine max, nitrogen, nitrogen fixation, nodulation