Carbohydrate Metabolism of Rice Seedlings Grown in Oxygen Deficient Solution

Atwell, B. J. and Greenway, H. 1987. Carbohydrate metabolismof rice seedlings grown in oxygen deficient solution.—J.exp. Bot. 38: 466–478. Rice seedlings (Oryza sativa L.) were grown in the dark forup to 4 d in solutions containing various concentrations ofO₂. The rate of depletion of the endosperm was most rapid inaerated solution (0·25 mol O₂ m^–3), largely dueto the inhibition of growth of seedlings at very low O₂ concentrations.Earlier suggestions that there is a deficit of sugars for growthand energy generation in O₂ deficient coleoptiles were tested. Coleoptiles, shaking in aerated solution, respired about one-thirdof the endogenous sugars to CO₂ and incorporated the rest intostructural compounds. In contrast, the proportion of carbonwhich went to growth in anoxia was very low. Consistent withthese results, endogenous sugar levels were generally highestat low O₂ concentrations. Even so, coleoptiles grown and testedas low as 0·03 mol O₂ m^–3 showed appreciable metabolismof exogenous ¹⁴C-glucose to CO₂, soluble and insoluble compounds,suggesting that a minimal O₂ supply was sufficient to sustainsome growth. Furthermore, glucose feeding caused little or norise in O₂ uptake or tissue sugar levels. Similarly, the specificactivity of the evolved CO₂ was not markedly different in coleoptilesgrowing at 0·03 and 0·25 mol O₂ m^–3 Further evidence was obtained to show that endogenous substrateswere adequate for growth and respiration at both low and highO₂ concentrations. Exogenous glucose and malate did not stimulateO₂ uptake at any stage of growth in aerated coleoptiles. Therewas sufficient endogenous substrate to sustain a 35–45%rise in O₂ uptake induced by uncoupling and enrichment withO₂. Exogenous glucose did not stimulate growth of intact seedlingsat any O₂ concentration. Key words: Rice seedlings, carbohydrate metabolism, oxygen deficient solution     

Dexamethasone treatment causes resistance to insulin-stimulated cellular potassium uptake in the rat

Patients treated with glucocorticoids have elevated skeletal muscle ouabain binding sites. The major Na⁺-K⁺-ATPase (NKA) isoform proteins found in muscle, ₂ and ₁, are increased by 50% in rats treated for 14 days with the synthetic glucocorticoid dexamethasone (DEX). This study addressed whether the DEX-induced increase in the muscle NKA pool leads to increased insulin-stimulated cellular K⁺ uptake that could precipitate hypokalemia. Rats were treated with DEX or vehicle via osmotic minipumps at one of two doses: 0.02 mg·kg^–1·day^–1 for 14 days (low DEX; n = 5 pairs) or 0.1 mg·kg^–1·day^–1 for 7 days (high DEX; n = 6 pairs). Insulin was infused at a rate of 5 mU·kg^–1·min^–1 over 2.5 h in conscious rats. Insulin-stimulated cellular K⁺ and glucose uptake rates were assessed in vivo by measuring the exogenous K⁺ infusion () and glucose infusion (G_inf) rates needed to maintain constant plasma K⁺ and glucose concentrations during insulin infusion. DEX at both doses decreased insulin-stimulated glucose uptake as previously reported. G_inf (in mmol·kg^–1·h^–1) was 10.2 ± 0.6 in vehicle-treated rats, 5.8 ± 0.8 in low-DEX-treated rats, and 5.2 ± 0.6 in high-DEX-treated rats. High DEX treatment also reduced insulin-stimulated K⁺ uptake. (in mmol·kg^–1·h^–1) was 0.53 ± 0.08 in vehicle-treated rats, 0.49 ± 0.14 in low-DEX-treated rats, and 0.27 ± 0.08 in high-DEX-treated rats. DEX treatment did not alter urinary K⁺ excretion. NKA ₂-isoform levels in the low-DEX-treated group, measured by immunoblotting, were unchanged, but they increased by 38 ± 15% (soleus) and by 67 ± 3% (gastrocnemius) in the high-DEX treatment group. The NKA ₁-isoform level was unchanged. These results provide novel evidence for the insulin resistance of K⁺ clearance during chronic DEX treatment. Insulin-stimulated cellular K⁺ uptake was significantly depressed despite increased muscle sodium pump pool size. skeletal muscle; sodium pump; Na⁺-K⁺-ATPase     

Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Short-term (< 12 h) uptake experiments were conducted with6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K⁺ andNH₄⁺. At equal solution concentrations (0.5 mol m^–3) netK⁺ uptake was inhibited 30–35% by NH₄⁺ and NH₄⁺ uptakewas decreased 9–24%. Removal of NH₄⁺ resulted in completerecovery in K⁺ uptake rate, but NH⁴⁺ uptake rate did not recoverwhen K⁺ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m^–3. The relative effectof K⁺-NH₄⁺ interactions was not altered when Cl- was replacedwith SO₄^2–, but the magnitudes of the uptake rates wereless in the absence of Cl-. The V_max for NH₄⁺ uptake was reducedfrom 128 to 105 µmol g^–1 dry wt. h^–1 in thepresence of 0.5 mol m^–3 K⁺ and the K_m for NH₄⁺ doubledfrom 12 to 27 mmol m^–3 in the presence of K⁺. The resultsof these K⁺-NH₄⁺ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K⁺ coupled to NH₄⁺influx via an antiporter cannot be ruled out as contributingto the decrease in net K⁺ uptake. Key words: Nicotiana tabacum, K⁺, NH₄⁺, Uptake interactions     

The Effects of Salinity upon Ion Content and Ion Transport of the Marine RedAlga Porphyra purpurea(Roth) C.Ag.

Ion contents and concentrations (K⁺, Na⁺, Cl^–, Ca²⁺, Mg²⁺,SO^2–₄, NO^–₃, HPOJ^2–₄, amino and organic acids)of P. purpurea have been studied in relation to salinity variation.Cells were shown to accumulate large amounts of K⁺ and Cl^–against their respective gradients of electrochemical potentialin all dilute and concentrated seawater media. Active influxof SOJ^2–₄, NO^–₃, and HPOJ^2–₄ is also suggested,while Na⁺ is actively excluded from cells under hyposaline andhypersaline conditions. The relative proportions of individualcomponents of the internal osmotic potential were found to changeaccording to the external salt concentration. KCL forms themajor fraction of _j} in concentrated seawater media while K⁺-aminoacids form the major fraction in dilute seawaters. Other intracellularsolutes comprise less than 15% oft_j, in all media. Unidirectional fluxes of K⁺ and Cl^– were studied by radioisotopicmeans. Fluxes of K⁺ and Cl^– are reduced in hyposalinemedia, as is absolute KCL content per cell. Intracelhilar KCLcontent was also found to be markedly dependent upon externalK⁺ concentration, rather than water potential. Changes in KC1levels induced by salinity variation occur over a 6 h period.     

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     

Sodium Exclusion from the Shoots by Roots of Zea mays (cv. LG 11) and its Breakdown with Oxygen Deficiency

The effects of sodium chloride salinity and root oxygen deficiency(anoxia) were studied in 11-12d old maize plants (Zea mays L.cv. LG 11) in nutrient solution culture. Transport of ²²Na bythe roots to the shoot in 24 h was markedly increased by anoxiawhen the external concentration of NaCl was in the range 0·1-10·9mol m^–3. Anoxia severely inhibited uptake of ⁴²K by rootsand its transport to the shoot, so that the ratio of Na⁺/K⁺moving into the shoot was increased by a factor of approximately10. When the external concentration of NaCl was increased to2.4 mol m^–3, the roots showed much less ability to excludeNa⁺ under aerobic conditions, and anoxia caused no further increasein the movement of Na⁺ to the shoot. It is concluded that atthe higher concentration the ability of the roots to excludeNa⁺, presumably through an active mechanism in the xylem parenchymacells or in the root cortex and transporting Na⁺ to the outersolution, is saturated by excessive inward diffusion of Na⁺.The ratio of Na⁺/K⁺ transported to the shoot increased by afactor of 600 when the concentration of NaCl was increased from2·4 mol m^–3 to 40 mol m^–3 and roots weremade anoxic. Such imbalances in the supply of cations to theshoot, particularly when roots are oxygen-deficient, may contributeto salinity damage. Key words: Anaerobic, Anoxic, Oxygen deficiency, Roots, Salinity, Salt stress, Sodium chloride, Zea mays     

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     

K+-Na+ Exchange and Selectivity in Barley Root Cells: Effect of Na+ on the Na+ Fluxes

Using the compartmental analysis the unidirectional Na⁺ fluxesin cortical cells of barley roots, the cytoplasmic and vacuolarNa⁺ contents Q_c and Q_v, and the trans-root Na⁺ transport R'have been studied as a function of the external Na⁺ concentration.Using the re-elution technique the effect of low K⁺ concentrationson the plasmalemma efflux _co of Na⁺ (K+-Na+ exchange) and onR' was investigated at different Na+ concentrations and correspondinglydifferent values of the cytoplasmic sodium content Qc. The relationof the K+-dependent Na+ efflux coK+-dep to Qc or to the cytoplasmicNa+ concentration obeyed Michaelis-Menten kinetics. This isconsistent with a linkage of co, K+-dep to K+ influx by a K⁺-Na⁺exchange system. The apparent Km corresponded to a cytoplasmicNa⁺ concentration of 28 mM at 0·2 mM K⁺ and about 0·2mM Na⁺ in the external solution. 0·2 mM K⁺ stimulatedthe plasma-lemma efflux of Na⁺ and inhibited Na⁺ transport selectivelyeven in the presence of 10 mM Na⁺ in the external medium showingthe high efficiency of the K⁺-Na⁺ exchange system. However,_co, K⁺-dep was inhibited at 10 mM Na¹ compared to lower Na¹concentrations suggesting some competition of Na¹ with K¹ atthe external site of the exchange system. The effect of theNa+ concentration on Na¹ influx _oc is discussed with respectto kinetic models of uuptake.     

Characteristics of Radial Solution Flow in Roots of Cucumber (Cucumis sativus L.)

Low salt roots of Cucumis sativus L. cv. Burpeeana Hybrid weresubjected to iso-ionic treatments in which the external solutionconcentration of K⁺ was maintained at 14 mM. Solution concentrationof varied from 0 to 14 mM, other anions compensating. When Cl^– was the compensating ion, its concentrationin the exudate increased during the first 4 h and thereafterwas nearly the same as that of the external solution in alltreatments containing I mM or more. After 8 h of equilibration the concentration in the exudate increased almost exactly as its concentrationin the external solution. Rates of exudation and K⁺ transportwere almost constant between I and 14 mM KNO₂. More Cl^–was transported from solutions of similar Cl– concentrationwhen was also present. When water transport was inhibited with mannitol in treatments containing both KNO₃and KCI, exudate concentrations of K⁺ and were increased, but exudate concentration of Cl^– was notsignificantly affected except at the highest Cl     

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.     

Effects of Anoxia on Wheat Seedlings: I. INTERACTION BETWEEN ANOXIA AND OTHER ENVIRONMENTAL FACTORS

Anoxia was imposed on 4–6-d-old, intact wheat seedlings,after the roots had first been exposed for 1 d to O₂ concentrationsbetween 0·016 and 0·06 mol m^–3. Apices ofthe main axis of the seminal roots were considered to have toleratedanoxia if elongation occurred after return from anoxia to air,hereafter called ‘retention of elongation potential’.During anoxia, elongation potential was retained longer in rootsof intact seedlings than in 0–5 mm excised root tips suppliedwith 50 mol m^–3 glucose. In intact seedlings, elongation potential was retained longerat 15°C than at 25°C, and at pH 50 and 60 than at pH40. These differences between treatments were maintained inthe presence of exogenous glucose, and glucose supply prolongedthe retention of elongation potential in all anoxic treatments. Elongation potential was retained much longer at very low 02concentrations (0006 to 00l mol m^–3) than under anoxia;this was established at pH 40. Anoxia inhibited the transport of sugars from the shoots and/orendosperm to the root by 79-97%, as assessed from experimentswith roots of intact plants exposed to anoxia at pH 60 and 15°C. Overall, the results demonstrate: (i) that the occurrence ofadverse effects of anoxia during waterlogging in the field mayinteract with other environmental factors and (ii) that thereare pronounced difficulties integrating data on tolerance toanoxia obtained in different laboratories. Key words: Anoxia, wheat seedlings, pH, temperature     

Isocitrate Lyase from Germinating Soybean Cotyledons: Purification and Characterization

Ruchti, M. and Widmer, F. 1986. Isocitrate lyase from germinatingsoybean cotyledons: purification and characterization.—J.exp. Bot. 37: 1685–1690. Isocitrate lyase (E.C. 4.1.3.1 [EC] ) was purified from the cotyledonsof 7-d-old soybean seedlings. Three molecular forms were detectedwith pi values of 6·46, 6·25 and 6·0. Themain form (pl = 6·46) had an approximate Mr of 130000,a pH optimum of 8·0, a K_m (isocitrate) close to 2·0mol m^–3 and a molecular activity of 615 min ^–1 at25 °C. The purified enzyme is not a glycoprotein and isheat labile. Key words: Isocitrate lyase, soybean     

Relationship between Coleoptile Elongation and Alcoholic Fermentation in Rice Exposed to Anoxia. II. Cultivar Differences

The relationship between coleoptile elongation and survivalvs. alcoholic fermentation of rice under anoxia is examinedusing eight cultivars differing in submergence tolerance. Anoxiawas imposed on either 1 or 4 d aerated seeds either by N₂ flushingsubmerged tissues or by incubating tissues in stagnant deoxygenatedagar at 0·1% w/v; the latter simulated the stagnant conditionsof waterlogged soil. Two cultivars that were most tolerant tosubmergence also had the greatest tolerance to anoxia, whilea submergence intolerant cultivar was also intolerant to anoxia. Coleoptile growth under anoxia was related to rates of ethanolsynthesis (R_E), however differences between growth during anoxiaand survival after anoxia indicated that post-anoxic injurymay also be important in rice seeds exposed to anoxia. The correlationbetween coleoptile growth and R_E measured on a tissue basisusing intact seeds was r² = 0·67 among six varietiesover 0-3 d anoxia. This correlation improved to about r² = 0·85using R_E of (embryos plus coleoptiles) over 0-3 d, or coleoptilesat 3 d after anoxia. Coleoptile growth of individual seeds wasusually poorly correlated to R_E in these cultivars at 2-3 dafter anoxia. When coleoptiles of similar lengths were obtainedfrom different cultivars using 4 d aerated seeds, there weredifferences in R_E and coleoptile growth which were related tocoleoptile growth during 3 or 5 d anoxia, either on a tissue(r² = 0·85) or a fresh weight basis (r² = 0·70-0·97respectively). Results are discussed in relation to factorswhich may limit ethanol synthesis in rice exposed to anoxiaand the importance of growth to the survival of seeds and matureplants during submergence in the natural environment.Copyright1994, 1999 Academic Press Anoxia, ethanol, alcoholic fermentation, Oryza sativa L., rice, submergence     

Hypoxia Induces Membrane Depolarization and Potassium Loss from Wheat Roots but does not Increase their Permeability to Sorbitol

This paper deals with the responses of roots of wheat {Triticumaestivum L.) to hypoxia with special emphasis on the effectsof severe O2 deficiency on membrane integrity, loss of K+ fromthe root and root membrane potentials. Seminal and crown roots of 26-d-old plants exposed to severehypoxia (0.003 mol O₂ m^–3) for 3 h or 10 d prior to excisionand subsequently exposed to hypoxic solutions, had slightlylower rates of sorbitol influx and a slightly smaller apparentfree space than roots in aerated solutions. These results indicatethat neither a few hours nor a 10-d exposure to hypoxia hadadverse effects on the membrane integrity of the bulk of thecells in the roots. However, both 6-d-old seedlings and 26-d-oldplants lost K⁺ from the roots following their transfer fromaerated to hypoxic nutrient solutions. In the 26-d-old plants,which were of high nutritional status, there was a net K⁺ effluxfrom the roots to the external solution. In contrast, with the6-d-old seedlings, which were of low nutritional status, thedecrease in the K⁺ content of the roots was smaller than thenet K⁺ uptake to the shoots. Exposure of excised roots to 0.008 mol O₂ m^–3caused arapid and reversible membrane depolarization from –120to ––80 mV. These data and the magnitude of thenet effluxes strongly suggest that K⁺ losses during the earlystages of hypoxia are due to membrane depolarization ratherthan to increases in the permeability of membranes to K ⁺. Key words: Hypoxia, membrane integrity, membrane potentials, seminal and crown roots     

The Relationship Between Growth and Oxygen Uptake in Hypoxic Rice Seedlings

Atwell, B. J. and Green way, H. 1987. The relationship betweengrowth and oxygen uptake in hypoxic rice seedlings.—J.exp. Bot. 38: 454–465. Rice seedlings (Oryza saliva L.) were grown in the dark forup to 4 d in solutions containing various concentrations ofO₂. Compared with seedlings grown at 0·250 mol O₂ m^–3,the dry weight of the growing seedling was 14% lower at 0·110mol O₂ m^–3 and 60% lower at 0 mol O₂ m^–3. Decreasesin fresh weight were similar but not identical to decreasesin dry weight, possibly because leaf growth was suppressed evenabove 0·110 mol O₂ m^–3. Oxygen deficiency inhibitedroot growth more severely than coleoptile growth. Coleoptiles from seedlings grown in aerated solution were exposedto an atmosphere of pure N₂ for 30 min. Anoxia caused a declinein ATP content and energy charge, suggestive of decreased oxidativephosphorylation. It is not clear whether the decline in oxidativephosphorylation was solely responsible for impaired growth inhypoxia. In seedlings growing at O₂ concentrations less than 0·110mol O₂ m^–3, significant amounts of ethanol were synthesized.The rate of O₂ uptake decreased markedly below 0·06 molO₂ m^–3; this was presumably near the external O₂ concentrationat which oxidative phosphorylation became limited by the supplyof O₂. The stage of development of the seedlings appeared toinfluence O₂ uptake, possibly through changes in conductanceof the tissue to O₂. Uncouplers were used to confirm that thecritical O₂ concentration was dependent on O₂ diffusion ratherthan enzyme kinetics. Impaired growth above 0·110 molO₂ m^–3 may have been due to a decreased activity of oxygenasesof relatively low affinity for O₂, which in turn altered cellmetabolism. Key words: Growth, oxygen uptake, rice seedlings, hypoxia     

Potassium Transport in Enteromorpha intestinalis (L.) Link: MEASUREMENT OF INTRACELLULAR K+, EXCHANGE FLUXES AND THERMODYNAMIC ANALYSIS

Potassium transport has been studied in the marine euryhalinealga, Enteromorpha intestimlis cultured in seawater and in low-salinitymedium (Artificial Cape Banks Spring Water, ACBSW; 25·5mol m^–3 Cl^–, 20·4 mol m^–3 Na⁺, 0·5mol m^–3 K⁺). K⁺ fluxes were measured using ⁴²K⁺ and ⁸⁶Rb⁺although ⁸⁶Rb⁺ does not act as an efficient K⁺ analogue in thisplant. ⁴²K⁺ experiments on seawater plants typically exhibiteda single protoplasmic exchange phase whereas ⁸⁶Rb⁺ exhibitedtwo exchange phases. Compartmental analysis of ⁸⁶Rb⁺ effluxexperiments on seawater-grown Enteromorpha plants were usedto deduce the intracellular partition of K⁺ between the cytoplasm(279±38 mMolal) and vacuole (405±68 mMolal). Theplasmalemma K⁺ flux in plants in seawater was greater in thelight than in the dark (563±108 nmol m^–2 s^–1versus 389±66·7 nmol m^–2 s^–1). Inlow-salinity plants, separate cytoplasmic and vacuolar exchangephases were apparent. Analysis of ⁴²K⁺ efflux experiments onlow-salinity plants yielded a cytoplasmic K⁺ of 222±38mMolal and a vacuolar K⁺ of 82±11 mMolal. The plasmalemmaand tonoplast flux was 23±4·5 nmol m^–2 s^–1. The Nernst equation showed that, although K⁺ was close to electrochemicalequilibrium, active accumulation of K⁺ across the plasmalemmaoccurred in plants in seawater and ACBSW both in the light anddark. K⁺ was also actively transported inwards across the tonoplastin low-salinity plants. The electrochemical potential for K⁺across the plasmalemma ranged from 2·41±0·60kJ mol^–1 in plants grown in seawater in the light to 5·79±0·87kJ mol^–1 for plants in ACBSW in the light. Although K⁺is close to electrochemical equilibrium, the flux of K⁺ in plantsin both seawater and ACBSW media is high, hence the power consumptionof K⁺ transport is high. The permeability of K⁺ (P_K⁺) was significantlyhigher in the light than in the dark in plants in seawater (about7·0 versus 2·5 nm s^–1) but in plants inlow-salinity (ACBSW) medium the permeability was independentof light (about 12 nm s^–1). The energy requirements ofactive K⁺ transport by ATP-dependent pumps is discussed. Key words: Enteromorpha, Potassium transport, Ionic relations, Saltwater, Low salinity, Thermodynamics     

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     

Nitrate Effects on Pre-emergence Growth and Emergence Percentage of Wheat (Triticum aestivum L.) from Different Sowing Depths

The effects of a range of applied nitrate (NO₃^–) concentrations(0–20 mol m3) on germination and emergence percentageof Triticum aestivum L. cv. Otane were examined at 30, 60, 90and 120 mm sowing depths. Germination percentage was not affectedby either sowing depth or applied NO₃^– concentration whereasemergence percentage decreased with increased sowing depth regardlessof applied NO₃^– concentration. Nitrate did not affectemergence percentage at 30 mm sowing depth, but at 60 to 120mm depth, emergence percentage decreased sharply with an increasedapplied NO₃^– concentration of 0 to 1·0 mol m^–3then decreased only slightly with further increases in appliedNO₃^– of about 5·0 mol m^–3. Root and shoot growth, NO₃^– accumulation and nitrate reductaseactivity (NRA) of plants supplied with 0, 1·0 and 1·0mol m^–3 NO₃^– at a sowing depth of 60 mm were measuredprior to emergence. The coleoptile of all seedlings opened withinthe substrate. Prior to emergence from the substrate, shootextension growth was unaffected by additional NO₃^– butshoot fr. wt. and dry wt. were both greater at 1·0 and1·0 mol m^–3 NO₃^– than with zero NO₃^–.Root dry wt. was unaffected by NO₃^–. Nitrate concentrationand NRA in root and shoot were always low without NO₃^–.At 1·0 and 10 mol m3 NO₃^–, NO₃^– accumulatedin the root and shoot to concentrations substantially greaterthan that applied and caused the induction of NRA. Regardlessof the applied NO₃^– concentration, seedlings which failedto emerge still had substantial seed reserves one month afterplanting. Coleoptile length was substantially less for seedlingswhich did not emerge than for seedlings which emerged, but wasnot affected by NO₃^–. It is proposed that (a) decreasedemergence percentage with increased sowing depth was due tothe emergence of leaf I from the coleoptile within the substrateand (b) decreased emergence percentage with additional NO₃^–was due to the increased expansion of leaf 1 within the substrateresulting in greater folding and damage of the leaf. Key words: Triticum aestivwn L., nitrate, sowing depth, seedling growth, seedling emergence     

Transnodal Transport of 14C in Nitella flexilis: I. TANDEM CELLS WITHOUT APPLIED PRESSURE GRADIENTS

Using NaH¹⁴CO₃ (0·01-0·03 mol m^–3) fed to5·0 mm of Nitella flexilis in artificial pond water at22–25°C, we have found that uptake in 5 min or in1 h varies with the streaming rate of the fed cell and is reducedby anoxia over the uptake position. We have also found thatthe %¹⁴C transported across the node between two internodalcells in tandem is highly sensitive to the streaming rate ineach cell, to the physiological state of the cells (summer versuswinter), to the method of feeding (5 min versus 1 h) and tocovering the node, particularly with winter cells or summercells preconditioned for 3 h in the dark and run in the dark.Transnodal transport by plasmodesmata is sensitive to anoxiaand seems to be at least partly active. Key words: Node, plasmodesmata, anoxia, active transport, influx     

Bafilomycin A1 is a non-competitive inhibitor of the tonoplast H+-ATPase of maize coleoptiles

When microsomal membranes from maize (Zea mays L. cv. Clipper)coleoptiles were separated by isopyc-nic centrifugation on acontinuous 10–45% sucrose gradient, bafilomycin A1-inhibitedATPase activity co-localized with the activities of the tonoplastmarker-enzymes, nitrate-Inhibited ATPase and K⁺-dependent pyrophosphatase.Thus, bafilomycin A1 is a specific inhibitor of the vacuolarH⁺-ATPase of maize coleoptiles. Inhibition of the vacuolar H⁺-ATPaseby bafilomycin A1 was strictly dependent upon the concentrationof the enzyme present in the assay medium, suggesting a stoichiometricassociation between bafilomycin A1 and the vacuolar H⁺-ATPase.In tonoplast-enriched preparations, half-maximal inhibitionwas obtained at 43 pmol bafilomycin A1 mg^–1 protein. BafilomycinA1 inhibited the vacuolar H⁺-ATPase in a simple non-competitivemanner: increasing bafilomycin A1 concentrations reduced theV_max, of the H+ -ATPase, but had no effect on its K_m towardsATP. Key words: Bafilomycin A1, coleoptile, H⁺-ATPase (vacuolar), maize, Zea mays L