Efficiency of Potassium Utilization by Barley Varieties: The Role of Subcellular Compartmentation

Memon, A. R., Saccomani, M. and Glass, A. D. M. 1985. Efficiencyof potassium utilization by barley varieties: The role of subcellularcompartmentation.?J. exp. Bot. 36: 1860–1876. The subcellulardistributions of K⁺ in roots of three barley (Hordeum vulgareL.) varieties, grown at 10 and 100 mmol m^–3 external K⁺([K⁺]_o) were estimated by compartmental analyses. In general,increased [K⁺]_o caused a 2–3 fold increase in vacuolar[K⁺], but cytoplasmic [K⁺] increased only slightly. Nevertheless,the three varieties, which had been selected for study on thebasis of their different rates of K⁺ utilization, showed distinctdifferences in the allocation of K⁺ between cytoplasm and vacuole.At 10 mmol m^–3 [K⁺]_o var. Betzes exhibited typical K⁺deficiency symptoms while var. Fergus and var. Compana did not,even though Betzes had higher [K⁺] in shoots and roots. Theinefficient utilization of K⁺ in this variety appears to beassociated with a failure to mobilize vacuolar K⁺ into the cytoplasmiccompartment (the ratio of vacuolar: cytoplasmic K⁺ contentsfor Betzes was 4.1 compared to 2.7 and 2.5, respectively, forFergus and Compana). Fergus and Betzes, which demonstrate pronouncedgrowth responses to increased [K⁺]₀ between 10 and 100 mmolm^–3, showed significant increases of cytoplasmic [K⁺]in this range of [K⁺]_o. By contrast, cytoplasmic [K⁺] in Compana,a variety whose growth is not stimulated by increased [K⁺]₀(from 10 to 100 mmol m^–3) showed virtually no increase.It is suggested that the efficiency of K⁺ utilization and thegrowth response to [K⁺]₀ in these varieties are functions ofthe subcellular distribution of this ion between cytoplasm andvacuole. Key words: Barley varieties, K⁺ subcellular compartmentation, utilization efficiency     

Rate of Uptake of Potassium by Three Crop Species in Relation to Growth

Barley, ryegrass, and fodder radish were grown in flowing nutrientsolutions at four potassium concentrations, [K_e⁺], from 0.05to 4 mg I^–1. During the first 2 weeks after germinationthe response to [K_e⁺] (fodder radish > barley > ryegrass)depended on the potential relative growth rate, the ratio ofroot surface area to plant weight, and on the K⁺ flux into theroots. Subsequently, there was no effect of [K_e⁺] on growthrate within the range tested. The K⁺ flux decreased from 4–23? 10^–12 mol cm^–2 s^–1 in the first 2 weeksafter germination, when it was concentration-dependent, to 2–5? 10^–12 mol cm^–2 s^–1 after 4–5 weeks,when it became independent of [K_e⁺] down to 0.05 mg 1^–1.The results explain the importance of high [K_e⁺] and rapid rootgrowth during the first 2 weeks after seed germination.     

Protective role of extracellular chloride in fatigue of isolated mammalian skeletal muscle

A possible role of extracellular Cl^– concentration ([Cl^–]_o) in fatigue was investigated in isolated skeletal muscles of the mouse. When [Cl^–]_o was lowered from 128 to 10 mM, peak tetanic force was unchanged, fade was exacerbated (wire stimulation electrodes), and a hump appeared during tetanic relaxation in both nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles. Low [Cl^–]_o increased the rate of fatigue 1) with prolonged, continuous tetanic stimulation in soleus, 2) with repeated intermittent tetanic stimulation in soleus or EDL, and 3) to a greater extent with repeated tetanic stimulation when wire stimulation electrodes were used rather than plate stimulation electrodes in soleus. In nonfatigued soleus muscles, application of 9 mM K⁺ with low [Cl^–]_o caused more rapid and greater tetanic force depression, along with greater depolarization, than was evident at normal [Cl^–]_o. These effects of raised [K⁺]_o and low [Cl^–]_o were synergistic. From these data, we suggest that normal [Cl^–]_o provides protection against fatigue involving high-intensity contractions in both fast- and slow-twitch mammalian muscle. This phenomenon possibly involves attenuation of the depolarization caused by stimulation- or exercise-induced run-down of the transsarcolemmal K⁺ gradient. potassium; skeletal muscle contraction; membrane potential; myotonia     

Regulation of K+ Influx in Barley: Evidence for a Direct Control of Influx by K+ Concentration of Root Cells

Siddiqi, M. Y. and Glass, A. D. M. 1987. Regulation of K⁺ influxin barley: Evidence for a direct control of influx by K⁺ concentrationof root cells.—J. exp. Bot. 38: 935–947. The kinetics of K⁺ (⁸⁶Rb⁺) influx into intact roots of barley(Hordeum vulgare L. cv. Fergus) seedlings having different combinationsof root and shoot [K⁺], different growth rates and differentroot:shoot weight ratios were studied. K⁺ influx was stronglycorrelated with root [K⁺]; shoot [K⁺], growth rates, and root:shoot ratios appeared to have little effect on K⁺ influx. Adetailed study showed that both V_max and K_m for K⁺ influx wereaffected by root [K⁺] but not by shoot [K⁺]. We have suggestedthat factors such as growth rates and root: shoot ratio mayaffect K⁺ influx indirectly primarily via their influence onroot factors such as root [K⁺]. We have reiterated that othertypes of kinetic control, e.g. increased or decreased synthesisof ‘carrier systems’, may operate in addition todirect (allosteric?) control of K⁺ influx by root [K⁺]. Thenegative feedback signal from root [K⁺] appeared to be the primeeffector in the regulation of K⁺ influx. Key words: Barley, K⁺ influx     

A Comparison between the Uptake of Potassium by Plants from Solutions of Constant Potassium Concentration and during Depletion

A comparison was made between two methods of measuring the relationshipbetween the external [K⁺] and the flux of K⁺ into whole plantsof Lolium perenne and Raphanus sativus. The values of flux obtainedfrom solutions of 1.2 µM K⁺ held constant around the rootswere three and six times greater for Lolium and Raphanus respectivelythan the values obtained at the same concentration in a depletionexperiment in which the solutions, initially 100 µM K⁺,were depleted to below 1.2 µM K⁺ by plant uptake. In thedepletion experiment with Lolium, the flux was higher into plantsgrown at low [K⁺] than into plants grown at 100 µM eventhough [K⁺] within the plant was about the same for all groupsof plants. It is suggested that Lolium grown at low [K⁺] hasan efficient mechanism for K⁺ uptake which continues to operatefor some time after the plants have been transferred to a higherconcentration. With both species, K_m was 15–20 µMin the depletion experiment and below 1 µM when concentrationswere held constant.     

Wild Oat/Barley Interactions: Varietal Differences in Competitiveness in Relation to K+ Supply

Eight cvs of barley (Hordeum vulgare L.) were separately plantedwith Wild Oats (Avena fatua L., genetically pure line CS40)in a sand culture with two external K⁺ concentrations. Substantialdifferences were observed among barley cvs in their abilityto compete with wild oat. The variety Fergus was highly competitiveat both high and low [K⁺]_e, whereas Steptoe was competitiveonly at high [K⁺]_e, and Compana was only weakly competitivewith wild oat. The differences between barley cvs were relatedto their previously reported efficiencies of K⁺ uptake and utilization. Hordeum vulgare L., Avena fatua L., barley, wild oat, competition, K⁺ nutrition, utilization efficiency     

Role of K+ and Cl- in Osmotic Adjustment in Roots and Hypocotyls of Intact Mung Bean Seedlings

It was confirmed that osmotic adjustment occurred in young intactmung bean (Vigna mungo (L.) Hepper) seedlings exposed to highosmotic pressure stress. Root growth was not affected by osmoticpressure of less than 200 mOsra in the external solution, althoughhypocotyl growth was conspicuously reduced. Under this moderateosmotic stress, intracellular K⁺ concentration, [K⁺]_i, increaseddramatically during the osmotic adjustment in all the regionsof the root, but the intracellular Cl^– concentration,[Cl^–]_i, increased only in the aged mature region of theroot (28–33 mm from the root tip). About a half of theintracellular osmotic pressure in the aged mature region ofthe root could be ascribed to the contributions of [K⁺]_i and[Cl^–]_i, but in the hypocotyl, [K⁺]_i only contributed slightlyto the osmotic adjustment. (Received June 18, 1986; Accepted August 26, 1986)     

Potassium Channels at Chara Plasmalemma

Exposure to high K⁺ medium transforms Chara plasmalemma into[K⁺]_osensitive state (K⁺ state). The current-voltage (I/V)characteristicsunder such conditions display a negative conductance region.This feature results from the complex time and voltage dependenceof K⁺ channel opening At potentials more negative than a thresholdp.d. the channels are closed and the I/V characteristics becomelinear with a low slope conductance of 0.8 S m² and only a weakdependence on [K⁺]_o. Such behaviour is usually associated witha non-specific leak current The threshold level for K⁺ channelclosing depends on [K⁺]_o. In 2.0 mol m^–3 and 5.0 mol m^–3K⁺ medium the membrane resting p.d. follows E_K, but hyperpolarizesgradually if the [K⁺]_o is lowered. The proton pump thus appearsto be non-operative, while the cell is in the K⁺ state, andrecovers slowly as the cell is returned to a low K⁺ medium.Excitation currents decline if the cells are kept in K⁺ statefor some hours. Key words: K⁺ channels, Chara corallina, Proton pump, Current/, oltage characteristics, Conductance     

K+ Channel in the Chora Plasmalemma: Estimations of K+ Channel Density and Single K+ Channel Conductance

The electromotive force E and the conductance G of the Characorallina plasmalemma were measured under voltage clamp conditions.In the depolarized voltage range less negative than –60mV, E changed according to the Nerhst equation for K⁺, and Gincreased with the external K⁺ concentration [K⁺]_o and alsowith the depolarization of the membrane potential. This is attributedto the voltage-dependent opening of the K⁺ channels in the largelydepolarized voltage region. The voltage-dependent increase ofG was due to the increase of the number of open K⁺ channelsper unit area. The density of the total K⁺ channels in the C. corallina plasmalemmawas estimated to be about 6.50/(10 µm)2. The single K⁺channel conductance _K changed with the external [K⁺]_o; it was79.3, 86.1, 105.9, 119.0 pS for external [K⁺]_o of 0.2, 0.5,2.0 and 5.0 mu respectively. (Received May 22, 1986; Accepted August 22, 1986)     

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     

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     

Contribution of Na(+)-K(+)-Cl(-) cotransporter to high-[K(+)](o)- induced swelling and EAA release in astrocytes

We hypothesized that highextracellular K⁺ concentration([K⁺]_o)-mediated stimulation ofNa⁺-K⁺-Cl cotransporter isoform 1 (NKCC1) may result in a net gain of K⁺ and Cland thus lead to high-[K⁺]_o-induced swellingand glutamate release. In the current study, relative cell volumechanges were determined in astrocytes. Under 75 mM[K⁺]_o, astrocytes swelled by 20.2 ± 4.9%. This high-[K⁺]_o-mediated swelling wasabolished by the NKCC1 inhibitor bumetanide (10 µM, 1.0 ± 3.1%; P < 0.05). Intracellular³⁶Cl accumulation was increased from acontrol value of 0.39 ± 0.06 to 0.68 ± 0.05 µmol/mgprotein in response to 75 mM [K⁺]_o. Thisincrease was significantly reduced by bumetanide (P < 0.05). Basal intracellular Na⁺ concentration([Na⁺]_i) was reduced from 19.1 ± 0.8 to16.8 ± 1.9 mM by bumetanide (P < 0.05).[Na⁺]_i decreased to 8.4 ± 1.0 mM under75 mM [K⁺]_o and was further reduced to5.2 ± 1.7 mM by bumetanide. In addition, the recovery rate of[Na⁺]_i on return to 5.8 mM[K⁺]_o was decreased by 40% in the presenceof bumetanide (P < 0.05). Bumetanide inhibitedhigh-[K⁺]_o-induced ¹⁴C-labeledD-aspartate release by ~50% (P < 0.05).These results suggest that NKCC1 contributes tohigh-[K⁺]_o-induced astrocyte swelling andglutamate release.

     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Modest dietary K+ restriction provokes insulin resistance of cellular K+ uptake and phosphorylation of renal outer medulla K+ channel without fall in plasma K+ concentration

Extracellular K⁺ concentration ([K⁺]) is closely regulated by the concerted regulatory responses of kidney and muscle. In this study, we aimed to define the responses activated when dietary K⁺ was moderately reduced from a control diet (1.0% K⁺) to a 0.33% K⁺ diet for 15 days. Although body weight and baseline plasma [K⁺] (4.0 mM) were not reduced in the 0.33% K⁺ group, regulatory responses to conserve plasma [K⁺] were evident in both muscle and kidney. Insulin-stimulated clearance of K⁺ from the plasma was estimated in vivo in conscious rats with the use of tail venous and arterial cannulas. During infusion of insulin·(50 mU·kg^–1·min^–1), plasma [K⁺] level fell to 3.2 ± 0.1 mM in the 1.0% K⁺ diet group and to only 3.47 ± 0.07 mM in the 0.33% K⁺ diet group (P < 0.01) with no reduction in urinary K⁺ excretion, which is evidence of insulin resistance to cellular K⁺ uptake. Insulin-stimulated cellular K⁺ uptake was quantitated by measuring the K⁺ infusion rate necessary to clamp plasma K⁺ at baseline (in µmol·kg^–1·min^–1) during 5 mU of insulin·kg^–1·min^–1 infusion: 9.7 ± 1.5 in 1% K⁺ diet was blunted to 5.2 ± 1.7 in the 0.33% K⁺ diet group (P < 0.001). Muscle [K⁺] and Na⁺-K⁺-ATPase activity and abundance were unchanged during the 0.33% K⁺ diet. Renal excretion, which was measured overnight in metabolic cages, was reduced by 80%, from 117.6 ± 10.5 µmol/h/animal (1% K⁺ diet) to 24.2 ± 1.7 µmol/h/animal (0.33% K⁺ diet) (P < 0.001). There was no significant change in total abundance of key renal K⁺ transporters, but 50% increases in both renal PTK cSrc abundance and ROMK phosphorylation in the 0.33% K⁺ vs. 1% K⁺ diet group, previously established to be associated with internalization of ROMK. These results indicate that plasma [K⁺] can be maintained during modest K⁺ restriction due to a decrease in insulin-stimulated cellular K⁺ uptake as well as renal K⁺ conservation mediated by inactivation of ROMK, both without a detectable change in plasma [K⁺]. The error signals inciting and maintaining these responses remain to be identified. potassium homeostasis; Na⁺-K⁺-ATPase; H⁺-K⁺-ATPase; protein tyrosine kinase; cSrc     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: III. POST-ANTHESIS KINETICS OF NET NITRATE UPTAKE AND THE ROLE OF THE RELATIVE ROOT SIZE IN DETERMINING THE CAPACITY FOR NITRATE ACQUISITION

Barley (Hordeum vulgare L., cvs Golf, Mette, and Laevigatum)was grown under nitrogen limitation in solution culture untilnear maturity. Three different nitrogen addition regimes wereused: in the ‘HN’ culture the relative rate of nitrate-Naddition (RA) was 0·08 d^–1 until day 48 and thendecreased stepwise to, finally, 0·005 d^–1 duringgrain-filling; the ‘LN’ culture received 45% ofthe nitrogen added in HN; the ‘CN’ culture was maintainedat RA 0·0375 d^–1 throughout. Kinetics of net nitrateuptake were measured during ontogeny at 30 to 150 mmol m^–3external nitrate. V_max (which is argued to reflect the maximuminflux rate in these plants) declined with age in both HN andLN cultures. A pronounced transient drop was observed just beforeanthesis, which correlated in time with a peak in root nitrateconcentration. Similar, but less pronounced, trends were observedin CN. The relative V_max (unit nitrogen taken up per unit nitrogenin plants and day) in all three cultures declined from 1·3–2·3d^–1 during vegetative growth to 0·1–0·7d^–1 during generative growth. These values are in HN andLN cultures 15- to more than 100-fold in excess of the demandset by growth rates throughout ontogeny. Predicted balancingnitrate concentrations (defined as the nitrate concentrationrequired to support the observed rate of growth) were below6·0 mmol m^–3 in HN and LN cultures before anthesisand then decreased during ontogeny. In CN cultures the balancingnitrate concentration increased during grain-filling. Apartfrom the transient decline during anthesis, most of the effectof ageing on relative V_max can be explained in terms of reducedcontribution of roots to total biomass (R:T). The loss in uptakeper unit root weight is largely compensated for by the declinewith time in average tissue nitrogen concentrations. The quantitativerelationships between relative V_max and R:T in ageing plantsare similar to those observed for vegetative plants culturedat different RAs. The data support the contention that the capacity for nitrateacquisition in N-limited plants is under general growth control,rather than controlled by specific regulation of the biochemicalpathway of nitrate assimilation. Key words: Barley, nitrogen concentration, root: total plant biomass ratio, V_max     

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     

Carbon Exchange Rate of Intact Individual Soya Bean Pods. 2. Ontogeny of Temperature Sensitivity

Diurnal temperature fluctuations induced change in soya bean-pod[Glycine max (L.) Merr.] carbon exchange rate (CER, where positiveCER represents CO₂ evolution). CER appeared to depend linearlyon temperature. Linear regressions of CER on temperature interceptedthe temperature axis at 5°C (i.e. zero CER at 5°C).Slopes of these regressions (i.e. temperature sensitivity) changedover the season. The CER-temperature sensitivity coefficient,K, (calculated from observed values of CER. pod temperatureand temperature intercept) rose from less than 0·02 mgCO₂ h^–1 pod^–1 °C^–1 during early pod-flll,peaked at over 0·04 mg CO₂ h^–1 pod^–1 °C^–1at mid pod-fill, and then declined during late pod-fill andmaturation. Glycine max (L.) Merr., Soya bean, carbon exchange rate, temperature     

Studies of the Uptake of Nitrate in Barley: III. COMPARTMENTATION OF NO3-

Compartmental analyses of intact roots of barley (Hordeum vulgareL. cv. Klondike) plants, grown with different levels of NO₃^–(up to 1·0 mol m^–3) in the external media, wereundertaken using ¹³NO₃^–. Two additional treatments, namelysodium dodecyl sulphate (SDS) or brief exposure to high temperature,designed to investigate the identity of the three NO₃^–compartments revealed by compartmental analyses, provided supportfor the identification of the latter as corresponding to superficialsolution, apoplasm, and cytoplasm. Half-lives for exchange ofthese compartments, 3 s, 30 s, and 7 mm, were unaffected bythe level of NO₃^– provided during growth. Independentestimates of ¹³NO₃^– fluxes obtained by direct methodsagreed well with values of fluxes calculated from the compartmentalanalyses. Cytoplasmic [NO₃^–], estimated from the compartmental analyses,were in the range from 1–37 mol m^–3, and increasedwith increasing [NO₃^–] of the medium. Such values forcytoplasmic [NO₃^–] are inconsistent with an earlier proposal(Siddiqi, Glass, Ruth, and Rufty, 1990; Glass, Siddiqi, Ruth,and Rufty, 1990) of passive NO₃^– uptake in the concentrationrange above 10 mol m^–3. A model, based upon localizeddistribution of nitrate reductase activity in epidermal cells,is proposed in which the proposed passive low affinity NO uptakeat high external [NO₃^–] is restricted to epidermal cells. During loading periods with ¹³NO₃^–, significant amountsof ¹³N were translocated to the shoot. Two pools of ¹³N, onebeing the root symplasm, appear to participate in the transferof labelled N to the shoot. Key words: Barley, compartmentation, nitrate, nitrate reductase, ¹³N     

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     

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