Relation between Ion Accumulation of Salt-Sensitive and Isolated Stable Salt-Tolerant Cell Lines of Citrus aurantium

Four selected NaCl-tolerant cell lines of Sour orange (Citrus aurantium) were compared with the nonselected cell line in their growth and internal ion content of Na⁺, K⁺, and Cl⁻ when exposed to increasing NaCl concentrations. No difference was found among the various NaCl-tolerant cell lines in Na⁺ and Cl⁻ uptake, and all these cell lines took up similar or even larger amounts of Na⁺ and Cl⁻ than the NaCl-sensitive cell line. Exposure of cells of NaCl-sensitive and NaCl-tolerant lines to equal external concentrations of NaCl, resulted in a greater loss of K⁺ from the NaCl-sensitive cell line. This observation leads to the conclusion that growth and ability to retain high levels of internal K⁺ are correlated. Exposure of the NaCl-tolerant cell lines to salts other than NaCl resulted in even greater tolerance to Na₂SO₄, but rather poor tolerance to K⁺ introduced as either K₂SO₄ or KCl; the latter has a stronger inhibitory effect. The NaCl-sensitive cell line proved to be more sensitive to replacement of Na⁺ by K⁺. Analyses of internal Na⁺, K⁺, and Cl⁻ concentrations failed to identify any particular internal ion concentration which could serve as a reliable marker for salt tolerance.     

Anion-Sensitive, H-Pumping ATPase of Oat Roots : Direct Effects of Cl, NO(3), and a Disulfonic Stilbene

To understand the mechanism and molecular properties of the tonoplast-type H⁺-translocating ATPase, we have studied the effect of Cl⁻, NO₃⁻, and 4,4′-diisothiocyano-2,2′-stilbene disulfonic acid (DIDS) on the activity of the electrogenic H⁺-ATPase associated with low-density microsomal vesicles from oat roots (Avena sativa cv Lang). The H⁺-pumping ATPase generates a membrane potential (Δψ) and a pH gradient (ΔpH) that make up two interconvertible components of the proton electrochemical gradient (μh⁺). A permeant anion (e.g. Cl⁻), unlike an impermeant anion (e.g. iminodiacetate), dissipated the membrane potential ([¹⁴C]thiocyanate distribution) and stimulated formation of a pH gradient ([¹⁴C]methylamine distribution). However, Cl⁻-stimulated ATPase activity was about 75% caused by a direct stimulation of the ATPase by Cl⁻ independent of the proton electrochemical gradient. Unlike the plasma membrane H⁺-ATPase, the Cl⁻-stimulated ATPase was inhibited by NO₃⁻ (a permeant anion) and by DIDS. In the absence of Cl⁻, NO₃⁻ decreased membrane potential formation and did not stimulate pH gradient formation. The inhibition by NO₃⁻ of Cl⁻-stimulated pH gradient formation and Cl⁻-stimulated ATPase activity was noncompetitive. In the absence of Cl⁻, DIDS inhibited the basal Mg,ATPase activity and membrane potential formation. DIDS also inhibited the Cl⁻-stimulated ATPase activity and pH gradient formation. Direct inhibition of the electrogenic H⁺-ATPase by NO₃⁻ or DIDS suggest that the vanadate-insensitive H⁺-pumping ATPase has anion-sensitive site(s) that regulate the catalytic and vectorial activity. Whether the anion-sensitive H⁺-ATPase has channels that conduct anions is yet to be established.     

Ae4 (Slc4a9) Anion Exchanger Drives Cl? Uptake-dependent Fluid Secretion by Mouse Submandibular Gland Acinar Cells

Transcellular Cl⁻ movement across acinar cells is the rate-limiting step for salivary gland fluid secretion. Basolateral Nkcc1 Na⁺-K⁺-2Cl⁻ cotransporters play a critical role in fluid secretion by promoting the intracellular accumulation of Cl⁻ above its equilibrium potential. However, salivation is only partially abolished in the absence of Nkcc1 cotransporter activity, suggesting that another Cl⁻ uptake pathway concentrates Cl⁻ ions in acinar cells. To identify alternative molecular mechanisms, we studied mice lacking Ae2 and Ae4 Cl⁻/HCO₃⁻ exchangers. We found that salivation stimulated by muscarinic and β-adrenergic receptor agonists was normal in the submandibular glands of Ae2^−/− mice. In contrast, saliva secretion was reduced by 35% in Ae4^−/− mice. The decrease in salivation was not related to loss of Na⁺-K⁺-2Cl⁻ cotransporter or Na⁺/H⁺ exchanger activity in Ae4^−/− mice but correlated with reduced Cl⁻ uptake during β-adrenergic receptor activation of cAMP signaling. Direct measurements of Cl⁻/HCO₃⁻ exchanger activity revealed that HCO₃⁻-dependent Cl⁻ uptake was reduced in the acinar cells of Ae2^−/− and Ae4^−/− mice. Moreover, Cl⁻/HCO₃⁻ exchanger activity was nearly abolished in double Ae4/Ae2 knock-out mice, suggesting that most of the Cl⁻/HCO₃⁻ exchanger activity in submandibular acinar cells depends on Ae2 and Ae4 expression. In conclusion, both Ae2 and Ae4 anion exchangers are functionally expressed in submandibular acinar cells; however, only Ae4 expression appears to be important for cAMP-dependent regulation of fluid secretion.     

Mineral ion contents and cell transmembrane electropotentials of pea and oat seedling tissue

The relationships of concentration gradients to electropotential gradients resulting from passive diffusion processes, after equilibration, are described by the Nernst equation. The primary criterion for the hypothesis that any given ion is actively transported is to establish that it is not diffusing passively. A test was made of how closely the Nernst equation describes the electrochemical equilibrium in seedling tissues. Segments of roots and epicotyl internodes of pea (Pisum sativum var. Alaska) and of roots and coleoptiles of oat (Avena sativa var. Victory) seedlings were immersed and shaken in defined nutrient solutions containing eight major nutrients (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻, H₂PO₄⁻ and SO₄²⁻) at 1-fold and 10-fold concentrations. The tissue content of each ion was assayed at 0, 8, 24, and 48 hours. A near-equilibrium condition was approached by roots for most ions; however, the segments of shoot tissue generally continued to show a net accumulation of some ions, mainly K⁺ and NO₃⁻. Only K⁺ approached a reasonable fit to the Nernst equation and this was true for the 1-fold concentration but not the 10-fold. The data suggest that for Na⁺, Mg²⁺, and Ca²⁺ the electrochemical gradient is from the external solution to the cell interior; thus passive diffusion should be in an inward direction. Consequently, some mechanism must exist in plant tissue either to exclude these cations or to extrude them (e.g., by an active efflux pump). For each of the anions the electrochemical gradient is from the tissue to the solution; thus an active influx pump for anions seems required. Root segments approach ionic equilibrium with the solution concentration in which the seedlings were grown. Segments of shoot tissue, however, are far removed from such equilibration. Thus in the intact seedling the extracellular (wall space) fluid must be very different from that of the nutrient solution bathing the segments; it would appear that the root is the site of regulation of ion uptake in the intact plant although other correlative mechanisms may be involved.     

The influence of ammonium and chloride on potassium and nitrate absorption by barley roots depends on time of exposure and cultivar

Net uptakes of K⁺ and NO₃⁻ were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K⁺ and NO₃⁻ uptake in the absence of NH₄⁺ or Cl⁻. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH₄⁺, K⁺, Cl⁻, or NO₃⁻ affected the cultivars very differently: (a) the presence of NH₄⁺ as NH₄Cl stimulated net NO₃⁻ uptake in Prato barley but inhibited net NO₃⁻ uptake in Olli barley; (b) Cl⁻ inhibited net NO₃⁻ uptake in Prato but had little effect in Olli; and (c) NH₄⁺ as (NH₄)₂SO₄ inhibited net K⁺ uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl⁻ initially inhibited net K⁺ uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH₄⁺ and Cl⁻ uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.     

Potassium Transport in Corn Roots : IV. Characterization of the Linear Component

A detailed examination was conducted on the linear, or first-order kinetic component for K⁺(⁸⁶Rb⁺) influx into root segments of both low- and high-salt grown corn seedlings (Zea mays [A632 × Oh 43]). In tissue from both low- and high-salt grown roots, replacement of Cl⁻ in the uptake solution by either SO₄²⁻, H₂PO₄⁻, or NO₃⁻ caused a significant (50-60%) and specific inhibition of the linear component of K⁺ influx. The anion transport inhibitor, 4,4′-diisothiocyano-2,2′-disulfonic acid, was found to abolish saturable Cl⁻ influx in corn roots while causing a significant (50-60%) and specific inhibition of the linear K⁺ uptake system; this inhibition was identical to that observed when Cl⁻ was replaced by other anions in the K⁺ uptake solution. Additionally, the quaternary ammonium cation, tetraethylammonium, which has been shown to block K⁺ channels in nerve axons, also caused a dramatic (70%) and specific inhibition of the linear component of K⁺ influx, but this was obtained only in high-salt roots. The reasons for this difference are discussed with respect to the differing abilities of low- and high-salt roots to absorb tetraethylammonium.

Our present results indicate that the linear component of K⁺ influx may occur by a passive process involving transmembrane K⁺ channels. Fluxes through these K⁺ channels may be partly coupled to a saturating Cl⁻ influx mechanism.

     

Calcium uptake by excised maize roots and interactions with alkali cations

Ca²⁺ uptake was studied in short-term experiments using 5-day-old excised maize roots. This tissue readily absorbs Ca²⁺, and inhibition by dinitrophenol and low temperature shows that the process is metabolically mediated. The uptake of Ca²⁺, like that of other cations, is influenced by the counter ion, the pH and concentration of the ambient solution, and the presence of other cations. The rate of uptake from various salts decreases in the following order: NO₃⁻ > Cl⁻ = Br⁻ > SO₄²⁻. K⁺ and H⁺ greatly interfere with Ca²⁺ absorption, while Li⁺ and Na⁺ have only slight effects.     

Active extrusion of protons into deionized water by roots of intact maize plants

The investigations were focussed on the question as to whether roots of intact maize plants (Zea mays L. cv Blizzard) release protons into deionized H₂O. Plants in the six to seven leaf stage depressed the pH of deionized H₂O from 6 to about 4.8 during an experimental period of 4 hours. Only one-third of the protons released could be ascribed to the solvation of CO₂ in H₂O. The main counter anions released were Cl⁻, NO₃⁻, and SO₄²⁻. At low temperature (2°C), the H⁺ release was virtually blocked while a relatively high amount of K⁺ was released. The presence of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the external solution increased the H⁺ secretion significantly. Addition of vanadate to the outer medium inhibited the H⁺ release while fusicoccin had a stimulating effect. Substituting the nutrient solution of deionized H₂O resulted in a substantial increase of the membrane potential difference from −120 to −190 millivolts. The experimental results support the conclusion that the H⁺ release by roots of intact maize plants is an active process driven by a plasmalemmalocated ATPase. Since the net H⁺ release was not associated with a net uptake of K⁺, it is unlikely to originate from a K⁺/H⁺ antiport.     

Intracellular Cl? Dependence of Na-H Exchange in Barnacle Muscle Fibers under Normotonic and Hypertonic Conditions

We previously showed that shrinking a barnacle muscle fiber (BMF) in a hypertonic solution (1,600 mosM/kg) stimulates an amiloride-sensitive Na-H exchanger. This activation is mediated by a G protein and requires intracellular Cl⁻. The purpose of the present study was to determine (a) whether Cl⁻ plays a role in the activation of Na-H exchange under normotonic conditions (975 mosM/kg), (b) the dose dependence of [Cl⁻]_i for activation of the exchanger under both normo- and hypertonic conditions, and (c) the relative order of the Cl⁻- and G-protein-dependent steps. We acid loaded BMFs by internally dialyzing them with a pH-6.5 dialysis fluid containing no Na⁺ and 0–194 mM Cl⁻. The artificial seawater bathing the BMF initially contained no Na⁺. After dialysis was halted, adding 50 mM Na⁺ to the artificial seawater caused an amiloride-sensitive pH_i increase under both normo- and hypertonic conditions. The computed Na-H exchange flux (J _Na-H) increased with increasing [Cl⁻]_i under both normo- and hypertonic conditions, with similar apparent K _m values (∼120 mM). However, the maximal J _Na-H increased by nearly 90% under hypertonic conditions. Thus, activation of Na-H exchange at low pH_i requires Cl⁻ under both normo- and hypertonic conditions, but at any given [Cl⁻]_i, J _Na-H is greater under hyper- than normotonic conditions. We conclude that an increase in [Cl⁻]_i is not the primary shrinkage signal, but may act as an auxiliary shrinkage signal. To determine whether the Cl⁻-dependent step is after the G-protein-dependent step, we predialyzed BMFs to a Cl⁻-free state, and then attempted to stimulate Na-H exchange by activating a G protein. We found that, even in the absence of Cl⁻, dialyzing with GTPγS or AlF₃, or injecting cholera toxin, stimulates Na-H exchange. Because Na-H exchange activity was absent in control Cl⁻-depleted fibers, the Cl⁻-dependent step is at or before the G protein in the shrinkage signal-transduction pathway. The stimulation by AlF₃ indicates that the G protein is a heterotrimeric G protein.     

Inhibition of anion transport in corn root protoplasts

The effects of several amino-reactive disulfonic stilbene derivatives and N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate on Cl⁻, SO₄²⁻, and inorganic phosphate (Pi) uptake in protoplasts isolated from corn root tissue were studied. 4-Acetamido-4′-isothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diamino-2,2′-stilbenedisulfonic acid, and NAP-taurine inhibited Cl⁻ and SO₄²⁻ but not Pi and K⁺ uptake in corn root protoplasts; whereas mersalyl inhibited Pi but not Cl⁻ or SO₄²⁻ uptake. The rate of uptake of all anions decreased with increasing external pH. In addition, these reagents markedly inhibited plasmalemma ATPase activity isolated from corn root tissue. Excised root segments were less sensitive to Cl⁻ and SO₄²⁻ transport inhibitors.     

Simulation of Cl Uptake by Low-salt Barley Roots as a Test of Models of Salt Uptake

Computing techniques are used to simulate the course of uptake of K⁺, Na⁺, and Cl⁻ by low-salt roots. Measurements of the fluxes of these ions in high-salt roots are used to calculate membrane permeabilities, which are then used to calculate cell uptake. In this way it is possible to test the predictive value of different models for the location of sites of salt uptake in the cell.     

Ion Transport in Isolated Protoplasts from Tobacco Suspension Cells: II. Selectivity and Kinetics

Protoplasts were enzymically isolated from suspension cultured cells of Nicotiana glutinosa L. and aspects of transport selectivity and kinetics were studied. In the presence of Ca²⁺, transport was selective for K⁺ (⁸⁶Rb) over Na⁺. ³⁶Cl⁻ transport was inhibited by Br⁻ or I⁻ but not by H₂PO₄⁻. The kinetic data for short term (30 minutes) K⁺ influx over the range of 0.05 to 100 millimolar KCl were complex but similar to those observed in other plant tissues. In contrast, the kinetic data for Cl⁻ and H₂³²PO₄⁻ over the same concentration range were different from those observed for K⁺, and could be accounted for by a single isotherm in the range of 0.05 to 4 millimolar and by an almost linear increase in influx rate above 4 millimolar. The kinetic data for Cl⁻ transport into intact cultured cells were identical in character to those observed for isolated protoplasts. The results support the view that enzymic removal of the cell wall produced no significant alteration in the transport properties of the protoplast.     

The Influence of Nitrate and Chloride Uptake on Expressed Sap pH, Organic Acid Synthesis, and Potassium Accumulation in Higher Plants

The influence of NO₃⁻ uptake and reduction on ionic balance in barley seedlings (Hordeum vulgare, cv. Compana) was studied. KNO₃ and KCl treatment solutions were used for comparison of cation and anion uptake. The rate of Cl⁻ uptake was more rapid than the rate of NO₃⁻ uptake during the first 2 to 4 hours of treatment. There was an acceleration in rate of NO₃⁻ uptake after 4 hours resulting in a sustained rate of NO₃⁻ uptake which exceeded the rate of Cl⁻ uptake. The initial (2 to 4 hours) rate of K⁺ uptake appeared to be independent of the rate of anion uptake. After 4 hours the rate of K⁺ uptake was greater with the KNO₃ treatment than with the KCl treatment, and the solution pH, cell sap pH, and organic acid levels with KNO₃ increased, relative to those with the KCl treatment. When absorption experiments were conducted in darkness, K⁺ uptake from KNO₃ did not exceed K⁺ uptake from KCl. We suggest that the greater uptake and accumulation of K⁺ in NO₃⁻-treated plants resulted from (a) a more rapid, sustained uptake and transport of NO₃⁻ providing a mobile counteranion for K⁺ transport, and (b) the synthesis of organic acids in response to NO₃⁻ reduction increasing the capacity for K⁺ accumulation by providing a source of nondiffusible organic anions.     

Influence of the level of nitrate nutrition on ion uptake and assimilation, organic Acid accumulation, and cation-anion balance in whole tomato plants

Tomato plants (Lycopersicon esculentum L. var. Ailsa Craig) were grown in water culture in nutrient solution in a series of 10 increasing levels of nitrate nutrition. Using whole plant data derived from analytical and yield data of individual plant parts, the fate of anion charge arising from increased NO₃ assimilation was followed in its distribution between organic anion accumulation in the plant and OH⁻ efflux into the nutrient solution as calculated by excess anion over cation uptake. With increasing NO₃ nutrition the bulk of the anion charge appeared as organic anion accumulation in the plants. OH⁻ efflux at a maximum accounted for only 20% of the anion charge shift. The major organic anion accumulated in response to nitrate assimilation was malate. The increase in organic anion accumulation was paralleled by an increase in cation concentration (K⁺, Ca²⁺, Mg²⁺, Na⁺). Total inorganic anion levels (NO₃⁻, SO₄²⁻, H₂PO₄⁻, Cl⁻) were relatively constant. The effect of increasing NO₃ nutrition in stimulating organic anion accumulation was much more pronounced in the tops than in the roots.     

Anion-sensitive, h-pumping ATPase in membrane vesicles from oat roots

H⁺-pumping ATPases were detected in microsomal vesicles of oat (Avena sativa L. var Lang) roots using [¹⁴C]methylamine distribution or quinacrine fluorescent quenching. Methylamine (MeA) accumulation into vesicles and quinacrine quench were specifically dependent on Mg,ATP. Both activities reflected formation of a proton gradient (ΔpH) (acid inside) as carbonyl cyanide m-chlorophenylhydrazone, nigericin (in the presence of K⁺), or gramicidin decreased MeA uptake or increased quinacrine fluorescence. The properties of H⁺ pumping as measured by MeA uptake were characterized. The K_mapp for ATP was about 0.1 millimolar. Mg,GTP and Mg, pyrophosphate were 19% and 30% as effective as Mg,ATP. MeA uptake was inhibited by N,N′-dicyclohexylcarbodiimide and was mostly insensitive to oligomycin, vanadate, or copper. ATP-dependent MeA was stimulated by anions with decreasing order of potency of Cl⁻ > Br⁻ > NO₃⁻ > SO₄²⁻, iminodiacetate, benzene sulfonate. Anion stimulation of H⁺ pumping was caused in part by the ability of permeant anions to dissipate the electrical potential and in part by a specific requirement of Cl⁻ by a H⁺ -pumping ATPase. A pH gradient, probably caused by a Donnan potential, could be dissipated by K⁺ in the presence or absence of ATP. MeA uptake was enriched in vesicles of relatively low density and showed a parallel distribution with vanadate-insensitive ATPase activity on a continuous dextran gradient. ΔpH as measured by quinacrine quench was partially vanadate-sensitive. These results show that plant membranes have at least two types of H⁺ -pumping ATPases. One is vanadate-sensitive and probably enriched in the plasma membrane. One is vanadate-resistant, anion-sensitive and has many properties characteristic of a vacuolar ATPase. These results are consistent with the presence of electrogenic H⁺ pumps at the plasma membrane and tonoplast of higher plant cells.     

Early effects of salinity on nitrate assimilation in barley seedlings

The effect of NaCl and Na₂SO₄ salinity on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO₃⁻ transporter was affected very little; the major effect of the salts was on its activity. Both Cl⁻ and SO₄²⁻ salts severely inhibited uptake of NO₃⁻. When compared on the basis of osmolality of the uptake solutions, Cl⁻ salts were more inhibitory (15-30%) than SO₄²⁻ salts. At equal concentrations, SO₄²⁻ salts inhibited NO₃⁻ uptake 30 to 40% more than did Cl⁻ salts. The absolute concentrations of each ion seemed more important as inhibitors of NO₃⁻ uptake than did the osmolality of the uptake solutions. Both K⁺ and Na⁺ salts inhibited NO₃⁻ uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.

Unlike NO₃⁻ uptake, NO₃⁻ reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO₃⁻ in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO₃⁻ reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO₃⁻ transporter, rather than that of the NO₃⁻ reduction system, may be a critical factor to plant survival during salt stress.

     

Potential-dependent anion transport in tonoplast vesicles from oat roots

Potential-dependent anion movement into tonoplast vesicles from oat roots (Avena sativa L. var Lang) was monitored as dissipation of membrane potentials (Δψ) using the fluorescence probe Oxonol V. The potentials (positive inside) were generated with the H⁺-pumping pyrophosphatase, which is K⁺ stimulated and anion insensitive. The relative rate of ΔΨ dissipation by anions was used to estimate the relative permeabilities of the anions. In decreasing order they were: SCN⁻ (100) > NO₃⁻ (72) = Cl⁻ (70) > Br⁻ (62) > SO₄²⁻ (5) = H₂PO₄⁻ (5) > malate (3) = acetate (3) > iminodiacetate (2). Kinetic studies showed that the rate of Δψ dissipation by Cl⁻ and NO₃⁻, but not by SCN⁻, was saturable. The K_m values for Cl⁻ and NO₃⁻ uptake were about 2.3 and 5 millimolar, respectively, suggesting these anions move into the vacuole through proteinaceous porters. In contrast to a H⁺-coupled Cl⁻ transporter on the same vesicles, the potential-dependent Cl⁻ transport was insensitive to 4,4′-diisothiocyano-2,2′-stilbene disulfonate. These results suggest the existence of at least two different mechanisms for Cl⁻ transport in these vesicles. The potentials generated by the H⁺-translocating ATPase and H⁺-pyrophosphatase were nonadditive, giving support to the model that both pumps are on tonoplast vesicles. No evidence for a putative Cl⁻ conductance on the anion-sensitive H⁺-ATPase was found.     

Differential Inhibition by Ferulic Acid of Nitrate and Ammonium Uptake in Zea mays L

The influence of the allelopathic compound ferulic acid (FA) on nitrogen uptake from solutions containing both NO₃⁻ and NH₄⁺ was examined in 8-day-old nitrogen-depleted corn (Zea mays L.) seedlings. Concurrent effects on uptake of Cl⁻ and K⁺ also were assessed. The presence of 250 micromolar FA inhibited the initial (0-1 hours) rate of NO₃⁻ uptake and also prevented development of the NO₃⁻-inducible accelerated rate. The pattern of recovery when FA was removed was interpreted as indicating a rapid relief of FA-restricted NO₃⁻ uptake activity, followed by a reinitiation of the induction of that activity. No inhibition of NO₃⁻ reduction was detected. Ammonium uptake was less sensitive than NO₃⁻ uptake to inhibition by FA. An inhibition of Cl⁻ uptake occurred as induction of the NO₃⁻ transport system developed in the absence of FA. Alterations of Cl⁻ uptake in the presence of FA were, therefore, a result of a beneficial effect, because NO₃⁻ uptake was restricted, and a direct inhibitory effect. The presence of FA increased the initial net K⁺ loss from the roots during exposure to the low K, ammonium nitrate uptake solution and delayed the recovery to positive net uptake, but it did not alter the general pattern of the response. The implications of the observations are discussed for growth of plants under natural conditions and cultural practices that foster periodic accumulation of allelopathic substances.     

Delta-endotoxin-induced leakage of 86Rb+-K+ and H2O from phospholipid vesicles is catalyzed by reconstituted midgut membrane

Brush border membrane from Heliothis virescens catalyzed delta-endotoxin-induced leakage of ⁸⁶Rb⁺-K⁺ and H₂O from phospholipid vesicles. Activated delta-endotoxin [CrylA(c)-55 kDa] from Bacillus thuringiensis kurstaki strain EG2244 producing a single CrylA(c) toxin, when incorporated into phospholipid vesicles, made these vesicles more leaky to ⁸⁶Rb⁺-K⁺ than phospholipid vesicles without toxin. This effect was assayed by following the movement of ⁸⁶Rb⁺ into the vesicles in response to a KCl gradient. When toxin was added to the outside of phospholipid vesicles, ⁸⁶Rb⁺ uptake was impeded. Vesicles prepared with H. virescens brush border membrane catalyzed the association of the toxin with the vesicle, and stimulated KCl gradient-induced ⁸⁶Rb⁺ uptake. Toxin did not catalyze the leakage of ³⁶Cl⁻, suggesting that the toxin created a cation-selective leak. Toxin enhanced the permeability of phospholipid vesicles to H₂O, demonstrated by the enhanced rate of vesicle shrinking under increased osmotic pressure. This was analyzed spectrophotometrically by following the rate of vesicle shrinking in response to a 10 mM KCl gradient. In the presence of concentrated phosphatidylcholine vesicles, toxin spontaneously associated with the vesicles so as to enhance the rate of vesicle shrinking in an osmotic gradient. The rate of vesicle shrinking the presence of KCl and toxin was catalyzed by the presence of brush border reconstituted into the vesicles, reducing the effective toxin concentration 1000-fold.     

Uptake and Assimilation of NO(3) and NH(4) by Nitrogen-Deficient Perennial Ryegrass Turf

Assimilation of NO₃⁻ and NH₄⁺ by perennial ryegrass (Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH₄⁺, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO₃⁻ had been reduced, whereas 97% of the NH₄⁺ had been assimilated. During the early stages (0 to 8 hours) of NO₃⁻ uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO₃⁻, NH₄⁺ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO₃⁻ uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.