Uptake and long-distance transport of phosphate,potassium and chloride in relation to internal ion concentrations in barley: evidence of non-allosteric regulation

The extent to which uptake and transport of either phosphate, potassium or chloride are controlled by the concentration of these ions within the root, perhaps through an allosteric mechanism, was investigated with young barley plants in nutrient solution culture. Plants were grown with their roots divided between two containers, such that a single seminal root was continuously supplied with all the required nutrient ions, while the remaining four or five seminal roots were either supplied with the same solution (controls) or, temporarily, a solution lacking a particular nutrient ion (nutrient-deficient treatment). Compared with controls, there was a marked stimulation of uptake and transport of labelled ions by the single root following 24 h or more of nutrient dificiency to the remainder of the root system. This stimulation, which comprised an increased transport to the shoot and, for all ions except Cl^-, increased transport to the remainder of the root system, took place without appreciable change in the concentration of particular ions within the single root. However, nutrient deficiency quickly caused a lower concentration of ions in the shoot and the remaining roots. The results are discussed in relation to various mechanisms, proposed in the literature, by which the coordination of ion uptake and transport may be maintained within the plant. We suggest that under our conditions any putative allosteric control of uptake and transport by root cortical cells was masked by an alternative mechanism, in which ion influx appears to be regulated by ion efflux to the xylem, perhaps controlled by the concentration of particular ions recycled in the phloem to the root from the shoot.     

The influence of potassium content on the kinetics of potassium influx into excised ryegrass and barley roots

The influx of K⁺ into excised roots of barley (Hordeum vulgare L.) and ryegrass (Lolium multiflorum L.) previously grown with or without K⁺ was measured in K⁺ solutions ranging in concentration from 0.01 to 50 mM. In both species the K⁺ influx was lower in the roots with high K⁺ content. The extent of reduction by high internal [K⁺] decreased with external concentration above 1 mM. These results support the contention that at high external concentrations passive diffusion makes significant contributions to observed fluxes.     

The regulation of K+ influx in excised barley roots

The electrochemical potential differences for potassium, between excised barley (Hordeum vulgare L.) roots and external media containing 0.05 mM KCl+0.5 mM CaSO₄, were determined over a 4-h period during which initially low-K⁺ roots accumulated K⁺ by pretreatment in 50 mM KCl plus 0.5 mM CaCl₂. This pretreatment resulted in increased internal [K⁺], decreased K⁺ influx (as measured from 0.05 mM KCl+0.5 mM CaSO₄) and decreased values of . These observations indicate that the decline of K⁺ influx associated with increased internal K⁺ concentration cannot be accounted for by passive adjustment to the electrochemical gradient for this ion.     

Uptake of Lucifer Yellow CH into intact barley roots: Evidence for fluid-phase endocytosis

Intact barley (Hordeum vulgare L.) roots have been shown to take up the highly fluorescent dye Lucifer Yellow CH (LYCH) into their cell vacuoles. In the apical 1 cm of root tip, differentiating and dividing cells showed a prolific uptake of LYCH into their provacuoles. The LYCH was retained during fixation, apparently becoming bound to electron-dense material in the vacuoles. The dye freely entered the apoplast of roots in which the Casparian band was not developed, being taken up into the vacuoles of cells in both the cortex and stele. However, when LYCH was applied to a 1-cm zone approx. 6 cm behind the root tip the Casparian band on the radial walls of the endodermis completely prevented the dye from entering the cells of the stele, only the cell walls and vacuoles of the cortical cells taking up the dye. The inability of LYCH to cross the plasmalemma of the endodermal cells and enter the stele via the symplast substantiates previous claims that the dye is unable to cross the plasmalemma of plant cells. The results are discussed in the light of recent demonstrations that LYCH is a particularly effective marker for fluid-phase endocytosis in animal and yeast cells. A calculation of the energetic requirements for LYCH uptake into barley roots supports the contention that LYCH is taken up into the vacuoles of plant cells by fluid-phase endocytosis.Abbreviation LYCH Lucifer Yellow CH     

Induction of a high-capacity nitrate-uptake mechanism in barley roots prompted by nitrate uptake through a constitutive low-capacity mechanism

Roots of nitrate-starved and nitrate-pretreated seedlings of Hordeum vulgare were used to investigate the induction of a high-capacity uptake mechanism for nitrate. When exposed to 0.2 mmol·l^-1KNO₃, nitrate-starved roots took up nitrate at a rate of approx. 1 mol·(g FW)^-1·h^-1; K⁺ was absorbed at a rate ten-times higher. Nitrate uptake accelerated after a lag of about 1 h, until it matched the rate of K⁺ uptake about 4 h later. p-Fluorophenylalanine (FPA), which prevents the synthesis of functioning proteins, suppressed the development of the high-capacity mechanism. Pretreatment of the roots with 0.2 mmol·l^-1 Ca(NO₃)₂ for 24 h established the high-capacity mechanism. Pretreated roots were able to absorb nitrate at high rates immediately upon exposure to 0.2 mmol·l^-1KNO₃, in the absence or presence of FPA. The high-capacity mechanism, once established, appeared to have a protein turnover as slow as that of the low-capacity mechanism or that of the mechanism involved in the uptake of K⁺. In contrast, the mechanisms for the transport of nitrate and K⁺ into the xylem vessels were completely blocked by FPA within 1 h of application, confirming earlier evidence for a rapid turnover of the transport proteins in the xylem parenchyma.Nitrate reduction proceeded at rates which were roughly one-tenth as large as the rates of the respective nitrate-uptake processes, indicating that nitrate-reductase activity was determined by the rate of nitrate uptake and not vice versa.We conclude that the formation of a high-capacity nitrate-uptake mechanism in barley roots occurs in response to nitrate uptake through a constitutive mechanism of low capacity which appears to function as a sensing mechanism for nitrate in the environment of the roots.Abbreviation FPA p-fluorophenylalanine     

Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings

The effect of NH ₄ ⁺ on the regulation of NO ₃ ^– and NO ₂ ^– transport systems in roots of intact barley (Hordeum vulgareL.) seedlings grown in NO ₃ ^– or NO ₂ ^– was studied. Ammonium partially inhibited induction of both transport systems. The inhibition was less severe in NO ₂ ^– -fed than in NO ₃ ^– -fed seedlings, presumably due to lower uptake of NH ₄ ⁺ in the presence of NO ₂ ^– . In seedlings pretreated with NH ₄ ⁺ subsequent induction was inhibited only when NH ₄ ⁺ was also present during induction, even though pretreated roots accumulated high levels of NH ₄ ⁺ . This indicates that inhibition may be regulated by NH ₄ ⁺ concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH ₄ ⁺ , suggesting that inhibition is caused by NH ₄ ⁺ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO ₃ ^– transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO ₃ ^– for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. Induction of the NO ₃ ^– transport system was about twice as sensitive to NH ₄ ⁺ as compared to the NO ₂ ^– transport system. This may relate to higher turnover rates of membraneassociated NO ₃ ^– -transport proteins.Abbreviations Mes 2(N-morpholino)ethanesulfonic acid - MSO L-methionine sulfoximine     

Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley)

Plants of Hordeum vulgare (barley) were grown initially in a solution containing 150 M phosphate and then transferred on day 6 to solutions with (+P) and without (-P) phosphate supplied. After various times plants from these treatments were supplied with labelled phosphate. Analysis of plant growth and rates of labelled phosphate uptake showed that a general enhancement of uptake and translocation was found, in plants which had been in the-P solution, several days before the rate of dry matter accumulation was affected. Subsequently a detailed analysis of phosphate uptake by segments of intact root axes showed that the enhancement of phosphate uptake by P-stress occurred first in the old and mature parts of the seminal root axis and last in the young zones 1 cm from the root apex. During this transition period there were profound changes in the pattern of P absorption along the length of the root. Most of the additional P absorbed in response to P-stress was translocated to the shoot, particularly in older zones of the axis. Enhancement of phosphate uptake in young zones of nodal axes occurred at an earlier stage than in seminal axes. The results are related to the P-status of shoots and root zones and discussed in relation to the general control by the shoot of phosphate transport in the root.     

Close coupling between extrusion of H+ and uptake of K+ by barley roots

Extrusion of H⁺ by intact barley (Hordeum vulgare L.) roots was automatically titrated. Simultaneously, uptake of K⁺ into the roots, transport of K⁺ through the roots, and (as a residual term) accumulation of K⁺ within the root tissue were determined. When no monovalent cation was present in the medium the steady rate of H⁺ release was close to zero. Addition of K⁺ stimulated H⁺ extrusion within less than 1 min. The stimulation of H⁺ release was apparently limited only by the movement of K⁺ through the apoplast of the roots. The steady rate of H⁺ extrusion depended on the availability of external K⁺ and saturated at a K⁺ concentration of about 100 mol· dm^-3. Half-maximum rates of net K⁺ uptake and H⁺ extrusion were reached at a K⁺ concentration of about 10 mol·dm^-3. With (slowly absorbable) sulfate as the only anion present, the stoichoimetry between H⁺ release and net K⁺ uptake was one. In conclusion, the uptake of K⁺ across the plasmalemma of the cells of the root cortex is electrically coupled to H⁺ extrusion.     

Sulphate deprivation depresses the transport of nitrogen to the xylem and the hydraulic conductivity of barley (Hordeum vulgare L.) roots

During the first 4 d after the removal of SO ₄ ^2- from cultures of young barley plants, the net uptake of ¹⁵N-nitrate and the transport of labelled N to the shoot both decline. This occurred during a period in which there was no measurable change in plant growth rate and where the incorporation of [³H]leucine into membrane and soluble proteins was unaffected. Reduced N translocation was associated with six- to eightfold increases in the level of asparagine and two- to fourfold increases in glutamine in root tissue; during the first 4 d of SO ₄ ^2- deprivation there were no corresponding increases in amides in leaf tissue. The provision of 1 mol · m^–3 methionine halted, and to some extent reversed the decline in NO ₃ ^- uptake and N translocation which occurred during continued SO ₄ ^2- deprivation. This treatment had relatively little effect in lowering amide levels in roots. Experiments with excised root systems indicated that SO ₄ ^2- deprivation progressively lowered the hydraulic conductivity, Lp, of roots; after 4 d the Lp of SO ₄ ^2- -deprived excised roots was only 20% of that of +S controls. In the expanding leaves of intact plants, SO ₄ ^2- deprivation for 5 d was found to lower stomatal conductance, transpiration and photosynthesis, in the order given, to 33%, 37% and 18% of control values. The accumulation of amides in roots is probably explained by a failure to export either the products of root nitrate assimilation or phloem-delivered amino-N. This may be correlated with the lowered hydraulic conductivity. Enhanced glutamine and-or asparagine levels probably repressed net uptake of NO ₃ ^- and ¹³NO ₃ ^- influx reported earlier (Clarkson et al. 1989, J. Exp. Bot. 40, 953–963). Attention is drawn to the similar hydraulic signals occurring in the early stages of several different types of mineral-nutrient stresses.Abbreviations Asn asparagine - Gln glutamine - Lp hydraulic conductivity J.L.K. is extremely grateful to the British Council for supporting his working visit to Long Ashton. We thank John Radin for helpful discussion and encouragement.     

The hydraulic safety zone at the base of barley roots

A hydraulic constriction of the vessels occurs at the base of the primary roots of barley (Hordeum vulgare L.). The constriction and consequent hydraulic protection result from an extreme shortening of the vessel elements, leading to the accumulation of perforation plates with simple, broad-rimmed perforations which are smaller than those in normal-length vessel elements. It is compensated for by a local increase in the number of tracheary elements and an increase in their diameter. A similar trend of development was observed both at the base of other seminal roots and at the base of stem-borne adventitious roots. The rate at which compensation for the hydraulic constriction occurs could be of crucial importance for the axial resistance of water transport.     

Potassium transport in salt-stressed barley roots

Salinization of the medium inhibits both K⁺ uptake by excised barley (Hordeum vulgare L.) roots and K⁺ release from their stele, as measured by short-term ⁸⁶Rb uptake and xylem exudation, respectively. Although inhibition was not specific to chloride, mannitol caused a different response from that of inorganic sodium salts, indicating that inhibition was at least partly the result of an ion effect. In roots previously exposed to low levels of NaCl, NaCl stress directly affected stelar K⁺ release, whereas in low-sodium roots stelar K⁺ release was much less salt-sensitive than K⁺ uptake.Abbreviation chCl choline chloride     

Phosphate transport across biomembranes and cytosolic phosphate homeostasis in barley leaves

Barley (Hordeum vulgare L.) plants were grown hydroponically with or without inorganic phosphate (P_i) in the medium. Leaves were analyzed for the intercellular and the intracellular distribution of P_i. Most of the leaf P_i was contained in mesophyll cells; P_i concentrations were low in the xylem sap, the apoplast and in the cells of the epidermis. The vacuolar concentration of P_i in mesophyll cells depended on P_i availability in the nutrient medium. After infiltrating the intercellular space of leaves with solutions containing P_i, P_i was taken up by the mesophyll at rates higher than 2.5 mol· (g fresh weight)^–1 · h^–1. Isolated mesophyll protoplasts did not possess a comparable capacity to take up P_i from the medium. Phosphate uptake by mesophyll protoplasts showed a biphasic dependence on P_i concentration. Uptake of P_i by P_i-deficient cells was faster than uptake by cells which had P_i stored in their vacuoles, although cytoplasmic P_i concentrations were comparable. Phosphate transport into isolated mesophyll vacuoles was dependent on their P_i content; it was stimulated by ATP. In contrast to the vacuolar P_i concentration, and despite different kinetic characteristics of the uptake systems for p_i of the plasmalemma and the tonoplast, the cytoplasmic p_i concentration was regulated in mesophyll cells within narrow limits under very different conditions of P_i availability in the nutrient medium, whereas vacuolar P_i concentrations varied within wide limits.Dedicated to Professor Wilhelm Simonis on the occasion of his 80th birthdayThis investigation was part of the research efforts of the Sonderforschungsbereich 176 of the Bayerische Julius-Maximilians-Universität Würzburg. We are grateful to Dr. Olaf Wolf for introducing us to the method for preparation of xylem sap of barley plants and to Mr. Yin Zuhua for fluorimetric experiments with the dye pyranine. T. Mimura is indebted to the Alexander-von-Humboldt-Stiftung for a postdoctoral research fellowship.     

Relationships between external nitrate availability,nitrate uptake and expression of nitrate reductase in roots of barley grown in N-limited split-root cultures

Despite the large number of studies of nitrate metabolism in plants, it remains undetermined to what extent this key plant system is controlled by overall plant N nutrition on the one hand, and by the nitrate ion itself on the other hand. To investigate these questions, V _max for nitrate uptake (high-affinity range), and nitrate reductase (NR) mRNA and activity, were measured in roots of N-limited barley (Hordeum vulgare L. cv. Golf) grown under conditions of constant relative addition of nitrate, with the seminal roots split between two culture compartments. The total amount of nitrate added per unit time (0.09·d^-1) was distributed between the two root parts (subroots) in ratios of 1000, 982, 955, 9010, 8020, and 5050. These nitrate-addition ratios resulted in nitrate fluxes ranging from 0 to 23 mol nitrate·g^-1 DW root·h^-1, while the external nitrate concentrations varied between 0 and 1.2 M. The apparent V _max for net nitrate uptake showed saturation-type responses to nitrate flux maintained during preceding growth. The flux resulting in half-maximal induction of nitrate uptake was approximately 4 mol nitrate·g^-1 DW root·h^-1, corresponding to an external nitrate concentration of 0.7 M. The activity of NR and levels of NR mRNA did not saturate within the range of nitrate fluxes studied. None of the parameters studied saturated with respect to the steady-state external nitrate concentration. At the zero nitrate addition — the 0%-root — initial uptake activity as determined in short-term ¹⁵N-labelling experiments was insignificant, and NR activity and NR mRNA were not detectable. However, nitrate uptake was rapidly induced, showing that the 0%-root had retained the capacity to respond to nitrate. These results suggest that local nitrate availability has a significant impact on the nitrate uptake and reducing systems of a split-root part when the total plant nitrate nutrition is held constant and limiting.Abbreviation NR nitrate reductase This work was supported by the Lars Hierta Memory Foundation, the Royal Swedish Academy of Sciences, and by the Swedish Natural Science Research Council via project grants (to C.-M.L. and B.I.) and visiting scientist grant (to W.H.C.). We thank Mrs. Ellen Campbell for technical advice, and Mrs. Judith V. Purves, Long Ashton Research Station, Long Ashton, UK, for analyses of ¹⁵N-labelling in tissue samples.     

Characterization of a voltage-dependent cation-channel from the plasma membrane of rye (Secale cereale L.) roots in planar lipid bilayers

Plasma-membrane vesicles were purified by aqueous-polymer two-phase partitioning of a microsomal membrane fraction from rye (Secale cereale L.) roots and incorporated into planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. A voltage-dependent cation-channel became incorporated into the bilayer with its cytoplasmic surface facing the trans compartment (which was grounded) and was characterized from single-channel recordings. The channel had a unitary conductance of 174 pS in symmetrical 100 mM KCl. The selectivity towards monovalent cations, determined from both conductance measurements in symmetrical 100 mM cation chloride and from permeability ratios in the presence of (cis: trans) 100 mM cation chloride: 100 mM KCl, was CsKRb>Na. The channel was also permeable to both Ba²⁺ and Ca²⁺. Although the unitary conductances in symmetrical 100 mM BaCl₂ and CaCl₂ were only 46 pS and 40 pS, respectively, the apparent permeabilities of the divalent cations relative to K⁺ were greater than expected (P_KP_BaP_Ca, 1.001.662.60). This anomaly might result from competition between divalent and monovalent cations for an intrapore binding site. The channel exhibited complex gating kinetics, which were modulated in response to changes in the zero-current (reversal) potential of the channel (E_rev). In symmetrical 100 mM KCl the channel inactivated at positive voltages greater than 100 mV and the activated channel exhibited a high probability of being in an open-state (P₀>0.90) at all voltages between ±100 mV. Channel P₀ approximated unity at voltages in the range -60 to +20 mV. As more-negative voltages were applied, P₀ decreased gradually. In contrast, as more positive voltages were applied, P₀ decreased initially to a local minimum (approaching P₀=0.90), then increased as the voltage was further increased before declining at extreme positive voltages. Under physiologically relevant ionic conditions, with 100 mM KCl plus contaminant Ca²⁺ on the trans (cytoplasmic) side and 1 mM KCl plus 2 mM CaCl₂ on the cis (extracellular) side of the channel, E_rev was 25.2 mV and the relative permeability P_Ca/P_K was 7.45. Thus, the channel would be activated by plasma-membrane depolarization in vivo and facilitate Ca²⁺ influx and net K⁺ efflux. A role in intracellular signalling is proposed for this channel. It could open in response to stimuli which depolarize the plasma membrane, allowing Ca²⁺ into the cytoplasm and, thereby, initiating a cellular response. The outward K⁺ current would act to stabilize the trans-plasma membrane voltage, preventing excessive depolarization during Ca²⁺ influx.Abbreviations and Symbols E_K Nernst (equilibrium) potential for potassium ions - E_rev zero-current (reversal) potential of the channel - _c apparent mean lifetime of the activated-channel closed-state - _o apparent mean lifetime of the activated-channel open-state - PE dephosphatidylethanolamine - P_O probability of finding the activated channel in an open-state This work was supported by the Agriculture and Food Research Council and by a grant from the Science and Engineering Research Council Membrane Initiative (GR/F 33971) to Prof. E.A.C. MacRobbie (University of Cambridge).     

Effects of Ca2+ on the salt-stress response of barley roots as observed by in-vivo 31P-nuclear magnetic resonance and in-vitro analysis

Calcium has been demonstrated to ameliorate the inhibitory effects of high salinity on nutrient transport in plants. Time-course experiments were carried out to study the effect of high Ca²⁺ (6 mM) supply under saline conditions (100 mM NaCl) on the regulation of intracellular pH in excised barley (Hordeum vulgare L. cv Arivat) roots. In-vivo ³¹P-nuclear magnetic resonance measurements showed an alkalinization of the vacuolar pH after salt treatment. In the presence of high Ca²⁺ the extent of salt-induced vacuolar alkalinization was lower. High Ca²⁺ partially mitigated the salt-induced increase in Na⁺ content and decrease in K⁺ content of the root. The pattern of change in the vacuolar pH paralleled that of Na⁺ accumulation in the root. This correlation is consistent with the involvement of a tonoplast Na⁺/H⁺ antiporter in Na⁺ transport and the role of Ca²⁺ in Na⁺ uptake. High salt appeared to decrease the Pi content of the vacuole while high Ca²⁺ increased this content irrespective of the salt treatment.Abbreviation NMR nuclear magnetic resonance We are grateful to Dr. T.W.M. Fan and R.M. Highasi (University of California, Davis, USA) for their valuable help with the NMR experiments. We also thank Dr. J. Norlyn for his technical assistance. V. Martinez was supported by a Fulbright fellowship.     

Effect of nitrate pulses on the nitrate-uptake rate,synthesis of mRNA coding for nitrate reductase,and nitrate-reductase activity in the roots of barley seedlings

Using pulses of nitrate, instead of the permanent presence of external nitrate, to induce the nitrate-assimilating system in Hordeum vulgare L., we demonstrated that nitrate can be considered as a trigger or signal for the induction of nitrate uptake, the appearance of nitratereductase activity and the synthesis of mRNA coding for nitrate reductase. Nitrate pulses stimulated the initial rate of nitrate uptake, even after subsequent cultivation in N-free medium, and resulted in a higher acceleration of the uptake rate in the presence of nitrate than in its absence.Abbreviations NR nitrate reductase     

Cytogenetic behaviour and phosphate and potassium content in desynaptic pearl millet

Summary Continued inbreeding by self pollination resulted in a proportion of sterile plants in some families of the inbred line IP 1475 of Pennisetum americanum (L.) Leeke. Cytological examinations of the sterile plants revealed mild to extreme desynapsis and also chromosome fragmentation in some plants. Segregation ratios in the selfed families did not fit into any simple Mendelian ratio; however, in one F₂ family of the cross desynaptic x normal, segregation into 15 normal: 1 desynaptic was observed. Plants from a segregating family were classified as normals, desynaptics with 2–6 univalents, desynaptics with 2–10 univalents, desynaptics with 10–14 univalents and desynaptics with chromosome fragmentation. Estimation of the content of phosphate and potassium from the flag leaves did not reveal significant differences between the five groups of plants.     

Regulation of phosphate influx in barley roots: Effects of phosphate deprivation and reduction of influx with provision of orthophosphate

Barley plants were grown in nutrient solutions, which were maintained at either 0 (-P) or 15 μ M orthophosphate (+P). After 11 days phosphate influx into the intact roots of the -P plants began to increase by comparison with +P plants. During this period differences became apparent between the treatments in absolute growth rates, as well as in the root:shoot ratios. Phosphate influx in the -P plants continued to increase as a function of time, to a maximum value of 2.4 μmol (g fresh wt)^-1h^-1 at 16 days after germination. This rate was 6 times higher than influx values for +P plants of the same age. During the period of enhanced uptake phosphate was strongly correlated (r²= 0.77) with root organic phosphate concentration. – The enhancement of inorganic phosphate influx into intact roots of -P plants was rapidly reduced by the provision of 15 μ M orthophosphate. Typically, within 4 h of exposure to this concentration of phosphate, influx values fell from 1.80 ± 0.20 to 0.75 ± 0.03 μmol (g fresh wt)^-1 h^-1, while inorganic phosphate concentrations of the roots increased from 0.12 to 1.15 μmol (g fresh wt)^-1 during the same period. Hill plots of the influx data obtained during this period, treating root inorganic phosphate as an inhibitor of influx, gave Hill coefficients close to 2. The rapidity of the reduction of influx associated with increased root inorganic phosphate together with the Hill plot data provide evidence for an allosteric inhibition of influx by internal inorganic phosphate.     

Metabolism of gibberellins by immature barley grain

Gibberellins A₁, A₄, A₉, A₁₂-aldehyde, A₂₀ and A₅₁, each labelled with both a radioactive and stable isotope were fed to immature barley grain by injection into the endosperm. After 7 d, extensive metabolism of all substrates had occurred, and metabolites were identified by combined capillary gas chromatography-mass spectrometry. A proposed scheme of gibberellin metabolism in immature barley grain is presented.Abbreviations GA_n gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography     

Effects of abscisic acid on sequestration and exchange of Na+ by barley roots

Excised Na⁺-starved barley roots were suspended in solutions of Na⁺ in combination with NO ₃ ^- , Cl^-, and SO ₄ ^2- , and effects of the added phytohormone, abscisic acid (ABA), to the medium were determined. Abscisic acid increased the rate of Na⁺ (²²Na⁺) accumulation and the amount of Na⁺ deposited in the vacuoles. These stimulating effects of ABA were modified by anions following the sequence NO ₃ ^- >Cl^->SO ₄ ^2- . Testing whether the magnitude of the pH gradient across the plasmalemma of the cells of the root cortex affects rates of Na⁺ accumulation and their dependence upon ABA, we observed that, in the pH range from 4 to 8, the ABA-induced stimulation was strongest at pH 5.8, and least at pH 4. Changes in pH during the experiment caused changes in the rates of Na⁺ accumulation in agreement with experiments performed at constant pH values. Simultaneously with ABA-enhanced accumulation, loss of Na⁺ occurred. Loss of Na⁺ was strongest at pH 4 and was affected by anions, being greatest with SO ₄ ^2- and following the sequence SO ₄ ^2- >Cl^->NO ₃ ^- . On the basis of the finding that initial acceleration of uptake as well as loss of Na⁺ depended on the pH of the medium we suggest that, in barley roots, ABA stimulates an exchange of Na⁺ for H⁺ at the plasmalemma of the cortical cells. The results indicate that ABA-stimulated expulsion of Na⁺, in combination with ABA-stimulated sequestration in the vacuoles, constitutes one of the mechanisms which enable barley plants to tolerate higher than normal levels of Na⁺.Abbreviations ABA abscisic acid - FW fresh weight