Shoot-Dependent Regulation of Sodium and Potassium Fluxes in Roots of Whole Barley Seedlings

Unidirectional fluxes and the cytoplasmic and vacuolar contentsof potassium and sodium in root cells of intact barley seedlings(Hordeum vulgare L., cv. Villa) were determined by use of compartmentalanalysis. In addition, the net vacuolar accumulation J_cv andthe xylem transport ø_cx of K⁺ and Na⁺ were measured.Both of these data were needed for the evaluation of the effluxdata. Fluxes and compartmental contents of K⁺ and Na⁺ were comparableto data obtained with excised roots. The effect of the shoot-to-rootratio—as varied by partial excision of the seedlings seminalroots—on the fluxes and contents was investigated. Highershoot-to-root ratios induced an increase in xylem transport,in plasmalemma influx, and also in the cytoplasmic content ofK⁺ and Na⁺. With potassium the plasmalemma efflux was almostunaltered while the tonoplast fluxes and vacuolar content weredecreased (in presence of Na⁺). With sodium, on the other hand,the plasmalemma efflux and the tonoplast fluxes were also increasedin the plants having one root and a high shoot-to-root ratio.These changes occurred even under conditions of low humidity,when transpiration was low and guttation occurred. The latterwas also increased at the high shoot-to-root ratio. The observedchanges could be due to a relieved feedback control of ion fluxesby the shoot and mediated in part by a relatively higher supplyof photosynthates in the plants having one root In addition,hormonal signals were suggested to participate. In particulara possibly decreased level of cytokinins in the plants havingonly one root could contribute to the signal. The observed changesappear to be responses of the plant to an alteration that canoccur under natural conditions when the root system is damaged.     

Unidirectional Ca2+ Fluxes in Roots of Rye (Secale cereale L). A Comparison of Excised Roots with Roots of Intact Plants

The unidirectional Ca²⁺ fluxes across the plasma membrane andtonoplast were determined in both excised roots and roots ofintact seedlings of rye (Secale cereale L. cv. Rheidol). Theunidirectional Ca²⁺ fluxes across the plasma membrane and tonoplastmeasured in excised roots were of a similar order of magnitudeto those determined in roots of intact plants. Influx and effluxof Ca²⁺ across the root plasma membrane were similar (estimatedto be between 0·7 and 3·4 µmol g     

Control of Sodium Transport in Sunflower Roots

Electrochemical potential differences (driving forces) for sodiumdistributed between the outside solution and the exuding sapof water-culture-grown sunflower plants (Helianthus annuius)have been determined. The results indicated that sodium wasmoving from the outside solution to the xylem against the electrochemicalpotential gradient at external concentrations below approximately0.30 mM Na. At higher external concentrations sodium appearedto be actively excluded from the xylem. An electrical potential difference between the exuding sap andthe external solution of approximately 30 mV was observed. Itwas unaffected by the external sodium concentration. Use ofa short-circuiting technique indicated that the trans-root potentialresides at the plasmalemma of the cortical cells. Driving forces on sodium distributed between the external solutionand the root and between the xylem sap and the root were calculated.They indicated that the root is able to accumulate sodium activelyboth from the external solution and the xylem sap. It is concludedthat sodium transport to the xylem in this species is controlledby the balance of these two opposing forces.     

P-NMR Spectroscopy of Roots of Intact Corn Seedlings

The application of ³¹P nuclear magnetic resonance spectroscopy to the study of metabolism in roots of intact corn seedlings is described. ³¹P-NMR spectra of developmentally distinct parts of primary roots of whole seedlings are presented. The spectra are of quality comparable to those of excised pieces of plant tissue.     

Acidification of Deionized Water by Roots of Intact Rice Seedlings

This investigation was designed to examine whether or not deionizedwater could be acidified by roots of intact rice seedlings.Roots of intact rice seedlings caused significant acidificationof the deionized water in which they were immersed and thisacidification could be repeated after replacement of acidifiedwater with fresh deionized water. The addition of K⁺, Na⁺, andMg²⁺ to the deionized water significantly increased the rateand extent of acidification. However, no such increase was foundwhen Ca²⁺ was present in the water. The inhibition of acidificationby vanadate and its promotion by fusicoccin indicated that theacidification of water by roots of intact rice seedlings originatedfrom an ATP-driven proton pump located in the plasmalemma. Ferricyanide was effectively reduced by the roots of intactrice seedlings. This reduction was associated with the acidificationof the bathing solution. 8-Hydroxyquinoline and p-nitrophenyl-acetateinhibited both the reduction of ferricyanide and ferricyanide-inducedacidification. Vanadate, although it slightly inhibited thereduction of ferricyanide, did not inhibit the ferricyanide-stimulateddecrease in pH. It seems that the involvement of redox activityassociated with the plasmalemma in the acidification of deionizedwater cannot be excluded. (Received August 30, 1989; Accepted April 5, 1990)     

Transport Interactions between Paraquat and Polyamines in Roots of Intact Maize Seedlings

Interactions between absorption of paraquat and the polyamines putrescine, cadaverine, and spermine in roots of intact maize (Zea mays L. cv 3377 Pioneer) seedlings were examined. Concentration-dependent kinetics for paraquat and putrescine influx were similar and both kinetic curves could be resolved into a linear and a saturable component. The linear component was previously shown to represent cell wall/membrane binding. The saturable components for paraquat and putrescine uptake, which represent influx across the plasmalemma, had K_m values of 98 and 120 micromolar, respectively, and V_max values of 445 and 456 nanomoles per gram fresh weight per hour, respectively. Lineweaver-Burk transformation of the saturable component of paraquat influx in the presence of varying concentrations of putrescine indicated that the diamine competitively inhibited the saturable component of paraquat uptake. Reciprocal experiments similarly demonstrated that paraquat competitively inhibited the saturable component of putrescine uptake. Competitive inhibition of both paraquat and putrescine influx could also be demonstrated with the diamine cadaverine, which has a charge distribution similar to that of paraquat and putrescine. In contrast, the larger, tetravalent polyamine spermine appeared to noncompetitively inhibit the influx of paraquat and putrescine. These results strongly suggest that paraquat enters maize root cells via a carrier system that normally functions in the transport of diamines with a charge distribution similar to that of paraquat.     

Cellular Cobalt Fluxes in Roots and Transport to the Shoots of Wheat Seedlings

Macklon, A. E. S. and Sim, A. 1987. Cellular cobalt fluxes inroots and transport to the shoots of wheat seedlings.—J.exp. Bot. 38: 1663–1677. From compartmental analysis of radioisotope elution measurements,concentrations and fluxes of cobalt were estimated for corticalcells of wheat (Triticum aestivum L. cv. Fenman) seedling roots,relative to a complete nutrient solution containing 20 mmolm³ Co^{2 +} . The water free space, Donnan free space and cytoplasmcontained little Co relative to the high levels accumulatedin the vacuole, resulting in time courses of Co uptake whichwere essentially linear in continuous light. In light/dark cyclesthere was almost complete inhibition of uptake during the darkperiods. Comparison of estimated concentrations of Co in cytoplasmand vacuole with values to be expected, if passive diffusionalone was operative, indicated that active transport was occurringin an outward direction at the plasmalemma and into the vacuoleat the tonoplast. Transport to the shoot, which was unaffectedby transpirational water flux through the plant, reached about10% of root absorbed Co, most of which was stored in the corticalcell vacuoles and removed from the transport pathway. Between0·02 and 20 mmol m ³ Co in external solution, uptakewas linearly related to concentration on a log/log plot, andthe proportion of transported Co was constant. The implicationsof these findings are discussed. Key words: Cobalt, transmembrane fluxes, transport.     

Sodium and Potassium Compartmentation and Transport across the Roots of Intact Spergularia marina

The Na⁺ and K⁺ transport characteristics of Spergularia marina (L.) Griseb. were considered in order to compare the systems by which these two physiologically different cations are managed during initial acquisition and subsequent partitioning in midvegetative plants. Uptake of ²²Na⁺ and ⁴²K⁺ and redistribution of labels in pulse-chase studies were compared under steady state growth conditions or with the concentration of one of the ions elevated. At high external concentrations, the initial ⁴²K⁺ accumulation and transport to the shoot was associated with a small, rapidly exchanging, cellular compartment similar to that previously indicated for Na⁺ (D Lazof, JM Cheeseman 1986 Plant Physiol 81: 742-747). At 1 mol m⁻³, K⁺ was conducted to the shoot through a root compartment, the specific activity of which rose much more slowly than the rapidly exchanging compartment. After a lag of approximately 5 minutes, ⁴²K⁺ translocation approached a constant rate with a half-time of 14 minutes compared to 5 minutes for ²²Na⁺ or for ⁴²K⁺ at higher external levels. At all external levels, prolonged translocation of ⁴²K⁺ was measured when a 10 minute pulse was followed by an unlabeled chase, again suggesting a conducting compartment distinct from that for Na⁺. It is suggested that the K⁺ conducting compartment, possibly the `bulk cytoplasm,' is associated with the active K⁺ transport system generally found in higher plants.     

Effects of Growth and Assay Temperatures on Unidirectional K+ Fluxes in Roots of Rye (Secale cereale)

The effects of growth and assay temperature on unidirectionalK⁺ fluxes in excised roots of rye (Secale cereale cv. Rheidol)were studied using ⁸⁶Rb⁺ as a tracer. Both K⁺ influx to thevacuole, estimated as K⁺ uptake between 3 and 12 h after transferof unlabelled roots to radioactive solution, and movement ofK⁺ to the xylem were determined directly. Other fluxes weredetermined on excised roots of plants, which had been labelledwith ⁸⁶Rb⁺ since germination, by conventional triple exponentialefflux analysis. When assayed at 20°C, roots of plants previously grown at20°C(WG roots) had lower rates of net K⁺ uptake than rootsof low temperature-acclimated plants, grown with a temperaturediferential between roots (87°C) and shoots (20°C) eithersince germination (DG roots) or for 3 d prior to experiments(DT roots). This resulted from a greater unidirectional K⁺ effluxacross the plasma membrane and a reduced K⁺ flux to the xylemin WG roots, compared to DG or DT roots, rather than a decreasein unidirectional K⁺ influx or a decrease in the net K⁺ fluxto the vacuole. Indeed, although WG roots had lower rates ofK⁺ influx and K⁺ efflux across the tonoplast at 20°C thanDG or DT roots, roots of plants from all growth temperaturetreatments showed an equivalent net K⁺ flux to the vacuole. Although all unidirectional K⁺ fluxes in roots from plants grownunder all temperature regimes were reduced by lowering the temperatureof the root, these fluxes were differentially affected in rootsof plants from contrasting growth temperature treatments. Rapidcooling to 8°C of WG roots resulted in a lower rate of K+influx and a transient increase in K⁺ efflux across both theplasma membrane and tonoplast, compared to DG and DT roots.Furthermore, since the K⁺ flux to the xylem was lower in WGroots, the net K⁺ uptake at 8°C into WG roots was considerablyreduced compared to DG and DT roots. These results suggest thatlow temperature-acclimation of K⁺ fluxes in rye roots may involvea reduction in the temperature sensitivity of K⁺ influx anda curtailment of K⁺ efflux across both the plasma membrane andtonoplast at low temperatures. Key words: K⁺influx, K⁺ efflux, low temperature, potassium, rye (Secale cereale cv. Rheidol)     

Potassium Fluxes in Excised Barley Roots

The method of the modified compartmental analysis for excisedroots has been adopted for measuring K⁺-fluxes and compartmentationin barley (Hordeum distichon) roots. Efflux of ⁴²K and ⁸⁶Rbindicated that more than two intracellular compartments wereinvolved in the tracer exchange; the ⁴²K data clearly showedthe components. On the basis of the efflux behaviour of theapical and more basal tissues of the roots, the three componentsof efflux were attributed to the cytoplasm of differentiated(fast) and meristematic tissues (intermediate) and to the vacuoles(slow exchange) of the roots. A model is proposed on the basisof which, the fluxes corresponding to the meristematic and differentiatedtissues of the root can be estimated. Additionally, fluxes ofthe differentiated root tissues were determined by using effluxdata obtained with root segments without apical tissues. Thedata obtained in both ways compare reasonably well and agreeto independent chemical measurements. Comparison of the ⁴²K and ⁸⁶Rb efflux data show strong discriminationof K^– in favour of Rb⁺ and indicate that ⁸⁶Rb is not suitableas a tracer for K⁺ in efflux measurements, at least with barleyroots.     

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)     

Regulation of Rubidium Uptake in Sunflower Roots

Uptake of Rb⁺ was investigated in 12-day-old intact plants of sunflower (Helianthus annum L. var. californicus) which had been cultivated or pretreated in nutrient solutions with various K⁺ concentrations. The relationship between Rb⁺ influx and K⁺ concentration of the roots indicated regulation of Rb⁺ uptake by allosteric inhibition of the uptake mechanism. A constant passive influx occurred contemporaneously with the active uptake as shown by experiments at 0°C or with 2,4-dinitrophenol. The allosteric regulation of ion carrier activity occurred after a time lag of up to 1 h after the change of external solution. In experiments involving Rb⁺ treatments of K⁺-deficient plants, the synthesis of carriers for transport of Rb⁺ could be demonstrated. A model including allosteric regulation of Rb⁺ uptake in roots is discussed.     

Sodium Fluxes in Excised Citurs Roots under Steady-State Conditions, with Particular Reference to Salinity Resistance

Sodium influx and efflux rates from excised Citrus roots werestudied. The sodium fluxes in the free spaces of the roots ofthe salt-sensitive variety Sweet Lime and of the salt-resistantvariety Cleopatra Mandarin were very close to each other. Onthe other hand, the sodium fluxes from intracellular spaceswere considerably lower in the roots of variety Cleopatra Mandarin.The experimental findings were evaluated by compartmental analysisand the rate constants of the sodium fluxes calculated. Thepossible significance of these procedures for the study of saltresistance is indicated.     

Ion Uptake Efficiency of Sunflower Roots

The term ion uptake efficiency is used for the rate of uptake of a particular ion from nutrient solutions holding a standard concentration of that ion (0.5 mM sulphate, 1.5 mM phosphate or 2.0 mM rubidium). The uptake efficiency for rubidium and phosphate in roots of intact sunflower plants depended on the salt status of the plants and on the concentration of the ion under investigation in pretreatment solutions. The effect of pretreatment was a rapid process causing differences of more than 300% in ion uptake efficiency within 1 h, depending on the composition of the pretreatment solution. At concentrations above 0.1 mM the rate of uptake of rubidium in the root was higher than the net potassium uptake necessary for adequate growth. The rate of sulphate uptake was related to potassium uptake but not to phosphate uptake. It is suggested that ion uptake of the roots is regulated by internal factors as well as by direct interactions between the medium and the absorbing surfaces.     

Leaf Abscission Induced by Ethylene in Water-Stressed Intact Seedlings of Cleopatra Mandarin Requires Previous Abscisic Acid Accumulation in Roots

The involvement of abscisic acid (ABA) in the process of leaf abscission induced by 1-aminocyclopropane-1-carboxylic acid (ACC) transported from roots to shoots in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings grown under water stress was studied using norflurazon (NF). Water stress induced both ABA (24-fold) and ACC (16-fold) accumulation in roots and arrested xylem flow. Leaf bulk ABA also increased (8-fold), although leaf abscission did not occur. Shortly after rehydration, root ABA and ACC returned to their prestress levels, whereas sharp and transitory increases of ACC (17-fold) and ethylene (10-fold) in leaves and high percentages of abscission (up to 47%) were observed. NF suppressed the ABA and ACC accumulation induced by water stress in roots and the sharp increases of ACC and ethylene observed after rewatering in leaves. NF also reduced leaf abscission (7-10%). These results indicate that water stress induces root ABA accumulation and that this is required for the process of leaf abscission to occur. It was also shown that exogenous ABA increases ACC levels in roots but not in leaves. Collectively, the data suggest that ABA, the primary sensitive signal to water stress, modulates the levels of ethylene, which is the hormonal activator of leaf abscission. This assumption implies that root ACC levels are correlated with root ABA amounts in a dependent way, which eventually links water status to an adequate, protective response such as leaf abscission.     

Modifying Effects of a Non-Toxic Level of Aluminium on Phosphate Fluxes and Compartmentation in Root Cortex Cells of Intact Ryegrass Seedlings

From compartmental analysis of ³²P elution measurements, concentrationsand fluxes of phosphate were estimated for root cortical cellsof intact Lolium perenne L. plants, when in complete nutrientsolution containing 0.1 mol m^–3 phosphate with and without37 mmol m^–3 Al during loading and elution. Failure ofthe data, when plotted as ct. min^–1 remaining in the tissueas a function of time, to meet the criteria for first orderkinetics led to a discussion of the relative importance of transportto the shoot, assimilation, and complexation in the vacuole,in causing this discrepancy. It was concluded that complexationwas the most important factor. Transformation of the data tomeet the criteria for first order kinetics gave corrected valuesfor compartmental concentrations and fluxes of phosphate, andestimates of the size of the otherwise unresolved slowly exchangingcompartment within the vacuole equated with condensed phosphate.In the control this was 2.5 mol m^–3 P but in Al treatmentsa much larger amount of phosphate was complexed with Al (6.5mol m^–3 P). Phosphate transport to the shoot was unaffectedby Al. Instead, levels of Al, common in solution in upland soils,sequestered in the root vacuoles quantities of phosphate significantfor stressing the phosphate economy of nutrient-poor grassland. Key words: Lolium perenne L. phosphate, aluminium, compartmentation, complexation     

Transport of Sodium in Intact Peanut Seedling

Peanut (Arachis hypogea L.) seedlings readily transported Nato the shoot. The amount of Na transported was linearly relatedto the absorption period (which ranged from 1 to 8 h) and alsoto the external Na level which varied from 0.05 to 1 mM.