Regulation of Sulphate Transport in a Tropical Legume, Macroptilium atropurpureum, cv. Siratro

When young plants of Macroptilium atropurpureum, cv. Siratrowere deprived of external sulphate (-S plants) growth of shootsand roots continued at rates comparable to those in plants wellsupplied with sulphate (control) for 3 d and 5 d respectively.Dilution of internal sulphur therefore took place and redistributionof sulphur occurred between inorganic and organic forms andbetween roots and younger leaves. Even when S-deficiency limitedgrowth, plants contained 16% of their total sulphur as sulphate,but most of this was retained in old leaves and redistributedslowly to growing zones. The capacity for sulphate uptake increased in roots of –Splants very soon after they were deprived of external sulphate;within 24 h the absorption from 0.25 mol m^–3 SO₄^2–was more than five times that of control roots. Maximum increasedcapacity was reached after 2–3 d stress when the V_maxof system 1 was 1948 nmol h^–1g^–1root fr. wt. in–S plants and 337 nmol h^–1g^–1root fr. wt.in controls. The K^mfor system 1 did not change significantlywith S-stress being between 5–8 µM in both setsof plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growthrate and the rate at which sulphate uptake capacity was enhancedby withholding sulphate from culture solutions. When –S plants were replaced in sulphate-containing solutiontheir capacity for SO₄^2– declined to the control levelwithin 24 h. Very marked repression of capacity was also foundwhen –S plants were treated with L-cysteine, but therewas no immediate effect with methionine. Roots of this species appear to have a very active system fordegrading L-cysteine to sulphate, 30% of the label in ³⁵S-cysteineabsorbed by roots was recovered in ³⁵SO₄^2– after 20 minor 2 h incubation. By contrast, roots had a very weak abilityto reduce sulphate. When part of the root system was in solution lacking sulphatethere was enhanced uptake of sulphate by other parts which themselveswere amply supplied with sulphate. This is seen as an exampleof compensatory absorption. The response to S-stress is specific and there were no positiveinteractions between S-stress and the absorption of phosphate,or P-stress and the uptake of sulphate. The results are discussed in relation to the close control ofsulphate uptake by internal sulphate concentration, redistributionof forms of sulphur during stress and mobility of sulphate inthe phloem. Key words: Kinetics, Amino-S, Sulpholipid, Repression;, Deficiency     

Dependence of Calcium Transport into Strawberry Leaves on Positive Pressure in the Xylem

Strawberry plants were cultured in concentrated and dilute nutrientsolutions by day and night, factorially. Guttation decreased,leaf tipburn increased and calcium in emerging leaves was reducedwhen solutions were more concentrated at night. Nutrient concentrationin the daytime was much less important for these responses.The effects were similar when the more concentrated solutioncontained additional Ca²⁺, NO₂^–, Cl^– and SO₄^2–or extra of all the major nutrients in the same proportions.When the solution was made stronger by adding extra Mg²⁺, NO₂^–,Cl^– and SO₄^2– the guttation, tipburn and calciumdifferences were basically similar to those in the previousexperiment but with an added effect explained by a competitivesuppression of calcium uptake caused by the extra magnesium.Thus, both nutrient strength and cation balance are importantfor preventing strawberry leaf tipburn. The results accord with the hypothesis that calcium translocationinto pre-emerged strawberry leaves depends on the attainmentof a positive water pressure in the xylem at night, when transpirationlosses are minimal. Fragaria ananassa Duchesne, Strawberry, calcium, tipburn, root pressure, nutrient concentrations     

Influence of the Form of Nitrogen Supply on Root Uptake and Translocation of Cations in the Xylem Exudate of Maize (Zea mays L)

Concentrations of inorganic cations are often lower in plantssupplied with NH₄⁺ as compared with NO₃^–. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m^–3 NO₃^–or NH₄⁺ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO₃^– and NH₄⁺ declined by more than 50%Compared with NO₃^– supply, NH₄⁺ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH₄⁺- than in NO₃^–-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH₄⁺-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH₄⁺ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH₄⁺-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation     

Sulphate supply and its regulation of transport in roots of a tropical legume Macroptilium atropurpureum cv. Sirato

During a period of sulphate deprivation, roots of Macroptiliumatropurpureum responded by increasing their uptake capacityat the plasma membrane. This effect was apparent both in intactplants and in tissues excised prior to uptake. In experiments using excised root systems previousy labelledwith ³⁵SO₄^2- the rate of tracer transport to the xylem was muchgreater in roots subsequently deprived of external sulphatethan in those supplied with unlabelled sulphate. Removing theexternal sulphate to the external solution. Additionally, compartmentalanalysis of tracer exchange kinetics showed that the flux ofsulphate from the cytoplasm to the xylem(     

A System for Measuring Effects of Sulphur Dioxide on Gas Exchange of Plants

Apparatus is described for exposing plants to low concentrationsof SO₂ (50–500µg m^–3 in air) and for measuringeffects on photosynthesis, dark respiration, and transpiration.Temperature, humidity, and irradiance in the chambers were controlledindependently, and fans ensured that leaf boundary layer resistanceswere low. Experiments with plants of Vicia faba in clean andpolluted air showed that: (i) a depression of net photosynthesisby 50 µg m^–3 SO₂ depended on boundary layer resistanceand on irradiance; (ii) stomatal resistance was increased ordecreased by 50 µg m^–3 SO₂ when relative humidityin the chambers was low (35% r.h., 22 °C) or high (50% r.h.,22 °C) respectively.     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

Hydraulic and osmotic properties of spruce roots

Hydraulic and osmotic properties of roots of 2-year-old Norwayspruce seedlings (Plcea abiea (L.) Karst) were investigatedusing different techniques (steady flow, pressure probe, andstop flow technique). Root pressures were measured using theroot pressure probe. Compared to roots of herbaceous plantsor deciduous trees, excised root systems of spruce did not developappreciable root pressure (-0.001 to 0.004 MPa or -10 to 40cm of water column). When hydrostatic pressure gradients wereused to drive water flows across the roots, hydraulic conductivities(Lp_r) were determined in two types of experiments: (i) rootpressure relaxations (using the root pressure probe) and (ii)steady flow experiments (pneumatic pressures applied to theroot system or xylem or partial vacuum applied to the xylem).Root Lp_r ranged between 0.2 and 810^–8m s^–1 MPa^–1(on average) depending on the conditions. In steady flow experiments,Lpr depended on the pressure applied (or on the flow acrossthe roots) and equalled (0.190.12) to (1.21.7)10^–8m s^–1 MPa^–1 at pressures between 0.2 and 0.4 MPaand (1.51.3)10^–8 m s^–1 MPa^–1 at appliedpressures between 0.8 and 1.0 MPa. When pressures or vacuumwere applied to the xylem, Lp_r values were similar. The hydraulicconductivity measured during pressure relaxations (transientwater flows) was similar to that obtained at high pressures(and water flows). Although there was a considerable scatterin the data, there was a tendency of the hydraulic conductivityof the roots to decrease with increasing size of the root system.When osmotic gradients were used to drive water flows, Lp_r valuesobtained with the root pressure probe were much smaller thanthose measured in the presence of hydrostatic gradients. Onaverage, a root Lp_r=0.01710^–8 was found for osmotic andLp_r=6.410^–8 m s^–1 MPa^–1 in correspondinghydrostatic experiments, i.e. the two values differed by a factorwhich was as large as 380. The same hydraulic conductivity asthat obtained in osmotic experiments using the pressure probewas obtained by the 'stop flow techniquel. In this technique,the suction created by an osmoticum applied to the root wasbalanced by a vacuum applied to the xylem. Lp_r values of rootsystems did not change significantly when measured for up to5 d. In osmotic experiments with different solutes (Na₂S0₄,K₂S0₄, Ca(NO₃)₂, mannitol), no passive uptake of solutes couldbe detected, i.e. the solute permeability was very low whichwas different from earlier findings on roots of herbs. Reflectioncoefficients of spruce roots (O were low for solutes for whichplant cell membranes exhibit values of virtually unity (     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

Metabolic Adaption in Mature Roots of Salt-stressed Zea mays

Respiratory gas exchange and incorporation of ¹⁴C-leucine intoprotein were studied in proximal root segments from 25-day-oldmaize plants grown for the last ten days in 50 mM Na₂SO₄. ¹⁴C-leucineincorporation, and oxygen uptake in the presence of glucose,were as large in Na₂SO₄-grown tissues tested under saline conditionsas in tissues exposed to non-saline solutions throughout Thisadaptation was attributed to an increased metabolic capacityof Na₂SO₄-treated tissues, because these tissues, when returnedto non-saline solutions, evolved oxygen and incorporated ¹⁴C-leucinefaster than tissues exposed continuously to non-saline solutions. These changes are interpreted as a ‘compensation’for the inhibitory effects found when non-adapted tissues wereexposed to 50 mM Na₂SO₄. Moreover, we have related them to ultrastructuralchanges observed previously in xylem parenchyma cells of thesetissues, and to the possible involvement of these xylem parenchymacells in the re-absorption of sodium from the ascending xylemfluid Zea mays L., maize, salt-stress, respiration, protein synthesis     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Ammonia emission from young barley plants: influence of N source, light/dark cycles and inhibition of glutamine synthetase

Barley (Hordeum vulgare L. cv. Golf) plants were grown at twodifferent relative addition rates; 0.1 and 0.2 d^–1 ofnitrate. Three to five days before measurements started theplants were transferred to a nutrient solution with 2 mM nitrateor ammonium. The ammonium-grown plants showed increased ammoniumlevels in both shoots and roots and also increased ammoniumconcentrations in xylem sap. Ammonia emission measured in cuvettes connected to an automaticNH₃ monitor was close to zero for nitrate-grown plants but increasedto 0.59 and 0.88 nmol NH₃ m^–2 S^–1 for plants transferredto ammonium after growing at RA=0.2 and 0.1 d^–1, respectively.In darkness, NH₃ emission decreased together with photosynthesisand transpiration, but increased rapidly when the light wasturned on again. Addition of 0.5 mM methionine sulphoximine (MSO) to the plantscaused an almost complete inhibition of both root and shootglutamine synthetase (GS) activity after 24 h. Ammonia emissionincreased dramatically and photosynthesis and transpirationdecreased in both nitrate- and ammonium-grown plants as a resultof the GS inhibition. At the same time plant tissue and xylemsap ammonium concentrations increased, indicating the importanceof GS in controlling plant ammonium levels and thereby NH₃ emissionfrom the leaves. Key words: Hordeum vulgare, ammonia emission, ammonium, glutamine synthetase, nitrogen nutrition, photosynthesis, transpiration     

The Import and Redistribution of Several Cations and Anions in Tomato Leaves

Wolterbeek, H. Th. and De Bruin, M. 1986. The import and redistributionof several cations and anions in tomato leaves.—J. exp.Bot. 37: 331–340. The upward movements in the xylem and redistribution from theleaf of Na⁺ , K⁺ , Rb⁺, Cs⁺ and four anions were examined insub-systems of tomato plants (Lycopersicon esculentum, Mill.cv. Tiny Tim). There was a delay with respect to the redistributionof newly imported elements from the source leaf of about 16–20h for all four alkali ions. This is considerably less than theapparent delay for the anions Sb(SO₄)^– WO₄^2– Mo₇O₂₄^6–and AsO₄^3– The prolonged delay for the anions is suggestedto be a consequence of metabolic transformation in the leaf.Reduction of the source-sink activity ratio did not decreasethe delay period from the source leaf, but apparently causedincreased Na⁺ transfer from the xylem. It is concluded thatthe application of a detailed mathematical descnption of upwardelement movement has considerable potential possibilities forunderstanding circulation of nutrients in the plant. Key words: Alkali ions, anions, xylem, phloem, redistribution, tomato     

Glucosinolate biosynthesis in oilseed rape (Brassicanapus L.): studies with 35SO2-4 and glucosinolate precursors using oilseed rape pods and seeds

³⁵SO^2–₄ and glucosinolate precursors were administeredto investigate the capacity of oilseed rape intact whole pods,isolated pod walls and seeds to carry out reactions of the glucosinolatebiosynthetic pathway. Pod walls incorporated ³⁵S-label from³⁵SO^2–₄ into both the thio-glucose and sulphonate-sulphurof glucosinolates. Pulse-labelling experiments showed that podwalls were the predominant source of the glucosinolates accumulatedby the seeds. Isolated immatureseeds were capable of reductiveassimilation of ³⁵SO^2–₄ and incorporated ³⁵S into sulphur-containingamino acids and the sulphonate moiety of glucosinolates, butwere not able to incorporate ³⁵S into the thio-glucose of glucosinolates. Key words: Biosynthesis, glucosinolates, oilseed rape, pod walls, seeds     

Compartmental analysis of 35SO2-4exchange kinetics in roots and leaves of a tropical legume Macroptilium atropurpureum cv. Siratro

Compartmental analysis of ³⁵SO^2-₄ tracer exchange kinetics hasbeen used to estimate unidirectional fluxes and compartmentcontents in excised root and leaf tissue of the tropical legume,Macroptilium atropurpureum. In excised root tissue only 5% ofthe sulphate taken up across the plasmalemma was reduced toorganic forms whereas in excised leaf tissue approximately 20%was reduced. It was necessary, therefore, to incorporate themetabolism of sulphate during the course of the experiment intothe compartmental models. In root tissue, wash-out data was fitted by three exponentials,assumed to correspond to exchange in the extracellular spaces,cytoplasm and vacuole, but in leaf tissue two large, slowlyexchanging compartments have been postulated in order to achievea fit to the data. It is likely that differences in leaf cellpopulations cause the ‘anomalous’ tracer exchangekinetics and the justification of this assumption is discussed. The fluxes of sulphate at the plasmalemma were greater thanthe corresponding fluxes at the tonoplast in both roots andleaves. The flux of SO^2-₄ from the cytoplasm to the externalsolution did not appear to limit the loss of SO^2-₄ from thevacuole. At an external SO^2-₄concentration of 0.25 mol m^-3 therate constants for exchange in the vacuole were two orders ofmagnitude greater in roots than in the slowest exchanging leafcell population. It is possible, therefore, that the slow lossof SO^2-₄ from leaf cell vacuoles may limit the redistributionof sulphate during S-stress. Key words: Compartmental analysis, sulphate, deficiency, Macroptilium atropurpureum     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

Transpiration and water uptake of Senecio medley-woodii and Aloe jucunda under changing environmental conditions: measurements with a potometric water-budget-meter

Transpiration, water uptake by the roots and CO₂ exchange oftwo leaf succulents, Senecio medleywoodii (Asteraceae) and Aloejucunda (Asphodeliaceae), were monitored simultaneously andcontinuously with a gas exchange cuvette combined with an apparatusto quantify water uptake (= waterbudget- meter). Measurements,which are primarily valid for plants with a sufficient watersupply, were made with the same plant for up to 29 consecutivedays. Ambient air temperature varied between 17 and 35 C witha constant dewpoint temperature of 13C of the ambient air anda 12 h photoperiod at 400–500mol m^–2s^–1 photonirradiance. The net water flux (J_w(net)=water uptake–transpiration)and the water balance (J_w(net) integrated for a timespan) werecalculated. Various tests were made to determine the accuracyof the measurements made with this rather complex equipment.In most cases the errors for transpiration and uptake rateswere much lower than 8% determined under the conditions of drastically(about 10 K per 30 min) increased or decreased ambient air temperatures.The experimental set-up proved to be a most valuable tool todetermine and analyse interactions between transpiration andwater uptake, changes in plant water status and the bufferingof negative J_wnet). Increasing the temperature of ambient air resulted, for bothspecies investigated, in a quick and considerably enhanced transpiration,but there was only a minor impact on water uptake. Water lossexceeding uptake was buffered by internal water reserves whichwere refilled within about 1 d after the plant was relievedof heat and drought stress caused by a period of high ambientair temperatures and high water vapour saturation deficits ofthe air. Repeated simulation of such stress periods showed thatthe absolute values of transpiration and the water uptake for24 h can vary, but the diurnal course of the values showed thesame pattern if the environmental conditions were identical.Such standardized diurnal transpiration and water uptake curvescould be very useful for the validation of mathematical modelsused to describe plant water relations. Key words: Plant water relations, water budget, drought stress, transpiration, water uptake     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

Effects of sulfite and pH on abscisic acid-dependent transpiration and on stomatal opening

In rice, alday, wheat and tobacco (Nicotiana tabacum L. Samsunand Samsun NN) plants which contained large amounts of ABA,the transpiration rate decreased rapidly with 2 ppm SO₂ fumigationand reached 20 to 65% of the initial level after 5- to 30-minexposure depending on their ABA contents. In the cases of broadbean and tobacco (N. glutinosa L.) with low ABA contents, therate slightly increased for 20 and 40 min, respectively, afterthe start of the fumigation and then decreased gradually. Thetranspiration rates of corn and sorghum, in spite of their extremelylow ABA contents, pronouncedly decreased with SO₂ fumigationand reached 65 and 50%, respectively, of the initial levelsafter 40-min exposure. Foliar application of 0.04 N HC1 to N.tabacum L. Samsun NN leaves remarkably depressed the transpirationrate, while the application of 0.04 M Na₂SO₃ decreased the rateonly to the same level as water treatment. Foliar applicationof either HCl or Na₂SO₃ to N. glutinosa L. leaves exerted littlechange in the transpiration rate. When 10^–4M ABA was appliedto broad bean leaves prior to HCl and Na₂SO₃ treatment, theirtranspiration rate was decreased by HCl, but not by Na₂SO₃ application.In sonicated epidermal strips peeled from broad bean leaves,Na₂SO₃ produced a slight increase in the stomatal aperture sizein the absence of ABA, but showed no effect in the presenceof ABA. The aperture size was identical in the pH range of 3.0to 7.0 in the incubation medium. In the presence of ABA in themedium, the aperture size was small in the acidic region ofpH with a minimal value at pH 4.0. ABA decreased the aperturesize at concentrations above 10^–9 M at pH 4.0 and 10^–6M at pH 7.0 in the medium. [2_–14C] ABA uptake by epidermalstrips was large in the acidic region, especially at pH 4.0. (Received February 28, 1980; )     

Ion Distribution in Salt-stressed Mature Zea mays Roots in Relation to Ultrastructure and Retention of Sodium

Ultrastructural features and the distribution of soluble ionshave been examined in mature roots of Zea mays plants grownin both NaCl and Na₂SO₄ salinities. When the plants were grown in either salt, the Na concentrationincreased proximally along the root with a concomitant declinein the K concentration. Both trends were reversed in the shoot. X-ray microanalysis of deep-frozen, fully hydrated specimensshowed that in salt-treated roots Na, and Cl, or S were distributedabout stoichiometrically in the cortex and endodermis. Na wasusually less concentrated than the anion in the lumens of thevessels, but was concentrated markedly relative to either Clor S in the adjoining xylem parenchyma cells. In the older, proximal parts of seminal roots of plants grownboth without salt (controls) and in the presence of either NaClor Na₂SO₄, wall developments occurred in xylem parenchyma cellsat the half-bordered pits in which the cell wall became markedlythicker and possessed a loosely packed fibrillar structure.These structures were not comparable with the transfer-celltype of protuberances reported in the roots of other species. In the xylem parenchyma of plants grown in the presence of Na₂SO₄there were dramatic increases in the quantities of rough endoplasmicreticulum, ribosomes, and mitochondria relative both to controlsand NaCl treatments. The results are discussed in relation to the possible functionof the xylem parenchyma of the mature root in the reabsorptionof Na from the xylem sap, which may mitigate adverse effectsof salinity in salt-sensitive glycophytes.