In-vitro-cultured subclover root can develop Fe-deficiency stress response

The Fe-deficiency stress response is induced in most plants under Fe-deficient conditions, but whether the shoot and/or the root control development of the stress response is not known. The objectives of the present study were to determine whether in-vitro-cultured subclover roots can develop Fe-deficiency stress response and to examine this approach as a possible screening technique for Fe-deficiency resistance. One-cm long root tips of subclover seedlings were cultured in modified White's medium without (-Fe) or with (+Fe) 100 μM Fe³⁺EDTA. Root Fe³⁺ reduction and H⁺ release were evaluated. On the first day after transfer to the -Fe medium, the Fe-deficiency-resistant cultivar Koala (Trifolium brachycalycinum Katzn. and Morley) started to release H⁺, resulting in a decrease in pH of the culture medium, while the susceptible cultivar Karridale (T. subterraneum L.) did not release H⁺ until the second day. The H⁺-release rate of the -Fe Koala was approximately twice as high as that of the -Fe Karridale for the first 4 days of -Fe treatment. Both Koala and Karridale reached their highest H⁺-release rates on the fourth day after -Fe treatment initiation. The +Fe Koala released H⁺ after several days of culture, but the H⁺ release of the -Fe Koala was severalfold greater than that of the +Fe Koala. The implicit correlation between H⁺ release and Fe-deficiency resistance was substantiated by using a series of subclover cultivars with a range of susceptibilities to Fe deficiency. The pH of the -Fe culture media of the series of cultivars was positively correlated to their Fe-chlorosis scores reported in previous research. The results of the present study indicate that root itself has the full ability to develop Fe-deficiency stress response and the response is dependent on the root Fe status. The results also suggest that root culture could be used as a simple and efficient alternative technique for screening germplasm for Fe-deficiency resistance.     

Plant growth and nutrient uptake characteristics of Fe-deficiency chlorosis susceptible and resistant subclovers

The objective of this study was to evaluate the growth and nutrient-uptake characteristics of Fe-deficiency resistant and susceptible subclover (Trifolium subterraneum L., T. yanninicum Katzn. and Morley, T. brachcalycinum Katzn. and Morley) cultivars on a calcareous soil. Ten subclover cultivars showing varying susceptibilities to Fe-deficiency chlorosis (Karridale, Nangeela, Geraldton, Mt. Barker, Woogenellup, Larisa, Trikkala, Rosedale, Koala and Clare) were grown on a low-Fe, calcareous soil (Petrocalcic Paleustoll) under moist (18% water content, 85% of water holding capacity) and water-saturated conditions using a Cone-tainer^® culture system. Chlorosis and its correlation with growth traits and mineral nutrition of the 10 cultivars were examined. The Fe-deficiency susceptibilities of the 10 cultivars decreased in the above order under the moist condition, but in slightly different order under the saturated condition. Shoot and root dry weights, total dry weight, and root-to-shoot ratio were each negatively correlated with chlorosis under both soil-moisture conditions, as was total shoot content of P, Ca, Fe, Mn and Zn. Shoot P and Fe concentrations were each positively correlated with chlorosis under the moist soil condition. Iron and Cu utilization efficiencies (biomass per unit weight of nutrient) in the shoot were each negatively correlated with chlorosis under the moist soil condition. These results suggest that there may be several characteristics of Fe-deficiency chlorosis resistance in subclovers, such as a more effective soil-Fe mobilizing mechanism(s), more balanced nutrition, lower required Fe concentration in the shoot, higher shoot-Fe utilization efficiency, and higher root/shoot ratio under Fe-deficiency stress conditions.     

Uptake of zinc from chelate-buffered nutrient solutions by wheat genotypes differing in zinc efficiency

Zinc-efficient Triticum aestivum (cv. Warigal) and Zn-inefficientTriticum turgidum conv. durum (cv. Durati) were grown in chelate-buffered,complete nutrient solutions providing either deficient or sufficientZn supply. When transferred to fresh chelatebuffered nutrientsolutions containing a wide range of Zn supplies (0–1.28µmol m^–3 Zn²⁺ activity) for 24–48 h, bothgenotypes increased net Zn uptake linearly with an increasein solution Zn²⁺ activities. Zincefficient Warigal accumulatedZn at a greater rate than Zn-inefficient Durati. The greaterrate of net Zn uptake was observed by plants of both genotypeswhen pretreated at deficient Zn supply. Net loss of Zn to thesolution was higher in plants pretreated with sufficient Znand was inversely related to Zn²⁺ activity in the external solution.When continuously supplied with 40 nmol m^–3 Zn²⁺, netZn uptake by Zn-efficient Warigal was significantly greaterthan that of Zn-inefficient Durati, but the difference diminishedwith plant age. Shoot concentrations of Fe, Mn and Cu were higherwhen plants were grown at deficient than at sufficient Zn supply.The Zn-efficient genotype transported less Zn and Fe to shootsand had higher Fe concentrations in roots than the Zn-inefficientgenotype, supporting the hypothesis that Zn efficiency may beconnected with inefficient transport of Fe from roots to shootsand thus initiation of the Fe-deficiency response resultingin increased release of Zn- and Fe-binding phytosiderophores.It is concluded that differential Zn efficiency of wheat genotypesis at least partly due to a greater ability of efficient genotypesto accumulate Zn. Key words: Chelate-buffering, genotypes, micronutrients, Triticum spp., uptake, zinc efficiency     

Rates of Hydrogen Ion Efflux by Nodulated Legumes Grown in Flowing Solution Culture with Continuous pH Monitoring and Adjustment

The net efflux of H⁺ from lucerne (Medicago saliva L.), redclover (Trifolium pratense L.) and white clover (Trifolium repensL.) growing in flowing solution culture and dependent upon symbioticfixation of atmospheric N, was measured over a 75 d experimentalperiod. Considerable and rapid increases in acidity of the nutrientsolution of up to 1.45 pH units were recorded when the pH wasriot held constant over a 30 h period. There was little differencein H⁺ efflux when solution pH was held constant at 4.75, 5.75or 6.75, but there was an immediate cessation when it was adjustedto 3.75. Differences in the daily net efflux of H⁺ closely followedthe pattern of daily differences in incoming radiation, andthere was also evidence of a diurnal pattern of H⁺ efflux. Althoughthere were initially distinct differences between the speciesin the calculated rate of net H⁺ efflux (µg H⁺ g^–1dry shoot d^–), by day 75 these had diminished. In allspecies, however, the maximum rate of efflux per unit of shootsoccurred during the earlier rapid phases of growth. The measuredefflux of H⁺ was well equated with the plant content of excesscations (as measured by ash alkalinity) and, on average, theratio of acidity produced to N assimilated (expressed as anequivalent) was 0-24. Medicago sativa L., Trifolium pratense L., Trifolium repens L., lucerne, red clover, white clover, acidification, cation/anion balance, flowing solution culture, H⁺ efflux, nitrogen fixation     

Calcium Inhibits Ion-Stimulated Stomatal Opening in Epidermal Strips of Commelina communis L.

Inoue, H. and Katoh, Y. 1987. Calcium inhibitsion-stimulatedstomatal opening in epidermal strips of Commelina communis L.—J.exp. Bot. 38: 142–149. Ca²⁺ suppressed both the ion-stimulated stomatal opening andH⁺ extrusion of pre-illuminated epidermal strips isolated fromCommelina communis L. In the absence of Ca²⁺, the rate of H⁺release was 18 nmol H⁺ cm^–2 h^–1 per epidermal stripunit area in 150 mol m^–3 KCL at pH 7?4. Half-maximum inhibitionof stomatal opening was observed with 220 mmol m^–3 ofCa²⁺. The hexavalent dye, ruthenium red, showed concentration-dependentprevention of the inhibition by Ca²⁺ of the ion-stimulated stomatalopening. The effect of ruthenium red was non-competitive, andthe K₁ for the calcium inhibition was found to be 3?6 mmol m^–3.The calcium inhibition of H⁺ extrusion was also prevented byruthenium red. These results suggest that Ca²⁺ inhibits theactivity of electrogenic H⁺ translocating ATPase of the guardcell plasma membrane and leads to the suppression of stomatalopening. Key words: Calcium, Commelina communis, ruthenium red, stomata     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

Transmembrane Ferricyanide Reduction and Membrane Properties in the Euryhaline Charophyte Lamprothamnium papulosum

The euryhaline charophyte Lamprothamnium papulosum has the abilityto reduce the extracellular electron acceptor ferricyanide (Fe³⁺Cy).Addition of 0.5 mol m^–3 Fe³⁺Cy stimulated H⁺-efflux ata rate of 0.8 H⁺/Fe³⁺Cy-reduced and increased K⁺-efflux intoa potassium-free medium at a rate of 0.66 K⁺/Fe³⁺Cy-reduced.0.5 mol m^–3 Fe³⁺Cy-induced maximum membrane depolarizationfor cells with resting potentials more negative than the diffusionpotential. The peak value of Fe³⁺Cy-induced depolarizationswas similar to the potential obtained by poisoning the electrogenicpump with DCCD. The value of maximum depolarization was determinedby (K⁺)₀. E_m tended to more positive values with increasing(K⁺)₀. Depolarizations coincided with a decrease in membraneresistance (R_m) from a resting value of 1.5 m² to 0.2 m² inthe depolarized state. Depolarization increased the sensitivityof the membrane potential (E_m) to (K⁺)₀. The resting potentialwas only slightly changed when (K⁺)₀ was increased from 3 to15 mol m^–3. The Fe³⁺ Cy-induced depolarized Em changedin a Nernstian fashion when (K⁺)₀ was increased. It is concludedthat Fe³⁺Cy reduction causes a net depolarization current flowacross the plasmalemma. The depolarization shifts the membranefrom a hyperpolarized pump dominated state into a depolarizedK⁺ diffusion state. Key words: Ferricyanide reduction, membrane potential, Lamprothamnium     

Nutrient Solution pH Control using Dipolar Buffers in Studies of Trifolium repens L. Nitrogen Nutrition

In studies of Trifolium repens nitrogen nutrition, the controlof nutrient solution pH using dipolar buffers, was evaluatedin tube culture under sterile conditions. Five buffers; MES,ADA, ACES, BES and MOPS with pK₂s (20 °C) of 6.15, 6.60,6.90, 7.15 and 7.20 respectively, at a concentration of 2.0mol m^–3, were provided to inoculated Trifolium repensgrowing in nutrient solution containing 7.13 mol m^–3 nitrogenas (NH₄)₂SO₄. Initial pH of each solution was adjusted to theappropriate buffer pK₂ Two buffers, ADA and ACES completelyinhibited plant growth. The remaining buffers had little effectin limiting pH change, although plant dry matter was higherand nodule numbers lower in the presence of these buffers. MESand MOPS were supplied to nutrient solutions with and without7.13 mol m^–3 (NH₄)₂SO₄, at concentrations ranging from0–12 mol m^–3. MES at 9 mol m^–3 and 12 molm^–3 reduced growth of plants reliant on the symbiosisfor providing nitrogen. The provision of MES to plants providedwith NH₄⁺ significantly increased plant yield and reduced nodulenumber at all concentrations. MOPS did not affect plant yieldor nodule number. The use of dipolar buffers in legume nitrogennutrition studies is considered in terms of buffering capacity,and the side effects on plant growth and symbiotic development. Key words: Ammonium, Dipolar buffer, Nitrogen nutrition, pH control, Symbiosis, Trifolium repens     

Characteristics of Fe‐deficiency‐induced acidification in subterranean clover

Iron-deficiency-induced acidification is one of the important reactions of plant Fe-deficiency-stress response, but the overall understanding of this reaction is limited. The characteristics of Fe-deficiency-induced acidification of subterranean clover (subclover) (Trifolium brachycalycinum Katzn. and Morley cv. Koala) were studied in this paper. Plants were grown hydroponically under -Fe conditions, and Fe-deficiency-induced acidification was determined using pH-stat, back-titration and chemical equilibrium procedures. Fe-deficiency-induced acidification was undetectable during the first day after Fe-deficiency stress initiation, but the maximum acidification rate was attained by the second day, when plants exhibited visual chlorosis symptoms. The acidification rate was relatively constant with increasing Fe-deficiency chlorosis, suggesting that a critical level of Fe deficiency was needed to trigger acidification, but that once the acidification process was initiated, the intensity of acidification was independent of severity of Fe deficiency. Net H⁺-release (PR) rate determined using a chemical equilibrium method and net acidity release (AR) rate determined using a back-titration method were practically identical, indicating that Fe-deficiency-induced acidification involved almost entirely the release of free H⁺, not organic acid. In the assay temperature range of 5 to 35°C, PR rate was highest at about 20°C. Net acidity release rate was almost totally inhibited at pH values ≤4.5 and increased with increasing assay pH up to pH 9. The pH effect occurred within 30 min of incubation initiation, implying that the effect of pH is probably on the activity of H⁺ transport through the plasma membrane, not on the quantity of responsible protein(s). Cations were required in the incubation solution for Fe-deficiency-induced acidification. Divalent cations in the assay solution resulted in a higher AR rate than monovalent cations, and essential cations resulted in a higher AR rate than non-essential cations, indicating that the relative effectiveness of cations is related to the efficiency of their absorption by plant roots. These results are discussed in relation to their practical significance and the mechanisms of Fe-deficiency-induced acidification.     

Effects of Cations on the Cytoplasmic pH of Chara corallina

Smith, F. A. and Gibson, J.–L. 1985. Effects of cationson the cytoplasmic pH of Chara corallina.—J.exp. Bot.36: 1331–1340 Removal of external Ca²⁺ from cells of Chara corallina lowersthe cytoplasmic pH, as determined by the intracellular distributionof the weak acid 5,5–dimethyloxazolidine2–,4–dione(DM0), when the external pH is below about 60. This effect isreversed, at least partially, by addition of the following cationsto Ca²⁺-free solutions: tetraethylammonium (TEA⁺) and Na⁺ at5 or 10 mol m^-3, Li⁺ and Cs⁺ (10 mol m^-3), or Mg²⁺, Mn²⁺ andLa³⁺ (02 or 05 mol m^-3). Under the same conditions, increasesin pH sometimes, but not always, occur in the presence of 10mol m^-3 K⁺ or Rb⁺ The results are discussed in relation to the major transportprocesses that determine pH and the electric potential differenceacross the plasma membrane, namely fluxes of H⁺ and of K⁺. Thesimplest explanation of the effects of the various cations testedin this study is that they primarily affect pHi_c via changesin influx of H⁺ but direct effects on the H⁺ pump or on K⁺ fluxesmay also be involved Key words: Chara corallina, cytoplasmic pH, cations, H⁺transport     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

Influence of Complexation on the Uptake by Plants of Iron, Manganese, Copper and Zinc: I. EFFECT OF EDTA IN A MULTI-METAL AND COMPUTER SIMULATION STUDY

After growing barley (Hordeum vulgare L.) in nutrient solutionscontaining EDTA, uptake of the nutrient metals was determinedat three harvests and concentrations of the various chemicalspecies of each metal in the growth solutions was modelled bycomputer simulation. Complexation with EDTA had different effectson the uptake of the ions Fe³⁺, Mn²⁺, Cu²⁺, and Zn²⁺. At thehighest EDTA level (EDTA/Fe=2/l) the plants were chlorotic andgrowth was inhibited. This is attributed to a deficiency inZn rather than in Fe. The critical level of free Zn²⁺ requiredin nutrient solutions for healthy growth was found to be approximately10^–1010^–10 mol dm^–3, which is consistent withthat found by earlier workers for other plant species. Barleytolerated much lower levels of the free ions of copper and ironwithout exhibiting any obvious adverse effects. Key words: EDTA, micronutrients, trace metals, computer simulation, deficiencies, absorption, iron, manganese, copper, zinc     

Electrogenic Pump Current and ATP-Dependent H+ Efflux Across the Plasma Membrane of Nitellopsis obtusa

The correlation between the pump current and the ATP-dependentH⁺ efflux was examined in internodal cells of Nitellopsis obtusa.To control the cytoplasmic pH and ATP concentration, the tonoplastwas removed by intracellular perfusion with an EGTA-containingmedium. Two groups of perfused cells were prepared, one with1 mM ATP (+ATP cells) and the other without ATP but with hexokinaseand glucose (–ATP cells). The ATP-dependent H⁺ effluxwas calculated as the difference in H⁺ efflux between the +ATPand –ATP cells. Based on an electrically equivalent circuitmodel of the plasma membrane, the pump current was calculatedfrom the membrane potentials and the membrane resistances ofboth +ATP and –ATP cells. When the membrane potentialwas not too high (–220 mV), the ATP-dependent H⁺ current(19 mA m^–2) was almost equal to the pump current (20 mAm^–2) calculated from the electrical data. This indicatesthat the electrogenic pump current across the plasma membraneof Nitellopsis obtuse was mostly carried by H⁺. But when themembrane potential was high (–280 mV), the H⁺ currentwas lower than the pump current. The possible cause of thisdiscrepancy is discussed. (Received November 5, 1984; Accepted February 28, 1985)     

H+ Efflux and Trans-Root Potential Measured while Increasing the Temperature of Solutions Bathing Excised Roots of Zea mays

Kennedy, C. D. and Gonsalves, F. A. N. 1988. H⁺ efflux and trans-rootpotential measured while increasing the temperature of solutionsbathing excised roots of Zea mays.—J. exp. Bot. 39: 37–49. Novel temperature-ramp procedures have been used to measureH⁺ efflux and trans-root potential of excised roots of Zea mays(var. Fronica). Two types of experiment were performed: (1),increasing temperature from 17°C, and (2), pre-cooling theroots to 1°C before starting the temperature ramp. The ratesof increase of temperature for H⁺ efflux and trans-root potentialexperiments were 0·5 and 2·1°C min^–1respectively The H⁺ scans revealed strong sharp maxima at 30°C and 32°C,for non-pre-cooled and pre-cooled roots respectively, the latterbeing significantly smaller. The trans-root potential scansfor the pre-cooled roots showed a corresponding maximum at 30°C,which was inhibited by KCN (1-0 mmol dm^–3) with or withoutSHAM (10 mmol dm^–3), or Hg²⁺ (1, 10, 100 µmol dm^–3)in the bathing solutions. Some of the evidence suggests thatthese maxima are associated with electrogenic H⁺ pumping, mediatedby a plasma membrane-bound ATPase. However, no correspondingmaximum was observed in the trans-root potential scans for non-pre-cooledroots, the potential remaining at about — 75 m V from20°C to 35°C. As there is a 7-fold increase in H⁺ effluxbetween 20°C and 30°C, the relationship between netH+ efflux and electrogenic proton pumping in these roots isby no means clear. Some possibilities are considered here. Pre-cooled and non-pre-cooled roots show clear maxima in thetrans-root potential scans at about 46°C, at which temperaturethere is a slight net H⁺ influx. This, and other less prominentfeatures observed, are briefly discussed. Key words: H⁺ efflux, trans-root potential, temperature-ramp procedure, Zea mays, roots     

Proton Translocation Associated with Denitrification in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

H⁺ translocation driven by NO₃^–, NO₂^– and N₂O reductionswith endogenous substrates in cells of Rhodopseudomonas sphaeroidesforma sp. denitrificans was investigated by the oxidant pulsemethod. Upon injection of nitrogenous oxides to anaerobic cellsin darkness, an alkaline transient in the external medium wasobserved, followed by acidification. The alkaline transientwas enhanced by carbonyl cyanide m-chlorophenylhydrazone. When a viologen dye was used as an electron donor in the presenceof 1 mM Af-ethylmaleimide and 0.1 mM 2-n-heptyl-4-hydroxyquinoline-N-oxideto preclude respiration-linked H⁺ extrusion, addition of KNO₃,KNO₂ and N₂O caused only a rapid alkalinization. The H⁺ consumptionstoichiometries, H⁺/2e^– ratios for NO₃^– reductionto NO₂^–, NO₂^– reduction to 1/2 N₂O and N₂O reductionto N₂ were –1.90, –3.18 and –2.04, respectively.These values agreed well with the fact that all reductions ofnitrogenous oxides in denitrification occur on the periplasmicside of the cytoplasmic membrane. When corrected for H⁺ consumption in the periplasm, the H⁺ extrusionstoichiometries, H⁺/2e^– ratios with endogenous substratesin the presence of K⁺/valinomycin for NO₃^– reduction toNO₂^–, NO₂^– reduction to 1/2 N₂O and N₂O reductionto N₂ were 4.05, 4.95 and 6.01, respectively. (Received August 4, 1982; Accepted January 13, 1983)     

Effects of cadmium on the behaviour of citric acid in isolated tomato xylem cell walls

Effects of cadmium on the sorption of citric acid In isolatedxylem cell walls were Investigated. 2.5 nM to 9.5 mM [1.5–¹⁴]crticacid solutions were perfused through columns of xylem cell wallmaterial, isolated from tomato plants (Lycoperslcon esculentumMill, cv. Tiny Tim). The anion exchange potential of the column was estimated byamino acid analysis as approximately 46 meq dm whereas the apparentanion exchange capacity (AEC) was estimated as 1.65±0.1810^–4(citric acId units). This low AEC was attributed toa ‘zipper’ effect, a mutual screening of fixed R^–and A⁺ charges. Pre-loading with ¹¹⁵Cd²⁺ did not affect citric acid sorption,indicating the absence of Cd-effects on the availability offixed A⁺ charges, and the absence of the formation of effectiveR^–-Cd²⁺ and Donnan tree space (DFS) (Cd(cit)H₂]⁺ complexes. Simultaneous application of both citric acid and ¹¹⁵Cd²⁺,⁴⁵Ca²⁺or ²⁸Mg²⁺ resufted in increased sorption of citric acid, probablydue to capacity improvement rather than changes in valence-dependentanion sorption; this may be due to the presence of bulk (M(cit)H₂]⁺,held in the column as [M(cit)H₂]⁺ after protonation in the DFS.Sorption of citric acid was greatest in the presence of Ca²⁺which was discussed in the light of the differences betweenCa, Cd and Mg in their characteristics as co-ordinative M-complexes of citric acid. The overall results indicate the potentialimportance of the presence of metal ions for the xylem transportbehaviour of organic acids in plants. Key words: Cadmium, citric acid, ion exchange, ligand exchange, tomato, xylem cell walls     

Proton Fluxes and the Activity of a Stelar Proton Pump in Onion Roots

The xylem vessels of excised adventitious roots of onion, Alliumcepa, were perfused with unbuffered nutrient solution adjustedinitially to either pH 9·3 or 3·9; the pH of thesolution after passage through the xylem, at rates not lessthan 2 xylem volume changes min^–1, was close to pH 6·5in both instances. The flux of H⁺ across the xylem/symplastboundary into mildly alkaline, phosphate-buffered solutionsperfusing the vessels could be increased greatly with increasingbuffer strength, up to a maximum value between 0·5–1·0pmol H⁺ mm^–2 s^–1. The apparent neutralization ofacidic malic acid buffers had a slightly lower maximum capacity,equivalent to –0·3 to –0·5 pmol H⁺mm^–2 s^–1. The addition of 5·0 pmol m^–3fusicoccin (FC) to the xylem perfusion solution stimulated theentry of H⁺ into the xylem; in unbuffered perfusion solutionsthe pH fell to pH 3·6 after a lag of 25–35 min.FC additions to phosphate-buffered solutions also stimulatedthe H⁺ flux to an extent similar to that in unbuffered solution,viz. 0·2–0·4 pmol mm^–2 s^–1. The release of K⁺ (³⁶Rb-labelled) into xylem sap transientlyincreased as the [K⁺] in weakly buffered perfusion solutionswas raised stepwise; a very marked increase being seen whenthe concentration was raised to 100 mol m^–3 from 40 molm^–3. The addition of 5·0 mmol m^–3 FC to theperfusing solution containing 100 mol m^–3 K⁺ rapidly decreasedthe K⁺ flux to the xylem as the H⁺ flux increased. Fusicoccinalso inhibited the flux of K⁺ into unbuffered perfusion solutionsbut the effect appeared reversible. Addition of 10 mmol m^–3abscisic acid (ABA) to the perfusion solution quickly producedtransient increases in both K⁺ and H⁺ fluxes into the xylem.In this and other experiments using weakly phosphate-bufferedperfusing solutions, H⁺ fluxes were comparable in size to thoseof K⁺ The results are consistent with the idea that the stele of onionroots contains a proton trarislocating ATPase whose activityresponds to the pH of the xylem sap. It is evident that theactivity of the proton secreting and proton neutralizing mechanismsin the xylem parenchyma control the movement of other ions acrossthe xylem/symplast boundary. Key words: Xylem perfusion, fusicoccin, abscisic acid, pH gradient     

Ion and Glycerol Concentrations in 12 Isolates of Dunaliella

Ginzburg, M., and Ginzburg, B. Z., 1985. Ion and glycerol concentrationsin 12 isolates of Dunaliella.—J. exp. Bot. 36: 1064–1074. Twelve isolates of Dunaliella with average cell volumes rangingfrom 50 to 1400x10^–18 m³ were grown in batch culture at0.5 M or 2.0 M NaCl. Glycerol and ions (Na⁺, K⁺, Mg²⁺, CI^–,phosphate) were measured in log-phase cultures. The contentsof Mg²⁺, K⁺ and phosphate per cell were found to be a functionof cell-volume. Cell glycerol, Na⁺ and Cl^– were functionsof cell-volume and of the NaCl concentration in the medium.Solute concentrations were calculated from the measured cell-volumesand from the ³H₂O content of pellets corrected for intercellularspace using Blue Dextran. Cell glycerol was found to accountfor about one-half of the expected osmolarity, the remainderbeing largely accounted for by Na⁺ and CI^–. Key words: —Dunaliella, isolates, glycerol, ion concentrations     

Plasma Membrane H+-ATPase in Guard-Cell Protoplasts from Vicia faba L.

The activity of plasma membrane H⁺-ATPase was measured withmembrane fragments of guard-cell protoplasts isolated from Viciafaba L. ATP hydrolytic activity was slightly inhibited by oligomycinand ammonium molybdate, and markedly inhibited by NO₃^–and vanadate. In the presence of oligomycin, ammonium molybdateand NO₃^–, the ATP-hydrolyzing activity was strongly inhibitedby vanadate. It was also inhibited by diethylstilbestrol (DES),p-chloromercuribenzoic acid (PCMB) and Ca²⁺, but slightly stimulatedby carbonyl cyanide m-chlorophenylhydrazone (CCCP). The acitivityhad higher specificity for ATP as a substrate than other phosphoricesters such as ADP, AMP, GTP and p-nitrophenylphosphate; theK_m was 0.5 mM for ATP. The activity required Mg²⁺ but was notaffected by K⁺, and it was maximal around pH 6.8. When guard-cellprotoplasts were used instead of membrane fragments, the ATPaseactivity reached up to 800µmol Pi.(mg Chl)^–1.h^–1in the presence of lysolecithin. These results indicate thatthe guard cell has a high plasma membrane H⁺-ATPase activity. (Received December 23, 1986; Accepted April 28, 1987)     

Absorption and Distribution of Copper in Plants with Sufficient or Deficient Supplies

Wheat, red clover and ryegrass were grown in flowing solutionculture with sufficient (+ Cu) and deficient (–Cu) suppliesof copper. The rates of Cu absorption (µg g^–1 dryroot day^–1) did not differ greatly between species ineither treatment. Wheat plants, when transferred from the –Cu to the +Cu treatment, absorbed Cu at a much slower rate thanthose which had remained throughout in the + Cu treatment. Inall plants considerable proportions of the absorbed Cu wereretained in the roots, even when the plants were Cu-deficient,and the concentration in roots usually exceeded that in anypart of the shoots in both treatments. Transferring wheat plantsfrom the +Cu to the –Cu treatment decreased the concentrationin all plant parts except old leaves; similarly, transferringfrom the –Cu to +Cu treatment increased the concentrationin all parts of the shoots, execept old leaves, and in the roots. Lolium perenne, Trifolium pratense, Triticum aestivum, ryegrass, red clover, wheat, absorption, copper, flowing solution culture