Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Xylem perfusion of tap root segments of Plantago maritima: the physiological significance of electrogenic xylem pumps

Abstract A method is described for perfusing xylem vessels in tap root segments of the halophyte P. maritima. Use of excised segments allowed recording of the trans-root potential (TRP) at both ends of a segment. It was shown that there can be a spatial variation of electrogenic ion pump activity along the xylem in one root segment. The pH of perfusion solutions, differing in buffering capacity, was adjusted by the root segment to pH 5.1–5.6 during How through the xylem. This pH range was similar to that of sap produced by root pressure. The K⁺ activity in the outflow solution (K⁺_out) was rather constant at 12–13 mol m^?l3 despite input K⁺ activities ranging from 8 to 20 mol m^?l3. Addition of fusicoccin (10^?l2 mol m^?l3) to the perfusion solution induced a strong acidification of the xylem sap, a decrease in K⁺_out and an increase in Na⁺_out. Inhibition of aerobic respiration through anoxia inhibited electrogenic proton pumping into the xylem and led to an increase in K⁺_out and a decrease in Na⁺_out. It is suggested that transport of K⁺ and Na⁺ to the shoot of the halophyte P. maritima is regulated in the tap root by means of ion exchange between xylem vessels and xylem parenchyma and that this exchange is energized by proton translocating ATPases.     

Proton co-transport of sugars in phloem loading

Loading of ¹⁴C-labelled sugars from the hollow petiole of Ricinus communis L. was stimulated by potassium and by low pH in that both the ¹⁴C-activity and the sugar concentration of phloem sap collected from a nearby incision increased. A pH drop was observed in the solution perfusing a hollow petiole. This pH drop was greater in the presence of potassium and less in the presence of sugars, while the uncoupler CCCP induced a pH rise in the perfusing solution. Sugars were detected in the perfusing solution when it was buffered at pH>9. A model is proposed for a proton co-transport of sugars from the free space driven by a linked proton efflux/potassium influx pump.     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.     

Effects of Cr on proton extrusion, potassium uptake and transmembrane electric potential in maize root segments

Abstract Proton extrusion of maize root Zea mays segments, was inhibited by the presence of Cr (o.n. + 6; present in solution as CrO₄^2-, Cr₂O₇^2-) in the incubation medium: the minimum inhibiting concentration was 2 × 10^?3 mol m^?3 and the inhibition progressively increased with Cr concentration. Cr inhibited proton extrusion. Also, when this activity was stimulated by the presence of K⁺ or fusicoccin (FC) in the incubation medium, the K⁺ and FC stimulating effect was still present when proton extrusion was inhibited by Cr. In addition, Cr inhibited K⁺ uptake. This inhibition was higher (50%) at K⁺ concentrations up to 1 mol m^?3 lower (15%) at higher K⁺ concentrations. This result indicates that the system responsible for K⁺ uptake operating at low K⁺ concentrations is more sensitive to Cr inhibition. Cr had no effect on transmembrane electric potential (PD). The depolarizing and hyper-polarizing effect of K+ and FC, respectively, were not affected by Cr; but Cr enhances the depolarizing effect of the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCP). These results indicate that Cr inhibited the proton translocating mechanism coupled with K⁺ uptake, but did not change the net transport of charges through the plasmalemma. The Cr effect is discussed, taking into account the possibility of a direct effect of Cr at the membrane level or, alternatively, of an effect on some metabolic processes controlling membrane function.     

Phloem Loading of Sucrose: pH Dependence and Selectivity

Autoradiographic, plasmolysis, and ¹⁴C-metabolite distribution studies indicate that the majority of exogenously supplied ¹⁴C-sucrose enters the phloem directly from the apoplast in source leaf discs of Beta vulgaris. Phloem loading of sucrose is pH-dependent, being markedly inhibited at an apoplast pH of 8 compared to pH 5. Kinetic analyses indicate that the apparent K_m of the loading process increases at the alkaline pH while the maximum velocity, V_max, is pH-independent. The pH dependence of sucrose loading into source leaf discs translates to phloem loading in and translocation of sucrose from intact source leaves. Studies using asymmetrically labeled sucrose ¹⁴C-fructosyl-sucrose, show that sucrose is accumulated intact from the apoplast and not hydrolyzed to its hexose moieties by invertase prior to uptake. The results are discussed in terms of sucrose loading being coupled to the co-transport of protons (and membrane potential) in a manner consistent with the chemiosmotic hypothesis of nonelectrolyte transport.     

Phloem loading of sucrose: involvement of membrane ATPase and proton transport

p-Chloromercuribenzenesulfonic acid markedly inhibited sucrose accumulation into sugar beet source leaves without inhibiting hexose accumulation. The site of inhibition is proposed to be the plasmalemma ATPase, since the ATPase-mediated H⁺ efflux was completely inhibited by p-chloromercuribenzenesulfonic acid under conditions where intracellular metabolism, as measured by photosynthesis and hexose accumulation, was unaffected. Fusicoccin, a potent activator of active H⁺/K⁺ exchange, stimulated both active sucrose accumulation and proton efflux in the sugar beet leaf tissue. These data provide strong evidence for the phloem loading of sucrose being coupled to a proton transport mechanism driven by a vectorial plasmalemma ATPase.     

Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2–3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80–100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K⁺ in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K⁺ concentration in the exudate. High concentrations of K⁺, however, strongly reduced the flow rate of exudation. Similar results were obtained with Na⁺ (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h^-1, a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.Abbreviations PTS 3-hydroxy-5,8, 10-pyrenetrisulfonate     

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     

Concentrations and Transport of Solutes in Xylem and Phloem along the Leaf Axis of NaCl-treated Hordeum vulgare

Hordeum vulgare cv. California Mariout was established in sandculture at two different NaCl concentrations (0.5 mol m^–3‘control’ and 100 mol m^–3) in the presenceof 6.5 mol m^–3 K ⁺. Between 16 and 31 d after germination,before stem elongation started, xylem sap was collected by useof a pressure chamber. Collections were made at three differentsites on leaves 1 and 3: at the base of the sheath, at the baseof the blade, i.e. above the ligule, and at the tip of the blade.Phloem sap was collected from leaf 3 at similar sites throughaphid stylets. The concentrations of K ⁺, Na⁺, Mg2⁺ and Ca²⁺were measured. Ion concentrations in xylem sap collected at the base of leaves1 and 3 were identical, indicating there was no preferentialdelivery of specific ions to older leaves. All ion concentrationsin the xylem decreased from the base of the leaf towards thetip; these gradients were remarkably steep for young leaves,indicating high rates of ion uptake from the xylem. The gradientsdecreased with leaf age, but did not disappear completely. In phloem sap, concentrations of K⁺ and total osmolality declinedslightly from the tip to the base of leaves of both controland salt-treated plants. By contrast, Na⁺ concentrations inphloem sap collected from salt-treated plants decreased drasticallyfrom 21 mol m^–3 at the tip to 7.5 mol m^–3 at thebase. Data of K/Na ratios in xylem and phloem sap were used to constructan empirical model of Na⁺ and K⁺ flows within xylem and phloemduring the life cycle of a leaf, indicating recirculation ofNa⁺ within the leaf. Key words: Hordeum vulgare, xylem transport, phloem transport, NaCl-stress     

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     

Subcellular concentrations of sugar alcohols and sugars in relation to phloem translocation in <Emphasis Type="Italic">Plantago major,Plantago maritima</Emphasis>, <Emphasis Type="Italic">Prunus persica</Emphasis>, and <Emphasis Type="Italic">Apium graveolens</Emphasis>

Sugar and sugar alcohol concentrations were analyzed in subcellular compartments of mesophyll cells, in the apoplast, and in the phloem sap of leaves of Plantago major (common plantain), Plantago maritima (sea plantain), Prunus persica (peach) and Apium graveolens (celery). In addition to sucrose, common plantain, sea plantain, and peach also translocated substantial amounts of sorbitol, whereas celery translocated mannitol as well. Sucrose was always present in vacuole and cytosol of mesophyll cells, whereas sorbitol and mannitol were found in vacuole, stroma, and cytosol in all cases except for sea plantain. The concentration of sorbitol, mannitol and sucrose in phloem sap was 2- to 40-fold higher than that in the cytosol of mesophyll cells. Apoplastic carbohydrate concentrations in all species tested were in the low millimolar range versus high millimolar concentrations in symplastic compartments. Therefore, the concentration ratios between the apoplast and the phloem were very strong, ranging between 20- to 100-fold for sorbitol and mannitol, and between 200- and 2000-fold for sucrose. The woody species, peach, showed the smallest concentration ratios between the cytosol of mesophyll cells and the phloem as well as between the apoplast and the phloem, suggesting a mixture of apoplastic and symplastic phloem loading, in contrast to the herbal plant species (common plantain, sea plantain, celery) which likely exhibit an active loading mode for sorbitol and mannitol as well as sucrose from the apoplast into the phloem.     

Accumulation and Conversion of Sugars by Developing Wheat Grains: V. THE ENDOSPERM APOPLAST AND APOPLASTIC TRANSPORT

The volume and composition of the endosperm apoplast of thedeveloping wheat grain, comprising endosperm cavity and intercellularfree-space, was examined in relation to kernel growth rate andsize. Samples of the cavity sap were collected by centrifugationof kernels during the linear phase of grain growth. The cavitysap contained 10–50 mM sucrose, a small amount of hexosesbut a high concentration of oligosaccharides (up to 9 timesthat of sucrose). In comparing cvs Yandilla King and Cleveland,high growth rate was associated with high cavity sap sucroseconcentration but with low K⁺ concentration. K⁺ concentrationin the endosperm cells (124 mM) was about 5 times higher thanin the cavity sap (10–40 mM). Cavity sap pH was 6.3–6.6.The uptake of sucrose by endosperm cells was partly inhibitedby PCMBS, an inhibitor of membrane-bound carriers. Several necessaryconditions for proton cotransport during sucrose uptake by endospermcells were met. The volume of the intercellular free-space, estimated by membranepermeating (¹⁴C-mannitol, ¹⁴C-sucrose) or non-permeating (³H-PEG900)markers averaged 2.2 µl or 5–7% of the water ingrains of cvs Yandilla King, Cleveland and SUN 9E. The cavityvolume was highly variable but tended to be larger in largergrains. Pulse labelling of ¹⁴CO₂ to flag leaves showed that ¹⁴C-sucrosewas the principal ¹⁴C-assimilate in the cavity sap and was convertedto insoluble compounds in the endosperm while the cavity sapoligosaccharides acquired negligible label in 6 h. Key words: Wheat, Endosperm apoplast, Sugars     

Transport of magnesium ions in the phloem of Ricinus communis L. seedlings

During growth of Ricinus communis seedlings, magnesium ions are mobilized in the endosperm, taken up by and accumulated to very high levels (150 μmol·g FW^?1) in the cotyledons, and translocated to hypocotyl and roots. The magnesium gain from days 6 to 7 in the cotyledons and the seedling axis necessitates a total up-take rate of 600 nmol·h^?1-seedling^?1 and the phloem translocation rate must amount to 200 nmol·h^?1. seedling^?1. The phloem loading of magnesium and the regulatory properties of this process were investigated, making specific use of the ability to collect pure phloem sap from the cut hypocotyl of 6-d-old Ricinus seedlings. The concentration of magnesium in sieve-tube sap (5 mM) was fairly constant under many incubation conditions, e.g. incubation in magnesium-free buffer, incubation with different cations (K⁺, Na⁺, NH ₄ ⁺ ) or anions (Cl^?, NO ₄ ^- , SO ₄ ^2- ), or incubation with sucrose and amino acids. Even addition of magnesium chloride to the cotyledons did not enhance phloem loading of magnesium ions. Therefore the high magnesium content of the cotyledons was sufficient for continuous phloem loading of magnesium, irrespective of external ionic conditions. Also, the flow rate of sieve-tube sap did not influence the magnesium concentration in the sap. Only the incubation with sulfate and phosphate ions increased the magnesium-ion concentration in the phloem. Magnesium sulfate offered to the cotyledons caused a threefold increase of magnesium ions in the sieve-tube sap, which was inhibited by Na⁺, NH ₄ ⁺ and Ca²⁺ in rising order, but not by K⁺. Incubation with phosphate for a prolonged period (8 h) led to an increased mobilization of intra-cotyle-donary magnesium and an enhanced phloem loading of mobilized magnesium. It is concluded that phosphate availability is a decisive factor for mobilization and translocation of magnesium ions within the plant.     

Sucrose Loading in Isolated Veins of Pisum sativum: Regulation by Abscisic Acid, Gibberellic Acid, and Cell Turgor

Enzymatically isolated vein networks from mature pea (Pisum sativum L. cv Alaska) leaves were employed to investigate the properties of sucrose loading and the effect of phytohormones and cell turgor on this process. The sucrose uptake showed two components: a saturable and a first-order kinetics system. The high affinity system (K_m, 3.3 millimolar) was located at the plasmalemma (p-chloromercuriphenylsulfonic acid and orthovanadate sensitivity). Further characterization of this system, including pH dependence and effects of energy metabolism inhibitors, supported the H⁺-sugar symport concept for sucrose loading. Within a physiological range (0.1-100 micromolar) and after 90 min, abscisic acid (ABA) inhibited and gibberellic acid (GA₃) promoted 1 millimolar sucrose uptake. These responses were partially (ABA) or totally (GA₃) turgor-dependent. In experiments of combined hormonal treatments, ABA counteracted the GA₃ positive effects on sucrose uptake. The abolishment of these responses by p-chloromercuriphenylsulfonic acid and experiments on proton flux suggest that both factors (cell turgor and hormones) are modulating the H⁺ ATPase plasmalemma activity. The results are discussed in terms of their physiological relevance.     

On the difference between washing-induced and fusicoccin-induced K+ uptake in maize root segments

A comparison between fusicoccin (FC)-induced and washing-induced increase of K⁺ uptake in maize root segments is reported.K⁺ uptake rate nearly doubles after 4 h of washing as compared to the rate in the unwashed tissue. The stimulation of K⁺ uptake by FC is the same (approx. 1.6 μmol × g fresh wt.⁻¹ × 30 min⁻¹) both in unwashed and in washed tissue.Washing-induced increase of K⁺ uptake is strongly inhibited if 10⁻² M KCl is present in the washing solution; on the contrary, FC-induced increase of K⁺ uptake seems independent of the presence of KCl in the medium during the washing period.The inhibition of protein synthesis completely prevents the stimulation of K⁺ uptake induced by washing, while it only partially inhibits the effect of FC on the same process.These results, taken as a whole, suggest that the activation of K⁺ uptake induced by washing and that induced by FC depend on two different mechanisms.     

Assimilation of gaseous ammonia and the transport of its products in barley and spinach leaves

The interactions between the assimilation and transport of nitrogenand carbon were investigated in barley and spinach leaves. Bothplants were fumigated with NH₃ (1 mg m^–3 and the contentof amino acids, sucrose and carbon intermediates of amino acidmetabolism were analysed in the leaves, apoplast and phloemsap. The following changes took place in the C- and N-metabolismof barley leaves during 5 h of fumigation with NH₃ (a) The contentsof amino acids, especially glutamine, largely increased andthe contents of sucrose, 2-oxoglutarate, phosphoenolpyruvate,and glycerate-3-phosphate declined. (b) A decrease in the phophoenolpyruvatecontent was accompanied by an increased activity of phosphoenolpyruvatecarboxylase. (c) The altered cytosolic concentrations of aminoacids and sucrose during NH₃ fumigation correlated with similarchanges in the apoplast and phloem sap. The altered percentageof each amino acid relative to the total amino acid concentrationin the cytosol, caused by NH₃ fumigation, is reflected in theapoplast and the phloem sap. The results indicate that the concentrations of amino acids in the cytosol determine their concentrationsin the phloem. Key words: Amino acids, ammonia fumigation, barley leaves, C: N partitioning, phosphoenolpyruvate carboxylase, phloem sap, spinach leaves     

Transmembrane electrical potentials in growing maize roots

The anti-auxin 4-chlorophenoxyisobutyric acid (PCIB) applied at a concentration of 10^-2 mol m^-3 to maize root segments was found to induce a transmembrane electrical potential of up to-130 mV (pd of 30 mV). The kinetics of this response were comparable to the time scale for PCIB-stimulated H⁺-extrusion. Both effects are eliminated by the addition of p-fluoromethoxycarbonyl cyanide phenylhydrazone (FCCP). Treatment with fusicoccin (FC) and PCIB together does not result in a hyperpolarization greater than with FC alone. Benzoic acid (10^-2 mol m^-3) had no effect on the transmembrane electrical potentials. These results are discussed in relation to a possible electrogenic proton pump which may be regulated by perturbations in the cellular auxin content or activity.Abbreviations ATPase adenosine triphosphatase - FC fusicoccin - FCCP p-fluoromethoxy carbonylcyanide phenylhydrazone - IAA indole-3yl-acetic acid - NAA naphthyl-lylacetic acid - PCIB 4-chlorophenoxyisobutyric acid - PD potential difference     

Abscisic acid in apoplastic sap can account for the restriction in leaf conductance of white lupins during moderate soil drying and after rewatering

We studied the effects of drought on leaf conductance (g) and on the concentration of abscisic acid (ABA) in the apoplastic sap of Lupinus albus L. leaves. Withholding watering for 5d resulted in complete stomatal closure and in severe leaf water deficit. Leaf water potential fully recovered immediately after rewatering, but the aftereffect of drought on stomata persisted for 2d. ABA and sucrose were quantified in pressurized leaf xylem extrudates. We assumed that the xylem sucrose concentration is negligible and hence that the presence of sucrose in leaf extrudates indicated that they were contaminated by phloem. To eliminate this interference, the concentration of ABA in leaf apoplast was estimated by extrapolation to zero sucrose concentration, using the regression between ABA and sucrose concentrations. The estimated apoplastic ABA concentration increased by 100-fold with soil drying and did not return to pre-stress values immediately following rewatering. g was closely related to the concentration of ABA in leaf apoplast. Furthermore, the feeding of exogenous ABA to leaves detached from well-watered plants brought about the same degree of depression in g as resulted from the drought-induced increase in ABA concentration. We therefore conclude that the observed changes in the concentration of ABA in leaf apoplast were quantitatively adequate to explain drought-induced stomatal closure and the delay in stomatal reopening following rewatering.     

No direct linkage between proton pumping and photosynthate unloading from seed coats of Phaseolus vulgaris L.

The energization of the active sucrose release from bean seed-coat halves was investigated. For this purpose, seed coat tissues adjacent to the apoplastic space were exposed to a variety of treatments and proton and photosynthate release were measured. Fusicoccin (10^–5 moll^–1) stimulated proton pump activities. Orthovanadate (2×10^–4 moll^–1) and abscisic acid (10^–5 moll^–1) diminished the proton extrusion evoked by fusicoccin. Fusicoccin inhibited sucrose release, whereas orthovanadate and abscisic acid stimulated it. Addition of 100 mmoll^–1 K⁺ had a promotory effect on photosynthate unloading, fading away with time. This extra unloading was linearly related to an enhanced proton loss. It was concluded that the photosynthate unloading apparently is not a proton/sucrose antiport and that a pump-leak system for photosynthate release is unlikely. A tentative model for photosynthate/proton symport not directly linked to proton pumping is presented as the mechanism of unloading.Abbreviations ABA abscisic acid - CCCP carbonyl cyanide m-chlorophenyl hydrazone - DTE diethioerythritol - FC fusicoccin - MES 2-(N-morpholino) ethanesulfonic acid monohydrate - NEM n-ethylmaleimide - PCMBS p-chloromercuriphenylsulfonic acid - TRIS 2-amino-2-(hydroxymethyl) propane-1,3 diol - VAN sodium orthovanadate