Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis III. Significance of the radial potential difference

The distribution pattern of the trans-root surface potentialalong the germ axis of Vigna sesquipedalis intimately dependedon the existence of oxygen as well as the potential difference(PD) between both cut ends of a cylindrical segment of the hypocotyl.In the grown-up region (basal region) of the hypocotyl, a largechange of the trans-root surface potential occurred with anoxia,but not in the elongating zone except in the transient phaseafter reaeration. The distribution pattern of the trans-root stump potential alongthe germ axis, measured just after die hypocotyl had been cutat various positions, was nearly flat. However, PD between diecut stump and a point on the hypocotyl surface at sufficientdistance from the stump had a large value. The results are not explainable based on the axial electromotiveforces alone. Consideration of equivalent circuits of the hypocotylhas led to die idea that radial electromotive forces largelycontribute to generation of die distribution pattern of thesurface electric potential. (Received June 30, 1977; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis I. Distribution of overall ion concentration and the role of hydrogen ion in generation of potential difference

Overall concentration of free inorganic ions distributes inthe hypocotyl of a bean seedling {Vigna sesquipedalis) at aconstant level (H⁺) or decreases monotonously from the cotyledonarynode towards the base (K⁺, Na⁺, Ca⁺⁺ and Mg⁺⁺, phosphate, NO₃^–).According to our theory, this is inconsistent with the distributionof electric potential having a definite minimum in the elongatingregion. The discrepancy can not be explained by regional variancein radial potential difference or histological differentiationin passive ionic permeability of the cell membrane. Short circuitcurrent observed through a hypocotyl segment corresponded toa net flux in ions of 10–24 pEq/cm².sec. It is questionable,however, whether this is due to active ion transport, whichcan be the source of electric potential difference, or is apassive flow due to histological heterogeneity in ion concentration. In order to investigate the latter possibility, pH of sap exudingfrom stumps made at various intervals along the hypocotyl axiswas measured, since H⁺ is the ion electro-osmotically most effective.pH Values of acropetal exudates distributed along the axis closelycorresponding to the distribution of electric potential. Thissuggests that potential distribution is determined by a passiveflow of H⁺ through a specific channel in the vascular system.The fact that H⁺ production and the uptake of ions and waterare most active at the elongating zone of hypocotyl is discussedfrom a physiological point of view. (Received December 3, 1969; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis IV. The effects of carbon monoxide and cyanide on the axial potential difference of hypocotyl segments

The effects of carbon monoxide and cyanide on the electric potentialdifference between both cut ends of a hypocotyl segment excisedfrom a bean embryo were investigated. Carbon monoxide definitely diminished the PD. This inhibitionwas photoreversible. The inhibition ratio was quantitativelyrelated to the [CO]/[O₂] ratio of the gas mixture. Prolongedexposure to a CO gas mixture induced spontaneous recovery ofthe PD, and this CO-resistant electrogenic activity was sensitiveto CN and also disappeared under anoxia. Cyanide also diminished the PD and often induced oscillationof the potential. The type of oscillation and the inhibitionratio varied with the concentration of CN. Sometimes CN pretreatmentcaused changes in the anoxia response of the potential. The time course of the spontaneous change in PD after cuttingof the hypocotylsegment also was investigated. (Received August 8, 1977; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis V. Electrogenic activity of the parenchyma cells in hypocotyl segments

Intracellular potential of parenchyma cells (V_ps) in the hypocotylsegment of Vigna sesquipedalis was initially low after excision,then gradually increased to a more negative level. ThereafterV_ps could be remarkably reduced under anoxia then recoveredwith reaeration, accompanied by several cycles of damped oscillations.Both the rapidity of the decrease of V_ps caused by anoxia andits temperature dependency suggest an electrogenic mechanism.No marked spatial differences along the germ axis and the radiusof hypocotyl were observed in the V_ps level and its electrogeniccomponent when the reference electrode was placed on the surfaceof the hypocotyl segment. V_ps also decreased rapidly in an atmospherecontaining 80% CO+20% O₂ in the dark, then was recovered immediatelyin the light or spontaneously but very slowly in the dark. Theextent of the decrease of V_ps caused by CO depended on the growthactivity of the cell and the time lapse after excision. Theseresults suggest the possible conversion or replacement of theterminal oxidase. Sometimes phenomena resembling those of actionpotentials occurred spontaneously or during the reduction ofV_ps due to inhibition of the energy metabolism. V_ps in the elongatingregion varied transiently with the change in illumination. Interrelationshipbetween V_ps and the surface resting potential is discussed. (Received July 20, 1977; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis VI. The dual structure of radial electrogenic activity

The electrophysiological structure in bean hypocotyl was investigatedby the intracellular electrode method in combination with surfaceelectric potential (V_s) measurement and respiratory inhibitionby anoxia, with special reference to the membrane transportof ions and the formation of an absorption centre in the elongating(E) zone. The radial potential difference (V_sx: electric potentialdifference between the organ surface and a xylem vessel), onwhich axial distribution of V_a was dependent, comprised twocomponents; V_ax=V_px–V_ps. |V_px| (the potential differencebetween the inside of a parenchyma symplast and a xylem vessel)was at a maximum in the E-zone, while |V_ps| (the intracellularelectric potential with respect to the organ surface) was largestin the G-zone (mature zone), resulting in the characteristicdistribution pattern of V_s with a minimum in the E-zone. Therewere two independent electromotive forces which were both partiallydependent on respiration; one corresponding to V_ps located atthe surface of the parenchyma symplast (P) and the other toV_px located between P and the xylem (X). The electrogenic componentof Vpx was relatively small both in the hook (H) zone and theG-zone, but maximal in the E-zone of the hypocotyl. This resultwas consistent with the emergence of a maximum pH differencebetween P and X in the E-zone, where accumulation of K⁺ andwater were at a maximum, suggesting maximum activity of an H⁺-pumpextruding protons from P into X in exchange for K⁺. (Received July 17, 1978; )     

Automatic recording of electric potential and ion transport in frog skin

Summary An apparatus for the automatic recording of the bioelectric potential of isolated frog skin and of the short-circuit current, which is a measure of active Na⁺ transport, is described. The equivalence of the uninterrupted short-circuit current and active Na⁺ transport has been checked. Current and Na⁺ transport agreed within 2%.The automatic ion transport recorder is particularly suited in studies where the exact time course of the effects of enzyme inhibitors, drugs and other chemicals on skin is of interest. This is illustrated by showing the effects of fluoroacetate, quinone and hydroquinone on spontaneous skin potential and short-circuit current.Attention is called to characteristic transients in the potential records which are probably related to changes in skin permeability to passively moving chloride ions.Supported by Public Health Grant RG 3545.     

Alveolar transepithelial potential difference and ion transport in adult rat lung

The complex morphology of the mammalian lung complicates characterization of solute transport across the intact alveolar epithelium. We impaled the subpleural alveolar epithelium with microelectrodes and measured the transepithelial potential difference (PD) of the liquid-filled vascular-perfused left lobe of the rat lung. When the air space was filled entirely with Krebs-Ringer-bicarbonate, the PD was 4.7 mV (lumen negative). The PD was not affected significantly by agents that modify either Na+ or Cl- transport, but replacement of luminal Cl- with gluconate resulted in a fourfold hyperpolarization, a response also noted for large airways. When the airways were blocked by an immiscible nonconducting fluorocarbon, basal PD was not different from unblocked lobes (4.0 mV) but was inhibited 73% by luminal amiloride. Cl(-)-free Krebs-Ringer-bicarbonate blocked in the alveoli with fluorocarbon did not induce hyperpolarization. This result suggests that 1) Cl- permselectivity of the alveolar epithelium is less than that of large airway epithelium and 2) airway PD dominates the voltage across the liquid-filled lung, even when measurements are made from alveoli. When airways are blocked by fluorocarbon, the PD across the alveolar epithelium is largely dependent on Na+ flow through a path with amiloride-sensitive channels.     

Effects of carbon dioxide on the spatially separate electrogenic ion pumps and the growth rate in the hypocotyl ofVigna sesquipedalis

Carbon dioxide, introduced into the gas phase of the experimental chamber, has distinct effects on two spatially separate membrane potentials and the rate of elongation growth in hypocotyl segments ofVigna sesquipedalis Wight. Both membrane potentials (V _ps andV _px=the electric potential difference between the parenchyma symplast and the surface of the hypocotyl, and that between the parenchyma symplast and the xylem, respectively) hyperpolarized rapidly but transiently at the introduction of CO₂. Prolonged exposure of the hypocotyl to high concentrations of CO₂ (above 10%) caused depolarization of membrane potentials above the level before CO₂ introduction. When CO₂ was replaced with air, the membrane potentials exhibited a distinct depolarization response of transient nature. The growth rate of the hypocotyl segments exhibited similar responses to CO₂ as did the membrane potentials (the increase and the decrease of the growth rate were corresponded to the hyperpolarization and the depolarization, respectively), but these responses always followed the changes of the membrane potentials. The CO₂-induced maximum hyperpolarization ofV _ps and the maximum increase of the growth rate were closely correlated. All these responses were strictly dependent on aerobic metabolism. These results indicate that CO₂ may regulate elongation growth in two ways: by affecting the activity of the electrogenic ion pump via intracellular acidification, and also by acting via apoplastic acidification as a wall-loosening acid.Symbols and abbreviations V _sx electric potential difference between the surface (S) and the xylem (X) of the hypocotyl - V _px electric potential difference between the inside of a parenchyma cell (P) andX - V _ps electric potential difference betweenP andS - V _ps (CO₂, max) the maximum value of CO₂-induced hyperpolarization ofV _ps - GR(CO₂, max) the maximum value of CO₂-induced increase of the growth rate - IAA indole-3-acetic acid     

Isolation of xyloglucans from etiolated Glycine max and Vigna sesquipedalis hypocotyls

Xyloglucans isolated from cell walls of etiolated Glycine maxand Vigna sesquipedalis hypocotyls were subjected to fragmentationanalysis with cellulase for structural comparison with thosederived from Phaseolus aureus hypocotyls. The xyloglucans fromG. max and V. sesquipedalis had glucose, xylose, galactose andfucose in the approximate molar ratio of 10:6:4:1 and 10:7:3:1,respectively. However, the results of cellulase fragmentationanalysis of xyloglucans from the three species suggested thatthe basic structure of the xyloglucans in the cell walls ofthese bean-hypocotyls is almost the same; the structure is basedon two repeating oligosaccharide units, one of which consistsof glucose and xylose and the other of glucose, xylose, galactoseand fucose. ¹ Present address: Toppan Printing Co., Ltd., Okaji, Sendai980, Japan. (Received February 3, 1977; )     

Radial electrogenic activity in the stem of Vigna sesquipedalis: involvement of spatially separate pumps

Abstract. The surface electric potential of Vigna sesquipedalis stem, relative to that of the root medium, was most negative in the elongating zone where the difference in pH between parenchyma (P) and xylem sap (X) was at a maximum. The surface electrical potential was found to be determined by the distribution of radial potential difference ( V _sx comprised two components; V_sx= V _px− V _ps.There were two electromotive forces, generating V _ps at the surface of the parenchyma symplast and V _px between P and X; both were partially dependent on respiration. The depolarization of V _px upon removal of O₂ was maximal in the elongating zone, suggesting that the activity of an H⁺-pump extruding protons from the parenchyma into the xylem in exchange for K⁺ was greatest in this zone.     

Development of Embryo and Endosperm and Nutrient Accumulation in Vigna sesquipedalis (L.) Fruwirth

The structure of embryo sac, fertilization and development of embryo and endosperm in Vigina sesquipedalis (L.) Fruwirth were investigated. Pollization occures 7–10h before anthesis, and fertilization is completed 10 h after anthesis. After fertilization, wall ingrowths are formed at the micropylar and chalazal ends of the embryo sac. Embryo development conforms to the Onagrad type, and passes through 2 or more celled proembryo, long stick-shaped, globular, heart shaped, torpedo, young embryo, growing and enlarging embryo and mature embryo. Wall ingrowths are formed on the walls of basal cells and outer walls of the cells at basal region of suspenser. The suspensor remains as the seed reaches maturity. The starch grains accumulate in the cells of cotyledons by 9–16 days after anthesis, and proteins accumulate by 12–18 days after. The endosperm development follows the nuclear type. The endosperm ceils form at the micropylar end, and remain free nuclear phase at chalazal end. The outer cells are transfer cells. Those cells at the micropylar end form folded cells with wall ingrowths. At heartembryo stage, the endosperm begins to degenerate and disintegrates before the embryo matures.     

Relationships between auxin- and acid-induced growth in Vigna radiata hypocotyl segments

The H⁺- and IAA-induced growth responses of isolated Vigna radiata (L.) Wilczek hypocotyl segments were investigated concurrently with IAA-induced H⁺ excretion. The effects of external pH on these reactions were also studied. Experiments were performed with intact, peeled and abraded segments. Only abraded segments reacted to H⁺ and to IAA. In short-term experiments, the cuticle prevented proton efflux and influx; however, it allowed gradual ion movements which become measurable after 1 h. Both phases of the IAA growth response reacted to external pH. The interactions between these two phases and their pH dependencies are discussed.     

Transmembrane electric potential difference in the protein-pigment complex of photosystem 2

The protein-pigment complex of photosystem 2 (PS2) localized in the thylakoid membranes of higher plants, algae, and cyanobacteria is the main source of oxygen on Earth. The light-induced functioning of PS2 is directly linked to electron and proton transfer across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). The major contribution to ΔΨ of the PS2 reaction center is due to charge separation between the primary chlorophyll donor P₆₈₀ and the quinone acceptor Q_A, accompanied by re-reduction of P ₆₈₀ ⁺ by the redox-active tyrosine residue Y_Z. The processes associated with the uptake and release of protons on the acceptor and donor sides of the enzyme, respectively, are also coupled with ΔΨ generation. The objective of this work was to describe the mechanisms of ΔΨ generation associated with the S-state transitions of the water-oxidizing complex in intact PS2 complex and in PS2 preparation depleted of Mn₄Ca cluster in the presence of artificial electron donors. The findings elucidate the mechanisms of electrogenic reactions on the PS2 donor side and may be a basis for development of an effective solar energy conversion system.