Blue Light Pulse-Induced Transient Changes of Electric Potential and Turgor Pressure in the Motor Cells of Phaseolus vulgaris L.

Blue light induces both depolarization of membrane potentialin the motor cell and turgor movement in the laminar pulvinusof bean plant. This paper describes the changes of electricpotential and turgor pressure induced in Phaseolus vulgarisL. by blue light pulses. A transient depolarization of membranepotential as large as 40 mV was induced by a short pulse of15 s blue light in motor cells of the laminar pulvinus. Thischange was not an action potential because of the absence ofa refractory period and threshold. The magnitudes of the responsewere dependent on the fluence of light. The response was long-lived,indicating that continuous input of light energy is not requiredfor a sustained response. The potential change was always followedby a transient turgor movement of the pulvinus. A molecular mechanism similar to a model postulated for theblue light response of stomata may operate in the motor cell.However, the direction of the electrical response to blue light(depolarization) in the motor cell was the opposite of thatin the guard cell (hyperpolarization). Turgor change of themotor cell by blue light was also opposite in direction (decrease). (Received February 19, 1988; Accepted June 28, 1988)     

Oscillations of the Membrane Potential of Pulvinar Motor Cells In Situ in Relation to Leaflet Movements of Desmodium motorium

The ultradian rhythmic movement of the lateral leaflets of Desmodiummotorium is accompanied by rhythmic changes of the extra- andintracellular electrical potentials in the pulvinus, which aremeasured in situ in the pulvinus against the bathing solutionof the petiole. Extra- and intracellular potentials oscillatewith 180'b0 phase difference to each other, as shown by simultaneousmeasurements of both types of potentials in the abaxial partof the pulvinus. Light-induced changes of these potentials movein opposite directions. The in situ membrane potential of themotor cells of the pulvinus was calculated from the differencebetween the extra- and intracellular potentials. It was foundto oscillate between –136 and –36 mV, in phase withthe intracellular and inverse to the extracellular potential.The phase relationship between the leaflet movement rhythm andthe in situ membrane potential rhythm was as follows: downwardmovement is preceded and accompanied by a strong depolarization,upward movement by hyperpolarization. Our results suggest that membrane depolarization in pulvinarmotor cells of Desmodium motorium drives and controls potassiumefflux and hyperpolarization potassium influx via potassiumchannels. Key words: Desmodium pulvinus, leaf movement, pulvinar motor cells, electrical potential     

Rhythmic and Blue Light-Induced Turgor Movements and Electrical Potential in the Laminar Pulvinus of Phaseolus vulgaris L.

The well-known circadian movement in Phaseolus leaves is usuallysuperimposed with small and somewhat irregular rhythmic changesof pulvinar movement. This study examines the relationship betweenthese small pulvinar movements and the membrane potential ofthe motor cell as well as the effect of light on them. Bluelight affected both the movements and potential while red andgreen lights of the same photon flux had little effect. Also,the difference in the membrane potential between the extensorand flexor cells was found to be closely related to the rhythmicturgor movement of the pulvinus. Changes in the potential differencealways preceded the movement. Sequential changes of the potentialdifference and turgor pressure in the motor cells, includingthe light-induced ones, are discussed in relation to the leafmovement. (Received August 8, 1985; Accepted November 11, 1985)     

Effects of Inhibitors on the Light-Induced Changes in Electric Potential in Pulvinar Motor Cells of Phaseolus vulgaris L.

To analyze the mechanism of the light-induced changes in electricpotential in motor cells of the pulvinus of Phaseolus vulgarisL., inhibitors were applied to the pulvinus by the xylem perfusionmethod. The membrane potential was –60 to –80 mV,which indicated that the polarization was less than that ofcells of a pulvinus in air. A pulse (30 s) of blue light (BL)induced transient depolarization of the membrane in the motorcells. Red light (RL) caused hyperpolarization of the membrane.The magnitude of BL pulse-induced transient depolarization wasgreater under the hyperpolarized state caused by the RL. The membrane was depolarized to –30 to –40 mV onperfusion with the respiratory inhibitor NaN₃ (1 mM) and a pulseof BL or irradiation with RL did not cause any change in thepotential in the depolarized state. Hyperpolarization of themembrane by RL was inhibited by perfusion with DCMU (50 µM),an inhibitor of electron transport in photosynthesis. However,the magnitude of the depolarization induced by the pulse ofBL was not affected. Perfusion with a proton ionophore CCCP(100µM) depolarized the membrane and no change in thepotential was induced by a pulse of BL or by RL in the depolarizedstate. The extent of the BL pulse-induced depolarization of the membranewas proportional to the magnitude of the membrane potentialat the time of which the pulse of BL was applied. It is suggestedthat the active component of the membrane potential was inhibitedby the pulse of BL. The experimental results further supportthe hypothesis that BL inhibits the activity of the proton ATPaseand, thus, causes loss of the electrogenic component of themembrane potential of the pulvinar motor cells. (Received June 22, 1992; Accepted August 24, 1992)     

Effects of blue light on electrical potential and turgor in pulvinar motor cells ofPhaseolus

Pulvinar motor cells ofPhaseolus vulgaris L display transient depolarization of the membrane potential and a turgor pressure decrease when exposed to a pulse of blue light. To analyze the mechanism of the transient depolarization, the effects of some factors such as anoxia, metabolic inhibitors and specific inhibitors of H⁺-ATPase have been examined. The findings have led to the conclusion that blue light inactivates the electrogenic H⁺-pumping ATPase in the plasma membrane of the motor cells. This inactivation seems to suppress ion uptake and decrease the turgor pressure of the motor cells.     

Effects of Anoxia and Red Light on Changes Induced by Blue Light in the Membrane Potential of Pulvinar Motor Cells and Leaf Movement in Phaseolus vulgaris L.

In the dark, the membranes of the pulvinar motor cells of Phaseolusvulgaris L. were rapidly depolarized under anoxic conditionsand repolarized with re-aeration. This change in potential mayhave been due to suppression by anoxia of a respiration-dependent,electrogenic ion pump in the motor cells. When the pulvinuswas irradiated with blue light (BL) in the depolarized stateunder anoxic conditions, no marked depolarization occurred.Furthermore, a short pulse of BL did not induce transient depolarization.On continuous irradiation with red light (RL), the motor cellunder anoxic conditions showed slow recovery of the depolarizedmembrane potential. When a pulse of BL was superimposed on theRL after the recovery, transient depolarization occurred again. The leaf showed a small downward movement under anoxic conditionsbut recovered with re-aeration. Upward movement of the leafcaused by continuous application of BL to flexor cells changedto a downward movement under anoxic conditions, and re-aerationled to a return to upward movement. Unidirectional irradiationby BL of the flexor side did not cause upward movement of theleaf under anoxic conditions. However, unidirectional irradiationof RL to the flexor side caused downward movement of the leafunder anoxic conditions, which could be changed to upward movementby superimposition of irradiation with BL. The experimentalresults clearly show that BL acts mainly to inhibit (depolarize)an electrogenic component of the membrane potential in pulvinarmotor cells which is dependent on a supply of energy from respirationor photophsphorylation. (Received November 1, 1989; Accepted April 12, 1990)     

Osmoregulation of leaf motor cells

"Osmotic Motors"--the best-documented explanation for plant leaf movements--frequently reside in specialized motor leaf organs, pulvini. The movements result from dissimilar volume and turgor changes in two oppositely positioned parts of the pulvinus. This Osmotic Motor is powered by a plasma membrane proton ATPase, which drives KCl fluxes and, consequently, water, across the pulvinus into swelling cells and out of shrinking cells. Light signals and signals from the endogenous biological clock converge on the channels through which these fluxes occur. These channels and their regulatory pathways in the pulvinus are the topic of this review.     

Change in potassium distribution in a Phaseolus pulvinus during circadian movement of the leaf

Phaseolus moves its leaves upward and downward with circadianperiod. This movement of the leaf results from the differentialchange in the turgor on opposite sides of the pulvinus. Concentrations of K⁺, Na⁺, Mg⁺⁺, and Ca⁺⁺ in the upper and lowerhalves of the pulvinus and the water content of cells on bothsides of it were analyzed in relation to the deformation ofthe pulvinus. The results showed that (1) the pulvinus was deformedby expansion and contraction of the cells on its opposite sides;(2) among the four cations, the K⁺ concentration was markedlyhigh in both halves of the pulvinus; (3) the osmotic pressureof the upper and lower halves were nearly equal during the rhythmicdeformation of the pulvinus; (4) the expansion and contractionof the cells on the opposite sides of the pulvinus have a positivecorrelation only with a change in the K+ concentration expressedin terms of µmoles per mg protein; (5) the concentrationsof other cations such as Na⁺, Mg⁺⁺, Ca⁺⁺, expressed in termsof µmoles per mg protein, did not change during the circadiandeformation of the pulvinus. Thus, the rhythmic K⁺ movementseems to be the basis for pulvinar turgor movements. With respectto the mechanism of K⁺ movement, three possibilities are discussed. (Received November 7, 1975; )     

HELIOTROPIC LEAF MOVEMENT RESPONSE TO H+/ATPase activation,H+/ATPase inhibition,AND K+ CHANNEL INHIBITION IN VIVO

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H⁺/ATPase and TEA-sensitive K⁺ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H⁺/ATPase inhibitors, vanadate and erythrosin-B, and the K⁺ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m^-2 s^-1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin-B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.     

Light-Induced Changes of Electrical Potential in Pulvinar Motor Cells of Phaseolus vulgaris L.

The sealing-off phenomenon of microelectrode tips has oftenbeen observed in plant cells during potential measurements.In motor cells of pulvinus, changes of the sealing-off potentialwere induced by blue light. These light-induced changes of electricalpotential were in the direction opposite to those of the changesin intracellular potential. The results were evaluated and discussedin relation to the changes of the potential difference of themotor cells and turgor movements of the pulvinus. (Received February 2, 1987; Accepted June 29, 1987)     

Rapid plant movements triggered by action potentials

Rapid bendings of the pulvinus inMimosa pudica, of the trap lobes inDionaea muscipula andAldrovanda vesiculosa, and of the tentacle in Drosera are triggered by action potentials in their motor cells. The action potential ofMimosa may be a C1⁻-spike, and that ofDionaea andAldrovanda may be a Ca²⁺-spike. Propagation of action potentials in the petiole or motor organ is thought to be electrotonie, cell-to-cell, transmission. The Ca 2+ release from unidentified organelles in the pulvinus or the Ca²⁺ influx of the cells in the trap with the action current and activation of contractile fibrillar network having ATPase activity in the cytoplasm must be involved in the rapid bending. Contractions of fibrils may open pores in the membrane of the motor cells upon activation. Outward bulk flow of the vacuolar sap through these pores, due to the pressure inside the cell, must result in turgor loss of the motor cells and then the bending of the organ.     

Vanadate and Dicyclohexylcarbodiimide Inhibit the Blue Light-Induced Depolarization of the Membrane in Pulvinar Motor Cells of Phaseolus

Transient depolarization of cell membranes precedes inductionby a pulse of blue light (BL) of pulvinar movement in Phaseolusvulgaris. The depolarization may involve the plasma membraneH⁺-ATPase since vanadate and dicyclohexylcarbodiimide decreasedor almost completely inhibited depolarization. BL may inactivatethe enzyme, thereby decreasing the turgor pressure in motorcells. (Received February 15, 1994; Accepted April 21, 1994)     

A pulse of blue light induces a transient increase in activity of apoplastic K+ in laminar pulvinus of Phaseolus vulgaris L

A pulse of blue light induced both a transient increase in activity of apoplastic K+ and membrane depolarization in laminar pulvinus of Phaseolus vulgaris L. This shows that blue-light-induced net efflux of K+ from motor cells is closely related to membrane depolarization.     

Dynamic aspects of the response of the pulvinus in the leaf of bean plants(Phaseolus vulgaris L.) to photoexcitation

The pulvinus of bean (Phaseolus vulgaris L) responds to unilateral photo-excitation by phototropic curvature. Osmotically active solutes and water are transported from its exposed to the opposite sector of its motor tissue, resulting in differential changes in turgor pressure in these sectors and generation of a trans-pulvinar torque. A null-point approach was used to non-invasively study these dynamic changes in the terminal leaflet of bean. A variable torque was applied perpendicular to the midrib, to restrain laminar movement by precisely and continuously counteracting the generated torque. This equilibrium prevented curvature of the pulvinus and the associated opposite axial changes in volume in the opposite sectors of its motor tissue. The laminar torques measured were used to estimate stresses (changes in turgor pressure) generated within the motor tissue. These stress values were used to derive the corresponding changes in osmotic pressures and in solute concentration. Skotonastically downfolded leaflets were excited with white light to study their combined dynamic response to photonastic and phototropic excitation. Photonastically unfolded (horizontal) leaflets were excited with blue and red light, alone and in combination, to determine the spectral dependence of the dynamic pulvinar responses tophototropic excitation by itself.     

Effects of vanadate, N2 and light on the membrane potential of motor cells and the lateral leaflet movements of Desmodium motorium

The lateral leaflets of Desmodium motorium (Houtt.) Merr. exhibit ultradian up- and down movements, which are paralleled by oscillations of the membrane potential of motor cells in the pulvinus. By different treatments we have tested the hypothesis that both that both oscillation-types are causally related. The reactions of the leaflet movement and the membrane potential were evaluated by the following approaches. (1) Application of vanadate. an inhibitor of the proton pump in the plasmalemma. and N₂ suppressed leaflet movements and finally arrested the leaflet in the lower position. Before the oscillations damped out, a strong lengthening in period was found. This indicates that the pump is part of the ultradian clock. A period lenthening and a final suppression of the rhythm by vanadate was also seen in the extracellular electric potential of the pulvinus. Intracellular recordings in situ showed that vanadate application depolarized the motor cells. (2) Light of high fluence rates diminished the amplitude of the oscillations of the membrane potential of single motor cells and shortened the period. The same effects were observed when monitoring the lateral leaflet movement. The leaflet always moved towards the direction of the light. whether it was applied from the abaxial or from the adaxial part of the pulvinus. (3) When light was applied to the pulvinus of lateral leaflets. which had spontancously stopped moving in an upper position. oscillations were induced transiently. This effect was also found for the membrane potential of motor cells in the pulvinus. - Our results thus provide further evidence that the membrane potential controls the volume state of the motor cells in the pulvinus of lateral leaflets of Desmodium motorium .     

A Simple Instrument for Measuring Leaf Extension in Grasses, and its Application in the Study of the Effects of Water Stress on Maize and Sorghum

The design of a simple instrument to monitor leaf expansionin grasses is described. The instrument was used to comparethe effects of water stress on leaf extension of two cultivarsof maize and sorghum. The effect of withholding water for 3days was an appreciable reduction in the rate of leaf expansionin both plants, particularly during the light period. In well-wateredplants of both species, leaf extension continued at a steadyrate even when leaf turgor fell to around 0.1 MPa. In water-stressedmaize plants, leaf turgor during the light period fell to zeroand leaf growth ceased. When turgor was restored, followingstomatal closure, leaf extension resumed at a slow rate. Inunwatered sorghum plants, leaf turgor remained at a value greaterthan 0.1 MPa but the rate of leaf extension was significantlyreduced. The reduction in leaf turgor in the unwanted plantsresulted partly from an increase in solute potential. Zea mays L, maize, Sorghum bicolor L, leaf expansion, leaf turgor, water stress     

Blue light inactivates plasma membrane H(+)-ATPase in pulvinar motor cells of Phaseolus vulgaris L

Unilateral blue light irradiation induces bending of pulvini of Phaseolus vulgaris towards the source of light. The pulvinar bending is caused by a decrease in turgor pressure of motor cells that are irradiated with blue light. Decrease in the turgor pressure is caused by the net efflux of K(+) and counter anions, accompanying membrane depolarization. In the present study the effect of blue light on the activity of plasma membrane H(+)-ATPase was studied in relation to the membrane depolarization. The activity of the plasma membrane H(+)-ATPase was measured using protoplast suspensions prepared from laminar pulvini from primary leaves. A pulse of blue light under continuous red light irradiation induced both a transient increase in the external pH and transient inhibition of the vanadate-sensitive ATPase. Continuous blue light irradiation under continuous red light irradiation induced both a sustained increase in the external pH and sustained inhibition of the vanadate-sensitive ATPase. These results show that blue light inhibits the activity of the plasma membrane H(+)-ATPase. Inactivation of the plasma membrane H(+)-ATPase supports the membrane depolarization induced by the blue light irradiation.     

Evaluation of a Non-Destructive Method for Measuring Turgor Pressure in Helianthus

The portable instrument described by Heathcote, Etherington,and Woodward (1979) for the non-destructive measurement of turgorpressure was evaluated in Helianthus annuus and Helianthus paradoxus.A good correlation was obtained between turgor pressure measuredwith the instrument and turgor pressure estimated by the pressure-volumetechnique for individual leaves allowed to dry after excision;however, variation in both the intercept and slope of the relationshipoccurred between leaves. Consequently, there was no correlationbetween the output of the instrument for individual leaves andthe turgor pressure of the same leaves estimated by conventionalmethods. Moreover, for a given leaf, the instrument had onlya limited ability to detect temporal variation in turgor pressurewhen compared with turgor pressure calculated from measuredvalues of leaf water potential and leaf osmotic potential. Theinstrument's output was influenced by its proximity to majorveins and by leaf thickness. We conclude that variability inleaf thickness and the presence of large veins limits its usefulnessfor measurement of turgor pressure in Helianthus. Key words: Leaf thickness, Turgormeter, Turgor pressure, Helianthus     

Potassium Flux and Leaf Movement in Samanea saman : I. Rhythmic Movement

Samanea leaflets usually open in white light and fold together when darkened, but also open and dose with a circadian rhythm during prolonged darkness. Leaflet movement results from differential changes in the turgor and shape of motor cells on opposite sides of the pulvinus; extensor cells expand during opening and shrink during closure, while flexor cells shrink during opening and expand during closure but change shape more than size. Potassium in both open and closed pulvini is about 0.4 N. Flame photometric and electron microprobe analyses reveal that rhythmic and light-regulated postassium flux is the basis for pulvinar turgor movements. Rhythmic potassium flux during darkness in motor cells in the extensor region involves alternating predominance of inwardly directed ion pumps and leakage outward through diffusion channels, each lasting ca 12 h. White light affects the system by activating outwardly directed K⁺ pumps in motor cells in the flexor region.     

Changes in the extracellular ion concentration in the main pulvinus of Mimosa pudica during rapid movement and recovery

A decrease in electric resistance and an increase in the extracellularCl^– concentration ([Cl^–]) in the main pulvinus ofMimosa occurred immediately after the action potential of themotor cells. The beginnings of both changes were almost coincidentalwith the beginning of the rapid movement. A remarkable increasein [Cl^–] was seen in the lower half of the pulvinus, butonly a slight increase in the upper half. Release of Cl^–,probably with K⁺; and other ions, from the motor cells impliesan ejection of liquid from the vacuole. When recovery of thepulvinus following rapid movement was fast in light, [Cl^–]decreased to its initial level within 20 min. When the recoverywas slow in darkness, [Cl^–] decreased at a slow rate andmaintained a higher level than its initial one for a long time.Photosynthetic inhibitors delayed recovery and the decreasein [Cl^–] even in light. These facts suggest that the re-entryof ions into motor cells during recovery partially requiresa photosynthetic energy supply. (Received February 12, 1980; )