The Role of Potassium in the Control of Turgor Pressure in a Gas-Vacuolate Blue-Green Alga

The contribution of K⁺ accumulation to cell turgor pressurewas investigated in the gas-vacuolate blue-green alga Anabaenaflos-aquae. The cell turgor pressure, measured by the gas vesiclemethod, drops in cells suspended in culture medium depletedof K⁺ but rapidly rises again, by 100 kPa or more, when K⁺ isresupplied. A similar though rather slower rise in turgor pressureis supported by an equivalent concentration of Rb⁺. The internalK⁺ concentration rose from 66 to 91 mM when K⁺ was suppliedat an external concentration of 0.4 mM. This rise was light-dependent.Greater increases in internal K⁺ concentration and turgor pressureoccurred when K⁺ was supplied at a higher concentration, 3.6mM. In both cases over 60% of the observed turgor pressure risecould be accounted for by accumulation of K⁺. The turgor pressurerise supported by light-stimulated K⁺ uptake can cause collapseof enough of the alga's gas vesicles to destroy its buoyancy.The effect of K⁺ availability on buoyancy regulation by planktonicblue-green algae is discussed.     

The Interrelations of Cell Turgor Pressure, Gas-vacuolation, and Buoyancy in a Blue-green Alga

The increase in pressure required to collapse gas vacuoles onsuspending the cells of the blue-green alga Anabaena flos-aquaein hypertonic sucrose solutions shows the turgor pressure tovary over the range of 265 to 459 KN m^–2 under differentculture conditions. The cell turgor increased at a rate of upto KN m^–2 h^–1 on transferring the alga from lowto high light intensity. This rise appears to be a result ofthe accumulation of photosynthate, as it is dependent on thepresence of carbon dioxide in the gas phase and is inhibitedby DCMU. Experiments using ¹⁴CO² indicate that the increasedrate of photosynthesis during the high light exposure is easilysufficient to account for the observed turgor rise. The rise in turgor can bring about collapse of sufficient ofthe alga's gas vacuoles to destroy its buoyancy. Higher turgorpressures, and consequently a lower degree of gas vacuolationand buoyancy, were maintained when the alga was kept at highlight intensitives for a week and more. The significance ofthis behaviour is discussed in relation to stratification ofplanktonic blue-green algae in natural habitats.     

Lamprothamnium, a Euryhaline Charophyte: I. OSMOTIC RELATIONS AND MEMBRANE POTENTIAL AT STEADY STATE

The charophyte Lamprothamnium papulosum (Wallr.) J. Gr. is foundat salinities varying from nearly fresh water to twice thatof sea water. It can maintain its turgor constant at 302 mosmolkg^–1 (0.73 MPa) when exposed to external osmotic pressuresof 550 to 1350 mosmol kg^–1 (1.3–3.3 MPa). Turgorshows a tendency to rise slightly at lower osmotic pressure(388 mosmol kg^–1 of turgor at 150 mosmol kg^–1 externalosmolality). K⁺ and Cl^– are the main solutes in the vacuole,and are most important in controlling internal osmotic pressure.Mg²⁺, Ca²⁺, and SO^2–₄ are present in significant amountsbut their concentrations do not change with changes in externalsalinity. Na⁺ is present in lower concentration than K⁺, andplays a minor role in regulating turgor. Sucrose is presentin significant concentrations, but changes little with changesin salinity. Two enzymes involved in sucrose metabolism, sucrosephosphate synthetase (EC 2.4.1.14 [EC] ), and sucrose synthetase (EC2.4.1.13 [EC] ) are active in whole cell extracts of Lamprothamnium.As in the fresh water charophytes, Lamprothamnium membrane potentialmay be depolarized (close to E_K) or hyperpolarized, and presumablyof electrogenic origin. Both types of potential are found atall salinities tested.     

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)     

Ionic Currents across the Plasmalemma of Chara inflata Cells: III. WATER-RELATIONS PARAMETERS AND THEIR CORRELATION WITH MEMBRANE ELECTRICAL PROPERTIES

The water-relations parameters of Chara inflata cells were determineddirectly using the micro pressure probe technique. The turgorpressure of cells in artificial pond water (₀ = 0.06 MPa) wasabout 0.65 MPa and the half-time (T_1/2) for water exchange wasabout 6.5 s. The calculated values of the hydraulic conductivity(L_P) were in the range 1–2 ? 10^–6m s^–1 (MPa)^–1.The volumetric elastic modulus () was 32.8 MPa for turgor rangingfrom 0.77 to 0.82 MPa. Large changes in the water-relations parameters and the electricalproperties of the membrane occurred when the turgor was decreasedto low values. These changes included: (i) a decrease in theT_1/2 for water exchange, (ii) an increase in L_P and (iii) depolarizationof the membrane potential difference (V_m). The micro pressure probe, which enabled the turgor pressureof the cell to be altered, was used in combination with thevoltage-clamp technique to determine the relationship betweenK⁺ and Cl^– conductances of the plasmalemma and the cellturgor. The K⁺ conductance increased reversibly as the turgorwas reduced in the range 0 to 0.6 MPa and the Cl^– -conductanceincreased as the turgor was reduced in the range 0.1 to 0.5MPa. It is suggested that these pressure-dependent K⁺ and Cl^–conductances may have a dual role in electrical events and thenon-electrical responses such as changes in the cell volume. Key words: Chara inflata, membrane conductances, ion channels, water-relations parameters     

Control of leaf expansion in sunflower (Helianthus annuus L.) by nitrogen nutrition

Seedlings of Helianthus annuus L. were grown at an initiallyhigh relative nitrate supply rate (0.27 mol N mol N^–1d^–1). The supply was subsequently reduced to a low rate(0.04 mol N mol N^–1 d^–1). The response of leaf areadevelopment to this abrupt decrease in nitrate availabilitywas characterized by following the expansion of the primaryand secondary leaf pairs. The timing of the drop in nitratesupply was when cell division in the epidermis of the primaryleaf pair was largely complete. Reducing the availability ofnitrate had a strong effect on leaf area expansion. The finalleaf size of the primary leaf pair was affected indicating aneffect of nitrate availability on cell expansion. By the endof the experiment the secondary leaf pair was only one-thirdthe area of that on control seedlings. The role of epidermalcell turgor pressure in this growth response was assessed bydirect measurements with a miniature cell pressure probe. Noreduction in cell turgor pressure following the decrease innitrate availability was detected. It is concluded that a reductionin turgor pressure was not responsible for the reduction inleaf area expansion and it is suggested that reduced cell expansionwas due to changes in cell wall properties. Concentrations ofleaf and root abscisic acid increased following the reductionin nitrate availability. Key words: Abscisic acid, cell size, cell turgor pressure, nitrate, nitrogen, relative rate of nitrate supply     

Nitrate Accumulation and its Relation to Leaf Elongation in Spinach Leaves

The leaf elongation rate (LER) of spinach leaves during theday was twice that during the night when grown at a photon fluxdensity of 145 µmol m^–2 s^–1. All leaves showedthe same LER-pattern over 24 h. Due to low turgor, LER was lowin the afternoon and in the first hours of the night until wateruptake restored full turgor. Osmotic potential remained constantdue to increased nitrate uptake and starch degradation in thisperiod. LER increased to high rates in the second part of thenight and in the morning. The lower rate in the dark comparedto the light was not caused by the lower night temperatures,as increased photon flux density during growth resulted in equalrates in the light and the dark. Increased relative humiditydecreased LER and afternoon rates were most sensitive to waterstress. A ‘low light’ night period did not changeLER-pattern during the night or on the following day. We concludethat nitrate is not an obligatory osmoticum during the nightand can be exchanged for organic osmotica without decreasingLER. During the night the turgor is first restored by increasingwater uptake, nitrate uptake and starch degradation. This resultedin increased leaf fresh weight in this period. Thereafter, elongationincreased by simultaneous uptake of nitrate and water. Nitrateconcentration was, therefore, constant in the older leaves.In the younger leaves nitrate concentration increased to replacesoluble carbohydrates. The vacuoles of the old leaves were filledwith nitrate before those of the young leaves. Key words: Spinacia oleracea L., nitrate accumulation, osmotic potential, organic acids     

Involvement of a Calcium-Dependent Protein Kinase in Hypoosmotic Turgor Regulation in a Brackish Water Characeae Lamprothamnium succinctum

Calcium ion is a key messenger in turgor regulation of internodalcells of Lamprothamnium succinctum in response to hypoosmotictreatment. An increase in the concentration of cytosolic freecalcium ion ([Ca²⁺]_c) is prerequisite for the turgor regulation[Okazaki and Tazawa (1990) J. Membr. Biol. 114: 189], We examinedwhether or not a calcium-dependent protein kinase (CDPK) isinvolved in the Ca²⁺-mediated turgor regulation of Lamprothamniumcells. A 53-kDa CDPK which phosphorylated preferentially histoneH1 but poorly myelin basic protein or casein, was detected inthe cell extract of Lamprothamnium by an in-gel protein kinaseassay. This protein kinase was detected by Western blottingand was immunoprecipitated using an anti-Dunaliella tertiolectaCDPK antibody which can neutralize the Dunaliella CDPK activity[Yuasa et al. (1995) Plant Cell Physiol. 36: 699]. The 53-kDaCDPK was partially purified from Lamprothamnium and its activitywas shown to be inhibited by the antibody and K-252a, a proteinkinase inhibitor. Microinjection of the antibody into the cytosblof Lamprothamnium cells inhibited the decrease in turgor pressurein response to hypoosmotic treatment. However, a transient increasein [Ca²⁺]_c, which was suggested by a transient reduction ofthe velocity of cytoplasmic streaming, was induced in antibody-injectedcells after hypoosmotic treatment. Turgor regulation upon hypoosmotictreatment was inhibited when the cells were treated with K-252a.These results imply that CDPK of Lamprothamnium functions ata down-stream position of Ca²⁺-mobilization in processing turgorregulation in response to hypoosmotic treatment. ² These authors contributed equally to the work.     

Red and blue light-stimulated proton efflux by epidermal leaf cells of the Argenteum mutant of Pisum sativum

Light stimulates leaf expansion in dicotyledons by increasingapoplastic acidification, cell wall loosening and solute accumulationfor turgor maintenance. Red and blue light enhance growth viadifferent photo-systems, but the cellular location and modesof action of these systems is not known. Here, the effect of red and blue light was studied on transportprocesses in epidermal cells of expanding leaves of the Argenteummutant of Pisum satlvum. Both red and blue light caused extraceiiuiaracidification by isolated epidermal tissue, which was stimulatedby extracellular K⁺ and inhibited by DCCD at 0.1 mol m^–3.Acidification induced by red compared with blue light showeddifferent saturating kinetics in fluence rate-response curves.Under near saturating light conditions the effects of red andblue light were additive. The red light-induced acidificationwas inhibited by far-red light while the blue light-inducedacidification was not. Light caused a hyperpoianzation of themembrane potential in epidermal strips, and stimulated ⁸⁶Rb⁺uptake by epidermal protoplasts. These results show that phytochromeand an additional blue light-photoreceptor function in isolatedepidermal cells to promote proton efflux, hyperpolarization,and cation uptake. Key words: Pisum sativum, light-induced acidification, ion transport, epidermis, photoreceptor     

Turgor Pressure and Phototropism in Sinapis alba L. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropismin Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls.Phototropism was induced by adding 0.27 µmol m^–2s^–1 unilateral blue light to a background of low pressuresodium (SOX) lamp light. Curvatures of some 6° from thevertical were reached by 60 min, the curvature rate between20 min and 60 min being 0.14° min^–1. From the axialgrowth rate and tissue geometry the local growth rates of illuminatedand shaded sides of the hypocotyl were calculated to be 1.5and 4.5 µmin^–1 respectively. Turgor pressures ofexpanding cells in control plants and in the shaded and illuminatedsides of the blue light illuminated hypocotyls were measuredto be 0.40-0.55 MPa with a pressure probe. No changes in turgorpressure were observed on initiation of curvature. The decayof pressure in the cells of non-transpiring plants followingexcision indicated that the yield stress threshold of the tissuemay be as low as 0.1 MPa. These results indicate that the phototropicgrowth response in this tissue is not mediated by changes inturgor pressure. Key words: Sinapis alba L., phototropism, turgor pressure     

Lamprothamnium, a Euryhaline Charophyte: IV. MEMBRANE POTENTIAL, IONIC FLUXES AND METABOLIC ACTIVITY DURING TURGOR ADJUSTMENT

The early time-course of turgor adjustment following a hyper-or hypo-osomotic shock was examined in the brackish-water charophyteLamprothamnium papulosum. The response to a reduction in turgorwas a five to seven-fold stimulation of the influxes of Cl^–,K⁺ and Na⁺. The distribution of radioactive tracers in the cellsuggested that the ionic composition of the cytoplasm was strictlycontrolled during turgor adjustment. Metabolic activity wasrelatively unaffected by the loss of turgor. high fluxes throughthe cytoplasm, and a cytoplasmic K^– concentration possiblyas high as 280 mol m^–3. Osmotic adjustment to a lower salinity was achieved by largeincreases in the passive effluxes of K⁺ and Cl^– ratherthan by decreases in their influxes. The membrane remained hyperpolarized during hyperosmotic adjustmentbut depolarized after a hypo-osmotic change. This result isdiscussed in relation to changes in the driving forces for ionmovements during osmotic transitions. Key words: Lamprothamnium, Turgor, Osmotic stress     

Measurement of Yield Threshold and Cell Wal Extensibility of Intact Wheat Roots under Different Ionic, Osmotic and Temperature Treatments

Yield stress threshold (Y) and volumetric extensibility () arethe rheological properties that appear to control root growth.In this study they were measured in wheat roots by means ofparallel measurement of the growth rate (r) of intact wheatroots and of the turgor pressures (P) of individual cells withinthe expansion zone. Growth and turgor pressure were manipulatedby immersion in graded osmoticum (mannitol) solutions. Turgorwas measured with a pressure probe and growth rate by visualobservation. The influence of various growth conditions on Yand was investigated; (a) At 27 °C.In 0.5 mol m^–3 CaCl₂ r, P, Y and were20.7±4.6 µm min^–1, 0.77±0.05 MPa,0.07±0.03 MPa and 26±1.9 µm min^–1MPa^–1 (expressed as increase in length), respectively.Following 24 h growth in 10 mol m^–3 KC1 these parametersbecame 12.3±3.5 µm min^–1, 0.72±0.04MPa, 0.13±0.01 MPa and 21±0.7 µm min^–1MPa^–1. After 24 h osmotic adjustment in 150 mol m^–3mannitol/0.5 mol m^–3 CaCl₂ r= 19.6±4.2 µmmin^–1, P = 0.68±0.05 MPa and Y and were 0.07±0.04MPa and 30±0.2 µm min^–1 MPa^–01, respectively.After 24 h growth in 350 mol m^–3 mannitol/0.5 mol m^–3CaCl₂ r= 13.3±4.1 µm min^–1, P= 0.58±0.07MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min^–1MPa^–1. During osmotic adjustment in 200 mol m^–3mannitol/0.5 mol m^–3 CaCl₂, with or without KCl, the recoveryof growth rate corresponded to turgor pressure recovery (t1/2approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influencedthe growth parameters which became r= 8.3±2.8 um min^–1,P=0.78 MPa, r=<0.2 MPa and =15±0.1 µm min^–1MPa^–1. Therefore, Y and are influenced by 10 mol m^–3 K⁺ ionsand low temperature. In each case the effective pressure forgrowth (P-Y) was large indicating that small fluctuations ofsoil water potential will not stop root elongation. Key words: Yield threshold, cell wall extensibility, wheat root growth, temperature, turgor pressur     

Injection of a Ca2+-Chelating Agent into the Cytoplasm Retards the Progress of Turgor Regulation upon Hypotonic Treatment in the Alga, Lamprothamnium

The turgor regulation induced by hypotonic treatment (hypotonicturgor regulation) in the brackish-water alga Lamprothamniumsuccinctum is accompanied by a transient increase in the electricalconductance of the membrane, membrane depolarization and a transientincrease in the cytoplasmic concentration of free Ca²⁺ ([Ca²⁺([Ca²⁺]_c) (Okazaki and Tazawa 1990). In the present study, weloaded a Ca²⁺-chelating agent, EGTA, into the cytoplasm by mechanicalinjection in order to suppress the increase in [Ca²⁺]_c thatoccurs during the hypotonic turgor regulation. The rate of thecytoplasmic streaming was taken as an indirect indicator of[Ca²⁺]_c, since cytoplasmic streaming has been shown to be inhibitedby high [Ca²⁺]_c in Lamprothamnium cells. The lag time for theinhibition of the cytoplasmic streaming upon hypotonic treatmentwas significantly prolonged in EGTA-loaded cells as comparedto that in intact cells. This result indicates that the loadedcytoplasmic EGTA functioned as a buffer of Ca²⁺ to retard theincrease in [Ca²⁺]_c. It took a longer time for the membraneconductance to reach the peak value in EGTA-loaded cells thanin intact cells. Membrane depolarization was affected to aninsignificant extent by the cytoplasmic EGTA. The regulationof turgor pressure itself was partially inhibited. These resultsstrongly support the idea that the net efflux of ions that occursduring the recovery from hy-potonically induced changes in turgorpressure is controlled by [Ca²⁺]_c. (Received August 22, 1990; Accepted December 6, 1990)     

Auxin Changes Both the Extensibility and the Yield Threshold of the Cell Wall of Vigna Hypocotyls

Elongation of plant stem is governed by two simultaneous processes:irreversible yielding of the cell wall and uptake of water.Among many candidates for the parameters that regulate and/or restrict growth, we focused on the mechanical propertiesof the cell wall and determined those parameters that governthe process of IAA-induced growth by means of the pressure-jumpmethod combined with the pressure-probe technique. The elongation growth of segments excised from the elongationzone of Vigna hypocotyls was accelerated by xylem perfusionwith 10^–4 M IAA. During the promotion of growth, boththe extensibility () of the cell wall and the effective turgor(Pⁱ–Y) increased while only a little or no change in theintracellular pressure (Pⁱ) occurred. These results indicate that IAA increases not only the extensibilityof the cell wall but also the effective turgor, i.e., the drivingforce for yielding of the cell wall. However, the driving forceis not increased by the increase in Pⁱ but by the decrease inthe yield threshold (Y). These results suggest that Y is adjustableduring the regulation of growth. ¹Present address: Department of Biology, Faculty of Science,Okayama University, Okayama, 700 Japan (Received September 20, 1990; Accepted November 27, 1990)     

The Effects of Temperature and Light Intensity on Embryo Numbers in Wheat Doubled Haploid Production through WheatxMaize Crosses

Triticum aestivumxZea mayscrosses are now widely used in theproduction of wheat doubled haploids to produce homozygous lines.Seasonal effects are known to influence the number of haploidembryos produced through wheatxmaize crosses, but the effectsof temperature and light have not been quantified. This studyinvestigated the effect of temperature and light intensity onhaploid embryo production. New Zealand wheat cultivars weregrown in a glasshouse until booting when they were transferredto growth cabinets at three temperatures (day/night; 17/12,22/17 or 27/22 °C at an irradiance of 250 µmol m^-2s^-1PAR).In another experiment, wheat lines were transferred to a growthcabinet at one of three light intensities (300, 500 or 1000µmol m^-2s^-1PAR at 22/17 °C day/night, with a photoperiodof 16 h). The temperature and light intensity at which pollinationswere made and subsequent fertilisation and embryo developmentoccurred, significantly (P<0.01) influenced the frequencyof haploid embryo production. The optimal temperature for embryorecovery was 22/17 °C. The greatest number of embryos wasproduced at a light intensity of 1000 µmol m^-2s^-1. Thesefindings will result in improvements in the overall efficiencyof the wheatxmaize system for wheat doubled haploid production.Copyright1998 Annals of Botany Company Intergeneric crossing, temperature, light intensity,Triticum aestivum,wheat,Zea mays,maize.     

Cell turgor, osmotic pressure and water potential in the upper epidermis of barley leaves in relation to cell location and in response to NaCl and air humidity

Previous single-cell studies on the upper epidermis of barleyleaves have shown that cells differ systematically in theirsolute concentrations depending on their location relative tostomatal pores and veins and that during NaCl stress, gradientsin osmotic pressure () develop (Fricke et al., 1995, 1996; Hinde,1994). The objective of the present study was to address thequestion to which degree these intercellular differences insolute concentrations and it are associated with intercellulardifferences in turgor or water potential (). Epidermal cellsanalysed were located at various positions within the ridgeregions overlying large lateral or intermediate veins, in thetrough regions between those veins or in between stomata (i.e.interstomatal cells). Turgor pressure of cells was measuredusing a cell pressure probe, and of extracted cell sap wasdetermined by picolitre osmometry. For both large and intermediatelateral veins, there were no systematic differences in turgorbetween cells located at the base, mid or top of ridges, regardlessof whether plants were analysed at low or high PAR (10 or 300–400µmol photons m^–2 s^–1). However, turgor withina ridge region was not necessarily uniform, but could vary byup to 0.14 MPa (1.4 bar) between adjacent cells. In 60 out of63 plants, turgor of ridge cells was either slightly or significantlyhigher than turgor of trough (lowest turgor) or interstomatalcells (intermediate turgor). The significance and magnitudeof turgor differences was higher in plants analysed under highPAR or local air flow than in plants analysed under low PAR.The largest (up to 0.41 MPa) and consistently significant differencesin turgor were found in plants treated for 3–9 d priorto analysis with 100 mM NaCl. For both NaCl-treated and non-treated(control) plants, differences in turgor between cell types weremainly due to differences in since differences in were negligible(0.01–0.04 MPa). Epidermal cell , in NaCl-treated plantswas about 0.38 MPa more negative than in control plants dueto higher . Turgor pressures were similar. Following a suddenchange in rooting-medium or air humidity, turgor of both ridgeand trough cells responded within seconds and followed the sametime-course of relaxation. The half time (T_1/2) of turgor relaxationwas not limited by the cell's T_1/2 for water exchange. Key words: Barley leaf epidermis, cell turgor, heterogeneity, NaCl stress, osmotic pressure, water potential     

Mechanism of Photoregulated Carotenogenesis in Rhodotorula minuta I. Photocontrol of Carotenoid Production

Rhodotorula minuta cells, which have only traces of carotenoidswhen grown in the dark, started carotenoid production with theonset of illumination and the amount increased almost linearlyuntil 70 hr then remained constant thereafter when incubationwas continued under illumination, with the number of cells continuingto increase. The rate of carotenoid production [Vc (µgg^–1 hr^–1)] depended on the intensity of light [I(ergcm^–2 sec^–1)], with the relationship of Vc=0.74 logI–1.46. The final carotenoid content [C(µg g^–1)]of cells incubated under continuous light was also controlledby the light intensity [I], with the relationship of C=52 logI–81. Control of carotenoid production by light occursas a two-phase process consisting of a temperatureindependentphotochemical reaction and light-independent biochemical reactions. (Received September 12, 1981; Accepted February 20, 1982)     

Turgor Regulation in a Brackish Charophyte, Lamprothamnium succinctum II. Changes in K$, Na$ and Cl- Concentrations, Membrane Potential and Membrane Resistance during Turgor Regulation

A brackish Characeae, Lamprothamnium succinctum, regulates intracellularosmotic pressure in response to changes in the external salinityand keeps the turgor pressure constant. The osmotic pressureof the vacuole was found to be mostly due to K^$, Na^$ and Cl^–.But in the cytoplasm, the sum of their concentrations was muchlower than the cellular osmotic pressure. Electroneutralitywas maintained among the analyzed inorganic ions in the vacuolebut a strong anion deficiency was detected in the cytoplasm,supporting the existence of organic anions to balance excesspositive charges. During turgor regulation, concentrations of inorganic ions inthe vacuole changed just enough to accommodate the osmotic pressurechange, while those in the cytoplasm remained almost constant.Since the cytoplasmic volume was almost constant during turgorregulation, some organic molecule(s) may have contributed tothe osmoregulation of the cytoplasm. The membrane potential and resistance at steady state underdifferent salinities were almost constant. Hypotonic treatmentcaused a sudden depolarization of the membrane potential anda drastic decrease in membrane resistance. Hypertonic treatmentcaused a slow hyperpolization of membrane potential but didnot significantly affect the membrane resistance. The energeticsof K^$ and Cl^– movements across the plasma membrane isdiscussed based upon the electrochemical potential gradients. (Received November 28, 1983; Accepted March 14, 1984)     

Cloning of the Gene Encoding a Protochlorophyllide Reductase: the Physiological Significance of the Co-Existence of Light-Dependent and -Independent Protochlorophyllide Reduction Systems in the Cyanobacterium Plectonema boryanum

Cyanobacteria have two protochlorophyllide (Pchlide) reductasescatalyzing the conversion of Pchlide to chloro-phyllide, a keystep in the biosynthetic pathway of chlorophylls (Chls); a light-dependent(LPOR) and a light-independent (DPOR) reductase. We found anopen reading frame (ORF322) in a 2,131-bp EcoRI fragment fromthe genomic DNA of the cyanobacterium Plectonema boryanum. Becausethe deduced amino acid sequence showed a high similarity tothose of various plant LPORs and the LPOR activity was detectedin the soluble fraction of Esche-richia coli cells over-expressingthe ORF322 protein, ORF322 was defined as the por gene encodingLPOR in P. boryanum. A por-disrupted mutant, YFP12, was isolatedby targeted mutagenesiss to investigate the physiological importanceof LPOR. YFP12 grew as well as wild type under low light conditions(10-25 µE m^–2 S^–1). However, its growth wassignificantly retarded as a result of a significant decreasein its Chl content under higher light conditions (85-130 µEm^–2 s^–1). Furthermore, YFP12 stopped growing andsuffered from photobleaching under the highest light intensity(170 µE m^–2 s^–1). In contrast, a chlL-dis-rupted(DPOR-less) mutant YFC2 grew as well as wild type irrespectiveof light intensity. From these phenotypic characteristics, weconcluded that, although both LPOR and DPOR contribute to Chlsynthesis in the cells growing in the light, the extent of thecontribution by LPOR increases with increasing light intensity;without it, the cells are unable to grow under light intensitiesof more than 130 µ Em^–2s-^–. (Received September 26, 1997; Accepted November 21, 1997)     

Modelling time-series of photosynthesis and comparisons with the fluorescence yield in Chlorella vulgaris: a study of adaptation, inhibition and recovery

Time-series of ¹⁴C uptake and fluorescence yield (i.e., thefluorescence enhancement after addition of the photosyntheticinhibitor DCMU) were measured in Chorella vulgaris at variouslight intensities. Adaptation and recovery processes after alterationof the light intensity were also studied. At a constant lightintensity, both the rate of ¹⁴C uptake and the fluorescenceyield decreased with time. Comparison of time-series data of¹⁴C uptake at different light intensities showed that this phenomenonconsisted of several processes (i.e., at low light intensitiessmall changes in uptake rates were mainly due to photoadaptation,while at higher light intensities relatively larger changesoccurred, as result of photoinhibition). Transfer of an algalsample to low light intensities after a period of exposure toinhibiting light intensities resulted in an exponential recoveryof the ¹⁴C uptake rate with time, coupled with an exponentialrecovery of the fluorescence yield. A mechanistic model is presented,which describes the algal ¹⁴C uptake rate as a function of timeand light intensity. The model includes adaptation, inhibitionand recovery. Six parameters, characterising the algal suspension,have to be estimated from the results of one P versus I curveand one time-series ¹⁴C uptake, which includes a period of recovery.Using these parameters the model can predict the time-courseof ¹⁴C uptake at every constant light intensity, as well aswhen the light intensity is changed during the experiment. Whenapplied to a culture of C. vulgaris, the theoretical valuesclosely approach the actual measurements. The resemblance betweenthe measured time-series of fluorescence yield and the rateof ¹⁴C uptake indicates, that the changes in the rate of ¹⁴Cuptake are due to changes in the photosynthetic apparatus, ratherthan to changes of diffusion of ¹⁴C into the cell.