Role of light, carbon dioxide and nitrogen in regulation of buoyancy, growth and bloom formation of Anabaena flos-aquae

The availability of light, CO₂ and NH₄-N interacted to controlbuoyancy and growth of the gas vacuolate blue-green alga, Anabaenaflos-aquae. At high light intensities algal growth rates werehigh; however, the alga was non-buoyant regardless of the availabilityof CO₂ or NH₄-N. The mechanism for buoyancy loss involved increasedcell turgor pressures at higher light intensities which resultedin collapse of gas vacuoles. At lower light intensities algalgrowth rates and cell turgor pressures were reduced and buoyancywas controlled by the availability of CO₂ and inorganic nitrogen.Carbon dioxide limitation increased buoyancy, while reducedinorganic nitrogen availability reduced buoyancy. Mechanismsfor buoyancy regulation at low light intensities involved changesin cellular C/N ratios which appeared to affect the rate ofsynthesis and accumulation of protein-rich gas vacuoles. Algalspecific growth rates were combined with buoyancy data to forma single index (µ_bloom) to the rate of surface bloom formationof A.flos-aquae as a function of the availability of light,CO₂ and NH₄-N. The bloom formation index was enhanced with decreasedavailability of light and CO₂, and increased availability ofNH₄-N.     

Diel Buoyancy Changes by the Cyanobacterium Aphanizomenon ovalisporum from a Shallow Reservoir

In the summer of 1999, a bloom (11 100 filaments ml^–1)of the gas vacuolate cyanobacteriumAphanizomenon ovalisporumdeveloped in a shallow (1.7 m deep) reservoir containing nutrient-enrichedwater from Lake Kinneret (Israel). During 4 days, A. ovalisporumshowed a marked diel periodicity in buoyancy: the proportionof floating filaments fluctuated between 76–84% from middayto evening and 94–98% at the end of the night, in bothsurface and bottom samples. Buoyant filaments were present throughoutthe water column, presumably due to wind-driven vertical mixing.Aphanizomenonfilaments collected from the reservoir were maintained undermean photon irradiances of 15 (LL), 150 (ML) and 1100 (HL) µmolm^–2 s^–1 in a computer-controlled set-up, which simulatedthe diel light changes at different depths in the reservoir.In the LL cultures, filament buoyancy showed no diel fluctuationpatterns during the 4 days of incubation, but ML and HL culturesshowed regular diel changes, with a higher proportion of filamentsfloating at the end of the night than during midday–evening.There was no evidence for either turgor-driven collapse of gasvesicles or dilution of gas vesicles by cell growth by any ofthe treatments. Gas vesicles of A. ovalisporum had a relativelylow mean critical pressure (p_c of 0.57 MPa), but the daytimerise in turgor pressure was too small to cause gas vesicle collapse.The observed diel buoyancy changes may be explained by accumulationof carbohydrate ballast during the day and decrease during thenight.     

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 properties and buoyancy-providing role of gas vacuoles in Trichodesmium Ehrenberg

All three species of the marine blue-green alga Trichodesmium collected in the Sargasso and Caribbean seas were found to possess gas vacuoles. The constituent gas vesicles were much stronger than those found in any freshwater blue-green alga, the mean critical collapse pressures being 12 bars in T. erythraeum, 34 bars in T. contortum and 37 bars in T. thiebautii. This great strength is obviously an adaptation to the hydrostatic pressures at the depths to which these organisms occur in the ocean. In each case the gas vesicles are far too strong to be collapsed by rising cell turgor pressure, though gas-vacuolation could be slowly regulated by the differential growth of gas vesicles and cells. Since the vesicles are of a similar shape and size to those in other species, the vesicle wall material must be stronger. The majority of Trichodesmium colonies collected were positively buoyant, and in all cases tested the buoyancy was dependent on the presence of gas vacuoles. The buoyancy is important in increasing the residence time of these slowly growing algae in the euphotic zone and it is responsible for the surface water-blooms which they form.     

Radial Turgor Pressure Profiles in Growing and Mature Zones of Wheat Roots--A Modification of the Pressure Probe

A modification of the pressure probe is described which allowsaccurate routine recording of the turgor pressure of singlecells at measured depth within a tissue. Measurements of radial profiles of turgor pressure in wheatroots grown in some simple salt solutions (0.5 mol m^–3CaCl₂, 0.5 mol m^–3 CaCI₂ plus 10 mol m^–3 NaCl, and0.5 mol m^–3 CaCl₂ plus 10 mol m^–3 KCI), are described.Turgor pressure was constant (approximately, 0.65 MPa) alonga radius within the elongation zone irrespective of the natureof the bathing solution. In mature root tissue turgor pressurein the cortex was lower than that of the growing zone in alltreatments and the pressure of the stele was on average 0.22MPa higher than that of the cortex. Potassium in the mediumbathing the root increased the turgor pressure in mature root(both cortex and stele) relative to low salt and sodium treatments. The results are discussed in relation to both root growth andion accumulation. Key words: Pressure probe, wheat roots, salt solution     

X-Ray Microanalysis and Ultrastructural Studies of Cell Compartments of Dunaliella parva

Hajibagheri, M. A., Gilmour, D. J., Collins, J. C. and Flowers,T. J. 1986. X-ray microanalysis and ultrastructural studiesof cell compartments of Dunaliella parva. -J. exp. Bot. 37:1725–1732. Ultrastructural studies of the unicellular green alga Dunaliellaparva showed the presence of cytoplasmic vacuoles. X-ray microanalysiswas performed on sections of cells which had been freeze substitutedin acetone. It was found that the concentrations of both Naand Cl were much higher in the vacuoles than in the cytoplasm.When cells were grown in 0·4 kmol m^–3 NaCl theNa and Cl concentrations in the vacuoles were 349 and 283 molm ^–3 respectively, whilst cytoplasmic Na and Cl concentrationswere 37 and 26 mol m^–3. Corresponding values for cellsgrown in 1·5 kmol m^–3 NaCl were 392 mol m^–3Na and 325 mol m^–3 Cl in the vacuoles and 36 mol m^–3Na and 30 mol m^–3 Cl in the cytoplasm. Immediately afterexposure to an increase in external salinity Na and Q concentrationsincreased in both vacuoles and cytoplasm. The results are discussedwith reference to compartmental models for the ionic relationsof Dunaiiella. Key words: X-ray microanalysis, ultrastructural studies, Dunaliella parva     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. II. Phloem in the Stem

Confirming a previous analysis by Lang (1974), it is concludedthat in tree trunks, phloem turgor and turgor gradients maybe estimated from osmotic pressure and osmotic-pressure gradients,respectively. The present analysis is an improvement becauseit is based on observed osmotic-pressure gradients rather thansupposed turgor gradients, and allowance is made for sucroseunloading and gradients of external water potential. It is concludedthat the rate of sucrose unloading in tree trunks must be lessthan 50 nmol m^–2 S m^–1. In small plants, higherrates of unloading (100 nmol m m^–2 S m^–1) and steeperconcentration gradients will lead to larger errors, but turgorpressures can still be estimated with acceptable accuracy. Oneshould be more cautious when considering turgor gradients insmall plants, although it seems likely that reasonable estimateswill still be obtained. Assuming plasmodesmatal transport throughan unconstricted cytoplasmic annulus, it is concluded that thesieve elements and their associated cells will sustain verysimilar turgor and osmotic pressures. Convection and diffusioncan both contribute significantly to plasmodesmatal sucroseunloading. Similarly, the plasmodesmatal volume flux will reflecta combination of pressure flow and osmosis. Water fluxes acrossthe sieve element plasmalemma and through the plasmodesmatacan be in opposite directions. It may be possible to assessthe extent of hydraulic coupling between the sieve elementsand their associated cells from studies of phloem water relations Phloem, turgor, osmotic pressure, plasmodesmata, phloem unloading, Munch hypothesis     

Regulation of Turgor Pressure by Suspension-Cultured Tobacco Cells

Turgor regulation and effects of high NaCl and water deficiton growth and internal solutes were studied after transferringtobacco cells from control culture medium (osmotic pressure= 0.13–0.15 MPa at time of transfer) to culture mediumcontaining either 82 mol m^–3 NaCl or 150 mol m^–3melibiose (osmotic pressure of media = 0.62 MPa). Followingtransfer to media with higher osmotic pressure, expansion rateand turgor pressure were reduced. Within 24 h of imposing thewater deficit, expansion rate had returned to that of cellsin control culture medium. However, by 24 h, turgor pressurehad only risen from 0.2 MPa to 0.65 MPa in the NaCl treatmentand to 0.53 MPa in the melibiose treatment, while it was 0.73MPa in the control treatment. Furthermore, turgor pressure remainedwithin 0.05 MPa of these respective values for the rest of the(75 h) experiment. These results suggest differences in bothcell wall properties (extensibility and/or threshold turgor)and the level at which turgor is maintained for cells in thevarious treatments. Solutes contributing nearly all (82–97%) of the osmoticpressure in cells were identified. The initial (up to 24 h)increases in turgor pressure were mainly due to increases insolute concentrations caused by relatively slow expansion rates.However, increased Na⁺ and Cl ^– uptake contributed toincreased turgor pressure in the NaCl treatment and caused turgorpressure of cells in this treatment to increase faster thanin the melibiose treatment. Likewise, expansion rate rose morequickly in the NaCl than in the melibiose treatment. After 24h, maximum expansion rate was reached and concentrations ofmost internal solutes began to decrease. Nevertheless, turgorpressure remained relatively constant. The constancy of turgorpressure was due to increased glucose uptake rates relativeto controls, with consequent increases in concentrations ofsucrose, glucose and fructose and, in cells in the melibiosetreatment, of organic acids. Glucose uptake was slower in theNaCl than in the melibiose treatment but higher turgor pressurewas maintained in the NaCl treatment due to high uptake of Na⁺and Cl^–. Glucose uptake appears to respond to a systemof turgor regulation, but further experiments are required toconfirm this and to determine whether Na⁺ and Cl^– uptakealso respond to a system of turgor regulation. Key words: Salinity, water deficit, growth     

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     

Effects of macronutrients upon buoyancy regulation by metalimnetic Oscillatoria agardhii in Deming Lake, Minnesota

Gas-vacuolate filaments of Oscillatoria agardhii form a metalimneticlayer in Oeming Lake, Minnesota. The environmental factors whichaffect buoyancy and the physiological processes which mediatechanges in buoyancy were determined. Buoyant filaments losttheir buoyancy in a few hours when incubated at light intensitiesabove those found in situ ({small tilde}15 µnol photonsm^–2 s^–1, or 1% of the surface value). The rate ofbuoyancy loss was accelerated by the addition of 10 µMphosphate at irradiances >200mol photons m^–2 s^–1.The effect of nutrient additions on buoyancy was also investigatedover a longer time period by incubating metalimnetic samplesin situ. The samples were deployed for 6 days at a depth wherethe irradiance was 8% of the surface value. As found in short-termexperiments, the addition of phosphate resulted in the largestdecrease in buoyancy. However, the addition of ammonia in additionto phosphate attenuated the buoyancy loss on day 2, and on day6 the filaments in these treatments were almost completely buoyant.The physiological status of the filaments in these treatmentswas assayed by analysis of elemental ratios of C, N and P, andby measurement of cellular chlorophyll, polysaccharide and protein.In addition, the cellular content of gas vesicles was determined.The construction of ballast balance sheets from these data indicatedthat changes in buoyancy were primarily due to differences inthe amount of polysaccharide ballast in the cells. However,in another set of in situ experiments, the increase in measuredballast molecules did not explain the observed loss of buoyancy.We hypothesized that another, undetected ballast-providing moleculehad accumulated in the cells.     

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.     

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     

Exposure of Dunaliella salina to Low Temperature Mimics the High Light-Induced Accumulation of Carotenoids and the Carotenoid Binding Protein (Cbr)

We report that growth of Dunaliella salina at either 13°C/150µmol m^–2s^–1 or 30°C/2,500 µmol m^–2s^–1 results in the accumulation of comparable levels ofcarotenoids and the zeaxanthin-binding protein, Cbr. We concludethat carotenoid and Cbr abundance in this green alga respondto changes in PSII ‘excitation pressure’ ratherthan to high light per se. (Received September 19, 1996; Accepted November 20, 1996)     

The critical pressures of gas vesicles in Planktorhrix rubescens in relation tothe depth of winter mixing in Lake Zrich, Switzerland

The vertical distribution of the cyanobacterium Planktothrir(Oscillazoria) rubescens in Lake Zrich was investigated fromMarch 1993 to June 1995 by collecting filaments on filters andmeasuring them by epifluorescence microscopy and computer imageanalysis. The initial population, which began to stratify inApril, decreased by up to 99% by June. During the summer, thepopulation peaked at depths of 8–15 m; it reached a maximumareal filament-volume concentration of -60 cm ^–3 of lakesurface in early September and was then entrained in the deepeningsurface layer. It became mixed progressively deeper, to thelake bottom in the cold winter of 1993–94, but less completelyin the milder winter of 1994–95. Most of the filamentsremained viable during the winter. At the end of the mild winterof 1994–5, 70% of filaments in the water column retainedbuoyancy, but after the cold winter of 1996–7 only 22%were buoyant. Few remained buoyant below 80 m, where the hydrostaticpressure caused gas vesicle collapse. The proportion that remainbuoyant decreases with the depth and duration of winter mixing,and increases with the critical collapse pressure (P_c) of thegas vesicles, which provide buoyancy. Strains of P.rubescensisolated from Lake Zrich differed in mean (P_c) of their gasvesicles, from 0.9 to 1.1 MPa, the highest values in freshwatercyanobacteria. Allowing for a turgor pressure of 0.2 MPa. thesestrains would remain buoyant at depths down to 70 and 90 m,respectively. Natural selection for gas vesicles of high (P_c)will operate by increasing the proportion of filaments thatremain buoyant in the upper parts of the water column aftercirculation to various depths during the winter because onlybuoyant filaments will form the inoculum for the following season.     

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     

Control of Wheat Root Elongation Growth: I. EFFECTS OF IONS ON GROWTH RATE, WALL RHEOLOGY AND CELL WATER RELATIONS

Pritchard, J., Tomos, A. D. and Wyn Jones, R. G. 1987. Controlof wheat root elongation growth. I. Effects of ions on growthrate, wall rheology and cell water relations.—J. exp.Bot. 38: 948–959. The nature of the ions in the bathing medium of hydroponicallygrown wheat seedlings strongly influenced root growth rate.In 0·5 mol m^–3 CaSO₄ the growth rate was 32 mm24 h^–1 (used as 100% control rate). K⁺ and SO^– ions(10 mol m^–3) each inhibited extension growth (to about40% and 70% of the control value respectively). In the absenceof K⁺, Cl^– greatly reduced the inhibition due to SO₄^2–.Measurement of tissue plasticity and elasticity in the expandingzone with an Instron-type tensiometer indicated that both werea function of growth rate although relationship of plasticityto growth rate was the steeper and the more pronounced. Turgor pressure at the proximal end of the expanding zone wasnot correlated to growth, being approximately 0·65 MPain all treatments. In mature tissue turgor pressure varied withtreatment, but was also not related to growth rate. Cell membranehydraulic conductivity (5 x 10^–7 ± 1·3 (10)m s^–1 MPa^–2) was not influenced by the presenceof K⁺. We propose that K⁺ and SO₄^{2 –} influence root growthrates by modulating the rheological properties of the wallsof the expanding cell. The physiological significance of these properties is discussed. Key words: Growth, wall extensibility, turgor pressure, wheat roots     

Xyloglucan Endotransglycosylase Activity, Microfibril Orientation and the Profiles of Cell Wall Properties Along Growing Regions of Maize Roots

A series of physical and chemical analyses were made on theexpanding zone of maize seedling roots grown in hydroponics.Comparison of longitudinal profiles of local relative elementalgrowth rate and turgor pressure indicated that cell walls becomelooser in the apical 5 mm and then tighten 5–10 mm fromthe root tip. Immersion of roots in 200 mol m^–3 mannitol(an osmotic stress of 0·48 MPa) rapidly and evenly reducedturgor pressure along the whole growing region. Growth was reducedto a greater extent in the region 5–10 mm from the roottip than in the apical region. This indicated rapid wall-looseningin the root tip, but not in the more basal regions. Following 24 h immersion in 400 mol m^–3 mannitol (an osmoticstress of 0·96 MPa) turgor had recovered to pre-stressedvalues. Under this stress treatment, growth was reduced in theregion 4–10 mm from the root tip, despite the recoveryof turgor, indicating a tightening of the wall. In the rootapex, local relative elemental growth rate was unchanged incomparison to control tissue, showing that wall properties herewere similar to the control values. Cellulose microfibrils on the inner face of cortical cell wallsbecame increasingly more parallel to the root axis along thegrowth profile of both unstressed and stressed roots. Orientationdid not correlate with the wall loosening in the apical regionof unstressed roots, or with the tightening in the region 5–10mm from the root tip following 24 h of osmotic stress. Longitudinal profiles of the possible wall-loosening enzymexyloglucan endotransglycosylase (XET) had good correspondencewith an increase in wall loosening during development. In thezone of wall tightening following osmotic stress, XET activitywas decreased per unit dry weight (compared with the unstressedcontrol), but not per unit fresh weight. Key words: Osmotic stress, turgor, growth, cell wall properties, microfibrils, XET     

Influence of Cytoplasmic Streaming and Turgor Pressure Gradient on the Transnodal Transport of Rubidium and Electrical Conductance in Chara corallina

In order to study the transnodal transport of Rb⁺ in internodalcells of Chara corallina, a low-temperature loading system wasestablished to separate the loading process from the transportprocess. Tandem cells, consisting of internode-node-internode,were isolated from algal plants. Treatment of a single internodewith 100 mM RbCl at 5°C for 30 min caused an accumulationof 43 mM Rb⁺ in the cytoplasm of this cell (= source cell),but no Rb⁺ was found in the other internode (= sink cell) ofthe tandem cells. In 40 min after a return to 25°C, about12% of the Rb⁺ loaded in the source cell was transported intothe sink cell. The apparent transnodal permeability of Rb⁺ wascalculated to be 4.6 x 10^–7 m.s^–1. Under the assumptionthat the total cross-sectional area of plasmodesmata occupies10% of the nodal area, the diffusion coefficient of RbCl throughplasmodesmata was calculated to be 2.3 x 10^–11 m².s^–1which is about 1% of the free diffusion coefficient in water(2 x 10^–9m².s^–1). The transnodal transport of Rb⁺ was intimately correlated withthe rate of cytoplasmic streaming. The rate of streaming inboth the source and sink cells was varied either by treatingthe cells with cytochalasin B (CB) or by lowering the temperature.The transport rate correlated with the streaming rate irrespectiveof the method used. Since the level of ATP was not influencedby CB or low temperature, the transnodal transport is assumedto be the result of passive diffusion process through plasmodesmata. A turgor pressure gradient across the node decreased both thenodal electrical conductance and the transnodal transport ofRb⁺. By contrast, the exposure of both internodal cells to asolution of sorbitol had no effect on either of them. A turgorpressure gradient of 240 mOsm decreased the transport of Rb⁺in the first hour to 3% of the control, while it decreased thenodal conductance to about 50%. The increase in the electricalresistance occurred on the junction side between the node andthe internode that was treated with sorbitol. Cytochalasin Ehad no effect on the nodal electrical resistance. It is assumedthat plasmodesmata are equipped with a valve-like mechanismwhich is sensitive to the gradient of turgor pressure acrossthe node and is not regulated by an actomyosin system. (Received February 15, 1989; Accepted April 20, 1989)     

The Effect of External pH on the Apparent CO2 Affinity of Chlorella saccharophila

The carbon dioxide compensation point of the unicellular greenalga, Chloretla saccharophila, was determined in aqueous mediumby a gas chromatographic method. Compensation points decreasedmarkedly from 63 cm³ m^–3 at an external pH of 4.0 to 3.2cm³ m^–3 at pH 8.0 and were not affected by the O₂ concentrationof the medium. The calculated CO² concentration required tosupport the half-maximum photosynthetic rate of the algal cellsranged from 6.0 mmol m_–3 at an external pH of 60 to 1.5mmol m^–3 at pH 8.0 and these values were not affectedby O₂ concentration. The K_m(CO₂) of nbulose-l,5-bisphosphatecarboxylase isolated from cells grown either at pH 4.0 or pH8.0 was determined to be 64 mmol m^–3. These results indicatethat loss of CO₂ by photorespiration does not occur in C. saccharophilacells at acid pH and the disparity between the apparent affinityfor CO₂ of the intact cells and that of the carboxylase indicatesthe operation of a ‘CO₂ concentrating mechanism’in this alga at acid pH. Key words: Acidophilic alga, bicarbonate transport, Chlorella saccharophila, compensation point, CO₂ affinity, PH, RuBP carboxylase