Quantitative Analysis of ATP-Dependent H+ Efflux and Pump Current Driven by an Electrogenic Pump in Nitellopsis obtusa

Internodal cells of Nitellopsis were made tonoplast-free byperfusion with a medium containing EGTA. Cytoplasmic concentrationsof solutes were controlled by a second perfusion with mediaof known composition. The electrogenic pump current (I_p), whichwas calculated from electrical data obtained from cells withand without ATP, was compared with the current carried by H⁺(I_H⁺) across the plasma membrane. A close correlation betweenI_p and I_H⁺ was found under various internal and external conditions.(1) I_p and I_H⁺ depended on the internal ATP and showed Michaelis-Mententype saturation curves. For I_p, K_m was 120 µM and themaximum current V_max was 15.1 mA m^–2, while for I_H⁺, K_mwas 160 µM and V_max was 16.6 mA m^–2. (2) I_p andI_H⁺ showed almost the same I_H²⁺ dependence. The Mg²⁺-dependentI_p was 19.5 mA m^–2, while the Mg²⁺-dependent I_H²⁺ was17.7 mA m^–2. (3) I_H²⁺ was maximal at an external pH of8 and decreased both in acidic and alkaline pH ranges. I_p wasnearly equal to I_H⁺ in the pH range between 8 and 5. (4) I_H⁺became maximal at an internal pH of 7.3, which is nearly thesame as the pH for maximal electrogenecity found by Mimura andTazawa (1984). All these facts support the idea proposed in our previous paper(Takeshige et al. 1985) that the electrogenic ion pump locatedin the plasma membrane of Nitellopsis is the H⁺ pump. ¹ Dedicated to Professor Dr. Erwin Bünning on the occasionof his 80th birthday. (Received June 21, 1985; Accepted December 20, 1985)     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.     

Carbonic Anhydrase of a Unicellular Red Alga Porphyridium cruentum R-l. II. Distribution and Role in Photosynthesis

Antibody was raised against Porphyridium carbonic anhydrase(CA) which was electrophoretically recovered from the gel afterSDS-polyacrylamide slab gel electrophoresis (SDS-PAGE) of thepartially purified enzyme. The antiserum reacted with CA ofPorphyridium, but not with that of Chlamydomonas reinhardtii.Even though the antiserum did not react with CA from P. cruentumR-l in Ouchterlony's double immunodiffusion, it blocked theenzyme activity in the presence of 1% Nonidet P-40 and 1% TritonX-100. After Western blotting and enzyme-linked immunostaining(ELIS), only one band which reacted with the antiserum was detectedin the extract of low-CO₂ cells (grown under ordinary air) ofP cruentum, while no significant band was detected in that ofhigh-CO₂ cells (grown under air enriched with 1–5% CO₂).Immunogold electron microscopy of low-CO₂ cells of P. cruentumR-l using this antibody revealed that most of the CA was localizedin the chloroplast, with some in the cytoplasm. No specificbinding of gold particles was observed in the high-CO₂ cells. ¹Present address: National Institute for Basic Biology, Myodaiji,Okazaki 444, Japan (Received May 18, 1987; Accepted September 7, 1987)     

Effects of Light and CO2 on Net Photosynthesis Rates of Stands of Aubergine and Amaranthus

Net photosynthetic rates per unit ground area for plant standsof Solanum melongena L. var. esculentum (aubergine) and Amaranthuscaudatus L. var. edulis (grain amaranth) were measured over10 min intervals in an airtight, glass, controlled-environmentcabinet for a range of light flux densities provided by thediurnal variation in daylight. Light response curves for photosynthesisof stands, grown at ambient CO₂ concentration, were definedat 400, 800 and 1200 vpm CO₂. Light compensation points for these stands were around 20-30J m^-2 s^-1 and decreased slightly at higher CO₂ concentrations.For aubergine, a C₃ species, the short-term effects of CO₂ enrichmentwere to increase the initial slope as well as the asymptoteof the light response curve, reducing light saturation at moderateto high light flux densities; but for amaranthus, a C₄ species,saturation was less apparent and CO₂ enrichment scarcely increasedphotosynthesis except at light flux densities above 150 J m^-2s^-1. The canopies intercepted 93-98% of incident light. The efficiencyof utilization of intercepted light in photosynthesis (µgCO₂ J^-1) increased from zero at the light compensation pointto a maximum at an optimum light flux density of about 100 Jm^-2 s^-1 (the optimum rose a little with CO₂ enrichment) anddecreased slightly with further increase in light. Maximum utilizationefficiencies at 400 vpm CO₂ were 8-9 µg CO₂ J^-1. Enrichmentto 1200 vpm did not affect the peak utilization efficiency ofthe C₄ amaranthus, but increased that aubergine to 12·2µg CO₂ J^-1 (equivalent to some 14% when using the heatof combustion of plant dry matter to convert to the dimensionlessform). This is among the highest recorded efficiencies of lightutilization for stands, and relates to the exceptionally favourableenvironment, with optimal control of CO₂ concentration, humidity,temperature, water supply and mineral nutrition.Copyright 1993,1999 Academic Press Amaranthus caudatus L. var. edulis, Solanum melongena L. var. esculentum, canopy photosynthesis, CO₂ enrichment, light interception, light utilization, photosynthetic efficiency     

S-Locus-Specific Glycoproteins Associated with Self-Incompatibility in Brassica campestris

Specific S-glycoproteins were isolated from three Brassica campestriscultivars homozygous with respect to the S-alleles S₈, S₉ andS₁₂. Amino acid sequences of various peptide fragments of theS-proteins were determined using a gas-phase protein sequencer,and a nearly complete amino acid sequence of the S₈-glycoproteinwas determined on the basis of the revised cDNA sequence ofthe B. oleracea S-specific glycoprotein. The lysyl endopeptidasefragments of S₉ and S₁₂-glycoproteins were aligned in comparisonwith the sequence of the S₈-glycoprotein. Although extensivesequence homology was evident among the three S-glycoproteins,the sequences of the middle part were relatively different fromeach other. The numbers and positions of N-glycosylation alsodiffered among the S-glycoproteins of Brassica species. (Received April 20, 1987; Accepted July 29, 1987)     

Riboflavin transport by isolated perfused rabbit renal proximal tubules

Rabbit renal proximal tubular transport of riboflavin(RF) was examined by using the in vitro isolated tubule perfusiontechnique. We found that proximal tubules actively reabsorbed(J_lb) and secreted (J_bl)RF. At 0.1 µM RF concentration, J_bl wassignificantly higher than J_lb, resulting in anet secretion. This net secretion of RF was decreased at 0.01 µM RFconcentration and increased at 1 µM RF concentration. BothJ_lb and J_bl wereinhibited by lowering temperature or by adding iodoacetate, a metabolicinhibitor, and lumichrome, an RF analog, suggesting the involvement ofcarrier-mediated transport mechanisms. J_bl wasinhibited by probenecid, an anion transport inhibitor, and bypara-aminohippuric acid, an organic anion, suggesting therelevance of RF secretion to renal organic anion transport.J_bl was also inhibited by alkaline pH (8.0) and by the calmodulin inhibitor trifluoperazine, indicating the influence of pH and Ca²⁺/calmodulin-dependent pathway on RFsecretion. Finally, we found that addition of chlorpromazine, aphenothiazine derivative, inhibited both J_lb andJ_bl, raising the concern about the nutritionalstatus in patients receiving such a type of medication.

     

The Permeability of Ammonia, Methylamine and Ethylamine in the Charophyte Chara corallina (C. australis)

Ritchie, R. J. 1987. The permeability of ammonia, methylamineand ethylamine in the charophyte Chara corallina (C. australis).—J.exp. Bot. 38: 67–76 The permeabilities of the amines, ammonia (NH₃), methylamine(CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the giant-celled charophyteChara corallina (C. australis) R.Br. have been measured andcompared. The permeabilities were corrected for uptake fluxesof the amine cations. Based on net uptake rates, the permeabilityof ammonia was 6?4?0?93 µm s^–1 (n = 38). The permeabilitiesof methylamine and ethylamine were measured in net and exchangeflux experiments. The permeabilities of methylamine were notsignificantly different in net and exchange experiments, norto that of ammonia (P_methylamine = 6?0?0?49 µm s^–1(n = 44)). In net flux experiments the apparent permeabilityof ethylamine was slightly greater than that of ammonia andmethylamine (P_{ethylamine, net} = 8?4?1?2 µm s^–1 (n= 40)) but the permeability of ethylamine based on exchangeflux data was significantly higher (P_{ethylamine, exchange} =14?1?2 µm s^–1 (n = 20)). Methylamine can be validlyused as an ammonium analogue in permeability studies in Chara. The plasmalemma of Chara has acid and alkaline bands; littlediffusion of uncharged amines would occur across the acid bands.The actual permeability of amines across the alkaline bandsis probably about twice the values quoted above on a whole cellbasis i.e. the permeability of ammonia across the permeablepart of the plasmalemma is probably about 12 µm s^–1. Key words: Chara, permeability, ammonia, methylamine     

Properties of the Cytochrome c Oxidase Activity of Cytochrome b561 from Photoanaerobically Grown Rhodopseudomonas sphaeroides

Cytochrome b₅₆₁ from Rhodopseudomonas sphaeroides had cytochromec (c2) oxidase activity and a pH optimum at 6.0 for this activity.The activity was affected by the ionic strength of the reactionmixture. The apparent K_m and maximal velocity (V_max) valuesin the absence of addea salts were 14 µM and 120 nmoloxidized per min per mg protein for horse heart cytochrome c.Reduced horse heart cytochrome c was reoxidized in first-orderkinetics by this cytochrome b₅₆₁. The specific activity was0.7 s^–1 per mg protein at 20°C at the concentrationof 30 µMM cytochrome c. Activity was inhibited by KCN and NaN₃, but not by antimycin.The addition of a low concentration of KCN to the cytochromeb₅₆₁ produced a change in the absorption spectrum, evidencethat KCN interacts with the heme moiety of cytochrome b₅₆₁.Results of this and preceeding studies show that the cytochromeoxidase (cytochrome "o") described earlier (Sasaki et al. 1970)is cytochrome b₅₆₁. (Received May 16, 1983; Accepted September 8, 1983)     

Scaling to a common temperature improves the correlation between the photosynthesis parameters Jmax and Vcmax

A strong correlation between the photosynthetic parameters J_maxand V_cmax was found by Wullschleger (1993) in a survey of 109plant species. Measurements were made at various leaf temperatures,but the temperature dependence of J_max and V_cmax differ. Oncevalues for J_max and V_cmax in Wullschleger's analysis were adjustedto a common temperature, using an equation for the temperaturedependence of these parameters, the slope of the linear regressionfor J_max versus V_cmax forced through the origin increased from1.97 to 2.68, and r² increased from 0.79 to 0.87. Key words: Temperature, photosynthesis parameters     

Protein kinase D inhibits plasma membrane Na+/H+ exchanger activity

The regulation of plasma membraneNa⁺/H⁺exchanger (NHE) activity by protein kinase D (PKD), a novel proteinkinase C- and phorbol ester-regulated kinase, was investigated. Todetermine the effect of PKD on NHE activity in vivo, intracellular pH(pH_i) measurements were made inCOS-7 cells by microepifluorescence using the pH indicator cSNARF-1.Cells were transfected with empty vector (control), wild-type PKD, orits kinase-deficient mutant PKD-K618M, together with green fluorescentprotein (GFP). NHE activity, as reflected by the rate of acid efflux(J_H), wasdetermined in single GFP-positive cells following intracellularacidification. Overexpression of wild-type PKD had no significanteffect on J_H(3.48 ± 0.25 vs. 3.78 ± 0.24 mM/min in control atpH_i 7.0). In contrast,overexpression of PKD-K618M increasedJ_H (5.31 ± 0.57 mM/min at pH_i 7.0;P < 0.05 vs. control). Transfectionwith these constructs produced similar effects also in A-10 cells,indicating that native PKD may have an inhibitory effect on NHE in bothcell types, which is relieved by a dominant-negative action ofPKD-K618M. Exposure of COS-7 cells to phorbol ester significantlyincreased J_H in control cells but failed to do so in cells overexpressing either wild-type PKD (due to inhibition by the overexpressed PKD) or PKD-K618M(because basal J_Hwas already near maximal). A fusion protein containing the cytosolicregulatory domain (amino acids 637-815) of NHE1 (the ubiquitousNHE isoform) was phosphorylated in vitro by wild-type PKD, but with lowstoichiometry. These data suggest that PKD inhibits NHE activity,probably through an indirect mechanism, and represents a novel pathwayin the regulation of the exchanger.

     

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

The capacity for C₄ photosynthesis in Panicum milioides, a specieshaving reduced levels of photorespiration, was investigatedby examining the activity of certain key enzymes of the C₄ pathwayand by pulse-chase experiments with ¹⁴CO₂. The ATP$P₁ dependentactivity of pyruvate,P₁ dikinase in the species was extremelylow (0.14–0.18 µmol mg chlorophyll^–1 min^–1).Low activity of the enzyme was also found in Panicum decipiensand Panicum hians (related species with reduced photorespiration)and in Panicum laxum (a C₃ species). The antibody to pyruvate,P₁dikinase caused about 70% inhibition of the ATP$P₁ dependentactivity of the enzyme in P. milioides. The activity of NAD-malicenzyme and NADP-malic enzyme in ^P. ^milioides was equally low(approximately 0.1–0.2 µmol mg chlorophyll^–1min^–1) and similar to the activity in P. decipiens, P.hians and P. laxum. Photosynthetic pulse-chase experiments underatmospheric conditions showed a typical C₃-like pattern of carbonassimilation including the labelling of glycine and serine asexpected during photorespiration. During the pulse with ¹⁴CO₂only about 1% of the labelled products appeared in malate and2–3% in aspartate. During a chase in atmospheric levelsof CO₂ for up to 6 min there was a slight increase in labellingin the C₄ acids. The amount of label in carbon 4 of aspartatedid not change during the chase, indicating little or no turnoverof the C₄ acid via decarboxylation. The results indicate thatunder atmospheric conditions P. milioides assimilates carbondirectly through the C₃ pathway. Photorespiration as indicatedby the CO₂ compensation point may be repressed in the speciesby a more efficient recycling of photorespired CO₂. (Received June 8, 1982; Accepted July 22, 1982)     

Activation of Electron Donation from Hydrogen Peroxide by Manganese in Non-oxygen evolving Photosystem II Particles

Analysis of the activation of H₂O₂-supported 2,6-dichloroindophenol(DCIP) photoreduction by MnCl₂ showed two Mn²⁺-binding sitesin non-oxygen evolving PS II particles, with large (0.4IM) andsmall (0.04 µm) K_m values for Mn²⁺. Photoreduction throughthe high-affinity Mn²⁺.-binding site was inhibited by treatmentwith H₂O₂. (Received April 20, 1987; Accepted July 13, 1987)     

Pyruvate and citric acid cycle carbon requirements in isolated skeletal muscle mitochondria

Carbohydrate depletion precipitates fatigue in skeletal muscle, but, because pyruvate provides both acetyl-CoA for mainline oxidation and anaplerotic carbon to the citric acid cycle (CAC), the mechanism remains obscure. Thus pyruvate and CAC kinetic parameters were independently quantified in mitochondria isolated from rat mixed skeletal muscle. Mitochondrial oxygen consumption rate (J_o) was measured polarographically while either pyruvate or malate was added stepwise in the presence of a saturating concentration of the other substrate. These substrate titrations were carried out across a physiological range of fixed extramitochondrial ATP free energy states (G_P), established with a creatine kinase energy clamp, and also at saturating [ADP]. The apparent K_m,malate for mitochondrial J_o ranged from 21 to 32 µM, and the apparent K_m,pyruvate ranged from 12 to 26 µM, with both substrate K_m values increasing as G_P declined. V_max for both substrates also increased as G_P fell, reflecting thermodynamic control of J_o. Reported in vivo skeletal muscle [malate] are >10-fold greater than the K_m,malate determined in this study. In marked contrast, the K_m,pyruvate determined is near the [pyruvate] reported in muscle approaching exhaustion associated with glycogen depletion. When data were evaluated in the context of a linear thermodynamic force-flow (G_P-J_o) relationship, the G_P-J_o slope was essentially insensitive to changes in [malate] in the range observed in vivo but decreased markedly with declining [pyruvate] across the physiological range. Mitochondrial respiration is particularly sensitive to variations in [pyruvate] in the physiological range. In contrast, physiological [malate] exerts very little, if any, influence on mitochondrial pyruvate oxidation measured in vitro. bioenergetics; fatigue; anaplerosis; Krebs cycle; fuel limitation; metabolic control analysis     

Membrane Potential Measurement of Protoplasts Isolated from Vigna mungo Hypocotyl Using a Fluorescent Probe, diS-C3-(5)

Membrane potentials of protoplasts isolated from Vigna mungohypocotyl segments were measured using the fluorescent probediS-C₃-(5). The fluorescence intensity changed in response tothe external K⁺ concentration. Membrane potential was estimatedto be inside negative (–85?8 mV at 0.1 mM KCl) from theNernst equation for K₊. The membrane potential was not affectedby DCCD (50 µM) or low temperature (5?C). Addition of0.5 mM Ca₂₊ to the protoplast suspension markedly depolarizedthe membrane potential, and subsequent EDTA treatment repolarizedit to the initial level. The Ca₂₊ effect on the membrane potentialmay be due to change in the permeability ratio of Cl^–to K₊. (Received December 16, 1986; Accepted April 22, 1987)     

Water Transport in Isolated Maize Roots

A simple model has been devised which predicts the concentration,C^x_s, of salt (e.g. KCl) in the exudate from isolated roots asa function of the salt concentration, C⁰_s, in the medium. Thechief assumption, made in deriving the relationship betweenC^x_s and C⁰₅, is that the exudation of water, J, from the rootsconsists of two components (one being osmotic, Ø, inorigin and the other, Ø⁰, flowing in the absence of anosmotic gradient). The exudation of salt, J_s, calculated asJ C^x_s, was found to be dependent on C⁰_s. Our investigationson maize roots were concerned with estimations of L_p and Ø^vand determinations of C^x_s as a function of C⁰_s. Satisfactoryagreement between prediction and experiment was found in thesepreliminary studies. It is considered that water movement inisolated roots can be explained by a simple osmotic model withthe additional possibility that a relatively small non-osmoticwater flow occurs.     

Respiratory Efflux in Relation to Temperature of Simulated Swards of Perennial Ryegrass with Contrasting Soluble Carbohydrate Contents

Fully light-intercepting simulated swards of S24 perennial ryegrasswere exposed to contrasting environmental conditions in a growthroom for 4 days. Half experienced 20 h days of 120 Wm^–2(400–700. nm) and 5 °C, and came to have a WSC (watersoluble carbohydrate) content of 235 mg g^–1 and half 4h days of 20 Wm^–2 and 25 °C leading to a WSC of 25mg g^–1. Their rates of CO₂ efflux were monitored at anumber of temperatures during an 8 h dark period; half experiencedincreasing (5–30 °C) and half decreasing (30–5°C) temperatures. The ‘high’ WSC swards hadrespiration rates of 3.7 mg CO₂ g^–1 (d. wt) h^–1at 15 °C, and the ‘low’ swards 0.8 mg CO₂ g^–1h^–1. The order in which the temperatures were experiencedwas immaterial. Even the ‘low’ WSC swards showedno evidence of a respiratory decline during the dark periodthat could be attributed to substrate shortage. The relationshipbetween temperature and CO₂ efflux was best represented by logisticcurves. Even so, a Q₁₀ of 2 fitted the data reasonably well,at least up to 20 °C, and has practical advantages wheninterpolating estimated between measured values of respirationin the construction of a carbon balance sheet. Lohum perenne L., ryegrass, respiration, temperature, Q₁₀, soluble carbohydrate content, simulated sward     

Light Interception and Photosynthetic Efficiency in Some Glasshouse Crops

Productivity of glasshouse crops is strongly limited by lightreceipt, and efficient interception and use of light in photosynthesisis correspondingly important. Mature row crop canopies of cucumberand tomato intercepted about 76% of the light incident on theirupper surfaces; about 18% was lost through gaps between therows. Light transmitted through the entire depth of the canopywas reflected back by white plastic on the ground, so that thelower surface of the canopy received approximately 13% of thelight incident on the upper surface. The light flux incidenton the sides of these canopies (c. 2m tall and 6m x 16m in area)amounted to some 20–30% of that incident on the uppersurface. About 32% of daylight falling on the glasshouse (c.9m x 18m in area) was intercepted by the glasshouse structureand glazing; of the 68% entering the house, some fell on headlandsoccupying 35% of the glasshouse area. The loss of light to headlands,and the gain from canopy side-lighting, would be relativelysmaller for larger glasshouses. At near-ambient CO₂ concentrations, net photosynthetic ratesof the cucumber canopy were comparable to those of closed canopiesof other glasshouse and field crops which have maximum lightconversion efficiencies of 5–8µg CO₂ J^–1 at50–200 W m^–2 incident light flux density. Efficiencydecreases only slightly with stronger light. Glasshouse cropswith CO₂ enrichment to 1200 vpm achieve conversion efficienciesof 7–10µg CO₂ J^–1. Efficiencies of utilizationof intercepted light, on an energy basis, reach 6–10%in various field and glasshouse crops with near-ambient CO₂,and reached an exceptional 11% for the cucumber canopy. Glasshousecrops with CO₂ enrichment achieve maximum efficiency of lightenergy utilization between 12% and 13%. Key words: Glasshouse cucumber and tomato, light interception and utilization, photosynthetic efficiency, row crops     

The Effect of Vapour Pressure Deficit on Carbon Isotope Discrimination in the Desert Shrub Larrea tridentata(Creosote Bush)

Larrea tridentata (creosote bush) seedlings were subjected tothree regimens of atmospheric humidity in a growth chamber experiment.Relative humidity was varied to achieve daytime vapour pressuredeficits (VPD) during growth of 29, 48 and 77 kPa. Photosyntheticgas exchange, carbon isotope composition and biomass productionwere measured after 8–10 weeks of treatment. Whereas stomatalconductance (g) declined linearly with increasing ambient VPD,CO₂ assimilation rate (A) was not measurably affected by changesin ambient VPD. This resulted in a decrease in intrinsic wateruse efficiency (ratio of CO₂ assimilation to stomatal conductance;A/_g) with increasing VPD. Leaf carbon isotope discrimination(A) was negatively correlated (r² = 088) with A/g ratios. Carbonisotope discrimination also correlated positively with ratiosof internal (C₁) to ambient (c_a) CO₂ levels determined by gasexchange measurements (c₁/c). The ratio of c₁ to c_a was lowerat higher VPD levels. Leaf biomass decreased with increasingambient VPD and correlated positively with. Root to leaf biomassratio increased at higher VPD levels and correlated negativelywith. Key words: Larrea tridentata, vapour pressure deficit, carbon isotope discrimination, intrinsic water-use efficiency     

Ca2+ Increases the Cl- Efflux of the Permeabilized Chara

An internode of Chara was permeabilized as described by Shimmenand Tazawa [(1983) Protoplasma 117:93]. The Cl^– effluxof the permeabilized cell increased when the extracellular Ca²⁺concentration was increased, and the degree of the increasewas dependent on the Ca²⁺ concentration. This suggests thatthe Cl^– channel in the tonoplast was activated by Ca²⁺. (Received May 22, 1987; Accepted October 21, 1987)     

Oxidative Damages and Repair in Euglena gracilis Exposed to Ozone. I. SH Groups and Lipids

Malondialdehyde, a product of lipid oxidation, increased graduallywhen Euglena gracilis cells were bubbled with 240 µ1.liter^–1ozone (delivery rate of 1µmolO₃.min^–1) for 120 min.Simultaneously, the sulfhydryl group content decreased by 36%during the treatment, which was mainly due to oxidation of proteinsulfhydryl groups. The molar amount of SH groups oxidized was3 times higher than that of fatty acid oxidized, indicatingthat sulfhydryl groups were more accessible or more easily oxidizedby O₃ than fatty acids. When Euglena cells were allowed to recoverunder autotrophic growth conditions following O₃ treatment,viable cells were incapable of dividing during the first 5 hof the recovery period but regenerated SH groups nearly to thecontrol level. The increase of SH content during this periodpreceded the resumption of cell division and the restorationof normal growth. These results suggest that the regenerationof SH groups by Euglena cells is a part of a mechanism involvedin the repair of oxidative damage caused by ozone and is anessential step for the initiation of cell division. (Received July 20, 1987; Accepted December 14, 1987)