Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture I. Some characteristics of the cell cycle

Cells of Chlamydomonas reinhardi Dangeard were grown synchronouslyunder a 12 hr light-12 hr dark regime. Time courses of nucleardivision, chloroplast division, "apparent cytokinesis" and zoosporeliberation were followed during the vegetative cell cycle inthe synchronous culture. Liberation of zoospores occurred atabout 23–24 hr after the beginning of the light periodat 25°C. Four zoospores were produced per mother cell underthe conditions used. At lower temperatures, the process of zoosporeliberation as well as length of the cell cycle was markedlyprolonged, but the number of zoospores produced per mother cellwas approximately the same. At different light intensities,lengths of the cell cycle were virtually the same, while thenumber of zoospores liberated was larger at higher rather thanat lower light intensities. During the dark period, nuclear division, chloroplast divisionand apparent cytokinesis took place, in diis order, and proceededless synchronously than did the process of zoospore liberation.When the 12 hr dark period was replaced with a 12 hr light periodduring one cycle, the time of initiation as well as the durationof zoospore liberation was litde affected in most cases, whereasnuclear division, chloroplast division and apparent cytokinesiswere considerably accelerated by extended illumination. Whenalgal cells which had been exposed to light for 24 hr were furtherincubated in the light, zoospore liberation started much earlierand proceeded far less synchronously, compared with that under12 hr light-12 hr dark alternation. (Received October 12, 1970; )     

Inductive and Inhibitory Effects of Light on Cell Division in Chattonella antiqua

The cell division of a red tide flagellate, Chattonella antiqua,was synchronously induced under light and dark regimes of 10L14D(a light period, L, for 10 h followed by a dark period, D, for14 h), 12L12D and l4L10D. In all regimes cell number began toincrease ca. 14 h after the onset of L and almost doubled duringone LD cycle. When the light-off timing of the last L was changedor the whole L was shifted, cells that had been synchronizedunder 12L12D invariably began to divide ca. 14 h after the onsetof L. This shows that the timing of cell division was determinedby the time of the onset of L. When cells were continuously exposed to light after a cell division,the subsequent cell division was inhibited. This effect waslimited to cells that had been synchronized under short-dayconditions. Thus it can be concluded that light has both inductive and inhibitoryeffects on cell division in this alga, the latter effect dependingupon the previously given light and dark regimes. (Received December 21, 1984; Accepted February 28, 1985)     

Time-Measuring Processes Involved in the Photoperiodic Response of Lemna paucicostata 441

Lemna paucicostata 441 exposed to a single dark period of variouslengths showed a rhythmic flowering response with a 22- to 24-hperiod, even when the dark period was preceded by continuouslight. The critical night length (about 12 h) was scarcely influencedby pretreatment with 8D–4L (8 h of darkness followed by4 h of light), 8D–8L or 8D–12L. However, the rhythmof the response in the second cycle was markedly damped by thepretreatment with 8D–4L or 8D–12L, and was slightlyamplified by 8D–8L. The flowering response to a red-light interruption given atdifferent times in the inductive dark period also showed circadianrhythmicity even when the dark period was preceded by continuouslight, and this rhythmicity was scarcely influenced by a dark-lighttreatment given prior to the inductive dark period. A red-lightinterruption given at the 6th or 14th hour of the dark periodmarkedly shifted the phase of the rhythm of the response tothe length of the following dark period (the former delayedand the latter advanced), but that given at the same phase markedlyweakened and disturbed the rhythmicity of the response to ared-light interruption given in the following dark period. (Received March 21, 1992; Accepted June 12, 1992)     

Synchronous Growth and Plastid Replication in the Naturally Wall-less Alga Olisthodiscus luteus

Olisthodiscus luteus is a unicellular biflagellate alga which contains many small discoidal chloroplasts. This naturally wall-less organism can be axenically maintained on a defined nonprecipitating artificial seawater medium. Sufficient light, the presence of bicarbonate, minimum mechanical turbulence, and the addition of vitamin B₁₂ to the culture medium are important factors in the maintenance of a good growth response. Cells can be induced to divide synchronously when subject to a 12-hour light/12-hour dark cycle. The chronology of cell division, DNA synthesis, and plastid replication has been studied during this synchronous growth cycle. Cell division begins at hour 4 in the dark and terminates at hour 3 in the light, whereas DNA synthesis initiates 3 hours prior to cell division and terminates at hour 10 in the dark. Synchronous replication of the cell's numerous chloroplasts begins at hour 10 in the light and terminates almost 8 hours before cell division is completed. The average number of chloroplasts found in an exponentially growing synchronous culture is rather stringently maintained at 20 to 21 plastids per cell, although a large variability in plastid complement (4-50) is observed within individual cells of the population. A change in the physiological condition of an Olisthodiscus cell may cause an alteration of this chloroplast complement. For example, during the linear growth period, chloroplast number is reduced to 14 plastids per cell. In addition, when Olisthodiscus cells are grown in medium lacking vitamin B₁₂, plastid replication continues in the absence of cell division thereby increasing the cell's plastid complement significantly.     

Environmental factors controlling the time of flower-opening in Pharbitis nil

Pharbitis nil, strain Violet, subjected to various photoperiods(24-hr cycle at 24?C) bloomed about 10 hr after light-off whenthe light period was 10 hr or longer, and about 20 hr afterlight-on when the light period was shorter. The higher the temperature(20–30?C) during the dark period, the later the time offlower-opening, with the temperature during the last half ofthe dark period having a stronger effect than that during thefirst half. In continuous dark or light, flower buds of Pharbitis openedabout every 24 hr at all temperatures tested between 20 and28?C, which suggests the participation of a circadian rhythmin determining the time of flower-opening. A light pulse given6–12 or 28–36 hr after the onset of the dark periodgreatly advanced the phase of this rhythm (8–10 hr). Phasedelay of this rhythm could not be obtained by light pulses givenat any time. (Received September 29, 1979; )     

Spectral Dependence of Night-break Effect on Photoperiodic Floral Induction in Lemna paucicostata 441

A single dark period of longer than 8 hr induced flowering inLemna paucicostata 441 cultured in E medium. Monochromatic lightsof 450, 550, 650 and 750 nm with a half-power bandwidth of 9nm given for 10 min at the 8th hour of a 14-hr dark period inhibitedflowering. The fluence rates required for 50% inhibition were10, 0.5, 0.1 and 3 µmol m^–2. sec^–1, respectively.When applied between the 4th and the 10th hour of the dark period,lights of 450, 550 and 650 nm were inhibitory showing a maximumeffect at the 8th hour. But 750-nm light completely inhibitedflowering when applied at any time during the first 8 hr ofthe dark period. The inhibitory effect of 750-nm light givenat the beginning of the dark period was totally reversed bya subsequent exposure to 650-nm light, and the fluence-responsecurves for the effect of 750-nm light given at the 0, 4th and8th hour were essentially the same. This suggests that the presenceof P_FR is crucial for the floral initiation throughout the first8 hr of the inductive dark period. The role of phytochrome inthe photoperiodic flower induction of L. paucicostata is discussed. (Received January 4, 1982; Accepted March 19, 1982)     

Photocontrol of Nuclear DNA Replication in Chattonella antiqua (Raphidophyceae)

The timing of nuclear DNA replication was examined in a synchronizedcell population of a red-tide flagellate, Chattonella antiqua,using fluorescence microspectrophotometry with a DNA-specificfluorochrome, 4',6-diamidino-2-phenylindole (DAPI). Under alternating12-h periods of light and dark (12L12D), nuclear DNA began toincrease synchronously ca. 10 h after the onset of light irradiation.Even when the light-off timing of the light period or the wholespan of the 12-h light period was shifted after synchronizationunder 12L12D cycles, the timing of the beginning of nuclearDNA replication was invariably ca. 10 h from the onset of lightirradiation. When irradiation was not given, there was no increaseof nuclear DNA. The conclusion reached was that light irradiationis necessary for nuclear DNA replication in Chattonella antiquaand that the timing of the replication is dependent upon onlythe timing of the onset of the last irradiation. In other words,a light-on signal induces the transition of cell nuclei fromthe G₁ into the S phase and also determines the timing of thisevent. When not irradiated, cells are arrested in the G₁ phase. ³ Present address: Department of Biology, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan. ⁴ Present address: Frontier Research Programs, RIKEN, Wako-city,Saitama 351-01, Japan. (Received February 28, 1987; Accepted June 5, 1987)     

Circadian variability in the photobiology of Phaeodactylum tricornutum: pigment content

Circadian variations of pigment content in the diatom Phaeodactylumtricornutum were analyzed in different light regimes. The studywas aimed at discerning the role of putative endogenous controlsfrom the constraint imposed by the alternation of light (L)and dark (D) periods. Our experiments showed that in a typicalLD cycle of illumination, pigment synthesis follows the somaticgrowth of the cell, both arresting during D periods. In particular,the diurnal increase of chlorophyll a content was proportionalto the increase in cell size and preceding cell division, occurringat night. By contrast, diadinoxanthin and ß–carotene displayed different phases, which is likely to be relatedto their involvement in photoprotection mechanisms. The experimentsalso showed that the synthesis of both photosynthetic and photoprotectivepigments was dependent not only on light availability and thephasing of somatic growth, but also responded to other internalregulation. Over the time scale of the experiments (hours todays), the removal of LD–DL triggers impaired cell physiology,whereas the circadian patterns in pigment synthesis persisted.Our results support the hypothesis that an internal regulationof cell biosynthetic machinery can improve phytoplankton fitness,even in high variable environments such as the oceanic mixedlayers. Therefore, we suggest that phytoplankton growth dependsnot only on the availability of external resources, but alsoon internal regulatory mechanisms whose unveiling would furtherour understanding of phytoplankton diversity and dynamics.     

Influence of light intensity on diel variations in rates of growth, respiration and organic release of a marine diatom: comparison of diurnally constant and fluctuating light

Effects of variations in light intensity on diel patterns ofgrowth, respiration and organic release of Skeletonema costatum(Grev.) Cleve in a cyclostat were evaluated. Light intensitywas either constant during the tight period at levels from 1500to 15 µEm^–2 s^–1 or fluctuated throughout thelight period from 500 to 10 µEm^–2 s^–1 at ratesof either 1 or 12 cycles day^–1. Periodicity in cell divisionwas observed only at light intensities of 130 Em^–2 s^–1and was decreased under diurnally varying tight. Under all lightconditions carbon and pigment growth were maximal during thelight period but well coupled throughout the 24 h period. Carbonassimilation during the dark period varied from 19 to 34% oftotal daily production and was a linear function of growth rate.Respiratory activity during the light period increased relativeto total daily respiration as growth rate increased. The increasein night-time carbon assimilation with growth rate interactedwith night-time respiration through the refixation of respiredcarbon, thus, influencing the pattern of respiratory loss ofcarbon. Rates of organic release (E^c) were maximal during thelight period and did variations consistently increased withtight intensity. Fluctuating light increased E^c relative toconstant light. Net growth efficiency was maximal at 130 µEm^–2s^–1 when cell division periodicity was greatest. Underother light conditions relatively higher rates of cell divisionoccorred at night and cell division periodicity was reducedas well as net growth efficiency. Cellular chemical fractionationindicated that under high or variable light conditions fixedcarbon was stored during the tight period for subsequent synthesisof protein and pigments, and division at night. Such an uncouplingof photosynthesis and other growth parameters resulted in greatermetabolic costs to the cell. ¹Present address: Marine Biology, Lamont Doherty GeologicalObservatory, Palisades, NY 10964, USA     

Effect of Benzyladenine on Diurnal Changes in DNA Content per Chloroplast and Chloroplast Replication in Intact Bean Leaves

The effect of benzyladenine (BA) on the diurnal changes in DNAand Chl contents per chloroplast and chloroplast replicationin primary leaves of bean plants (Phaseolus vulgaris L.) grownunder a 16 h light/8 h dark cycle was studied. Experiments weremade on primary leaves in the early expansion phase, where celldivision had been completed but chloroplasts were replicating.In untreated controls, chloroplast number, Chl content and freshweight per leaf showed daily periodic changes. Chl content perchloroplast increased in the light period every day, and freshweight per leaf increased most rapidly in the early dark period.Chloroplast number per leaf increased rapidly in the early darkperiod on day 9, though the increase began a little earlierand was less sharp on days 8 and 10. During these periods, DNAcontent per chloroplast was decreasing due to chloroplast divisionas chloroplast DNA (ctDNA) per leaf remained unchanged throughoutthe experimental period. BA induced increases in Chi contentper chloroplast, ctDNA content and fresh weight per leaf within6 h of its application, regardless of whether it was appliedat or 10 h after the beginning of the light period. Applicationof BA at 10 h in the light period shifted the start of chloroplastreplication by 6 h compared to that in untreated controls. However,when BA was applied at the beginning of illumination, the startof chloroplast replication showed the same relative change intime as above. 5-Fluorodeoxyuridine (5-FdU) promptly preventedBA-induced increase in Chl content and chloroplast number perleaf as well as ctDNA content per leaf.     

Nucleic Acid Metabolism in Chlamydomonas moewusii Undergoing Daily Cell Doubling

Nucleic acid metabolism was studied in synchronized cultures(12 h light, 12 h dark) of Chlamydomonas moewusii Gerloff inwhich cell number approximately doubled each 24 h. Analysisfor DNA showed a small but significant rise between 5 to 7 hand a major increase from 11 to 16 h after first exposure tolight. Cellular RNA levels increased from 0·5 to 1·1pg within 1 h after exposure to light, dropped after 2 h toa plateau value of about 0·9 pg, declined from the 10thto the 17th hour, and subsequently increased a second time duringthe dark period. Investigations using gel electrophoresis showedthat RNAs of chloroplastic ribosomes were only a minor componentin extracts obtained in the first 8 h of light. It is suggestedthat the time of initiation of synthesis and degradation aswell as the nature and amounts of nucleic acids produced inChlamydomonas are regulated by conditions of growth.     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

The Effects of {delta}-Aminolevulinic Acid and Inhibitors of RNA and Protein Synthesis on Phytochrome Mediated Chlorophyll Accumulation in Sorghum bicolor

In 5-d-old etiolated seedlings of Sorghum bicolor, 12 h of darknessafter 5 min in red light eliminated a lag before the accumulationof chlorophylls in subsequent continuous white light. Increasingthe dark period to 24 h and 36 h, increased the rate of chlorophyllaccumulation in the later stages of greening. Exogenous -aminolevulinicacid neither completely removed the lag, nor increased the rateof chlorophyll accumulation. Cycloheximide (25 µg ml^–1)and 6-methyl purine (5.0 µg ml^–1), given continuouslyor only until the 12 h dark period following the red light irradiation,restored the lag and decreased the rate of chlorophyll accumulation.D-threo-chloramphenicol (400µg ml^–1) also decreasedthe rate of chlorophyll accumulation but did not restore thelag. Addition of these inhibitors even 12 h after red lightirradiation decreased the rate of chlorophyll accumulation.Rifampicin (Rifamycin SV, 400 µg ml^–1) did not havesuch effects. Key words: Chlorophylls, Phytochrome, -Aminolevulinic acid, Sorghum bicolor     

Factors Affecting Chloroplast Replication in Spinach

Chloroplast replication has been studied in discs cut from thebase of young spinach leaves and cultured on sterile nutrientagar. In discs grown in a growth cabinet chloroplast numbersper cell increased logarithmically with time over a 7-day cultureperiod. Chloroplast replication proceeds in a similar way incultured discs and in intact leaves. Cytokinins do not affect chloroplast replication in this systembut they stimulate the fresh-weight growth of discs. Chloroplastreplication is temperature dependent, having an optimum at 25°C. By contrast chloroplast size is at a maximum in discscultured at 12 °C. Light stimulates chloroplast replication, a linear relationshipoccurring between chloroplast number per cell and the dailyquantity of light given to discs up to a saturating value of250 J d^–1. Daylength does not affect chloroplast formationin spinach. In a number of experiments a general relationship was establishedbetween chloroplast number per cell and cell size but no evidenceis available to suggest that this correlation is causal. Theresults of experiments in which discs were transferred fromdark to light suggest that some of the events which precedechloroplast replication may occur at similar rates in both lightand dark.     

Cell Cycle Timing of Replication of the Nuclear Gene Encoding Chloroplastic NADP-Specific Glutamate Dehydrogenase Isoenzymes in Chlorella sorokiniana

In synchronized Chlorella sorokiniana cells, the NH₄⁺ inducibleNADP-specific glutamate dehydrogenase enzyme (NADP-GDH) accumulatedin a linear manner throughout the first cell cycle. Early inthe following second cell cycle, an increase in its rate ofaccumulation occurred that was proportional to the increasein total cellular DNA in the previous cell cycle. In synchronizedbacterial cells, increases in rate of linear accumulation ofinducible enzymes coincide with the time of replication of theirstructural genes. To determine whether the rate change in NADPGDHaccumulation resulted from a delay in replication of its nuclearstructural gene (gdhN) in fully induced C. sorokiniana cells,the cell cycle timing of replication of this gene was comparedto that of another nuclear gene, nitrate reductase (nia), andof a chloroplast gene, ribulose bisphosphate carboxylase large-subunit(rbcL), in synchronized cells cultured in NH₄⁺ or NO₃^–(uninduced) medium. The gdhN and nia genes replicated withinthe period of nDNA synthesis and rbcL within the period of ctDNAsynthesis in cells growing in either nitrogen source. Therefore,the delayed rate change in enzyme accumulation results froma process that regulates expression of the gdhN gene after itsreplication. (Received July 16, 1994; Accepted November 28, 1994)     

Flower Opening in Asiatic Lily is a Rapid Process Controlled by Dark-light Cycling

In commerce, Asiatic lilies are picked in bud, each stem holdingseveral buds. We found flower opening was rapid, taking lessthan 4 h both on the stem and for excised buds. Opening wasalso strongly synchronous. For a 12 h day-night cycle, openingbegan late in the dark period, reaching a mid-point after 11h of darkness. This was equally true of buds that were excisedwhen nearly ready to open, and those with 3–4 d of developmentto complete. Reversing day and night reversed the time of opening,and red light was as effective as white light in providing ‘day’conditions. A 15 min light break during the night did not affectthe opening. Lengthening the night (8, 12, 16 h) and shorteningthe day delayed opening from 9, to 11, to 13 h after the startof darkness, respectively. In continuous light and continuousdark, synchronicity was lost. If opening flowers were held inextended darkness, two phases of opening could be discriminated.In a ‘dark phase’, petals opened to approx. 40°,and anthers remained intact. When such flowers were returnedto light, there was a ‘light phase’, where petalsopened further, became more pigmented and began to recurve,and the anthers dehisced, these events taking only 2–3h. The net result was that flowers became fully open and anthersdehisced approx. 2 h after dawn, regardless of daylength. Copyright2000 Annals of Botany Company Asiatic lily, Lilium hybrid, flower opening, timing, endogenous rhythm, synchronicity     

Synchronous division and the pattern of diel vertical migration of Heterosigma akashiwo (Hada) Hada (Raphidophyceae) in a laboratory culture tank

Heterosigma akashiwo (Hada) Hada (Raphidophyceae) causes red tides in Osaka Bay (Japan). A clonal culture of the alga was grown in a 2 m tall culture tank on a 12: 12 LD cycle to determine patterns of vertical migration and cell division. A specific growth rate of 0.43 ln unit · day⁻¹ was obtained during complete mixing conditions. Under weakly stratified conditions (≈ΔT = 3–4°C/1.5 m), H. akashiwo in the tank grew and showed a similar pattern of vertical migration to that observed in the field for at least 6 days. Cell concentration, mean cell volume, and photosynthetic capacity, estimated by DCMU-induced fluorescence increase of H. akashiwo, were monitored in the stratified tank at 2-h intervals over 24 h at three levels in the water column. Cell ascent began shortly before the light period and vertically swimming cells were smaller in size than those sampled near the bottom of the tank. The cell division cycle and the pattern of vertical migration were phased individually by the light regime and were well synchronized with each other. This synchrony must be due to the interrelation between these two processes or the existence of a clock which controlled endogenous rhythms of both processes and was entrained by a light: dark cycle. The relative increase of fluorescence with DCMU was higher for migrating cells than for non-migrating cells.     

Pathway of dark inorganic carbon fixation in two species of diatoms: influence of light regime and regulator factors on diel variations

Planktonic algae submitted to vertical mixing with a short periodicitycommute many times a day from low to high irradiance levels.To study the influence of this light periodicity, two diatoms,Skeletonema coslatum and Nitzschia turgiduloides, were cultivatedunder alternating conditions of 2 h light/2 h dark (2 h/2 h),simulating vertical mixing in the natural environment. Two otherlight regimes were used: continuous light (CL) and alternatecycles of 12 h light/12 h dark (12 h/12 h). Products synthesizedin the dark by S.costmum during 60 s incubation for 2 h/2 hculture or during 5 min for 12 h/12 h culture were determined.They were essentially sugars, malate, aspartate and glyceratefor 2 h/2 h cells and 12 h/12 h cells taken at the beginningof the light period. In contrast, 12 h/12 h cells taken duringthe darkness or in the middle of the light period and set inthe dark synthesized only amino acids. Our results corroborateprevious reports on dark CO₂ fixation via phosphoenolpyruvatecarboxykinase (PEPCKase, enzyme allowing the fixation of CO₂on PEP and the synthesis of amino acids) with involvement ofa substrate synthesized during the light period, but demonstratethat incorporation also occurs by the C-3 pathway (pathway responsiblefor the major CO₂ fixation in the light) in the very early stagesof the dark period. Another important result highlighted bythis study is the appreciable rate of dark fixation: on average6.7, 8.3 and 12.7% of photosynthesis at saturating photon fluxdensity for N.turgiduloides cultivated under 2 h/2 h, CL and12 h/12 h regime respectively and nearly 12% for S.costatumin the 2 h/2 h light regime. Variation of dark fixation wasinvestigated as a function of hour in the two species. Skeletonemacostatum cells submitted to the 2 h/2 h cycle show a constantrate of light-independent assimilation throughout the day. Bycontrast, both N.turgiduloides grown under the 12 h/12 h or2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycleundergo fluctuations in the rate of dark fixation over the light/darkcycle. The mean dark fixation rate is controlled by the lengthof the photoperiod or the frequency of light fluctuations, dependingon species. We argue that this phenomenon must be taken intoconsideration in primary production calculations. Dependingon whether they are synthesized at the beginning or at the endof the light period, products are somewhat different and therate of fixation varies. This leads us to suggest that the pathwayof dark fixation may be regulated by at least two factors: amountof available substrate and enzyme (RuBPCase and PEPCKase) activityand/or amount.     

Environmental Anoxia is Unnecessary for Inhibiting Chloroplast Photomorphogenesis in Rice Coleoptiles (Oryza sativa L.)

High population densities of germinating rice seedlings in initiallyair-saturated sealed aquatic environments exhibited d^–seedling growth consisting solely of coleoptile emergence inlight and dark environments. Residual oxygen tensions of 17–23%of the initially air-saturated water containing the d^–seedlings were evident after 15 d in both the light and dark.Coleoptiles of all d^– seedlings were stark white in appearance,lacked protochlorophyllide, and contained proplastids and amyloplasts,there being no evidence of normal etioplast development in thelight or dark and no chloroplast development in the light. Thus,complete environmental anoxia was observed to be unnecessaryfor inhibiting normal chloroplast photomorphogenesis in coleoptilesof light-germinated rice seedlings. Increasing the oxygen tensionsof the 15-d-old aquatic environments of light- and dark-germinatedd^– seedlings placed in the light resulted in normal chloroplastphotomorphogenesis in coleoptiles, shoots, and roots. Key words: Oryza sativa, environmental anoxia, chloroplast photomorphogenesis, rice coleoptiles     

SYNCHRONIZATION OF CHLOROPLAST DIVISION IN THE ULTRAMICROALGA CYANIDIOSCHYZON MEROLAE (RHODOPHYTA) BY TREATMENT WITH LIGHT AND APHIDICOLIN

A system of highly synchronized chloroplast divisions was developed in the unicellular red alga Cyanidioschyzon merolae De Luca, Taddei, & Varano. Chloroplast divisions were examined by epifluorescence microscopy following treatments with light and inhibitors. When the cells during stationary phase were transferred into a new medium under a 12:12 h LD cycle, chloroplasts, mitochondria, and cell nuclei divided synchronously in that order soon after the initiation of dark periods. More than 40% of the cells contained dividing chloroplasts. To obtain a system of highly synchronized cell division and chloroplast division, the cells synchronized by a 12:12 h LD cycle were treated with various inhibitors. Nocodazole and propyzamide did not affect cell and organelle divisions, whereas aphidicolin markedly inhibited cell-nuclear divisions and cytokinesis and induced a delay in chloroplast division. More than 80% of the cells contained dividing chloroplasts when cells synchronized by light were treated with aphidicolin for 12 h. This synchronized system will be useful for studies of the molecular and cellular mechanisms of organelle divisions .