The Relationship between the Growth of the Primary Leaf and of the Coleoptile in Seedlings of Avena and Triticum

Partial inhibition of extension growth of the primary leaf occurswhen whole Triticum seedlings are immersed in aerated solutionsof IAA but is replaced by growth promotion when sucrose is addedto the external solution. In seedlings in which the coleoptilehas been excised, IAA increases the growth of the leaf bothwith and without additional sucrose. Inhibition of the leaf by moderate concentrations of IAA nolonger occurs when the seedling is detached from the endosperm.Sucrose added to the external solution raised the percentageelongation of the coleoptile almost to the level of that attainedin intact seedlings without additional carbohydrate. It alsoenabled the leaf to show a positive growth response with IAA. The results indicate that in intact seedlings treated with IAAthe growth of the primary leaf is markedly diminished owingto diversion of carbohydrate to the coleoptile if the growthof the latter is promoted as a result of the treatment. Whenthe competition of the coleoptile for carbohydrate is diminishedor eliminated, acceleration of the growth of the primary leafby IAA becomes apparent. In addition to the endogenous rhythm, with a period close to24 hours, induced in the growth-rate of the coleoptile whenseedlings of Avena are transferred from red light to darkness,a similar rhythm, with a slightly longer period, is inducedin the growth-rate of the primary leaf. This rhythm persistsin elongating leaves so long as they remain within the coleoptile.It can be recorded for at least 100 hours in deseeded seedlings. When intact seedlings of Avena are immersed for one hour inrelatively high concentrations of IAA and then transferred todistilled water for 18 hours, the elongation of the coleoptileis greater and the inhibition of the leaf is less than whenthey are transferred to humid air. Sections of the leaf of Triticum showed a slight increase inelongation in concentrations of IAA up to 5 mg./l., but no evidencewas obtained that sections of leaf and coleoptile exert any.influenceon each other's elongation when floated together on solutionsof IAA.     

The Effects of Decapitation, Lack of Oxygen, and Low Temperature on the Endogenous 24-hour Rhythm in the Growth-rate of the Avena Coleoptile

Further work on the endogenous rhythm in the growth-rate ofthe Avena coleoptile was carried out by the methods previouslydescribed. Decapitation of the coleoptile does not prevent induction ofthe rhythm, nor does it affect the phase of a rhythm previouslyestablished. When seedlings are transferred to darkness and are simultaneouslydeprived of oxygen by replacing the surrounding air with nitrogen,a rhythm is induced, but the first peak is delayed for a periodapproximately equal to that of the nitrogen treatment. Providedthe period in nitrogen is not excessively prolonged, the retardationof the second peak is less than that of the first. This suggeststhat the rate of the time-keeping mechanism has become accelerated.Since only a part of the initial retardation in the rhythm isthus eliminated, a change in phase persists. The younger theseedlings at the time of treatment, the smaller is this persistentchange. When the nitrogen treatment is commenced some hoursafter the transfer to darkness, the retardation of the secondpeak is greater than when treatment and transfer are simultaneous.In each series the retardation increases exponentially as thelength of the period in nitrogen is extended. Retardation followedby acceleration was also observed after the seedlings had beenexposed to low temperature. It seems probable that induction of the rhythm involves thesynchronization of independent time-keeping mechanisms alreadyin existence, rather than the establishment of an entirely newsystem.     

The Occurrence of Endogenous Rhythms in the Coleoptiles in Various Cereal Genera

The rate of growth of the coleoptiles was determined from photographstaken by infra-red radiation. CO₂ output was measured by meansof an infra-red gas analyser. The rhythm of CO₂ output from the coleoptile of Avena was inducedby a change from red light to darkness. It has a period of about24 hours and agrees in timing with the growth-rate rhythm previouslyrecorded. Some degree of rhythmicity in the growth-rate was found in Triticumvulgare (var. ‘Eclipse’) and in Secale cereale (var.Petkus). Very slight indications of rhythmicity were found inTriticum spelta and in Hordeum vulgare. Negative results wereobtained with Oryza sativa and with Zea mays. Where rhythmicityin the coleoptile is less strongly developed, the peaks comecloser together, the interval being about 18–20 hours.Cereals cannot be sharply separated into two groups accordingto the presence or absence of rhythmicity in the coleoptile.Of the genera examined, the most marked endogenous rhythms occurin Avena. It is doubtful if the ability of the coleoptile toexhibit an endogenous rhythm has any beneficial effect on thedevelopment of the seedling. Under normal conditions of germinationinduction of the rhythm would not occur.     

A Technique for taking Successive Shadowgraphs of the Same Seedlings in Growth and Curvature Experiments with Oat Coleoptiles

A method is described for taking successive shadowgraphs ofthe same Avena seedlings, using an orange-sensitive paper andvery low-intensity orange light such that whereas shadowgraphscan be made with one minute's exposure no photo-tropic or growthresponse can be detected as a result of one hour's unilateralexposure of intact seedlings. Phototropic curvatures producedby somewhat higher intensities of red light emphasize the importanceof specifying not only the wave-length but also the intensityand duration of illumination used in experiments with Avenaseedlings. When not subjected to the unilateral illuminationthe seedlings were grown under deep red light (wave-length >0.63 µ) at a total intensity of less than 1/6 foot-candle;it must not be assumed that the technique would be satisfactoryfor seedlings grown in total darkness.     

An Endogenous Rhythm in the Rate of CO2 Output of Bryophyllum: II. THE EFFECTS OF LIGHT AND DARKNESS ON THE PHASE AND PERIOD OF THE RHYTHM

The effects of light, darkness, and changes in light intensityon the phase and period of the endogenous rhythm in the rateof CO₂ output of excised leaves of Bryophyllum fedtschenkoihave been examined. The duration, intensity, and wavelength of a short light treatment,and the point in the cycle at which it is administered, determinethe degree of phase shift induced in a rhythm persisting indarkness. When light treatments of 3 and 6 hours' duration,at an intensity of 3,000 lux, are applied between the peaksthe phase is completely reset, the first post-treatment peakoccurring 18–19 hours after the end of the treatment.The degree of phase shift is therefore determined not by theduration of the treatment but by the time at which the treatmentterminates. One hour's illumination has little or no effect.The phase is unaffected when light treatments of up to 5 hours'duration at an intensity of 3,000 lux are applied at the crestof a peak. Over the range 8-3,000 lux the intensity of lightduring a 6-hour treatment applied between the peaks does notaffect the efficiency with which that treatment completely resetsthe phase. At an intensity of 2 lux, however, the phase delayis equal to the duration of the treatment. A 6-hour red-light treatment (850 ergs/cm.²/sec.) applied betweenthe peaks completely resets the phase whereas blue light (10,860ergs/cm.²/sec.) has no effect on the phase but induces a slightprotraction of the period. Moreover, continuous red light inhibitsthe rhythm, which recommences in blue light. A rhythm is induced in illuminated leaves when the light intensityis either gradually or suddenly reduced by at least 80 per cent.Whether a given intensity of illumination inhibits or permitsthe persistence of a rhythm depends upon the light intensityby which it is immediately preceded. A rhythm will persist in illuminated leaves for approximatelyas long as in leaves in darkness and the phase shows no correlationwith time of day. The period is unaffected by the intensityof white light (from 0-500 lux) to which the leaves are subjected.The duration of a short dark treatment, and the point in thecycle at which it is applied, determine the degree of phaseshift induced in a rhythm in illuminated leaves. The phase isreset when 3-, 6-, and 9-hour dark treatments are applied atthe crest of a peak, the amount of phase shift increasing toa maximum with 9 hours' darkness. The phase shift is not equalto the duration of the treatment. The phase is unaffected when3- and 6-hour dark treatments are applied between the peaks. The variation in the sensitivity of the phase of a rhythm persistingin darkness to short light treatments is in the opposite senseto that of a rhythm persisting in light to short dark treatments.The phase of a rhythm in illuminated leaves is completely resetwhen the leaves are transferred to continuous darkness commencingeither at the crest of, or between, the peaks. The results are discussed and compared with those of other authors.     

The Effects of Indole-3-Acetic Acid and 2:4-Dichlorophenoxyacetic Acid on the Growth Rate and Endogenous Rhythm of Intact AvenaColeoptiles

The rates of elongation of the coleoptiles of Avena seedlings,subjected to intermittent immersion in solutions of IAA or 2:4-Dfor various total periods, were determined from measurementsof photographs taken every hour by infra-red radiation. Immersion in 17·5 mg./l. IAA for 1–5 hours causeda large increase in the growth rate followed by a depression.When the seedlings were immersed in 8·75 mg./l. IAA forperiods of 12 or 24 hours the depression was partially overcomeso long as the treatment was continued. Absorption of additionalIAA by the coleoptiles reduced their geotropic sensitivity. Penetration of 2:4-D (sodium salt) into the coleoptiles wasslower than that of IAA and the resulting stimulation of thegrowth rate was less, particularly in unbuffered solutions.After the treatment the growth rate declined slowly to aboutthe normal value. Results with coleoptiles were very similar to those previouslyobtained with rhizomes of Aegopodium and suggest that inhibitionof growth following stimulation by IAA may be of general occurrence.Possible causes of the inhibition are discussed and a comparisonis made between the results with intact coleoptiles and observationsmade by others on coleoptile sections. Temporary immersion of the seedlings in auxin solutions depressedthe rate of elongation of the primary leaf while it increasedthat of the coleoptile. It caused little disturbance of theendogenous rhythm induced by change from light to darkness.The suggestion that such rhythms can be explained in terms ofvariation in concentration of IAA-oxidase is not supported.     

Investigation of the endogenous rhythm of flowering inChenopodium rubrum L.

Under the conditions applied in our laboratory 4 1/2 days old plants ofChenopodium rubrum require 2–3 photoperiodic cycles for maximal flowering response, whereas 2 1/2 days old plants are able to flower after having obtained a single inductive cycle. The period length of the free-running rhythm of flowering observed in 2 1/2 days old plants after a single transfer from light to darkness is 30h and the first peak of flowering occurs at about hour 12 in darkness. When a cycle consisting of 16h darkness and 8h light or of 8h darkness and 8h light precedes the long dark period the rhythm is rephased. Rephasing is greater when the light commenced to act on the positive slope of the first peak of the free running rhythm than when it impinged on the negative slope. With an 8h interruption of darkness by light rhythm phase is controlled by the light-on, as well as by the light-off signal. Feeding 0.4 M glucose during the long period of darkness enhanced the amplitude of the flowering response and, moreover, substituted for one photoperiodic cycle.     

Die Steuerung einer gezeitenparallelen Schlüpfrhythmik nach dem Sanduhr-Prinzip

Summary The arctic population of the intertidal midge Clunio marinus (location: Tromsö, Norway) shows a tidal rhythm of emergence (period: 12.4 hours) in midsummer. The emergence time exactly coincides with the initial exposure of the habitat during the ebb.When the animals were bred in a 24 hour light-dark cycle involving 16 hours of light, emergence occurred mainly 10–11 hours after light-on. When the animals were subsequently placed under constant light, no rhythm could be detected. When constant light was interrupted by a single period of 6 hours of darkness, only one peak of emergence was evoked, again 10–11 hours after light-on. Similar results were observed when in constant darkness a single period of 6 hours of lower temperature was offered.The mechanism of control differs from that of a southern population (location: St. Jean-de-Luz, France) which has a circadian clock mechanism and shows a free-running rhythm of emergence in constant light. It is postulated that the tidal rhythm of the Tromsö population is controlled by an hour-glass mechanism which starts its run of at least 10–11 hours during the preceding ebb.     

A Critical Study of the Auxin Theory of Growth Regulation in the Mesocotyl of Avena sativa

A method of growing Avena seedlings is described, which allowsthem to be handled individually in darkness. Mesocotyls of seedlinge from which the tip of the coleoptileis repeatedly removed are as long as those of control plantsnot so decapitated. Mesocotyls of seedlings which are deseeded on the 3rd day ofgrowth, followed by decapitation of the cleoptile tip on the4th day, are, at 7 days old, as long as those of controls notso decapitated. When deseeded plants are decapitated, regeneration of auxinproduction occurs at the tip of the coleoptile stump. Where a reduction in the length of the mesocotyl results fromdecapitation, a wound reaction is probably concerned in additionto any auxin changes. Removal of the coleoptilar node causes a sharp decrease in thefinal length of the mesocotyl. Heating intact seedlings at 40° C. for 3 hours causes areduction in the length of the mesocotyl but not of the coleoptile.The effect of heating is not reversed by subsequent treatmentat low temperature, which instead appears to augment these effects. When seedlings are exposed to the action of KCN, iodoacetate,or anaerobic conditions, and illuminated while so exposed, perceptionof light takes place, resulting in a reduction in the lengthof the mesocotyl. Perception of light takes place in seedlings germinated at normaltemperatures, but maintained at low temprature during illuminationand also in seedlings grown for 6 weeks at 2° C. withoutany previous growth at normal temperature. Light perception takes place in embryos excised from dry grainand grown on a culture medium. No difference in free amino-acid content is apparent betweendark grown and illuminated seedlings. The effects of illumination survive a period of drying downand become apparent upon subsequent germination of the grainin darkness. The drying process itself causes an additionalreduction in mesocotyl length. It is concluded that auxin itself is not the primary reactantin the perception process, and that the growth of the mesocotylis probably controlled by the coleoptilar node and plumulargrowing point, rather than by auxin diffusing downward fromthe tip of the coleoptile.     

CIRCADIAN GROWTH RHYTHM IN JUVENILE SPOROPHYTES OF LAMINARIALES (PHAEOPHYTA)1

A circadian rhythm in growth was detected by computer-aided image analysis in 3–4-cm-long, juvenile sporophytes of the kelp species Pterygophora California Rupr. and in seven Laminaria spp. In P. californica, the free-running rhythm occurred in continuous white fluorescent light, had a period of 26 h at 10°or 15°C, and persisted for at least 2 weeks in white or blue light. The rhythm became insignificant in continuous green or red light after 3 cycles. Synchronization by white light-dark regimes, e.g. by 16 h light per day, resulted in an entrained period of 24 h and in a shift of the circadian growth minimum into the middle of the light phase. A morning growth peak represented the decreasing portion of the circadian growth curve, and an evening peak the increasing portion. The circadian growth peak was not visible during the dark phase, because growth rate decreased immediately after the onset of darkness. At night, some growth still occurred at 16 or 12 h light per day, whereas growth stopped completely at 8 h light per day, as in continuous darkness. During 11 days of darkness, the thallus area became reduced by 3.5%, but growth rate recovered in subsequent light–dark cycles, and the circadian growth rhythm reappeared in subsequent continuous light.     

Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

An Endogenous Rhythm in the Rate of Carbon Dioxide Output of Bryophyllum: I. SOME PRELIMINARY EXPERIMENTS

A techinique is described for recording automatically, withthe aid of an infrared gas analyzer, the rate CO₂ output orabsorption by plant material under controlled conditions. An examination of the rate of CO₂ output by excised leaves of16 species of succulent plants in darkness and in a CO_-2-freeatmosphere revealed clearly defined rhythms in only Bryophyllumfedtschenkoi, B. daigremontianum and B. calycinum (pinnatum). Further investigation of the rhythm in leaves of B. fedtschenkoirevealed that: (1) daylength has no effect upon the period ofthe rhythm in subsequent darkness, the phase being set at thetime the lights are extinguished; (2) normal air suppressesthe rhythm; (3) removel of the epidermis and cutting the mesophyllinto pieces 1 cm² does not effect either the phase or periodof the rhythm; (4) continuous illumination at an intensity of3,000 lux inhibits the rhythm which restarts when the lightsare extinguished. The phase of the rhythm can be set at anytime of day according to the time at which the lights are extinguished.The time which elapses between the onset of darkness and thefirst peak decreases as the length of the light treatment isincreased. The endogenuous nature of the rhythm is fully established. Theresults are compared with of other researches.     

Modes of control by the circadian oscillator and the hourglass mechanism of the activities of cytoplasmic and chloroplast glyceraldehyde 3-phosphate dehydrogenases in Lemna gibba G3

Two types of clocks, i.e., the circadian oscillator and thehourglass mechanism, which under continuous light and darknessrespectively control the mutually inverse temporal changes inthe activities of Cyt-NAD-GPD and Chl-NADP-GPD of Lemna gibbaG3, were studied. Both clocks controlled the apparent Km values,not the Vmax values, of the GPD reactions for their substrateand coenzymes. A red light pulse inserted 3 hr after the onsetof the dark period eliminated the sigmoidal changes in darkness,but evoked rhythmical changes which otherwise did not occurin continuous darkness. Thus, the photosynthetic rhythm, ifpresent, would not sustain the GPD rhythms. This effect of ared light pulse was not nullified by a subsequent far red lightpulse. A far red light pulse given at the 3rd hour of an extendeddark period made conspicuous the sigmoidal changes in activityof GPDs in the dark period, and its effect was nullified bya subsequent red light pulse, suggesting that phytochrome isinvolved in the hourglass mechanism. (Received September 26, 1978; )     

Studies on the Control of the Rhythm of Photosynthetic Capacity in Synchronized Cultures of Euglena gracilis (Z)

Synchronous cell division in Euglena gracilis (strain Z) was obtained in 24-hour light cycles consisting of 10 hours of light and 14 hours of darkness; cell division was restricted to the dark period. Photosynthetic capacity was found to vary in a cyclic manner during the cell cycle, reaching a peak 2 hours before the onset of darkness. Light reactions were investigated during the cell cycle to determine what role they played in the control of the observed rhythmic changes in capacity. Light-saturation curves showed no major change in the light-limited region. No fluctuations were found in Hill reaction activity or photoreduction of methyl red during the cell cycle. These results imply that the reactions comprising photosystems I and II do not generate the capacity rhythm.     

Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

The effects of blue light (B) pretreatments on internode extensiongrowth and their possible interaction with phytochrome mediatedresponses were examined in Sinapis alba seedlings grown for11 d under 280 µmol m^–2 s^–1 of continuousblue-deficient light from low pressure sodium lamps (SOX). SupplementaryB (16 µmol m^–2 s^–1) caused no detectable inhibitionof the first internode growth rate under continuous SOX, butgrowth rate was inhibited after transfer to darkness. This effect,and the growth promotion caused by far-red bend-of-day' lightpulses were additive. The addition of B at 16 µmol m^–2s^–1 during 11 d, or only during the first 9 or 10 d orthe latest 0.75, 1 or 2 d of the SOX pretreatment caused approximatelythe same extent of inhibition after the transition to darkness.A single hour of supplementary B before darkness caused morethan 50% of the maximum inhibition. However, 24 h of lower fluencerates of B (4 or 7 µmol m^–2 s^–1) were ineffective.Covering the internode during the supplementary B period didnot prevent the response to B after the transition to darkness.Far-red light given simultaneously with B (instead of the SOXbackground) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly inthe leaves inhibits extension growth in subsequent darkness,provided that high phytochrome photo-equilibria are presentduring the irradiation with B. Once triggered, this effect doesnot interact significantly with the ‘end-of-day’phytochrome effect. Key words: Blue light, extension growth, phytochrome     

Is the far-red-absorbing form of Avena phytochrome A that is present at the end of the day able to sustain stem-growth inhibition during the night in transgenic tobacco and tomato seedlings?

Avena phytochrome A (phyA) overexpressed in tobacco (Nicotiana tabacum L.) and tomato (Lycopersicon sculentum Mill) was functionally characterised by comparing wild-type (WT) and transgenic seedlings. Different proportions of phytochrome in its far-red-absorbing form (Pfr/P) were provided by end-of-day (EOD) light pulses. Stem-length responses occurred largely in the range of low Pfr/P (3–61%) for WT seedlings and in the range of high Pfr/P (61–87%) for transgenic seedlings. A similar shift was observed when the photoperiod was interrupted by short light pulses providing different Pfr/P ratios and followed by 1 h dark incubation. In other experiments, Avena phyA was allowed to re-accumulate in darkness and subsequently phototransformed to Pfr but no extra inhibition of stem extension growth was observed. In transgenic tomato seedlings the response to EOD far-red light was faster and the response to a far-red light pulse delayed into darkness was larger than in the WT. Avena phyA Pfr remaining at the end of the photoperiod appears intrinsically unable to sustain growth inhibition in subsequent darkness. Avena phyA modifies the sensitivity and the kinetics of EOD responses mediated by native phytochrome.Abbreviations EOD end-of-day - FR far-red light - Pfr/P pro-portion of phytochrome in its FR-absorbing form - phyA phyto-chrome A - phyB phytochrome B - R red light - RFR R to FR ratio - WT wild type We thank Dr Brian Thomas for providing the antibodies used in this work, and Federico Guerendiain for his excellent technical assistance. This work was financially supported by grants UBA AG 040 and Fundacion Antorchas A-12830/1-19 (both to J.J.C.), PID-CONICET (to R.A.S. and J.J.C.), United States Department of Energy DE-FG02-88ER13968 (to R.D.V.).     

Further studies on potassium uptake rhythm in the long-day duckweed Lemna gibba G3 with special reference to vegetative growth

Some properties of the circadian rhythm in potassium uptakeof flow medium culture of the long-day duckweed Lemna gibbaG3 were examined.

1. In total darkness, the rhythm faded out in ca. 48 hr; it restartedon transfer to continuous light. Under low-intensity light (below700 lux), the rhythm was damped rapidly
2. The rhythm appearedregardless of the potassium concentrationin the culture medium(from 10/m to 2 HIM). The amplitude, butnot the period, ofthe rhythm was influenced by the ambientpotassium concentration.
3. Alteration in the light intensity or medium composition causeda change in the growth rate without modifying the period ofthe rhythm.
4. These results indicate that potassium uptake rhythmin thisduckweed is typical light-on rhythm, which has no directrelationwith the rate of vegetative growth and requires lightenergyfor its duration.

¹Present address: Aichi-Gakuin University, Chikusa-ku, Nagoya464, Japan. ²Present address: National Institute for Basic Biology, Okazaki,Aichi 444, Japan. (Received February 1, 1979; )     

Kinetics of the Unrolling of the Second Leaves Detached from Rice Seedlings by Blue Light

Unrolling due to blue light (B) irradiation of the second leavesdetached from 8-day-old rice (Oryza saliva L.) seedlings wassimilar to that reported previously for nondetached leaves.The effect of B was counteracted by irradiation with red light(R). The counteracting effect of R was reversed by subsequentirradiation with far-red light (FR). When the detached leaf was irradiated with B passed througha 1-mm-wide slit 5, 8, 10, 12 or 15 mm from the leaf tip, irradiation10 mm from the leaf tip was the most effective. The effect of a 1 mm-wide-B irradiation 10 mm from the leaftip was counteracted by a 1 mm-wide-R irradiation at the sameposition, but not by irradiations at the other points. The counteractingeffect of R was reversed by a 1 mm-wide-FR irradiation at thesame position. This suggests that the excitation or the reactionof the B photoreceptor(s) is affected directly by the PFR formof phytochrome. The dose-response curve for the unrolling caused by B showeda simple Bunsen-Roscoe relation without two peaks, which differsfrom that for the phototropism in Avena caused by B. (Received August 21, 1980; Accepted December 20, 1980)     

Persistence of the Potassium Uptake Rhythm in the Presence of Exogenous Sucrose in Lemna gibba G3

The potassium uptake rhythm in a flow medium culture of Lemnagibba G3 persisted in darkness for 3 days, when the flow mediumcontained sucrose (1%). The rhythm was damped out after thatin darkness but it persisted longer when the plants were keptunder continuous weak light (80 lux). The rhythm was not dampedout when a daily light pulse (4,200 lux for 15 min) was applied.A single light pulse (4,200 lux for 15 min) at hour 48 of theprolonged dark period caused the rhythm to start again. DCMU(1 µM) slightly reduced the amplitude of the rhythm butdid not nullify the effect of the inserted light pulse. (Received September 16, 1981; Accepted February 2, 1982)