Flowering of the long-day plant, Lemna gibba, under short-day schedules composed of red and far-red light

Flowering in Lemna gibba, a long-day duckweed, can be inducedunder a short-day condition when the photoperiodic regimes areR₇FR₃ (7 hr red followed by 3 hr far-red), R₅FR₅ and R₃FR₇.This indicates the necessity of a proper balance between redand far-red effects for flowering. The flowering induced bythese regimes is inhibited by a brief exposure to red givenat the start of darkness and this inhibition is reversed bysubsequent exposure to far-red. Thus, the red/far-red reversibleeffect is found only at the beginning of darkness for floweringof L. gibba. However, flowering of L. gibba is promoted by a red light breakgiven near the middle of a 14 hr dark period. The promotiveeffect is not reversed by subsequent exposure to far-red, i.e.,the effect of the red break converts from inhibition to promotionas when given later in the dark period, which suggests the involvementof a timing mechanism. (Received July 21, 1973; )     

Photoperiodic requirements for flowering of the long-day duckweed, Lemna gibba G3

The min-LD estimation by the log. flower number vs. the cultureperiod curve provides a unique method of judging whether a givenphotoperiodic schedule is a long day or not for Lemna gibbaG3. Duckweeds in M-sucrose medium are exposed to the test scheduleat 26°C on either the first or second day of a continuouslight culture. If the min-LD (48 hr for control cultures) isnot changed, the inserted schedule is considered to have functionedas a long day. If, however, the min-LD is extended by 24 hr,the inserted schedule is judged to have functioned as a shortday. Examinations using this method of orderly designed light-darkschedules disclosed two critical phases in the light requirement;the initial and terminal 1 hr portions (designated the L1- andL2-phases) of the subjective day. Thus, a given day became along or short day when both the L1- and L2-phases were illuminatedor when either or both of the two phases were darkened. Thecritical daylength (11.5 hr) was just long enough to cover boththe L1- and L2-phases and the inductive phase (L2-phase) waslocated at the end of the subjective day. (Received June 9, 1975; )     

Comparison of the flowering behavior of the long-day plant Lemna gibba G3 from different laboratories

In an effort to determine the cause for the wide discrepanciesin the level of flowering response reported for the long-dayplant Lemna gibba L., strain G3, cultures of L. gibba G3 wereobtained from the laboratories of W. S. Hillman (G3-H), R. Kandeler(G3-K), Y. Oota (G3-O) and and A. Pieterse (G3-P) and comparedto the L. gibba G3 (G3-C) from this laboratory. Under continuouslight all cultures gave FL% values of 77 or above, and on a9L:15D short-day treatment, all cultures were completely vegetative.However, on daylengths of 10 to 12 hr, small but statisticallysignificant differences were obtained for the different cultures.The critical daylength curves for G3-G, which showed the shortestcritical daylength, and G3-K, which showed the longest criticaldaylength, differed by approximately one hour. Salicylic acidtreatment caused flower promotion in each culture, but statisticallysignificant differences were obtained between some of the culturesin their response to salicylic acid. It is concluded that the large discrepancies in the floweringresponses of L. gibba G3 that have been reported are due primarilyto differences in culturing methods and counting proceduresin the different laboratories. However, the results also indicatethat there may be distinct cultures of L. gibba G3 that exhibitsmall physiological and/or genetic differences that would makeprecise quantitative comparison between different laboratoriesvery difficult. (Received January 23, 1979; )     

Effects of some metabolic inhibitors on flowering of Lemna gibba G3, a long-day duckweed

Flowering of Lemna gibba G₃, a long-day duckweed, was inhibitedby adding CuSO₄, AgNO₃, HgCl₂, Na₂WO₄ or iodoacetamide to themedium at the concentrations inducing long-day flowering inLemna paucicostata 6746, a short-day duckweed. This suggeststhat these metabolic inhibitors affected the photoperiodic sensitivityrather than directly affecting flower initiation. Ferricyanidepromoted flowering in both of these short-day and long-day duckweeds. (Received July 7, 1977; )     

Inhibition of flowering in the long-day plant Lemna gibba G3 by Hutner's medium and its reversal by salicylic acid

Flowering in the long-day plant Lemna gibba L., strain G3 ispoor or absent in Hutner's medium even under continuous light,an effect generally ascribed to the ammonium content of themedium. However, flowering is also inhibited in ammonium-freemodifications of Hutner's medium, particularly in the presenceof sucrose, but is restored to high levels by the presence of10 µu salicylic acid. These results link two of the leastunderstood chemical effects in Lemnaceae flowering, and theyprovide a system in which large effects of salicylic acid canbe readily obtained. ²Present address: Lab. of Applied Bot., Fac. of Agric, KyotoUniv., Kyoto 606, Japan. (Received January 27, 1979; )     

Effect of interruption of the nyctoperiod with an R/FR-mixture of various ratios of red and far-red light on flowering in Lemna gibba G3

A brief R pulse inserted in the 15-hr nyctoperiod of the 9L15D regime almost always promoted flower formation of a long-dayduckweed, Lemna gibba G3. This R effect was partially reversedby subsequent FR, which suggests that phytochrome is involvedin the floral processes occurring in the nyctoperiod. The null% R value (% of R in the R/FR-mixture that exerts no effecton flowering) was high during the initial 7.5 hr of the nyctoperiod,then rapidly decreased. The starting time of the rapid decreasein the null % R value was hardly affected by a change in temperaturein the nyctoperiod. A similarly high initial level of the null% R value followed by a rapid decrease was observed when thenyctoperiod was extended to 39 hr. At hour 15 of the 39-hr nyctoperiod,flower promotion by the R puke was not reversed by subsequentFR, although the spectral dependence of the light pulse effectdid not exclude the possible involvement of phytochrome. Atabout hour 21 of the 39-hr nyctoperiod, the null % R value beganto increase rapidly, and almost reached the initial high levelat hour 24 when R-FR reversibility was also restored. These results suggest that the Pfr-level remained high duringthe initial hours of the nyctoperiod, followed by a rapid decreaseirrespective of the temperature of the nyctoperiod. This rapiddecrease in the Pfr-level may play a role in the time measurementof the photoperiodic floral induction of L. gibba G3. (Received February 13, 1979; )     

Effect of light on the metabolism of thymidine in the long-day duckweed, Lemna gibba G3

Metabolism patterns of exogenous thymidine as disclosed by ECTEOLAcellulose column chromatography, were examined with a long-dayduckweed, Lemna gibba G3, under dark and light conditions. Whenthymidine-6-³H was applied, the pattern of thymidine metabolismin the light was not very different from that in the dark. However,when thymidine (methyl-³H) was used, incorporation of radioactivityinto two major ( and ß) and one minor components ofboth B and D fractions separated by column chromatography, wasstrikingly stimulated by the light, through photosynthetic activity.Component a of fraction B was tentatively concluded to be ß-ureidoisobutyricacid by paper chromatography. As the radioactivity from thymidine-6-³Hwas hardly recovered in the a component of fraction D, the partof the thymidine molecule incorporated into this component wasnot the pyrimidine ring, but a methyl residue. (Received March 29, 1973; )     

On the rhythm of sensitivity to light interruption in a long-day duckweed, Lemna gibba G 3

1. The rhythm of sensitivity to light interruption in a long-dayduckweed, Lemna gibba G 3, was examined. The rhythm was circadianand was suggested to be under the control of a physiologicalclock. Light given in the trough between the first and secondpeaks reset the rhythm. Five to 7 cycles of non-circadian photoperiodicregimes given entrained the rhythm to external periodicity,and this entrained rhythm persisted even after the plants weretransferred to continuous darkness. 2. It was suggested that the induction period is not determinedby the physiological clock disclosed here, but by a periodicalternation of dark-sensitivity, or by a periodic change inactivity of SS (system sensitive to IPEF, induction period extendingfactor,) as postulated previously. (Received November 28, 1967; )     

Reversal of the dark inhibition of flowering in a long-day duckweed, Lemna gibba G3, by thymidine and related nucleosides

Effects of several nucleosides on flowering in Lemna gibba G3inhibited by darkness inserted just before an inductive lightperiod were investigated. Thymidine and deoxyuridine could reversethe inhibition, but deoxycytidine, cytidine, uridine, thymineand deoxyribose could not. Since interruption of die darknesswith a brief light period was effective similarly to additionof thymidine and deoxyuridine, the light-stimulated step inthymidine biosynthesis is probably the reaction of deoxyuridinesynthesis. Furthermore, maintenance of thymidine content isprobably required for the progress of floral induction. (Received March 29, 1973; )     

Uptake of uridine by a long-day duckweed, Lemna gibba G3

Uptake of uridine by a long-day duckweed, Lemna gibba G₃ wasexamined. Km and Vmax for uptake were in the range of 1 to 2x10^–5 M and of 5 to 10 x10^–8 moles/g fresh weight/2hr, respectively. Uptake rate depended on temperature, and theoptimum pH was 5.0. Uridine uptake was competitively inhibitedby some compounds structurally analogous to uridine. However,the activity of uridine kinase was not affected by these compounds,except for cytidine. Uridine uptake was inhibited by metabolicinhibitors, in which uridine taken up was left unconverted toother forms, especially in the presence of DNP. These resultssuggest that uridine was taken up into the duckweed celb bya specific transport system and immediately phosphorylated byuridine kinase. Phosphorylation of uridine was not associatedwith the uridine transport reaction. (Received November 15, 1976; )     

Chemical control of flowering in the long-day plantLemna gibba G3

Lemna gibba G3 is an ideal system for studying the chemical control of flowering in a photoperiodic plant due to its small size and aquatic growth habit which allow substances to be taken up continuously and rapidly distributed throughout the plant. Each of the known plant growth regulators has been tested onL. gibba G3 and only the gibberellins appear to be important for flowering, although they are not the limiting factor for flowering on short days. Salicylic acid (SA) and ferricyanide will both induce flowering inL. gibba G3 with ferricyanide being most effective on short days where flowering is daylength limited and SA most effective where flowering is limited by factors other than daylength. The ferricyanide action is probably due to HCN and it may act during photoperception or photoinduction. SA is most effective when reversing the inhibition caused by various parameters including copper and agar, and its effect is always strongly daylength dependent. It is postulated that SA may interact with the flowering stimulus to promote flowering and thus that SA acts at some point following photoinduction and the formation of the flowering stimulus.     

Spectral dependence of the critical photoperiod in the long-day duckweed, Lemna gibba G3

Spectral dependence of the light sensitive L1- and L2-phasesof the critical photoperiod of Lemna gibba G3 was examined bythe min-LD method for wavelengths ranging from ca. 420 to 800nm. The L1-phase had a sharp peak of sensitivity at 650–680nm and a gentle slope of sensitivity ascending from 500 nm towardshorter wavelengths. The L2-phase showed remarkable sensitivityaround 750 nm and in the blue-violet regions (500 nm). Red/far-redphotoreversibility was confirmed for the LI-phase, but not forthe L2-phase. The properties of the light receptors involvedare discussed briefly. ¹Present address: National Institute for Basic Biology, Myodaiji,Okazaki 444, Japan. (Received May 14, 1979; )     

Endogenous light-on rhythm in respiration of a long-day duckweed, Lemna gibba G3

1. The rate of O₂-uptake of Lemna gibba G₃ changed with a dampeddiurnal rhythm under continuous illumination given after shortdays. The rhythm was started by a light-on stimulus with a 6hr lag period and is thought to be under the control of a biologicalclock. 2. The 6 hr lag period was replaceable with a 6 hr dark periodinterrupted twice (at 0 and 3 hr) by a brief illumination withred light. The effect of red light was removed by immediateexposure to far-red light. This effect of far-red was reversedby subsequent red light. The 6 hr lag may involve a phytochrome-mediatedreaction which may be preparatory to the induction of this rhythm. (Received December 13, 1969; )     

Gibberellin-EDDHA interaction in flowering and gibbosity of Lemna gibba G3

The effect of gibberellic acid (GA₃), in the presence of EDDHA,on the flowering and gibbosity of Lemna gibba G3 was studied.At 10 ppm and at higher concentrations of GA₃ the EDDHA-effect,i.e. profuse flowering and conspicuously gibbous fronds, wascompletely nullified. (Received July 15, 1974; )     

Induction of flowering in Lemna gibba G 3 by Fe-EDDHA

Fe-EDDHA (iron salt of ethylenediamine-di-o-hydroxyphenylaceticacid) induced profuse flowering in Lemna gibba G 3 culturedin HUTNER'S medium. The maximum number of flowering plants wasobserved in a medium supplemented with 5 ppm of this chelate. (Received April 20, 1970; )     

Suppression of long-day flowering by nitrogenous compounds in Lemna perpusilla 6746

The long-day flowering of Lemna perpusilla 6746 on an SH inhibitor-containingmedium was inhibited by the application of ammonium ion to themedium. Ammonium ion not only suppressed long-day flowering,but relieved the inhibition of vegetative growth caused by theinhibitors. Nitrite, casamino acids, glutamine and asparaginehad a similar effect, suggesting that the inhibition of long-dayflowering by ammonium ion is not a direct effect of the ion.Most amino acids, with the exception of glutamate and aspartate,also prevented long-day flowering, but their effects on vegetativegrowth varied. No qualitative differences in amino acid compositionwere observed among plants cultured on media containing nitrate,nitrite or NH4₄NO₃as the sole nitrogen source. However, theamounts of free and total amino acids werehigher in plants fedwith nitrite or NH₄NO₃ than in those fed with nitrate. Thissuggests that the inhibition of long-day flowering by ammoniumand nitrite can be ascribed to increased nitrogen metabolism. Though decreased activity by SH inhibitors of nitrate reductase(SH enzyme) is assumed to result in long-day flowering by loweringthe nitrogen metabolism, lowering the nitrogen level in M mediumdid not bring about floral initiation in the absence of SH inhibitors. (Received January 7, 1975; )     

Spectral dependences of flowering in Lemna perpusilla and L. gibba

Floral induction in Lemna perpusilla and L. gibba was determinedunder continuous irradiation with monochromatic light in spectralranges from 396 to 765 nm. In the former it was induced underwavelengths from about 400 to 550 nm and longer than 700 nm,while in the latter with wavelengths near 400 nm and from about550 to 650 nm. The patterns of these spectral dependences werenearly mirror images and corresponded to the P_fr level in thephotostationary states of phytochrome. (Received December 3, 1974; )     

Induction of flowering in Lemna gibba G3 under short-day conditions

In the presence of the chelating agent EDDHA, long-day duckweedLemna gibba G3 was induced to flower under a short-day scheduleof 9 hr of light and 15 hr of darkness in a 24-hr cycle. Weconcluded that EDDHA creates effects very similar to those ofsalicylic acid. When EDDHA or salicylic acid was added to thenutrient medium in combination with BA, flowering was inducedeven under conditions of 8 hr of light and 16 hr of darkness.Under a photoperiod of 9 hr, BA markedly enhanced the effectof EDDHA as well as salicylic acid. On the other hand, BA alonewas ineffective as far as flowering was concerned. By quantitativeinteractions, BA seems to complement the modifying effect ofEDDHA or salicylic acid on flowering in this duckweed strain. (Received June 25, 1976; )     

Short-day flowering of Lemna gibba G3 induced by salicylic acid

Salicylic acid, probably as a chelating agent of the EDTA-salicylaldoximetype, can eliminate the light requirement during the inductivephase of Lemna gibba G3, and thus is able to induce short-dayflowering of this long-day plant. (Received September 4, 1975; )     

Further studies on the diurnal rhythm of uridine incorporation into RNA in the long-day duckweed, Lemna gibba G3

Using the long-day duckweed Lemna gibba G3, the changes in theactivities of RNA synthesis in isolated nuclei and chloroplastsand of the reaction prerequisite for the incorporation of exogenousuridine into RNA were examined. When the duckweed was exposedto either a light-dark cycle or to continuous light, the activityof RNA synthesis in the nuclear and chloroplast fractions changeddiurnally and reached its highest levels during the night phase.The changes coincided with uridine incorporation into RNA invivo. However, the amount of radioactive uridine taken up intothe acid-soluble fraction remained unchanged during the wholeday. The proportion of radioactivity incorporated into phosphorylateduridine compounds as well as UTP+UDP to the radioactivity inthis fraction remained constant. Thus, the diurnal rhythm ofuridine incorporation into RNA was related to the diurnal rhythmof RNA synthesis in isolated nuclei and chloroplasts. The loweractivity of uridine incorporation into RNA under continuousdarkness may be determined by the activity of RNA synthesisin nuclei and chloroplasts as well as the uptake rate of uridineinto the duckweed cells, not by the activity of its phosphorylation. (Received August 30, 1977; )