Developmental and photobiological factors affecting photoperiodic induction in Arabidopsis thaliana Heynh. Landsberg erecta

We have tested whether the promotion of flowering by long days(LD) in Arabidopsis thaliana is a consequence of photoperiodicinduction. To achieve this, the flowering responses of Arabidopsisthaliana (L.) Heynh. Landsberg erecta (Ler) and the long-hypocotylmutants hy2, hy3 and hy4 were determined with respect to age,daylength and light quality. Ler was capable of distinguishingbetween short days (SD) and long days (LD) from about 4 d aftersowing at 20 C, the time at which cotyledons were expandingand greening. At this stage, the critical daylength was between8 h and 10 h. At 7 d, seedlings required five LD for inductionand, as the seedlings aged, they became more sensitive so thatby day 20, one LD was fully inductive. The response to SD innewly germinated seedlings was to delay flowering without alteringleaf number, but after about 10 d, delay of flowering by SDwas accompanied by extra leaves. In light quality experiments,blue light (B) was inductive for 5-d-old plants and in all subsequenttreatments, far-red (FR) caused induction in treatments at 12d and 18 d and low pressure sodium, equivalent to red, was notinductive at 5 d and 12 d, but partially inductive at day 18.Hence, both a specific blue-light photoreceptor and phytochromeA in High Irradiance Response mode promote floral induction.In daylength transfer experiments all three hy mutants respondedto LD by earlier flowering. Both hy2 and hy3 produced substantiallyfewer leaves than Ler in SD and hy3 flowered slightly earlierthan Ler. The hy4 mutants flowered later than Ler in SD andhad a higher leaf number. A scheme is proposed in which photoperiodicinduction depends on the ability of the plant to sense photoperiod,the stage of development and the photobiological input. We alsopropose that phytochrome A and the blue photoreceptor promoteflowering whereas phytochrome B promotes vegetative development. Key words: Arabidopsis thaliana, blue-absorbing photoreceptor, flowering, photoperiodic induction, phytochrome     

The transition to reproductive phase inchenopodium murale L. ecotype 197 - Early flowering long-day plant

Five days of suitable continuous light induced flowering in the majority ofChenopodium murale L. ecotype 197 plants as early as at the phase of the first pair of leaves. At the time of initiation of the 2nd to 4th pairs of leaves the capacity of plants to flower was reduced, the number of flowering plants being significantly lower under the same inductive light treatment. The capacity to flower increased again at the phase of the 5th and the 6th pairs of leaves. Inductive light treatment brought about a marked growth activation of organs present before induction, shoot apex elongation, precocious formation of new leaves and activation of axillary meristems. The course of these changes in plants of different age is demonstrated. The terminal flower developed during 5 short days following inductive light treatment. The paper shows similarities and differences between long-daymutale L. ecotype 197 and short-day C.rubrum L. ecotype 374 grown under practically uniform conditions.     

Studies on the Photoreceptors for the Promotion and Inhibition of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy

Three-day-old etiolated seedlings of Pharbitis nil were exposedto red light for 10 min and sprayed with N⁶-benzyladenine beforetransfer to a 48-h inductive dark period, after which they weregrown under continuous white light. A second red irradiationpromoted flowering when given at the 5 and 24th hour of theinductive dark period but inhibited flowering at the 10 and15th hour. Far-red light inhibited flowering when given at anytime during the first 24 h of the dark period. Red/far-red reversibilitywas clearly observed at the 0, 5, 10 and 24th hour, but notat the 15th hour when both red and far-red lights completelyinhibited flowering. The action spectrum for the inhibition of flowering at the 15thhour of the inductive dark period had a sharply defined peakat 660 nm and closely resembled the absorption spectrum of theP_R form of phytochrome. The photoreceptors involved in thesephotoreactions are discussed. (Received June 10, 1983; Accepted July 6, 1983)     

Juvenility and flowering of Brunonia australis (Goodeniaceae) and Calandrinia sp. (Portulacaceae) in relation to vernalization and daylength

Background and Aims

The time at which plants are transferred to floral inductive conditions affects the onset of flowering and plant morphology, due to juvenility. Plants of Brunonia australis and Calandrinia sp. were used to investigate whether Australian native ephemeral species show a distinct juvenile phase that can be extended to increase vegetative growth and flowering.

Methods

The juvenile phase was quantified by transferring seedlings from less inductive (short day and 30/20°C) to inductive (vernalization or long day) conditions at six different plant ages ranging from 4 to 35 d after seed germination. An increase in days to first visible floral bud and leaf number were used to signify the end of juvenility.

Key Results

Brunonia australis was receptive to floral inductive long day conditions about 18–22 d after seed germination, whereas plants aged 4–35 d appeared vernalization sensitive. Overall, transferring plants of B. australis from short to long day conditions reduced the time to anthesis compared with vernalization or constant short day conditions. Calandrinia sp. showed a facultative requirement for vernalization and an insensitive phase was not detected. Floral bud and branch production increased favourably as plant age at time of transfer to inductive conditions increased. Younger plants showed the shortest crop production time.

Conclusions

Both species can perceive the vernalization floral stimulus from a very young age, whereas the photoperiodic stimulus is perceived by B. australis after a period of vegetative growth. However, extending the juvenile phase can promote foliage development and enhance flower production of both species.     

Promotive effect of abscisic acid in flowering ofChenopodium rubrum as the result of decreasing apical dominance

Abscisic acid (ABA) was applied in a concentration of 1. 10^?3 M and 1. 10^?4 M to the quantitative SD plantChenopodium rubrum under various light regimes. ABA did not influence flowering in plants under continuous illumination, enhanced flowering in plants subjected to long days and inhibited it in plants induced by short days. It was concluded that ABA can not substitute for inductive treatment but its action may be additive to initial stages of reproductive morphogenesis (enhanced growth rate and branching of the apical meristem) as evoked by long days.     

Promotion and Inhibition of Inflorescence Growth by Long Days in Short-day Plants

Results of previous investigators have indicated that long periodsof light intercalated between inductive short-day cycles havean inhibitory effect on inflorescence growth in short-day plants.The present experiments show that such light periods can eitherpromote or inhibit inflorescence growth in Xanthium pemtsylvanicumand Chenopodium amaranticolor depending on their previous degreeof induction. Intercalated light exerts an inhibitory influence on the inductiveprocesses occurring during the dark period which follows itwhen unifoliate Xanthium plants have been previously exposedto not more than one short day and when fully foliated Chenopodiumplants have been previously exposed to not more than one ortwo short days. When plants are more strongly induced initially,an intercalated light period has a very marked promoting effecton the dark period succeeding it. In Xanthium this stimulatoryeffect increases with the duration of the light period up toan optimum of approximately 80 hours. It is suggested on the basis of available evidence that thepromotive effect of such intercalated light possibly affectsthe sensitivity of the apex to inductive stimuli and that itsinhibitory effect acts on the inductive processes occurringin the leaves.     

Effect of growth substances on flowering and sex expression in isolated apical buds of Spinacia oleracea

Apical buds of Spinacia oleracea L. cv. Matador were isolated from 7-day-old vegetative seedlings and grown in sterile culture under inductive long, or non-inductive short photoperiods. Flowering of isolated apices was inducible in long days in approximately 75% of the plants, and in short days by gibberellin treatment (about 40%) or by raising the temperature to 30–32°C (13%). In long days the percentage of flowering was further increased by gibberellin treatment (up to 90%), while it was unaffected by abscisic acid and was strongly decreased by Amo 1618 (55%). In long days the ratio of male to female plants was near 1. The percentage of female plants in long days was increased by gibberellins, and the percentage of male ones decreased by kinetin; as a consequence, the ratio of male to female plants was lowered to about 0.50 in both cases. Abscisic acid and Amo 1618 had the opposite effect, probably because they decreased the flowering in female plants, so that the sex ratio was shifted to 2.6 and 6.8, respectively. Simultaneous treatment with GA₃ reversed the effect of abscisic acid on the sex ratio, but the reversal of the shift to maleness induced by Amo 1618 was only partial. In short days, gibberellins also stimulated the flowering in female plants more than in male. However, when the flowering was induced by higher temperature, most flowering plants were male and kinetin increased their percentage further. The above results have been discussed in terms of different requirements for flowering in male and female plants.     

Evidence for a Flowering Inhibitor Produced in Long Days in Kalanchoe blossfeldiana

Experiments with Kalanchoe blossfeldiana are described in whichperiods of short-day treatment were interrupted by intercalatedlong days or light breaks during long dark periods. The effectsof 24-hour dark periods preceding and following such intercalatedlong days were also investigated. The results of these experiments have shown that: Single longdays intercalated between numbers of short days have a positiveinhibitory effect on flower initiation and are not merely ineffective.The inhibitory effect expressed as the number of inductive cyclesannulled is approximately additive, provided the long days areinterspersed with short days, but not if several long days aregiven consecutively. On the average 1 long day is capable ofannulling the flower-promoting effect of about 1 short days.To a first approximation flower numbers in Kalanchoe increaseexponentially with the number of inductive cycles given—upto at least 12 short days; the inhibitory effect of long daysinterspersed with short days also fits an exponential curve;i.e. the inhibition is roughly proportional to the amount ofprevious photo-periodic induction. A light break of as littleas 30 seconds' duration given in the middle of a long dark periodis as inhibitory as a long day. If followed by a long dark periodthe inhibition of an intercalated long day is almost completelyneutralized; a long dark period preceding it has no such effect. These results have been interpreted as due to the interactionof a flowering inhibitor with a reaction leading to flowering.A mechanism involving competitive inhibition of an adaptivelyformed enzyme has been described as a possible example of thekind of reaction which could account for the results presented.     

Correlations between Growth and Flowering in Chenopodium amaranticolor: II. Leaf and Stem Growth

Plants of Chenopodium amaranticolor grown in long photoperiodsfor 26 days were exposed to 0, 2, 6, or continuous short days(SD) and the resultant changes in leaf and stem growth on returnto long days recorded. Growth of the leaves, the main stem,and the axillary branches was initially strongly stimulatedby short days. Continued growth of leaves was inhibited by thedevelopment of flowers to anthesis; strong inhibition was observedafter 6 and continuous SDs, but not in the 2-SD or control groups,neither of which reached anthesis during the investigation. In the groups which flowered strongly, the intensity of inhibitionwas sufficient to mask completely the effect of the initialstimulation on final leaf and stem size. In the 2-SD group,however, lack of inhibition allowed full expression of the effectsof stimulation on the final leaf and stem dimensions. The system of stimulators and inhibitors controlling leaf growthprobably also brings about the changes in leaf shape associatedwith the onset of flowering. The results are discussed in relation to previous findings andit is suggested that similar stimulations of growth correlatedwith the induction of flowering might occur widely in both short-and long-day plants.     

Floral and growth responses in Chenopodium rubrum L. to an extract from flowering Nicotiana tabacum L.

Flowering of Chenopodium rubrum seedling plants was obtained in continuous light after application of fractions of a partially purified extract from leaves of flowering Maryland Mammoth tobacco (Nicotiana tabacum). The stage of flowal differentiation was dependent on the age of the Chenopodium plants used for the bioassay. Apices of plants treated with the extract at the age of four or seven days showed an advanced branching of the meristem or the beginning of formation of a terminal flower; treatment with the extract of plants 12 d old resulted in rapid formation of flower buds in all assay plants. Non-treated control plants kept in continuous light remained fully vegetative. The effects of the extract on flowering were associated with pronounced growth effects. Floral differentiation was preceeded by elongation of the shoot apex. Extension of all axial organs occurred, while growth of leaves, including leaf primordia, was inhibited. The pattern of growth after application of the flower-inducing substance(s) did not resemble the effects of the known phytohormones, but showed some similarities to growth changes resulting from photoperiodic induction of flowering.     

Apical Growth and Modification of the Development of Primordia During Re-flowering of Reverted Plants of Impatiens balsamina L

Impatiens balsamina L. was induced to flower by exposure to5 short days and then made to revert to vegetative growth byreturn to long days. After 9 long days reverted plants wereinduced to re-flower by returning them to short days. Petalinitiation began immediately and seven primordia already presentdeveloped into petals instead of into predominantly leaf-likeorgans. However, the arrangement of primordia at the shoot apex,their rate of initiation and size at initiation remained unchangedfrom the reverted apex, as did apical growth rate and the lengthof stem frusta at initiation. The more rapid flowering of thereverted plants than of plants when first induced, and the lackof change in apical growth pattern, imply that the revertedapices remain partially evoked, and that the apical growth patternand phyllotaxis typical of the flower, and already present inthe reverted plants, facilitate the transition to flower formation. Impatiens balsamina, flower reversion, partial evocation, shoot meristem, determination, leaf development     

Inhibition imposed by developing flowers on further flower-bud initiation in Chamelaucium uncinatum Schau

In Chamelaucium uncinatum, an Australian woody perennial, flower initiation ceases under continuous inductive short-day (SD) conditions after the first flowering flush. The developing flowers were found to be the prime cause of the cessation in flower initiation. Removal of flowering shoots or lowers as soon as the buds appeared resulted in continuous flower formation. Pruning the plants below the young flower buds at the same stage also caused increased flower formation at the tips of the new growth. If pruning was delayed until flower buds were approx. 3 mm in diameter, however, nor further flower initiation took place and the plants, although still under inductive conditions, shifted to vegetative growth. The inhibiting factor is translocated from one branch to another. At least a six-week rest period (a vegetative growth period under long-day conditions) is needed before the plants are able to respond to further SD stimuli.Abbreviations LD long day - SD short day - SE standard error     

Flowering of Stylosanthes guianensis in Relation to Juvenility and the Long-Short Day Requirement

Seedlings of Stylosanthes guianensis var. guianensis were grownin long (14 h) days in five temperature regimes for varyingperiods before transfer to short (11 h) days at 30 ?C/21 ?C.The juvenile phase before seedlings responded to inductive conditionswas c. 45–50 d, 50–60 d and 60–70 d for cv.Schofield, cv. Cook and C.P.I. 34906 respectively, which ispositively related to their critical photoperiod for flowering.Temperatures favourable for growth (e.g. 30 ?C/26 ?C) reducedthe juvenile phase in C.P.I. 34906 and in Cook, which did notflower in 11 h days unless previously exposed to more than 18long days. In a second experiment cv. Cook was confirmed as a long-shortday plant. Seedlings were grown for 50 d in a glasshouse withnatural daylength extended to 13, 14, 16 or 24 h before transferto 12 h photoperiods. Cook floral development was positivelyrelated to daylength provenance before transfer and plants incontinuous 12 h did not flower. Shortening daylength after 48 cycles of 12 h to 11.75 h didnot result in continued floral development in Cook plants butcv. Graham plants were initiated or transitional by 75 d. Key words: Stylosanthes guianensis, Photoperiod, Temperature, Flowering     

Changes in RNA levels in the shoot apex of Silene during the transition to flowering

Changes in RNA concentration in the shoot apical meristem during induction and the transition to flowering were measured histochemically in Silene coeli-rosa (L.) Godron, a long-day plant. In the apices of plants induced by 7 long days the RNA concentration increased to about 25 per cent higher than in non-induced plants. Three long days did not induce flowering but resulted in a transient rise in RNA concentration. When plants were given long days interrupted by varying numbers of short days successful induction was accompanied by a sustained increase in RNA concentration but those treatments which were not inductive gave only transient increases in RNA. Gibberellic acid had no effect on induction or apical growth rates but increased the RNA concentration by 50 per cent or more in both induced and non-induced plants. Plants induced to flower at 13° C had the same RNA concentration and growth rate at the apex as in non-induced plants at 20° C. Since changes in RNA concentration in the apex could occur without changes in growth rate and without flowering, and induction could occur without a change in RNA concentration or growth rate, it is suggested that the increase in RNA and growth rate which normally occur at the transition to flowering might not be essential for the formation of a flower but may be more closely related to the rapid growth associated with the formation of the inflorescence.Abbreviations LD long day - SD short-day     

Oak Seedlings Grown in Different Light Qualities

Seedlings of oak (Quercus robur) were germinated in darkness for 3 weeks and then given continuous light or short pulses of light (5–8 min every day). The morphological development was followed during 25 days. In continuous white, blue, and red light the stem growth terminated after about 10 days by formation of a resting bud. At that time the seedlings were about 100 mm high. In con tinuous long wavelength farred light (wavelength longer than 700 nm) the stem growth including leaf formation was continuous without the formation of resting buds, and the stem length was about 270 mm after 25 days. The number of nodes developed became twice that of the seedlings grown in while light. The leaves became well developed in all light colours, but leaf areas were largest in plants cultivated in white light. Compared to dark grown seedlings the mean area per leaf was increased about five times in continuous long wavelength far red light. A supplement with short (5 min) pulses of red light each day increased the leaf area up to 20 times. The stem elongation showed a high energy reaction response, i.e. the stem length increased only in continuous long wavelength far-red light but was not influenced by short pulses of red light or far-red light. The leaf expansion, however, was increased by short pulses of red light with a partial reversion of the effect by a subsequent pulse of far-red light. The fraction of the plant covered with periderm was higher in plants given continuous light. In respect to periderm inhibition continuous long wavelength far red light was the most effective. The transfer of seedlings from darkness to continuous white light gave anthocyanin formation in the stem 10–20 mm below the apex. This formation took place in the cortex and was evident in plants grown in darkness or under short pulses of light. Plants grown in continuous red, blue or long wavelength Far red light showed only traces of anthocyanin.     

Photoperiod Sensitivity during Flower Development of Opium Poppy (Papaver somniferum L.)

Photoperiod is a major factor in flower development of the opiumpoppy (Papaver somniferum L. ‘album DC’) which isa long-day plant. Predicting time to flower in field-grown opiumpoppy requires knowledge of which stages of growth are sensitiveto photoperiod and how the rate of flower development is influencedby photoperiod. The objective of this work was to determinewhen poppy plants first become sensitive to photoperiod andhow long photoperiod continues to influence the time to firstflower under consistent temperature conditions. Plants weregrown in artificially-lit growth chambers with either a 16-hphotoperiod (highly flower inductive) or a 9-h photoperiod (non-inductive).Plants were transferred at 1 to 3-d intervals from a 16- toa 9-h photoperiod andvice versa . All chambers were maintainedat a 12-h thermoperiod of 25/20 °C. Poppy plants becamesensitive to photoperiod 4 d after emergence and required aminimum of four inductive cycles (short dark periods) beforethe plant flowered. Additional inductive cycles, up to a maximumof nine, hastened flowering. After 13 inductive cycles, floweringtime was no longer influenced by photoperiod. These resultsindicate that the interval between emergence and first flowercan be divided into four phases: (1) a photoperiod-insensitivejuvenile phase (JP); (2) a photoperiod-sensitive inductive phase(PSP); (3) a photoperiod-sensitive post-inductive phase (PSPP);and (4) a photoperiod-insensitive post-inductive phase (PIPP).The minimum durations of these phases forPapaver somniferum‘album DC’ under the conditions of our experimentwere determined as 4 d, 4 d, 9 d, and 14 d, respectively. Anthesis; days to flowering; flower bud; opium poppy; Papaver somniferum L.; photoperiod; photoperiod sensitivity; predicting time to flowering; transfer     

Ethylene and ABA interactions in the regulation of flower induction in Pharbitis nil

Hormones are included in the essential elements that control the induction of flowering. Ethylene is thought to be a strong inhibitor of flowering in short day plants (SDPs), whereas the involvement of abscisic acid (ABA) in the regulation of flowering of plants is not well understood. The dual role of ABA in the photoperiodic flower induction of the SDP Pharbitis nil and the interaction between ABA and ethylene were examined in the present experiments. Application of ABA on the cotyledons during the inductive 16-h-long night inhibited flowering. However, ABA application on the cotyledons or the shoot apices during the subinductive 12-h-long night resulted in slight stimulation of flowering. Application of ABA also resulted in enhanced ethylene production. Whereas nordihydroguaiaretic acid (NDGA) - an ABA biosynthesis inhibitor - applied on the cotyledons of 5-d-old seedlings during the inductive night inhibited both the formation of axillary and of terminal flower buds, application of 2-aminoethoxyvinylglycine (AVG) and 2,5-norbornadiene (NBD) - inhibitors of ethylene action - reversed the inhibitory effect of ABA on flowering. ABA levels in the cotyledons of seedlings exposed to a 16-h-long inductive night markedly increased. Such an effect was not observed when the inductive night was interrupted with a 15-min-long red light pulse or when seedlings were treated at the same time with gaseous ethylene during the dark period. Lower levels of ABA were observed in seedlings treated with NDGA during the inductive night. These results may suggest that ABA plays an important role in the photoperiodic induction of flowering in P. nil seedlings, and that the inhibitory effect of ethylene on P. nil flowering inhibition may depend on its influence on the ABA level. A reversal of the inhibitory effect of ethylene on flower induction through a simultaneous treatment of induced seedlings with both ethylene and ABA strongly supports this hypothesis.     

Effect of High-intensity Light Given Prior to Low-temperature Treatment on the Long-day Flowering of Pharbitis nil

Pharbitis nil, strain Violet which had been exposed to high-intensitylight (18,000 lux at 23?C) for 7 days followed by a low-temperaturetreatment (13–14?C) for 7 days initiated flower buds evenunder continuous light, but plants given these treatments inreverse order failed to bud. Three days of high-intensity lightat 23?C was most effective in promoting the flower-inducingeffect of the subsequent low-temperature period. Six days oflow temperature following the 3-day high-intensity light periodinduced near-maximum flowering response. DCMU (5?10^–6M) given during the high-intensity light period inhibited flowering,but when given during or after the low-temperature period itwas ineffective. DCMU at the same concentration given before,during or after an inductive 16-hr dark period at 26?C did notinhibit flowering. Sucrose, ATP, NADPH and some other reducingagents tested did not nullify the DCMU effect nor substitutefor the effect of high-intensity light. But, the high-intensitylight effect could be substituted, at least partly, by 5-chlorosalicylicacid, 3,4-dichlorobenzoic acid and some other benzoic acid derivatives,which are highly effective in inducing long-day flowering inthe short-day plant, Lemna paucicostata. (Received October 20, 1981; Accepted February 3, 1982)     

Determination of the Length of the Vegetative and Pre-floral Stages in the Day-Neutral Plant Helianthus annuus by Chilling Pulses

Helianthus annuus seedlings grown in an 18 h day at 28 ?C wereexposed to one 6 d chilling pulse of 12 ?C, at spaced timesduring the first 21 d from sowing. At 2 d intervals, the terminalbuds of 5 plants were dissected to determine leaf number andto score the vegetative or flowering state of the shoot apex.It was found that, while the rate of leaf initiation was reducedequally by each chilling pulse, pulses commencing on days 9or 12 reduced the total leaf number from 30 to 26, while pulsesapplied earlier had little effect. This variation is interpretedin terms of the time available for leaf production. The apicesof control plants commenced the visible transition to flowering16 d after sowing. Chilling pulses applied from days 3 or 6delayed this transition by about 5 d, whereas later pulses causedonly a 1•5 d delay. In a second experiment, where the chillingwas reduced to 2 d duration, it was again found that chillingdelayed flowering during the first 8 d and was progressivelyless effective when applied later. From this variation in temperaturesensitivity it is proposed that chilling sunflower plants immediatelyafter sowing delays flowering by extending the vegetative phaseof growth and so delaying the attainment of a ‘ripenessto flower’ state that appears to coincide with the expansionof the first pair of leaves. From day 8 onwards processes leadingto flowering that are relatively temperature insensitive apparentlybecome dominant in the apex and result in visible signs of flowering8 d later, although during this transitional stage leaf primordiacontinue to be initiated on the flanks of the apex.     

Rhythmicity of Flowering in Pharbitis nil

When young seedlings of Pharbitis nil are grown under continuous light, except for a single inductive dark period, they flower to a varying degree, depending on when this dark period is given. Plants become sensitive to this induction approximately three days after the seedlings emerge from the soil. The expression of flowering varies in a rhythmic fashion for three or more cycles, when an inductive dark period is given at progressively later times. The time between maximum expression of flowering is 24 hours or somewhat longer. It appears necessary that the inductive dark period be of sufficient duration, to only partially induce the plants to flower for this rhythm to be expressed. Under the conditions employed in this study, this duration is 12 hours. If this rhythm is endogenous, it exists at least from when the plants emerged from the soil since no environmental cues are given after that time, and it raises questions of the interpretations of data from previous studies with this organism.