Control of Flowering of Xanthium pensylvanicum by Red and Far-red Light

A study was made of the effects of various durations, intensities and combinations of red and far-red light interruptions on the flowering responses of Xanthium pensylvanicum Wallr. A dual response to treatments of far-red light was observed. In short dark periods, far-red light alone did not greatly affect flowering but was able to overcome the inhibition of flowering caused by red light. In dark periods longer than 15 hours, far-red inhibited flowering and added to rather than overcame the inhibition by red light. The dark period length required for far-red inhibition remained the same whether far-red was given at the start or at the eighth hour of darkness.

In 48-hour dark periods Xanthium showed 3 responses to additions of red and far-red light breaks: A) response to red light; B) response to far-red light; and C) response to red followed by far-red light. Red light given any time in the first 30 hours of darkness overcame the inhibitory effect of far-red light given at either the start or the eighth hour of darkness. Red light given later than the thirtieth hour did not overcome the far-red effect.

Approximately the same energy of red light was required to overcome the inhibitory effect of far-red at the second hour of darkness as was required to produce maximum red light inhibition at the eighth hour. Although far-red light was most inhibitory when given early in a long dark period, approximately the same energy of far-red light was required to saturate the far-red response at the fourth, eighth and sixteenth hours.

The results are discussed in relation to other reports of far-red inhibition of flowering in short-day plants.

     

Studies of the Involvement of an Endogenous Rhythm in the Photoperiodic Response of Hyoscyamus niger

An attempt was made to determine the involvement of an endogenous circadian rhythm in the flowering response of the long-day plant Hyoscyamus niger L. grown in a modified White's medium. Both variable-cycle-length and light interruption experiments were employed in this attempt. In the variable-cycle experiments, plants were subjected to light periods of 6, 12, or 18 hours followed by varying lengths of darkness. The total lengths of the cycles varied from 12 to 72 hours. In experiments utilizing a 6-hour photoperiod, a high level of flowering occurred in cycle lengths of 12, 36, and 60 hours. Flowering was suppressed in the 24-, 48-, and 72-hour cycles. When a 12-hour photoperiod was used the flowering response was low between 24 and 36 hours and flowering did not indicate a rhythmic response. When an 18-hour photoperiod was used, the flowering response was suppressed in the 36- and 60-hour cycles.

Light-break experiments were conducted to study further the flowering response in Hyoscyamus. These experiments consisted of a 6-hour main photoperiod followed by varying lengths of darkness to make cycles of 24, 48, and 72 hours. At given intervals the dark period was interrupted by 2-hour light breaks. In a 24-hour cycle, flowering was promoted when a light break was given at either the twelfth or eighteenth hour of the cycle. In a 48-hour cycle, flowering was strongly promoted by light breaks given near the beginning or at the end of the dark period. In a 72-hour cycle, light breaks given at the eighteenth, forty-second, and sixty-sixth hour of the cycle stimulated flowering as compared with light breaks given at the thirtieth and fifty-fourth hour. These results are indicative of the involvement of an endogenous rhythm in the flowering response of Hyoscyamus niger.

     

Effect of Low-intensity Red and Far-red Light and High-intensity White Light on the Flowering Response of the Long-day Plant Lemna gibba G3

The long-day plant Lemna gibba L., strain G3 exhibits a relatively low sensitivity to short, white-light interruptions given during the dark period of a short-day cycle. However, the plants are fairly sensitive to low-intensity red light treatments given during a 15-hour dark period on the third day of a 2LD-(9L:15D)-2LD-7SD schedule. Far-red light is almost as effective as red light, and attempts to reverse the red light response with subsequent far-red light treatments have not been successful. Blue light proved to be without effect. When plants were grown on a 48-hour cycle with 15 minutes of red light every 4 hours during the dark period, the critical daylength was reduced from about 32 hours to slightly less than 12 hours.

Continuous red light induced a fairly good flowering response. However, as little as 1 hour of white light each day gave a significant improvement in the flowering response over that of the continuous red light control. White light of 600 to 700 ft-c was more effective than white light of 60 to 70 ft-c. The white light was much more effective when divided into 2 equal exposures given 8 to 12 hours apart. These results suggest an increase in light sensitivity with regard to flower induction about 8 to 10 hours after the start of the light period.

     

Photoperiodic Control of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy : The Effect of Skeleton and Continuous Light Photoperiods

The control of night-break timing was studied in dark-grown seedlings of Pharbitis nil (Choisy cv. Violet) following a single continuous or skeleton photoperiod. There was a rhythmic response to a red (R) interruption of an inductive dark period, and the phasing of the rhythm was influenced by the preceding light treatment.

Following a continuous white light photoperiod of 6 hours or less, the points of maximum inhibition of flowering were constant in real time. Following a continuous photoperiod of more than 6 hours, maximum inhibition occurred at 9 and 32.5 hours after the end of the light period. The amplitude of the rhythm during the second circadian cycle was much reduced following prolonged photoperiods.

Following a skeleton photoperiod, the time of maximum sensitivity to a R interruption was always related to the second pulse of the skeleton, R₂, with the first point of maximum inhibition of flowering occurring after 12 to 18 hours and the second after 39 hours. Without a second R pulse, the time of maximum sensitivity to a R interruption was related to the initial R₁ pulse. A `light-off' or dusk signal was not mimicked by a R pulse ending a skeleton photoperiod; such a pulse only generated a `light-on' signal and initiated a new rhythm.

It is concluded that the timing of sensitivity to a R interruption of an inductive dark period in Pharbitis nil is controlled by a single circadian rhythm initiated by a light-on signal. After 6 hours in continuous white light, the phase of this rhythm is determined by the transition to darkness. Following an extended photoperiod, the timing characteristics were those of an hourglass; this seemed to be due to an effect on the coupling or expression of a single circadian timer during the second and subsequent cycles, rather than to the operation of a different timing mechanism.

In addition to the effects on timing, the photoperiod affected the magnitude of the flowering response.

     

Aspects of clock resetting in flowering of xanthium

Flowering is induced in Xanthium strumarium by a single dark period exceeding about 8.3 hours in length (the critical night). To study the mechanism which measures this dark period, plants were placed in growth chambers for about 2 days under constant light and temperature, given a phasing dark period terminated by an intervening light period (1 min to several hrs in duration), and finally a test dark period long enough normally to induce flowering. In some experiments, light interruptions during the test dark period were given to establish the time of maximum sensitivity.

If the phasing dark period was less than 5 hours long, its termination by a light flash only broadened the subsequent time of maximum sensitivity to a light flash, but the critical night was delayed. In causing the delay, the end of the intervening light period was acting like the dusk signal which initiated time measurement at the beginning of the phasing dark period.

If the phasing dark period was 6 hours or longer, time of maximum sensitivity during the subsequent test dark period was shifted by as much as 10 to 14 hours. In this case the light terminating the phasing dark period acted as a rephaser or a dawn signal.

Following a 7.5-hour phasing dark period, intervening light periods of 1 minute to 5 hours did not shift the subsequent time of maximum sensitivity, but with intervening light periods longer than 5 hours, termination of the light acts clearly like a dusk signal. The clock appears to be suspended during intervening light periods longer than 5 to 15 hours. It is restarted by a dusk signal. There is an anomaly with intervening light periods of 10 to 13 hours, following which time of maximum sensitivity is actually less than the usual 8 hours after dusk.

Ability of the clock in Xanthium to be rephased, suspended, restarted, or delayed, depending always upon conditions of the experiment, is characteristic of an oscillating timer and may confer upon this plant its ability to respond to a single inductive cycle. It is suggested that phytochrome may influence only the phase of the clock and not other aspects of flowering such as synthesis of flowering hormone.

     

Floral Inhibition of Biloxi Soybean During a 72-hour Cycle

The inhibitory effect of light interruptions given during the photophobe phases of a 72-hour cycle was studied with Biloxi soybean [Glycine max (L.) Merr.]. The basic 72-hour cycle consisted of 8 hours of light followed by 64 hours of darkness and was repeated 7 times. Supplementary white light treatments given at the twenty-fourth and/or forty-eighth hour of the cycle (photophil phases) promoted the flowering levels of the controls and kept light treatments given at the most inhibitory points from inhibiting flowering completely. Such supplementary light treatments did not affect the time of maximum sensitivity to light interruptions. When 30-minute light breaks were used, maximum inhibition occurred at the 16-, 43-, and 63-hour points. The duration of the light breaks affected the time of maximum inhibition when given during the second photophobe phase. The time of maximum inhibition occurred earlier with 4-hour light breaks than with either 3-minute or 2-hour light interruptions.

Three-minute red light interruptions produced essentially the same effect as 3-minute white light interruptions. Such treatments inhibited flowering completely in the first photophobe phase, inhibited flowering to only a small degree in the second photophobe phase, and inhibited flowering to an intermediate degree in the third photophobe phase. Far-red light interruptions strongly inhibited flowering in the first photophobe phase, especially when given early in the dark period. Three minutes of supplementary white light given at the twenty-fourth or forty-eighth hour of the cycle partially overcame the inhibitory effect of far-red light. Four hours of supplementary white light at these times completely overcame the far-red inhibition.

     

Dark reactions in the flowering ofLemna perpusilla 6746

Summary This study concerns the effects of red and far-red light on flowering in the short day plantLemna perpusilla 6746. The critical day length for maximum flowering was found to be 10 hours. Exposure to red light near the middle of the dark period inhibited flowering, and the time of maximum sensitivity to red light occurred 9 hours after the beginning of dark periods of either 14 or 17 hours. The inhibition by red light was not reversible by far-red light, which also inhibited flowering, especially when given early in the dark period. Flowering inhibited by exposure to far-red light at the beginning of the dark period could be restored by subsequent exposure to red light. It appears that two photoperiodic partial processes in some plants may be controlled by the red, far-red reversible pigment system.With 5 Figures in the Text     

Formation of chlorophyll B, and the fluorescence properties and photochemical activities of isolated plastids from greening pea seedlings

Chlorophyll b was first detectable after 10 minutes of illumination of etiolated pea seedlings (Pisum sativum L. var Greenfeast) with continuous white light. The chlorophyll a/b ratio decreased from 300 at 10 minutes to 15 after 1 hour. There was little change in the chlorophyll a/b ratio between 1 and 2 hours, and it declined to 3 between 2 and 5 hours of illumination. In red light, the time courses of total chlorophyll synthesis and chlorophyll a/b ratio were similar to those in white light for the first 5 hours of illumination. But with increasing time of illumination with red light, there was an increase in the chlorophyll a/b ratio to 7 after 30 hours. Illumination with white light of very low intensity also gave high chlorophyll a/b ratios. Seedlings which had been illuminated for varying periods and then returned to darkness always showed an increase in chlorophyll a/b ratio during the dark period. It is concluded that the synthesis of chlorophyll b is controlled by light.     

Light Rquirements for Photoperiodic Sensitivity in Cotyledons of Dark-grown Pharbitis nil

The light requirements for induction of flowering by a long dark period were investigated in dark-grown seedlings of Pharbitis nil Chois, cv. Violet. The cotyledons bcame photoperiodically sensitive to a 24 h dark period by two 1 min red irradiations (6.3 μmol m⁻² S⁻¹) separated by a 24 h dark period. The reversibility of the effect of brief red irradiations, and the effectiveness of low energies of red irradiation suggest the involvement of phytochrome in the induction of photoperiodic sensitivity. Partial de-etiolation occurred after these brief periods of red irradiation but the seedlings were not capable of net CO₂ uptakeeven 7 h after the start of the main light period that followed the critical dark period. A changing response to the duration of the priod of darkness given between the two short red irradiations showed the the correct phasing of an endogenous photoperiodic rhythm is needed for the attainment of photoperiodic snsitivity.     

A Persistent Daily Rhythm in Photosynthesis

The luminescent marine dinoflagellate, Gonyaulax polyedra, exhibits a diurnal rhythm in the rate of photosynthesis and photosynthetic capacity measured by incorporation of C¹⁴O₂, at different times of day. With cultures grown on alternating light and dark periods of 12 hours each, the maximum rate is at the 8th hour of the light period. Cultures transferred from day-night conditions to continuous dim light continue to show the rhythm of photosynthetic capacity (activity measured in bright light) but not of photosynthesis (activity measured in existing dim light). Cultures transferred to continuous bright light, however, do not show any rhythm. Several other properties of the photosynthetic rhythm are similar to those of previously reported rhythms of luminescence and cell division. This similarity suggests that a single mechanism regulates the various rhythms.     

Rhythms as photoperiodic timers in the control of flowring in Chenopodium rubrum L.

Summary In C. rubrum, the amount of flowering that is induced by a single dark period interrupting continuous light depends upon the duration of darkness. A rhythmic oscillation in sensitivity to the time that light terminates darkness regulates the level of flowering. The period length of this oscillation is close to 30 hours, peaks of the rhythm occurring at about 13, 43 and 73 h of darkness.Phasing of the rhythm by 6-, 12- and 18-h photoperiods was studied by exposing plants to a given photoperiod at different phases of the free-running oscillation in darkness. The shift in phase of the rhythm was then determined by varying the length of the dark period following the photoperiod; this dark period was terminated by continuous light.With a 6-h photoperiod the timing of both the light-on and light-off signals is shown to control rhythm phasing. However, when the photoperiod is increased to 12 or 18 h, only the light-off signal determines phasing of the rhythm. In prolonged periods of irradiation-12 to 62 h light—a durational response to light overrides any interaction between the timing of the light period and the position of the oscillation at which light is administered. Such prolonged periods of irradiation apparently suspend or otherwise interact with the rhythm so that, in a following dark period, it is reinitiated at a fixed phase relative to the time of the light-off signal to give a peak of the rhythm 13 h after the dusk signal.In daily photoperiodic cycles rhythm phasing by a 6-h photocycle was also estimated by progressively increasing the number of cycles given prior to a single dark period of varied duration.In confirmation of Bünning's (1936) hypothesis, calculated and observed phasing of the rhythm controlling flowering in c. rubrum accounts for the photoperiodic response of this species. Evidence is also discussed which indicates that the timing of disappearance of phytochrome P_fr may limit flowering over the early hours of darkness.     

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)     

Circadian Rhythm in Amino Acid Uptake by Synechococcus RF-1

In the prokaryote Synechococcus RF-1, circadian changes in the uptake of l-leucine and 2-amino isobutyric acid were observed. Uptake rates in the light period were higher than in the dark period for cultures entrained by 12/12 hour light/dark cycles. The periodic changes in l-leucine uptake persisted for at least 72 hours into continuous light (L/L). The rhythm had a free-running period of about 24 hours in L/L at 29°C. A single dark treatment of 12 hours could initiate rhythmic leucine uptake in an L/L culture. The phase of rhythm could be shifted by a pulse of low temperature (0°C). The free-running periodicity was “temperature-compensated” from 21 to 37°C. A 24 hour depletion of extracellular Ca²⁺ before the free-running L/L condition reduced the variation in uptake rate but had little effect on the periodicity of the rhythm. The periodicity was also not affected by the introduction of 25 mm NaNO₃. The uptake rates for 20 natural amino acids were studied at 12 hour intervals in cultures exposed to 12/12 hour light/dark cycles. For eight of these amino acids (l-Val, l-Leu, l-Ile, l-Pro, l-Phe, l-Trp, l-Met, and l-Tyr), the light/dark uptake rate ratios had values greater than 3 and the rhythm persisted in L/L.     

Effect of far-red light pulse on induction and production of flowers in Lemna paucicostata 6746 in darkness

A short-day duckweed, Lemna paucicostata 6746, was exposed tocontinuous darkness at 26^?C, and the changes in the floral parameters(3) due to far-red and/or red light pulse given at various timesof the dark period were studied. Parameters a (vegetative growth rate) and (flowering ratio)were respectively decreased and increased with a far-red lightpulse given at the outset of the dark period. The decreaseda and the increased remained almost unchanged until the 7thhour, but returned to their initial levels thereafter. The far-redlight actions on a and were reversed by subsequent exposureto red light. Parameter P₁ (pre-flower induction period) wasextended by 1 day when far-red and/or red pulse was given atabout the 7th hour of the dark period. A far-jed pulse givenat the outset of the dark period only affected parameter P₂(flower induction period). Although the sensitivity of P₂ tored light increased with time, its sensitivity to far-red lightremained constant and at about the 7th hour was equally sensitiveto far-red and red lights. Both red and far-red pulses givenlater than the 7th hour were increasingly ineffective on P₂.The red/far-red reversibility occurred only for the action onP₂ of the far-red pulse applied during the early dark period.Parameter P₄ (flower production period) varied rhythmicallyin length with a far-red puke, the maximum shortening and extensionbeing induced by the pulse given at about the 7th and 19th hours,respectively. The sensitivity of P₄ to red light also changedrhythmically with an inverse phase angle to the rhythmic responseto farred light, and the far-red and red light actions werereversed respectively by subsequent red and far-red lights. These findings suggested that multiple timing devices includingan hourglass-type clock and a circadian clock are involved induckweed flowering. (Received October 25, 1978; )     

Different circadian rhythms regulate photoperiodic flowering response and leaf movement in Pharbitis nil (L.) Choisy

The phase shifting effect of red light on both the leaf movement rhythm, and on the rhythm of responsiveness of photoperiodic flower induction towards short light breaks (10 min red light), has been studied in Pharbitis nil, strain Violet, and comparisons between the two rhythms have been made. The phase angle differences between the rhythms after a phase shift with 2 or 6 h of red light given at different times during a long dark period were not constant. The results indicate the involvement of two different clocks controlling leaf movement and photoperiodic flower induction.Abbreviations DD continuous darkness - l:D x:y light/dark cycles with x hours of light and y hours of darkness - PPR rhythm of photoperiodic responsiveness towards light break     

Circadian rhythms and the induction of flowering in the long-day grass Lolium temulentum L.

Plants of Lolium temulentum L. strain Ceres were grown in 8-h short day (SD) for 45 d before being exposed either to a single long day (LD) or to a single 8-h SD given during an extended dark period. For LD induction, the critical photoperiod was between 12 and 14 h, and more than 16 h were needed for a maximal flowering response. During exposure to a single 24-h LD, the translocation of the floral stimulus began between the fourteenth and the sixteenth hours after the start of the light period, and was completed by the twenty-fourth hour. Full flowering was also induced by one 8-h SD beginning 4 or 28 h after the start of a 40-h dark period, i.e. by shifting 12 h forward or beyond the usual SD. The effectiveness of a so-called ‘displaced short day’ (DSD) was not affected by light quality and light intensity. With a mixture of incandescent and fluorescent lights at a total photosynthetic photon flux density of 400 μmol m⁻² s⁻¹, a 4-h light exposure beginning 4 h after the start of a 40-h dark period was sufficient to induce 100% flowering. The flower-inducing effect of a single 8-h DSD was also assessed during a 64-h dark period. Results revealed two maxima at a 20-h interval. This fluctuation in light sensitivity suggests that a circadian rhythm is involved in the control of flowering of L. temulentum.     

Twilight effect: initiating dark measurement in photoperiodism of xanthium

Six experiments studied the effects of low levels of red and far-red light upon the initiation of measurement of the dark period in the photoperiodic induction of flowering in Xanthium strumarium L. (cocklebur), a short-day plant, and compared effects with those of comparable light treatments applied for 2 hours during the middle of a 16-hour inductive dark period. Red light, or red plus far-red, at levels that inhibit flowering when applied during the middle of the inductive dark period, either had no effect on the initiation of dark measurement (i.e., were perceived as darkness), or they delayed the initiation of dark measurement by various times up to the full interval of exposure (2 hours). Far-red light alone had virtually no effect either at the beginning or in the middle of the dark period. These results confirm that time measurement in the photoperiodic response of short-day Xanthium plants is not simply the time required for metabolic dark conversion of phytochrome. Results also suggest that the pigment system (phytochrome?) and/or responses to it may be significantly different as they function during twilight (initiation of dark measurement), and as they function during a light break several hours later. Possible mechanisms by which cocklebur plants detect the change from light to darkness are discussed.Comparing experimental results with spectral light measurements during twilight and with measurements of light from the full moon led to two conclusions: First, light levels pass from values perceived by the plant as full light to values perceived as complete darkness in only about 5.5 to 11.5 minutes, although twilight as perceived by the human eye lasts well over 30 minutes. Second, cocklebur plants probably do not respond to light from the full moon, even when most sensitive, 7 to 9 hours after the beginning of darkness.     

Shifts in the semidian rhythm of flowering do determine night-break timing and critical night length in Pharbitis nil

There is a semidian (≈12 h) rhythm in the flowering response of the short-day plant Pharbitis nil Choisy following 90 min exposure to either far-red light/darkness or a temperature drop (27 °C to 12 °C) given at various times in constant conditions before an inductive dark period. This semidian rhythmic response to the temperature-drop pretreatments in the light is also evident through the inductive dark period without change of phase. Furthermore, those pretreatments which increase flowering also advance the time of maximum sensitivity to red light (R) interruptions of the dark period by up to 1.5 h and shorten the critical night length. Conversely, pretreatments which reduce flowering delay the time of maximum R inhibition by up to 1.5 h and increase the critical night length by the same amount. However the phase of a circadian rhythm of flowering response had no effect on either the time of maximum R inhibition or the critical night length. Thus, the semidian rhythm determines both the time of maximum R inhibition and the critical night length in Pharbitis. Received: 8 November 1997 / Accepted: 7 January 1998     

Action Spectrum for Resetting the Circadian Phototaxis Rhythm in the CW15 Strain of Chlamydomonas: I. Cells in Darkness

We have developed protocols for phase shifting the circadian rhythm of Chlamydomonas reinhardtii by light pulses. This paper describes the photobiology of phase-resetting the Chlamydomonas clock by brief (3 seconds to 15 minutes) light pulses administered during a 24 hour dark period. Its action spectrum exhibited two prominent peaks, at 520 and 660 nanometers. The fluence at 520 nanometers required to elicit a 4 hour phase shift was 0.2 millimole photon per square meter, but the pigment that is participating in resetting the clock under these conditions is unknown. The fluence needed at 660 nanomoles to induce a 4 hour phase shift was 0.1 millimole photon per square meter, which is comparable with that needed to induce the typical low fluence rate response of phytochrome in higher plants. However, the phase shift by red light (660 nanometers) was not diminished by subsequent administration of far-red light (730 nanometers), even if the red light pulse was as short as 0.1 second. This constitutes the first report of a regulatory action by red light in Chlamydomonas.     

Phase Shifting of the Circadian Clock by Diethylstilbestrol and Related Compounds in Neurospora crassa

Phase shifts of the circadian conidiation rhythm in Neurospora crassa were induced by 3-hour treatments of mycelia in liquid medium with diethylstilbestrol (DES), dienestrol (DIE), hexestrol (HEX), diethylstilbestroldipropionate (DESP), and dienestroldiacetate (DIEA). Over a 24-hour period beginning 24 hours after the transition from light to constant dark, maximum phase shifts occurred about 36 hours. DES was the most effective of the drugs tested, giving 10-hour phase advances at 20 micromolar. DIE and HEX caused similar phase shifts as DES at 40 micromolar. The two derivatives of the last, DESP and DIEA, were much less effective in shifting phase; only a few hours of phase advance result from treatments at 80 micromolar concentrations.

The activity of isolated plasma membrane ATPase was inhibited by DES and partially by HEX, but not by DIE, DESP, or DIEA. O₂ consumption of the mycelia was inhibited equally by DES, DIE, and HEX, while DIEA and DESP had little effect. Phase-shifts by DES cannot be interpreted as evidence that plasma membrane ATPase is a component of the circadian clock.