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)     

A Semidian Rhythm in the Flowering Response of Pharbitis nil to Far-Red Light: I. Phasing in Relation to the Light-Off Signal

Evidence is presented of an endogenous rhythm in flowering response to far-red (FR) irradiation, with a period of about 12 h (hence semidian rhythm), which persists through at least three cycles in constant conditions of continuous light at 27°C and has a marked influence on the flowering response in Pharbitis nil to a subsequent inductive dark period. The phase of the rhythm is not influenced by real time nor by the time from imbibition or from the beginning of the light period. Rather, it is fed forward from the beginning of the FR interruption to the beginning of the inductive dark period. The period of the rhythm is not affected by irradiance but is longer at cooler temperature. When there are two FR interruptions during the preceding light period, it is primarily the later one which determines the phase of the rhythm, although some interactions are evident. There appears to be an abrupt rephasing of the rhythm at the beginning of the inductive dark period. No overt rhythms which could be used as “clock hands” for the semidian rhythm were detected in photosynthesis, stomatal opening, or translocation.     

Effect of Daylength on Phenol Metabolism in the Leaves of Salvia occidentalis

A method of phenol determination in plant leaves has been developed which is based on the in situ oxidation of these compounds in an atmosphere containing ammonia, followed by difference spectrophotometry. The development of the phenol pattern has been studied in each separate leaf of a Salvia occidentalis plant grown in short and in long days. During the light period the phenol content (mainly chlorogenic acid and isochlorogenic acids) increases in proportion to the length of this period, whereas during the subsequent dark period the phenol content decreases. This decrease does not continue during the second part of a dark period if that period is interrupted by a light break with red light. Instead a small increase is observed. This effect of red light can be reversed with far red light. It is argued that a correlation with flower induction in this short day plant can be construed if it is assumed that the continuous presence of certain o-dihydroxyphenols in the cytoplasm of leaf cells inhibits the synthesis or the transport of a flowering hormone.     

The Negative Response of Photoperiodic Floral Induction in Chenopodium rubrum L. to Preceding Growth

In Chenopodium rubrum there exists a correlation between theage of the seedlings and the effectiveness of photoperiodicinduction. The younger the plants the more effective was photoperiodictreatment. In three-day-old seedlings one short day was sufficientto promote incomplete flowering, while two short days broughtabout 100 per cent flowering. With six-, eight-, and ten-day-oldplants exposed to two or three short days quantitative differenceswere observed in the earliness of flowering and the percentageof flowering plants. The effects of continuous light and ofshort days with a light break preceding the inductive treatmentwere compared. The results obtained indicate that the inhibitoryeffect of plant age cannot be attributed solely to the appearanceof inhibitors under continuous light but changes of growth patternin plants of different age should also be taken into consideration. The inhibition of RNA synthesis in shoot apices brought aboutby 6-azauridine resulted also in a flowering stimulation, providedthat the inhibitor was applied one or two days prior to inductionand the inductive process itself remained undisturbed. Thisstimulation was accompanied by inhibition of vegetative growthand by a decrease of RNA concentration in the cytoplasm as estimatedcytophotometrically. The competition between growth of vegetative organs and floraldifferentiation affects the response to inductive treatment.The suppression of growth can result in enhancement of flowering.     

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.     

Rhythmic changes in ribonuclease activity in relation to nucleic acid synthesis in cotyledons ofChenopodium rubrum L. and to floral induction of the plants

Ribonuclease (RNAse) activity was investigated in cotyledons ofChenopodium rubrum plants subjected to various conditions of illumination (photoperiodic induction, continuous light, induction cancelled by interrupting the dark period by a light-break). At the end of the dark period of the single inductive cycles RNAse activity of induced plants was inferior to that of plants grown in continuous light. At the end of the first two cycles the activity was lowest after the interruption of the dark period by light. The investigation of the enzyme in 6h intervals showed rhythmic changes in activity to occur in induced plants. Enzyme activity followed a pattern opposed to this of nucleic acid (NA) synthesis in the cotyledons. In plants from continuous light the enzyme activity did not show any rhythm and in plants having obtained a light-break during the inductive period the rhythm was less distinct than in the induced ones. The period length of the endogenous rhythm of NA synthesis in the cotyledons is about half as long as this of flowering and the peaks of flowering coincide with the throughs of NA synthesis.     

Kinetics and time dependence of the effect of far red light on the photoperiodic induction of flowering in wintex barley

Flowering in the long day plant Hordeum vulgare L. var. Wintex barley was enhanced by the addition of far red light to the main light portion of the photoperiod. Far red energy was provided to produce quantum flux ratios (660/730 nm) and phytochrome photoequilibria (Pfr/total phytochrome) equivalent to those reported both beneath a leaf canopy and outside a canopy at twilight. The photoperiodic requirement for long days can be completely eliminated by the addition of far red light. However, both the effect of extending the photoperiod without far red and the addition of far red to 12-hour photoperiods were suboptimal. Maximal stimulation was achieved only when far red was added to continuous light. The duration of the period of maximal apex elongation rate, as well as the reduction of the time required for floral initiation, were saturated by three inductive cycles. When far red energy was provided intermittently during 3 days of continuous light, the ability to respond varied in a circadian manner. This enhancement of flowering by far red appears to be mediated by the “high irradiance response” of phytochrome.     

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.     

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.     

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.     

Esters hydroxycinnamiques et induction photopériodique florale in vitro chez Cichorium intybus

Hydroxycinnamic acid esters and photoperiodic flowering induction in vitro of Cichorium intybus.
In root tissues of Cichorium intybus L. cv. Witloof cultured in vitro, the photoinductive period occurs between the 8th and the 16th day after the start of the culture. The promotive light conditions are either long days (16 h photoperiod) or daily cycles of 9 h white + 15 h red light applied during the photoinductive period in an otherwise short day treatment. During the photoinductive period and under suitable irradiation conditions, the endogenous hydroxycinnamic acid esters (especially chlorogenic acid) show a regular development as from the 8th day, reaching a maximal level by the 16th day. The amount of these molecules then decreases rapidly, preceding the external expression of floral induction. When we apply non-inductive conditions [short days (9 h) or daily cycles of 9 h white + 15 h (far-red + red) light supplied from the 8th to the 16th day in otherwise short day conditions], the metabolic changes indicate the same pattern during the inductive period as mentioned above. However, a fundamental difference exists between the inductive and non-inductive conditions, so that the production of hydroxycinnamic acid esters is particularly high towards the end of the second week of in vitro development in the induced state. This increase is not primarily due to increased photosynthetic activity in long day conditions, since it occurs both in the long day treatments and in the treatments with daily cycles of 9 h white + 15 h red light, thus revealing the morphogenetic action of light via phytochrome. This accumulation of hydroxycinnamic acid esters is correlated with floral induction, which appears to be ineffective unless there is a certain minimum amount of these molecules in the tissues.     

Serine Involvement in the Flowering of Lemna during Photoperiodic Induction

The levels of the endogenous free amino acid serine in Lemnaperpusilla are found to fluctuate during the course of shortdays. Serine levels do not change in plants receiving long daysor continuous light. The levels of serine decrease graduallyin the dark reaction of short days reaching their lowest aboutthe middle of the dark period, indicating a utilization of serineduring the dark reaction. In the second half of the dark periodthe serine level rises. The decline and rise of serine duringthe dark period is observed in successive short-day cycles.During the light period the serine level increases, reachinga maximum after 6 h from the beginning of the light exposure.When the light period is eliminated and a continuous dark periodis given the serine level remains unchanged. It is suggestedthat serine is involved in the floral initiation as a precursorto a flowering substance.     

The association of carbohydrate changes in the shoot tip of cauliflower with flowering

Changes in levels of sugars and starch in the shoot tip of cauliflower, Brassica oleracea L. var. botrytis D. C. cv. Main Crop were studied during periods of growth which were inductive or non-inductive to flowering. Flowering was induced by growing plants for 2 weeks under 16 hr of light at 5°. During this period of floral induction there was a significant increase in sugar and starch content compared to that in vegetative plants grown at 20 to 26°. Sugar and starch content did not increase and flowering was prevented when light and CO₂ were excluded during growth at 5°. A 3-day dark period at 20° or a high temperature treatment at 33° with light following growth at 5° reduced the carbohydrate level and prevented flowering.     

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.     

Seed and Embryo Germination in Syringa vulgaris and S. reflexa as Affected by Temperature during Seed Development

A nitrogen source was needed for the flowering of Lemna gibba L., a long-day plant, and L. perpusilla Torr., a shortday plant. The level of endogenous amino acids analyzed by an Amino Acid Analyzer, rose during the first few inductive cycles, but was reduced during later stages of the flowering process. Serine and threonine levels increased during the light period and decreased during the dark period in L. perpusilla. Exogenous serine and threonine added to the culture medium at 10^?6M increased the rate of flowering by more than 35% over the controls. Cysteine inhibited flowering, while other amino acids had little or no promotive effect on flowering. Serine and threonine increased flowering rate in L. perpusilla only when added during a dark period of the inductive cycle. The addition of amino acids during a light period not followed by a dark period had no effect on flowering.     

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.

     

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.     

The required day number and timely induction of diapause in geographic strains of the mosquito Aedes atropalpus

Diapause is triggered in A. atropalpus from 14°N lat when 9 or more short-day photoperiod cycles occur during the photosensitive 4th instar and pupa. Temperatures of 22°C and higher cause part or all of this population to complete the photosensitive period in less than 9 days thereby avoiding diapause. In contrast, 4 short-day cycles during the photosensitive period are sufficient to trigger diapause in A. atropalpus from 45°N lat. This population experiences the 4 required days at temperatures as high as 28°C. A strain from an intermediate location of 34°N lat must “see” 7 short days during the photosensitive stages for induction of diapause. This requirement is met at temperatures of 24°C and less. These results suggest that southern strains of A. atropalpus have evolved a means of avoiding short-day induced diapause if temperatures are high and other requirements for growth are available.A correlation between critical daylength for induction of diapause and geographic origin has been shown for several insect species. The evolution of a requirement for different numbers of diel cycles during the photosensitive period is another means of modifying the diapause response in populations from different latitudes and geographic areas.     

Action spectra for the light inhibition of flowering and its reversal inLemna paucicostata T-101

Lemna paucicostata Hegelm. T-101, a short-day plant, flowers when plants preirradiated with red light (R) for 24 h are subjected to inductive darkness for 72 h followed by two short-day cycles (6 h R+ 18 h dark). However, flowering is inhibited by blue-or far-red-light pulses applied at the beginning of the inductive dark period. These inhibitory light effects are fully reversible by a R pulse. The action spectra for the inhibitory light effect and for its reversal show that the light pulses act exclusively through phytochrome. It is concluded that a low level of Pfr at the beginning of the inductive dark period prevents flowering.Abbreviations R red (light) - B blue (light) - FR far-red (light)     

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)