Photoreactions Controlling Flowering of Chrysanthemum morifolium (Ramat. and Hemfi.) Illuminated with Fluorescent Lamps
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Plant Physiol. (1970) 45, 235-239
Photoreactions Controlling Flowering of Chrysanthemum
morifolium (Ramat. and Hemfi.) Illuminated with
Fluorescent Lamps
Received for publication October 15, 1968
H. M. CATHEY AND H. A. BORTHWICK
Crops Research Division, Agricultural Research Service, United States Department of Agriculture,
Beltsville, Maryland 20705
ABSTRACT MATERIALS AND METHODS
Flowering of chrysanthemum plants under short photo- The cultivars of chrysanthemum used in these experiments were
periods, as is well known, is prevented when the plants are Improved Indianapolis Yellow and White Pink Chief. They were
illuminated near the middle of the long night. Such illumi- selected because they are responsive to short-day treatment, dif-
nation inhibits flowering whether it is given continuously fering in degree of response to artificial light in the night and in
or intermittently, and whether it comes from incandescent the number of short days required for flowering. The plants were
or from fluorescent lamps. We discovered, however, that grown in the greenhouse from rooted cuttings and were main-
fluorescent light applied intermittently (cyclically) tained on photoperiodic conditions that assured their remaining
throughout the entire 16-hour long night was far less in- in a vegetative condition until used experimentally. These condi-
hibitory than when applied during only part of this dark tions consisted of natural photoperiods and 4 hr of incandescent
period. By contrast, incandescent filament illumination is light of 20 ft-c from 10 PM to 2 AM daily.
strongly inhibitory under these conditions. The cycles of Plants were brought from the greenhouse to plant growth
fluorescent light usually lasted 15 minutes, 1.5 minutes of rooms where they received treatment for 10 consecutive days.
light followed by 13.5 minutes of dark. When such cycles During this 10-day experimental period, the general routine con-
were applied for only 12 hours, leaving 4 hours of uninter- sisted of a daily 8-hr period of illumination, a dark period, often
rupted darkness in each long night, inhibition of flowering of 4 hr, but sometimes longer or shorter, and the remainder of
was complete again. each 24-hr period, usually 12 hr, of cyclic fluorescent light. The
cycles of the fluorescent light and darkness were 15 min except in
Table II, where they were 24 min. In all cases, 10% of the cycle
was light and 90% dark. Intensity of the fluorescent light was
about 80 ft-c. In previous experiments with other short-day
plants, cyclic lighting as applied here was completely inhibitory
to flowering, but in chrysanthemum it is sometimes completely
Flowering of Chrysanthemum morifolium (Ramat. & Hemfl.) is and sometimes incompletely inhibitory, depending on such condi-
promoted by subjecting the plants to several daily dark periods of tions as length of a preceding uninterrupted dark period and kinds
more than 12 hr and is inhibited by illuminating them near the of interrupting irradiations applied during the dark period.
middle of each long night for a few hours with continuous or During the 8-hr photoperiods, the plants received about 2000
intermittent low intensity light (cyclic light) from fluorescent or ft-c of illumination from cool white fluorescent lamps and about
incandescent lamps (3). When applied cyclically throughout the 80 ft-c from incandescent ones. The night temperature was main-
entire 16-hr "dark" period, however, fluorescent illumination is tained at a minimum of 17°. At the end of the 10-day treatment
far less inhibitory than when applied during only part of the period, the plants were returned to the greenhouse and non-
dark period. We discovered this peculiar ineffectiveness of pro- photoinductive conditions for a further period of growth and
longed periods of cyclic fluorescent light when we included the development. Most of the plants were dissected 2 weeks after the
short-day chrysanthemum in an experiment designed primarily 1st day of treatment. A few were continued in the greenhouse
to investigate light responses of several long-day species. In- until certain of them bloomed (Fig. 1).
candescent filament illumination is strongly inhibitory under both As in previous work with chrysanthemums (3), we used either
conditions. two or three plants per treatment. Use of such small numbers
Other differences between the actions of fluorescent and in- was permitted by the great uniformity of the cuttings which were
candescent filament illuminations on chrysanthemums were previ- from selected clonal stocks of the respective cultivars. Three
ously encountered (3). They were interpreted on the basis of plants per lot re uesed in Tables I, II, and III and two per lot
differences between the two kinds of illumination in the red and elsewhere.
far-red parts of the spectrum, coupled with conditions of leaf Experimental treatments involved the use of filtered and un-
structure that might peculiarly affect screening by chlorophyll of filtered light from both fluorescent and incandescent lamps. Un-
radiation absorbed by phytochrome in the chrysanthemum plant. filtered
Discovery of the incomplete inhibitory effectiveness of cyclic illumination from quartz-iodide lamps, which was used
fluorescent illumination applied for daily 16-hr periods, although in one experiment, is qualitatively very similar to that from
of little immediate horticultural interest, again raised questions ordinary incandescent filament lamps. Quartz-iodide lamps were
as to the photoreactions involved in controlling flowering of used because illumination intensities of 4000 ft-c from them were
chrysanthemum and led to the work herein reported. more readily attainable than from the ordinary incandescent
235
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Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.236 CATHEY AND BORTHWICK Plant Physiol. Vol. 45, 1970
FIG. 1. Inhibition of flowering of White Pink Chief chrysanthemum by cyclic lighting. Plants grown at 17 C and 8-hr photoperiods (8 AM to
4 PM) and treated as follows during a 10-day experimental period: Above (left to right): 1.5 min of incandescent filament light (about 60 ft-c)
every 15 min from 11 PM to 1 AM (2 hr); from 9 PM to 3 AM (6 hr); and from 4 PM to 8 AM (16 hr). Below (left to right): 1.5 min of fluorescent
light (about 80 ft-c) every 15 min from 11 PM to 1 AM (2 hr); from 9 PM to 3 AM (6 hr); and from 4 PM to 8 AM (16 hr). All lots received uninter-
rupted long nights after the treatment period. Photographed 9 weeks after the start of the experiment.
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Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.-P-Ia-nt Physiol. Vol. 45, 1970 FLOWERING MUMS WITH FLUORESCENT LAMPS 237
ones. Further details of the illumination treatments are presented Table I. Effects of Illumination from Inzcandescent Filament and
with results of the individual experiments. Cool White Fluorescent Lamps Applied for Various Periods in
The terminal bud of each plant was dissected, and the stage the Middle of Daily 16-hr Dark Periods on Flowering of
of influorescence development was observed and assigned a White Pink Chief Chrysanthemum
number as previously described (2). These stages in brief were
as follows: Kind of Illumination and Period of Mean Stage of
Method of Application Treatment Flowering'
0: terminal meristem flat, plants vegetative
1: terminal meristems dome-shaped hr
2: terminal meristem becoming globose, first bracts present Incandescent:
3: receptacle spherical with 12 or more bracts around its rim Cyclic 6 2.0
4: receptacle flattened on top, many bracts but no floret primordia Cyclic 16 1.0
5: two or three rows of floret primordia on rim of receptacle Continuous 16 1.0
6: about six rows of floret primordia receptacle
on Fluorescent:
7: all but center of receptacle covered with floret primordia
8: receptacle completely covered with floret primordia Cyclic 6 0.0
9: central florets without perianth primordia Cyclic 16 7.0
10: all florets with perianth primordia Continuous 16 4.0
Dark control ... 10.0
EXPERIMENTAL RESULTS 1 Meaning of numbers given in "Materials and Methods."
A significant feature of the results presented in this paper is
the fact that nearly all treatments caused a marked reduction of proved Indianapolis Yellow and nearly so in White Pink Chief.
flowering below that of unirradiated controls. Some treatments These changes were prevented and, as a consequence, flowering
reduced flowering to stage 0. Most other treatments reduced it was only incompletely inhibited when the dark period was inter-
well below the stage 10 of the dark controls but not to 0. This rupted by 1 min of high intensity (4000 ft-c) cool white fluorescent
paper is mainly concerned with differences between these low illumination. Moreover, it was also incompletely inhibited when
levels to which the various treatments reduced flowering. the blue was removed by red cellophane from the 1-min, 4000
The incomplete inhibitory effectiveness of daily 16-hr periods ft-c illuminance that interrupted the 4-hr period of initial darkness
of cyclic fluorescent light on flowering of chrysanthemum is il- (Table VI). However, when red was removed by blue cellophane,
lustrated by results in Table I and in treatment 1 of Tables II, flowering was then completely prevented by the subsequent CFL.
III, IV, VI, and VII. Plants that received cyclic fluorescent light That is, the interruption of the dark period had no apparent
throughout the 16-hr "dark period" flowered at a low level. effect because, like the uninterrupted controls, the plants failed
That is, inhibition of flowering was not complete. In contrast, to flower. When a copper sulfate filter of a density adequate to
treatments in which the 16-hr periods consisted of 6 hr of dark- remove the far red (but not the red) was used, the plants flowered
ness and 10 hr of CFL1 were completely inhibitory, reducing the as well as when interrupted with unfiltered fluorescent illumina-
stage of flowering from 10 to 0. In other experiments, insertion tion. A 1-min interruption by 4000 ft-c of illumination from a
of 4-hr instead of 6-hr dark periods in the 16-hr periods of CFL quartz-iodide lamp, however, did not prevent change during the
also resulted in complete prevention of flowering. dark period. This is evident because the CFL inhibited flowering
The position of the short dark period in the 16-hr period in- as completely as in plants that received no interruption of the
fluenced the inhibitory effectiveness of the remaining CFL (Table dark period.
II). In general, greatest inhibition resulted when the dark period Response to far red and red radiations applied after a 1-min
occurred early instead of late in the 16-hr period. The two interruption of the 4-hr dark period with 4000 ft-c of fluorescent
cultivars were similarly responsive to these treatments, but inhibi- illumination was clearly reversible (Table VII). Inhibition of
tion in comparable lots was greater for Improved Indianapolis flowering of the interrupted plants was complete when far red
Yellow than for White Pink Chief. immediately preceded the 12 CFL but incomplete when the 1 min
The duration of darkness inserted in daily 16-hr periods of of cool white or red illumination immediately preceded 12 CFL.
CFL influenced the inhibitory effectiveness of the treatment In other experiments not shown here, reversibility of response to
(Table III). About 4 hr were required for inhibition of Improved far red and red radiations did not occur when such radiations
Indianapolis Yellow and slightly more for White Pink Chief. were not preceded by 1 min of high intensity fluorescent illumina-
When the inserted short dark period was interrupted even so tion during the 4-hr dark period.
briefly as for 1 min with high intensity (4000 ft-c) cool white
fluorescent illumination, the inhibitory effectiveness of the treat- DISCUSSION
ment was markedly reduced (Table IV). As the dark periods were
lengthened further, however, this effect of a brief light interrup- This paper is mainly concerned with a failure of fluorescent
tion was not expressed; that is, the treatments remained fully illumination to prevent flowering of chrysanthemums when it is
inhibitory. applied throughout the daily 16-hr dark periods between succes-
Interruptions of the dark period with fluorescent illuminances, sive 8-hr photoperiods. The failure is exhibited when illumination
both greater and smaller than 4000 ft-c for 1 min, were tested. is applied cyclically, but it occurs to some extent even when
One half minute was as effective as 1 min at 4000 ft-c (Table V). applied continuously.
Illuminances longer than 1 min at 1000 and 500 ft-c were also It is remarkable, however, that the cyclic illumination becomes
effective, but precision of the experiment did not clearly show completely inhibitory if preceded by about 4 hr of uninterrupted
whether reciprocity held. darkness. Evidently a change occurs, during the 4-hr dark period,
Apparently, during the 4-hr dark period, changes occur in the which increases the inhibitory effectiveness of the Pfr converted
plant that permit CFL to inhibit flowering completely in Im- during the subsequent 12 hr of CFL. Attention, therefore, turns
to the nature of the change during the dark period.
Since the change is prevented by a high intensity fluorescent
Abbreviations: CFL: cyclic fluorescent light; Pir: active form of interruption of the dark period, the effects of which are photo-
phytochrome. reversible by red and far red (Table VII), it is evidently involved
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Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.238 CATHEY AND BORTHWICK Plant Physiol. Vol. 45, 1970
Table II. Effects of Illuminiationt from Stanidard Cool White Fluorescen?t Lamps Applied Cyclically durinig Various Periods and at Various
Inttenisities in Daily 16-hr Dark Periods oni Flowerinig of Two Chrysanithemum Cultivars
AMean Stage of Flowering' at Indicated Levels of Illumination (ft-c)
Treatment Treatment
No. Improved Indianapolis Yellow Wi'hite Pink Chief
1
7.5 7. 15 30 60 80 7.3 15 30 60 80
- ~~ - 1 -
1 Ir 5.5 5.0 3.7 3.0 3.0 5.5 6.0 5.0 3.0 5.0
2 2.3 1.0 0.0 0.0 0.0 5.0 2.3 0.0 0.0 0.0
3 'I _I 1.3 1.3 0.0, 0.0 0.0 4.3 3.0 0.6 0.0 0.0
4 2.0 2.0 0.0 0.0 0.0 4.1 4.3 1.0 0.0 0.0
5 3.0 2.6 0.0 0.0 0.0 7.0 7.0 3.0 0.0 0.0
6 1
6 6 .0 3 .5 1.0 0.0 0.0 10.0 7.0 3.0 0.6 0.3
0 2 4 6 8 10 12 14 16
Hours
1 Meaning of numbers given in "Materials and Methods."
Table III. Effect of Various Durationzs of Darkness Preceding Cyclic Table IV. Effect of Variolus Durationts of Darkniess Ending with or
Fluorescent Illuminiation durintg Daily 16-hr Periods following 8-hr without Initerruptions with Cool White Fluorescenit Illuminationz
Photoperiods on Flowering of Two Chrysantthemum Cultivars anid Precedintg Cyclic Fluorescent Illuminiation during Daily
16-hr Periods between Successive 8-hr Photoperiods otn
Mean Stage of Flowerinig of Two Chrysanthemum Cultivars
Flowering'
Treat-
ment Schedule of Treatments AMean Stage of Flowering'
No. Improved
Indiana- White
Pink Treatment during
ols 16-hr Periods Indianapolis Yellow White Pink Chief
Chief
DC C\\' DC cwl
1 16 CFL 4
3
16 CFL 2 3
ID + 15 CFL 2.5 3 4.5 5.5
2 1 dark 15 CFL 2.3 4 2D + 14 CFL 2 3 3.5 6
3D + 13 CFL i
2.5 3 S
0
3 2 cIrk 14 CFL 2.3 4 4D+12CFL 3 2.5 3.5
SD + 11 CFL 0 0.5 1.5 2
4 3 darkj 13 CFL 2
3 6D + 10 CFL 0 0 0 1
7D + 9 CFL 0 0 0 0.5
5 4 dark 12 CFL 8D + 8 CFL 0 0 0 0
0.5
I DC: Dark control; CW: 1 min 2000 ft-c of
illumination from
6 S dark 11 CFL 0 cool white fluorescent lamps given at end of indicated dark periods
and preceding cyclic fluorescent illumination. Meaning of numbers
7 6 dark 10 CFL 0 given in "Materials and Methods."
8 7 dark 9 CFL 0
4-hr dark period by a high intensity fluorescent illuminance is
9 8 dark 8 CFL 0
completely reversible by far red. Action of Pfr is thus detected in
at least two parts of the 16-hr periods between successive 8-hr
I
photoperiods; and the actions are opposite.
0 2 4 6 8 10 12 14 16 The change that occurs during the uninterrupted 4-hr dark
Hours period apparently involves disappearance of Pfr. It seems that
successful inhibition of flowering by CFL requires that the Pfr
I Meaning of numbers given in "Materials and Methods." left in the plant at the close of the photoperiod be allowed to
drop to a low level for a time, and that Pfr then again be increased
by application of CFL. A 1-min high intensity fluorescent light
with an action of phytochrome. However, the Pfr produced by interruption of the dark period at any time during the 4 hr
CFL following a 4-hr dark period completely inhibits flowering if prevented the response (failure of flowering) that otherwise
the dark period is uninterrupted but not if it is interrupted by would be displayed.
fluorescent light. Moreover, response to an interruption of the Most of the findings of these investigations are thus explainable
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Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.Plant Physiol. Vol. 45, 1970 FLOWERING MUMS WITH FLUORESCENT LAMPS 239
Table V. Effects of Cool White Fluorescent Interruptionts ntear End on the basis of phytochrome action. The results obtained with
of 4-hr Dark Periods That Are Followed by 12-hr Periods of cellophane filters are completely compatible if Pfr is considered
Cyclic Fluorescent Light on Flowering of Improved to be the active agent. The result with high intensity quartz-
Inidiantapolis Yellow Chrysanthemum iodide illumination was also to be expected on the basis of earlier
investigations (3) in which very high intensity 1-min illuminances
Mean Stage of Flowering, at from a quartz-iodide lamp in the middle of 16-hr dark periods
Duration of Dark Period Level of Illumination did not cause a response (inhibition of flowering). In the current
Interruption experiments, a similar treatment likewise did not cause a response
500 ft-c 1000 ft-c 4000 ft-c
(in this case, the promotion of flowering). Although the results
min of these two different experiments were opposite in nature (that
0.5 0.5 2 3 is, failure to prevent and failure to promote flowering, respec-
1.0 0.5 2 2.5 tively), the explanations are probably the same.
2.0 1 3 3 The finding that Pfr action during different parts of daily 16-hr
8.0 2 3 3 dark periods leads to opposite results is consistent with observa-
16.0 3 3 3 tions on other plants. Flowering of Pharbitis, for example, is
Dark control = 0 inhibited by removal of Pfr by far red at the beginning of the
dark period or by its reintroduction by red light in the middle of
I Meaning of numbers given in "Materials and Methods." the dark period (5). Xanthium exhibits similar responses but
only when the photoperiod is reduced to about 2 hr (1). Opposite
Table VI. Effects of 1-mimi Initerruptions of 4-hr Dark Periods with actions in Eragrostis seed germination of Pfr introduced by red
Illumination of 4000 ft-c from Different Sources, Filtered as radiation at different times during dark inhibition have also been
Itndicated, oni Flowering of Two Chlrysanthemum Cultivars observed (7).
A crucial point in these results is that high intensity interrup-
Mean Stage of tions of the 4-hr dark period with fluorescent and incandescent
Flowering2
light lead to opposite results (flowering and nonflowering, re-
Dark CFL Kind of
Illumination ind offFle
Kn Filter m hite spectively). Fluorescent light is essentially red and incandescent
India- Pink
is a mixture of red and far red. The far red in the presence of red
napolis Chief thus prevents display of a response (flowering) that appears with
Yellow
red light alone. This action does not result directly from response
hr hr to the Pfr torm of phytochrome which probably exceeds 50%O P
0 16 None None 2 6 with both types of radiation. It rather appears to involve far red
4 12 None None 0 2 radiation per se. The phenomenon resembles the opening of
4 12 Cool white None 2 3.5 Mimosa pudica leaflets in light against the closing action of Pfr
4 12 Cool white Red cellophane 2 3.5 (4) or the inhibition of germination of seed of Poa pratensis and
4 12 Cool white Blue cellophane 0 1.5 Amaranthus arenicola by far red but not by red alone (6). The
4 12 Cool white CuS043 2 4 two responses were attributed to a high energy reaction having a
4 12 Quartz-iodide None 0 2.5 maximum in the far red near 720 mu.
1 Illumination of 4000 ft-c for 1 min from cool white fluorescent The difference in response of chrysanthemums to the two kinds
or quartz-iodide lamps. of light was ascribed in an earlier paper to effects of light screening
2 Meaning of numbers given in "Materials and Methods." by chlorophyll (3). Similarities of the chrysanthemum response
3 CuS04 = 5 cm of 20 g/liter CuS04 in 0.5% H2SO. . to the above mentioned leaf movement and seed germination re-
sponses make consideration of a high energy response in chrysan-
Table VII. Effects of I min of 4000 ft-c of Cool White Fluorescent themum necessary. Although it seems unwise to decide in the
Illumination Followed by Far Red and Red at End of a 4-hr Dark
Period and before a 12-hr Period of Cyclic Cool White absence of an action spectrum whether or not a high energy
Fluorescent Illumination ont Flowering of Two reaction is involved, the participation of the ordinary phyto-
Chrysanithemum Cultivars chrome reaction is clearly evident.
Mean Stage of
Flowering2 Acknowledgments-We thank S. B. Hendricks for many helpful discussions during
the course of this work and the preparation of the manuscript. The chrysanthemum
Treatment' Im- cuttings were provided by Yoder Brothers, Inc., Barberton, Ohio.
proved White
Indian- Pink
apolis Chief
Yellow LITERATURE CITED
16 CFL 2 5.5 1. BoRTHwicK, H. A. AND R. J. DowNs. 1964. Roles of active phytochrome in
4D + 12 CFL 0 0 control of flowering of Xanthium pennsylvanicum. Bot. Gaz. 125: 227-231.
4D+CW+ 12CFL 1 4.0 2. CATHEY, H. M. AND H. A. BORTHWICK. 1957. Photoreversibility of floral initia-
4D + CW + FR + 12 CFL 0 0 tion in chrysanthemum. Bot. Gaz. 119: 71-76.
1 3.5 3. CATHEY, H. M. AND H. A. BORTHWICK. 1964. Significance of dark reversion of
4D + CW + FR + R + 12CFL phytochrome in flowering of Chrysanthemum morifolium. Bot. Gaz. 125: 232-236.
4D + CW + FR + R + FR + 12 CFL 0 0.5 4. FONDEVILLE, J. C., M. J. SCHNEIDER, H. A. BORTHWICK, AND S. B. HENDRICKS.
4D + CW + FR + R + FR + R + 12 CFL 1 3.5 1967. Photocontrol of Mimosa pudica leaf movement. Planta 75: 228-238.
5. FREDERICQ, H. 1964. Conditions determining effects of far-red and red irradia-
1 Hours of darkness (D) and cyclic fluorescent light (CFL). tions on flowering response of Pharbitis nil. Plant Physiol. 39: 812-816.
CW: 1 min of 4000 ft-c illumination from a cool white fluorescent 6. HENDRICKS, S. B., V. K. TOOLE, AND H. A. BORTHWICK. 1968. Opposing action
lamp; FR: 1 min far red; R: 4 min red from standard red and far of in Poa pratensis and Amaranthus arenicola. Plant Physiol. 43: 2023-2028.
red sources. 7. TooLE, V. K. AND H. A. BORTHWICK. 1968. The photoreaction controlling
2 Meaning of numbers given in "Materials and Methods." seed germination in Eragrostis curvula. Plant Cell Physiol. 9: 125-136.
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