Annals of Botany 82 : 67–70, 1998
Environmental Factors Controlling Flower Opening and Closing in a
Portulaca Hybrid
K A Z U O I C H I M U RA and K E N I C H I S U T O
National Research Institute of Vegetables, Ornamental Plants and Tea, Ano, Mie, 514-2392 Japan
Received : 12 December 1997
Returned for revision : 26 January 1998
Accepted : 26 March 1998
To examine flower opening and closing of a Portulaca hybrid, flower buds were placed in darkness for 12 h
(2030–0830 h) at 20 °C and then exposed to various light-temperature conditions. Flower buds exposed to light at 25,
30 or 35 °C opened within 1 h, and wilted 10–14 h later. Flower buds exposed to light at 20 °C started to open after
4 h but opened slowly and not completely. Flower buds subjected to 25, 30 or 35 °C in darkness also opened rapidly,
but did not reach full opening. Flowers opened at 30 °C in light, and partially closed and opened repeatedly in
response to cycles of a 2-h exposure to 20 °C and a 2-h exposure to 30 °C at any time between 1000 to 1600 h. Similar
phenomena were observed when the flowers opened at 30 °C in light and then were subjected to darkness and light
alternately at 30 °C, although the effect of light was less obvious than that of alternating temperature. Flower opening
and closing were not affected by relative humidity. These results indicate that a rise in temperature is required for
rapid flower opening in the buds kept at 20 °C, and that light intensifies the effect of high temperature. Exposure to
light without a temperature change delayed and slowed flower opening which was never complete. The involvement
of an endogenous rhythm in flower opening by Portulaca is indicated.
# 1998 Annals of Botany Company
Key words : Portulaca hybrid, flower opening, flower closing, temperature shift, endogenous rhythm.
INTRODUCTION
Portulaca cv. ANR1 is considered to be an interspecific
hybrid between P. grandiflora and P. oleracea. Flower and
leaf morphologies of the hybrid resemble those of P.
grandiflora and P. oleracea, respectively (Skirvin, Carlson
and Gorske, 1982). The hybrid grows more vigorously than
P. grandiflora under the very hot and dry conditions typical
of a japanese summer. In addition, the flower of the hybrid
is larger than that of P. grandiflora and is grown as a showy
bedding plant in Japan. The flower of Portulaca is ephemeral
and generally opens early in the morning and wilts in the
afternoon. The flower does not fully open on cloudy or
relatively cool days.
The environmental factors that control opening and
closing of Portulaca flowers have not yet been investigated
in detail, although Iwanami and Tsuji (1962) reported that
flowers of P. grandiflora open in response to a temperature
rise. Flower opening of Anemone, Crocus and Tulipa is also
controlled by a temperature rise (Andrews, 1929 ; Wood,
1953 ; Horovitz, Galil and Zahary, 1975). However, in
Taraxacum, flower opening in response to a warming
temperature is greatly accelerated by exposure to light
(Tanaka et al., 1987 ; Tanaka, Tanaka and Wada, 1988). On
the other hand, flower opening of Pharbitis is promoted by
low temperature (20 °C) and inhibited by high temperature
(30 °C) (Kaihara and Takimoto, 1981). In Oryza satiŠa,
flower opening is affected by relative humidity (Kobayashi,
1927). It is also well-known that, in KalanchoX e and Pharbitis,
the time of flower opening is controlled by an endogenous
rhythm (Bunsow, 1953 ; Oltmanns, 1960 ; Karve! , Engelmann
0305-7364}98}070067­04 $30.00}0
and Schoser, 1961 ; Kaihara and Takimoto, 1979, 1980,
1981).
In the present study, we investigate the effects of
temperature, light and relative humidity on flower opening
and closing of the Portulaca hybrid, and discuss the
involvement of an endogenous rhythm in flower opening.
MATERIALS AND METHODS
Portulaca hybrid cv. ANR1 was grown at 26}23 °C
(day}night). Day length was 12 h (0830–2030 h). Light
intensity at plant level was about 200 µmol m−# s−" (PAR)
provided by cool white fluorescent lamps. Flower buds
expected to open the next morning were cut from the plant,
along with 10 cm of stems, at 2000 h, and placed with the
stalks in distilled water. At 2030 h they were transferred to
20 °C in darkness until 0830 h the next morning. They were
then exposed to various experimental conditions, and the
angle between the two opposite petals (opening angle) was
measured with a protractor at 1- or 2-h intervals until
flowers wilted. In darkness, the opening angle was measured
under a weak green light that had no effect on the opening
of Portulaca flowers. We confirmed that there was no
difference in flower opening and closing between detached
and attached flowers. The term ‘ flower closing ’ in this paper
means the closing of petals without wilting.
At 0830, flower stems kept in darkness at 20 °C were
transferred to temperature-controlled chambers at 20, 25,
30 or 35 °C, and exposed to designated light}dark regimes
described in each experiment. Light intensity at plant level
was about 30 µmol m−# s−" provided from cool white
bo980642
# 1998 Annals of Botany Company
68
Ichimura and Suto—EnŠironmental Factors and Portulaca Hybrid Flowers
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Angle (°)
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F. 1. Effect of light at various temperatures on flower opening of
Portulaca hybrid. Flower buds placed in darkness at 20 °C for 12 h
were subjected to 20, 25, 30 or 35 °C in light at 30 µmol m−# s−", and
opening angles were measured. Values are means of eight buds³s.e.
200
20 °C
25 °C
30 °C
35 °C
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30° C
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Angle (°)
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Time of day
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F. 2. Effect of change in temperature on flowers opened at 30 °C in
light. Flowers opened at 30 °C in light were transferred to 20, 25, 30 or
35 °C in light at 30 µmol m−# s−" and opening angles were measured.
Values are means of eight flowers³s.e.
fluorescent lamps. Humidity was not controlled, but the
relative humidity in the chambers, monitored with a
hygrometer, was about 30 % (low relative humidity).
To examine the effect of humidity of ambient atmosphere
on flower opening, flower stems kept in darkness at 20 °C
were wrapped in a transparent polyethylene bag together
with moistened filter paper, and transferred to 30 °C in light
at 0830 h. The relative humidity in the bag was above 95 %
(high relative humidity). To examine the effect of humidity
on flower closing, flower stems opened at 30 °C in light
(after being transferred to 30 °C in light at 0830 h) were
wrapped in the bag together with moistened filter paper,
and transferred to 20 °C in light at 1030 h.
RESULTS
Effect of temperature on flower opening and closing
Flower buds which had been kept dark for 12 h at 20 °C
opened within 2 h when exposed to light at 25, 30 or 35 °C
(opening angle was more than 150°) and wilted 10–14 h
later. The higher the temperature, the earlier the time of
wilting within this temperature range (Fig. 1). On the other
50
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Time of day
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F. 3. Effect of alternating exposure to 30 °C and 20 °C on flower
opening and closing in light. Flowers fully opened at 30 °C in light were
exposed to 20 °C, and 30 °C alternately, each for 2 h in light, starting
at 1000 (A), 1200 (B) or 1400 h (C). Light intensity was 30 µmol m−# s−".
Values are means of seven flowers³s.e.
hand, flower buds exposed to light at 20 °C (without a
change in temperature) started to open after 4 h, opened
slowly and wilted without attaining full opening (opening
angle, about 100°).
The flower buds which had been kept in darkness for 12 h
at 20 °C were transferred to 30 °C in light. The fully opened
flowers were then transferred to 20, 25, 30 and 35 °C in light
at 1000 h (1±5 h after the start of the treatment). When the
flowers were transferred to 20 °C, they closed within 2 h to
70–80° opening angle, but did not close completely until
they wilted (Fig. 2). The flowers transferred to 25–35 °C in
light wilted 8–10 h later (Fig. 2). In this experiment too, the
higher the temperature, the earlier the time of wilting.
In the next experiment, the flowers fully opened at 30 °C
in light were transferred at 1000, 1200 or 1400 h, to cycles of
a 2-h exposure to 20 °C in light followed by a 2-h exposure
to 30 °C in light (Fig. 3). Flowers transferred to 20 °C at
Ichimura and Suto—EnŠironmental Factors and Portulaca Hybrid Flowers
Angle (°)
150
100
A
200
Angle (°)
25 °C light
30 °C light
35 °C light
25 °C dark
30 °C dark
35 °C dark
200
Light
Dark
Light
Dark
Light
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Light
Dark
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Dark
Light
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F. 4. Comparison of the effects of light and temperature on flower
opening of Portulaca hybrid. Flower buds placed in darkness at 20 °C
for 12 h were exposed to 25, 30 or 35 °C in light at 30 µmol m−# s−" or
in darkness, and opening angles were measured. Values are means of
eight buds³s.e.
1000 or 1200 h partially closed within 1 h (opening angle,
50–60°), and opened again within 1 h after transfer to 30 °C.
They started to close again when transferred to 20 °C a
second time, but did not open thereafter even when returned
to 30 °C. Finally they wilted. When the flowers fully opened
at 30 °C in light were transferred to 20 °C in light at 1400 h,
they partially closed within 1 h, but did not reopen when
exposed to 30 °C, or subsequent 20}30 °C cycles.
Effect of light on flower opening and closing
The flower buds kept in darkness for 12 h at 20 °C were
exposed to light or kept in darkness at 25, 30 or 35 °C (Fig.
4). The buds exposed to light opened within 2 h, confirming
the results shown in Fig. 1. Buds subjected to 25–35 °C in
darkness also opened rapidly, although opening at 25 °C
was slightly later than that at 30 or 35 °C. Flower buds did
not fully open in darkness (opening angle was about 100°),
and the time of flower wilting in darkness was clearly later
than in light (Fig. 4).
In the next experiment, flowers fully opened at 30 °C in
light were transferred to darkness at the same temperature
for 2 h starting at 1000, 1200 or 1400 h, and again transferred to light. Alternate exposure to light and darkness,
each for 2 h, was repeated at 30 °C until wilting (Fig. 5).
When fully opened flowers were transferred to darkness at
1000 h, they partially closed rapidly (opening angle, about
100°), and opened again when exposed to light at 1200 h
(Fig. 5). When flowers opened at 30 °C in light were
transferred to darkness at 1200 h, they partially closed but
opened slightly when exposed to light again at 1400 h.
Flowers transferred to darkness at 1400 h also closed
rapidly, but failed to open again when exposed to light at
1600 h, and then wilted.
Effect of relatiŠe humidity on flower opening
Flower buds placed in high relative humidity opened
when exposed to 30 °C in light, as did those kept at a low
relative humidity. Flower closing at 20 °C was not affected
B
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Angle (°)
8
Light
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C
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Angle (°)
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Light
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Time of day
F. 5. Effect of alternating treatment with light and darkness on flower
opening and closing of Portulaca hybrid. Flowers fully opened at 30 °C
in light were placed in darkness and light alternately, each for 2 h,
starting at 1000 (A), 1200 (B) or 1400 h (C). Light intensity was
30 µmol m−# s−". Values are means of eight flowers³s.e.
by humidity treatment (data not shown). Flower opening of
Portulaca was not influenced by humidity.
DISCUSSION
Flower buds of Portulaca cv. ANR1, kept at 20 °C, opened
rapidly in response to a rise in temperature (Fig. 1), in
agreement with the report of Iwanami and Tsuji (1962) who
concluded that the flower opening of P. grandiflora is
mainly controlled by a warming temperature. However,
flowers did not fully open in darkness even when the
temperature was raised from 20 °C to 30–35 °C. Full opening
was observed only when flower buds were exposed to light
and a higher temperature.
Exposure to light without a temperature change (Fig. 1,
20 °C) resulted in delayed and slowed flower opening which
70
Ichimura and Suto—EnŠironmental Factors and Portulaca Hybrid Flowers
was only partial. The effect of light was quite different at
high temperatures. For example, flowers half-open in
darkness at 30 °C reopened again within 1–2 h when exposed
to light at the same temperature (Fig. 5). Thus, a rise in
temperature is required for rapid flower opening in the bud
kept at 20 °C, and light given together with a high
temperature intensifies the high temperature effect.
When flowers fully opened in light at 30 °C were exposed
to 20 °C and 30 °C alternately, each for 2 h, they repeatedly
closed and opened in response to 20 °C and 30 °C,
respectively (Fig. 3). Thus, to keep flowers open, the flowers
must be exposed to a high temperature together with light
continuously. The energy level of light used in the present
experiment was very low, suggesting that supply of
photosynthate from leaves to flower stems is not the major
factor influenced by light in the present experiment. The
mechanism by which light intensifies the effect of high
temperature is uncertain.
One possibility is that exposure to light (light-on signal)
resets the endogenous rhythm which controls the time of
flower opening. Flowers of Portulaca open slightly every
24 h even when kept in continuous darkness at 25 °C and
closing in response to temperature and light shifts clearly
differed depending (data not shown). In addition, the degree
of flower opening on the time of day (Figs 3 and 5),
suggesting the involvement of an endogenous circadian
rhythm. We assume that an endogenous rhythm is involved
in the control of flower opening of Portulaca, in addition to
the direct effect of temperature and light, although further
experiments are necessary to confirm this.
According to the endogenous rhythm hypothesis, flower
opening should be stimulated 4–10 h after the light-on
signal (see curve for 20 °C in Fig. 1). However, flowers of
Portulaca opened and closed rapidly in response to a
temperature shift from 20 to 30 °C (Fig. 3) and exposure
to light, at least at 30 °C (Fig. 5), during the daytime. Thus,
flower opening of Portulaca under natural conditions may
be controlled largely by the direct effects of temperature and
light.
Under the present experimental conditions, a rise in
temperature was essential for rapid flower opening in the
buds kept at 20 °C, and light intensified the effect of high
temperature. However, in all experiments reported here,
only buds kept at 20 °C for 12 h were used. Whether the
temperature rise is essential for flower opening in buds
previously exposed to different temperatures or different
periods of dark treatment is unknown.
A C K N O W L E D G E M E N TS
We are grateful to Dr S. Kaihara for her critical reading
and correction of this manuscript. We also thank Dr M.
Roh, Professor T. Ando, Dr M. Shibata and Dr S. Kubota
for their valuable advice.
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