Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol83.Issue3-4.March2000:
Scientia Horticulturae 83 (2000) 265±274
Environmental regulation of flowering and growth
of Cosmos atrosanguineus (Hook.) Voss
E.A.G. Kanellos, S. Pearson*
The Department of Horticulture and Landscape, School of Plant Sciences,
The University of Reading, Reading RG6 6AS, UK
Accepted 5 May 1999
Abstract
This study investigated the factors affecting the emergence and subsequent flowering and growth
of the tuberous perennial Cosmos atrosanguineus. A first experiment showed the time of emergence
of overwintered plants raised from micro-propagated tubers was highly related to temperature, but
not photoperiod, such that at 11.58C shoots emerged 17 days later than those at 27.28C. Subsequent
growth was also significantly affected by temperature. Plant height doubled and flower area halved
as temperature increased from 138C to 268C. However, the response of time to flowering from
emergence to temperature was small, increasing temperature from 138C to 21.58C only advanced
flowering by 9 days. In terms of the overall response to photoperiod, flowering was advanced by
long-days; plants at a daylength of 17 h per day flowered 33 days earlier than those at 8 h per day.
Photoperiod also dramatically affected plant morphology, with long photoperiods (17 h per day)
leading to a greater than 7-fold increase in plant mass compared to short-days (8 h per day). The
experiments described suggest that out of season forcing of Cosmos is horticulturally attainable at a
relatively small cost. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cosmos atrosanguineus; Temperature; Tubers; Flowering; Plant height; Flower area;
Emergence
1. Introduction
Cosmos is a genus within the family Asteraceae. They are late-flowering
annuals or tuberous perennials. Cosmos bipinattus (Cav.) and Cosmos sulphureus
(Cav.) are the most widely studied species of the genus, and are generally
* Corresponding author. Tel.: +44-118-9-316379; fax: +44-118-9-750630.
E-mail address: [email protected] (S. Pearson).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 8 1 - 3
266
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
considered to be short-day plants (Molder and Owens, 1985). The photoperiod for
optimal flowering in C. bipinnatus is less than 14 h per day. At longer daylengths
flower development is delayed and flower buds appear irregularly (Molder and
Owens, 1985). The response of C. bipinnatus is typical of a facultative SDP,
whereas C. sulphureus seems to be an obligate SDP.
C. atrosanguineus (Hook.) Voss has received little if any research attention. It is
a tuberous perennial, popular for its highly `chocolate' scented maroon-crimson
flowers, but is thought to be extinct in the wild. In nurseries, the tubers of C.
atrosanguineus are grown from micro-propagated mini-tubers. These can be
produced throughout the year, and in the UK generally between March and
August. Plants propagated in the first season are acclimated from the micropropagation environment and grown in plugs. Small quantities from the early
batches are sold in September but the majority of the crop suspends growth late in
autumn and overwinters in plugs as tubers. The following season the plants are
potted up and sold after the shoots have emerged and produced the first flower in
late spring. Chilling seems not to be involved in either the shoot emergence of
tubers or through vernalization, as tubers can be acclimated and grown to form
flowering plants in the late summer of the same season.
It would be highly advantageous if flowering could be advanced further, if only by
a few weeks, since it would allow the plants to be sold earlier than or during the time
of peak garden plant demand in the spring/early summer, when bedding plants are on
the market. This could be achieved either by bringing forward the emergence of the
plants from tubers or shortening the time from emergence to flowering.
The objectives of this study were therefore to investigate the role of
temperature and photoperiod on the emergence, subsequent growth and flowering
of overwintered C. atrosanguineus. Effects of photoperiod on emergence were
examined as it has been shown to influence the emergence of other subterranean
organs including the geophyte Colchicum tunicatum (Gutterman and Boeken,
1988). Temperature is also known to affect the rate of emergence of a number of
bulb, corm and tuber species (Rees, 1992).
2. Materials and methods
Two experiments were conducted to investigate the effects of temperature, as
well as photoperiod in one, on shoot emergence, growth and flowering of C.
atrosanguineus. A third experiment examined specific effects of photoperiod.
2.1. Plant material
For experiments 1 and 2, 300 C. atrosanguineus tubers in plug-trays (P80,
Plantpak, UK), raised from tissue culture, were received on the 15th January 1997
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
267
from Wyevale Nurseries (Herefordshire, UK), where they had been grown under
protection in a polyethylene tunnel. Plants were received on that date as they were
just beginning to senesce and, presumably, tubers were entering a temporary
suspension of growth. On average, plants had three to four stems, each 10±15 cm
in height with 15 leaves. Once received, and following commercial practice, all
the shoots were trimmed off. For experiment 3, similar plants were received on 12
February 1998. Plants were potted up in 2l pots with peat-bark (SHL, W. Sinclair
Horticulture Ltd., Lincoln, UK) compost. Three weeks after potting, a 6-monthrelease 5 g tablet of OsmocotePlus (15 N : 10 P : 12 K) controlled release
fertilizer was inserted into each pot. They were watered as required with tap
water.
The number of days to flowering (i.e. when the corolla of the first flower had
fully opened) were recorded for each plant. At flowering, the plant was harvested
to assess final height to the first flowering node.
2.1.1. Experiment 1: The effect of temperature and photoperiod on emergence,
subsequent growth and time to flower of C. atrosanguineus
The aim of the experiment was to assess the role of temperature and
photoperiod on shoot emergence and subsequent growth. The experiment was
carried out in the winter and spring of 1997. It started on 21st January (first batch)
and was repeated 6 weeks later starting on 11th of March (second batch). For the
6 weeks between the first and second batch, the plugs were maintained in a cold
store (58C).
At the start of each batch, the plugs were placed on movable trolleys in five, of
a linear array of eight, 2.7 m 7.2 m temperature controlled glasshouse
compartments. The experimental compartments used, had set-point heating
temperatures of 108C, 148C, 188C, 228C and 268C. The coolest compartment was
equipped with air conditioning units in order to maintain temperatures throughout
the experimental period. Ventilation occurred at temperatures 48C higher than the
set point. Mean diurnal temperatures within each compartment were calculated
from temperatures recorded with a data-logger (Datataker, DT500, Data
Electronics, Letchworth Garden City, UK) scanned every 15 s but recording
hourly averages, using aspirated PT100 temperature sensors. Each compartment
was equipped with four photoperiod controlled chambers, sealed from exterior
light sources. For this experiment, two of the chambers were used. Thus, the
plugs were kept on trolleys in the different glasshouse compartments from 08:00
hours to 16:00 hours receiving natural daylight. Thereafter, they were wheeled in
the photoperiod garages for the photoperiod treatment. In one compartment no
day extension lighting was used, giving a daylength of 8 h per day. In the second,
from 16:00 hours, the day was extended to 24:00 hours (16 h per day
photoperiod) by an irradiance of 11 mmol mÿ2 sÿ1 PAR, using a 40 W tungsten
and a 15 W compact fluorescent light bulb. Eight replicate plugs were maintained
268
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
on each trolley within each compartment; in total 10 treatments. From the start of
the experiment, data on emergence were collected daily. Three weeks after
emergence plants were potted on and transferred to an unheated glasshouse
(15 38C) until flowering when they were recorded.
2.1.2. Experiment 2: The effect of different temperatures on the emergence,
subsequent growth and time to flower of C. atrosanguineus
The aim of the experiment was to assess the role of different forcing
temperatures on the growth of C. atrosanguineus. It began on 21st January 1997
and was repeated on a further occasion; with the repeat batch removed from a
cold store (58C) after 6 weeks.
The plugs were placed in four out of a 2 3 array of six, 7 m 7 m
temperature controlled glasshouse compartments. The experimental compartments used had set-point heating temperatures of 148C, 188C, 228C and 268C.
There were six replicate plants per batch per treatment. A 16 h per day daylength
(to avoid potential affects of changes in natural photoperiod between repeated
batches) was provided by SON-T lamps, with an irradiance of 120 mmol mÿ2 sÿ1
centred at mid-day.
At the start of the experiment, data on emergence were collected daily. The
plants were grown for four months in total. Data on the flower area of the fully
expanded corollas were also collected.
2.1.3. Experiment 3: The effects of photoperiod on the flowering and morphology
of C. atrosanguineus
The aim of this third experiment was to assess the role of photoperiod on
flowering and growth of C. atrosanguineus. The experiment was carried out in
the winter and spring of 1998, starting on 12th February. Plug plants were placed
on movable trolleys in a glasshouse compartment containing four photoperiodic
chambers, similar to experiment 1.
Inside the chambers, the extension of the daylength (from 16:00 hours
onwards) was provided by an irradiance of 10 mmol mÿ2 sÿ1 from a 60 : 40
(calculated on the basis of nominal wattage) mixture of tungsten lamps and warm
white fluorescent tubes. Treatments were arranged in a completely randomized
design of four photoperiods (8, 11, 14 and 17 h per day), with five replicate plants
monitored per treatment. To minimize positional effects, treatments were rerandomized between chambers twice during the experiment. Plants were grown
and recorded as in the first two experiments. The average daytime air temperature
(08:00 hours till 16:00 hours) for the duration of the experiment was 188C and the
chamber temperature was maintained via air conditioning units at 12 18C
throughout. Cultural treatments were as in experiments 1 and 2.
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
269
3. Results
3.1. Experiment 1: The effect of temperature and photoperiod on emergence,
subsequent growth and time to flower of C. atrosanguineus
For the 21st January batch, time to shoot emergence was highly dependent
upon temperature, and not affected by photoperiod (Fig. 1). As temperature
increased, the time to emergence decreased curvi-linearly (r2 0.975), such that
at 11.58C on average 23.2 days were required for emergence, compared to 6.3
days at 27.28C. Similar responses were found for the second batch of plants (data
not shown).
The temperature and photoperiod treatments applied over the first three weeks
had no significant effects on the time from emergence to flowering (data not
shown) or the node of first flowering. However, effects on plant height were still
noted at flowering (Fig. 2), such that high emergence temperatures reduced final
quality by increasing height; plants which emerged at 11.58C were 10 cm at
flowering compared to 14.1 cm at 27.28C (P < 0.05). There was a trend towards
increasing plant height with long photoperiods (P < 0.01); however, it was not
clear whether this was a response to photoperiod per se or to the far-red light
provided by the tungsten lamps used to provide the daylength extension.
Fig. 1. The effect of mean temperature on the time to shoot emergence of tubers of C.
atrosanguineus forced on the 21st January 1997. Tubers forced under 8 h per day (*) and 17 h per
day (*). The relationship was fitted by regression analysis where, days to shoot
emergence 65.06(4.67) ÿ 4.8126(0.5)T 0.098(0.013)T2, r2 0.975, 9 d.f. where T,
represents the mean temperature from the start of the experiment to shoot emergence. Standard
errors of the mean are shown where bigger than the point.
270
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
Fig. 2. The effect of temperature on plant height at first flowering of C. atrosanguineus. The
relationship was fitted by regression analysis where, plant height 6.19(1.59) 0.29(0.08)T,
r2 0.819, 4 d.f., T represents the mean temperature from the start of the experiment to first
flowering. Standard errors of the mean are shown where bigger than the point.
3.2. Experiment 2: The effect of different forcing temperatures and time of
forcing commencement on emergence, subsequent growth and time to flowering
of C. atrosanguineus
In general, there were relatively few differences between batches, notably there
were no significant differences between batches in terms of the mean time to
emergence, averaged over all treatments. Temperature had a small but significant
effect on the time to flowering from emergence, such that at 21.58C plants
flowered after 80 days, compared to a mean of 89 at 138C. Fig. 3 shows that for
the first batch, increasing temperatures above 21.28C led to slight delays in
flowering. The largest effects of temperature on flowering were in terms of flower
area, where high temperatures led to a substantial decrease in final flower size
(Fig. 3b). However, the most striking effects were in terms of plant height at first
flowering, which increased linearly with temperature (Fig. 4), increasing
temperature from 138C to 268C doubled plant height at flowering (P < 0.05).
However, all plants from all treatments flowered on the 7±8th internode.
3.3. Experiment 3: Effects of photoperiod on flowering and plant morphology
Time to flowering was significantly (P < 0.001) affected by photoperiod (see
Table 1), with plants grown at 17 h per day flowering 33 days earlier than those at
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
271
Fig. 3. (a) The effect of mean temperature on the time to flowering of C. atrosanguineus. The days
to flower, as well as the mean temperature, were determined from shoot emergence. Plants of the
first batch ( ) forced on 21st January 1997, plants of the second batch (*) forced three weeks later.
The relationship was fitted by regression analysis where days to flower 122.8(13.95)
ÿ 3.86(1.49)T 0.089(0.038)T2, r2 0.747, 7 d.f., T represents the mean temperature from
shoot emergence to the first flower. (b) The effect of mean temperature on the flower size of
C. atrosanguineus. The relationship was fitted by regression analysis where flower area
26.496(2.97) ÿ 0.673(0.142)T, r2 0.789, 7 d.f. where T represents the mean temperature
from shoot emergence to the first flower. Standard errors of the mean are shown where bigger than
the point.
272
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
Fig. 4. The effect of temperature on plant height at first flower of C. atrosanguineus. Plants of the
first batch (*) forced on 21st January 1997, plants of the second batch (*) forced three weeks
later. The relationship was fitted by regression analysis where plant height ÿ1.309(5.1)
1.062(0.243)T, r2 0.759, 7 d.f., where T represents the mean temperature from shoot emergence
to the first flower. Standard errors of the mean are shown where bigger than the point.
8 h per day. However, the node at which the first flower was formed was not
significantly affected by photoperiod. All plants flowered after the formation of
between 7 and 9 nodes. Plant height recorded at flowering was also significantly
reduced (P < 0.001), by twofold, under the short- (8 h per day) compared to the
long-day (>14 h per day) treatments. This reflected a dramatic change in plant
and leaf morphology with photoperiod (see Plate 1), which led to greater than
sevenfold reduction in plant fresh weight under short-day treatments.
Table 1
The effects of photoperiod on the days to flowering and plant morphology and growth of C.
atrosanguineus
Treatment
Days to
flowering
Plant
height (cm)
8 h per day
11 h per day
14 h per day
17 h per day
SED
157.2
150.2
130.8
124.0
2.4***
11.4
19.6
21.0
25.0
1.2***
***
Significantly different at the 0.001 level.
Branch
number
7
7.6
5.2
8
1.1 (NS)
Fresh
weight (g)
12.4
13.9
71.5
95.0
4.3***
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
273
Plate 1. Effect of photoperiod on growth C. atrosanguineus. From left to right, plants grown under
17, 14, 11 and 8 h per day treatments.
4. Discussion
This study has shown that the time to emergence of C. atrosanguineus can be
advanced by using high temperature, and that cool temperatures considerably
delay time to emergence. The advance in time to emergence using warm
temperatures would be in the order of 17 days if 278C was used compared to
118C. This would give growers considerable opportunity to extend the current
season, especially as most C. atrosanguineus are grown in cold frost-protected
greenhouses. However, very high emergence temperatures may have a deleterious
effect on final plant height.
The time to flowering was relatively insensitive to temperature, an 88C increase
in temperature from 138C only shortened time to flowering by 9 days. This
insensitivity to temperature may explain why overwintered C. atrosanguineus
typically flowers at the same time between years (as noted by growers, A.
Johnson, personal communication), even though temperature may vary
substantially.
In this study, C. atrosanguineus were shown to be quantitative long-day plants.
This is in complete contrast to other Cosmos species studied, since C. bipinnatus
and C. sulphureus are SDP's (Molder and Owens, 1985). In this instance, the
effects of photoperiod were most likely to have occurred post flower initiation,
since node numbers below the flower were similar between all treatments. It is
not clear, however, whether the effects of the long-day treatments had a direct or
indirect effect on subsequent flowering, since photoperiod led to a dramatic
change in plant morphology and a seven-fold reduction in plant growth. Such
changes in plant morphology with photoperiod are not uncommon and have been
274
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
widely reported in the literature (Thomas and Vince Prue, 1995). Here, it is not
unreasonable to assume that the changes in plant morphology with photoperiod
may be associated with the plant attempting to form tubers under short-days,
although final tuber weights were not measured.
Post emergence temperature also had a dramatic effect on final plant
appearance, with high temperatures leading to undesirable increases in plant
height and reductions in final flower size. Increasing plant height and reductions
in flower size with high temperature have been widely reported in other species
(see, for example, Langton and Cockshull, 1997; Pearson et al., 1995). Thus, in
terms of overall plant quality and flowering, there seems to be little advantage to
the grower to use high forcing temperatures. A two-phase production system,
therefore, seems to be the most efficient; during the first phase temperature
should be warm, until the shoots emerge, and then reduced in conjunction with
long-day treatments, to prevent subsequent final plant quality loss and the
maximum advancement of flowering.
Acknowledgements
We wish to thank Andy Johnson and Wyevale Nurseries for providing guidance
and the plant material. EAGK was funded by the University of Reading Research
Endowment Fund.
References
Gutterman, Y., Boeken, B., 1988. Flowering affected by daylength and temperature in the leafless
flowering desert geophyte Colchicum tunicatumits annual life-cycle and vegetative propagation.
Bot. Gaz. 149, 382±390.
Langton, F.A., Cockshull, K.E., 1997. Is stem elongation determined by DIF or by absolute day and
night temperatures? Sci. Hortic. 59, 91±106.
Molder, M., Owens, J.N., 1985. Cosmos. In: Halevy, A.H. (Eds.), CRC Handbook of flowering.
CRC Press, Boca Raton, FL.
Pearson, S., Adams, S.R., Hadley, P., May, D.R., Parker, A., 1995. The effects of temperature on the
flower size of pansy (Viola X wittrockiana Gams). J. Hortic. Sci. 70, 183±190.
Rees, A.R., 1992. Ornamental Bulbs, Corms and Tubers. CAB International, Wallingford, 220 pp.
Thomas, B., Vince Prue, D., 1995. Photoperiodism in Plants. Academic Press, London, UK.
Environmental regulation of flowering and growth
of Cosmos atrosanguineus (Hook.) Voss
E.A.G. Kanellos, S. Pearson*
The Department of Horticulture and Landscape, School of Plant Sciences,
The University of Reading, Reading RG6 6AS, UK
Accepted 5 May 1999
Abstract
This study investigated the factors affecting the emergence and subsequent flowering and growth
of the tuberous perennial Cosmos atrosanguineus. A first experiment showed the time of emergence
of overwintered plants raised from micro-propagated tubers was highly related to temperature, but
not photoperiod, such that at 11.58C shoots emerged 17 days later than those at 27.28C. Subsequent
growth was also significantly affected by temperature. Plant height doubled and flower area halved
as temperature increased from 138C to 268C. However, the response of time to flowering from
emergence to temperature was small, increasing temperature from 138C to 21.58C only advanced
flowering by 9 days. In terms of the overall response to photoperiod, flowering was advanced by
long-days; plants at a daylength of 17 h per day flowered 33 days earlier than those at 8 h per day.
Photoperiod also dramatically affected plant morphology, with long photoperiods (17 h per day)
leading to a greater than 7-fold increase in plant mass compared to short-days (8 h per day). The
experiments described suggest that out of season forcing of Cosmos is horticulturally attainable at a
relatively small cost. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cosmos atrosanguineus; Temperature; Tubers; Flowering; Plant height; Flower area;
Emergence
1. Introduction
Cosmos is a genus within the family Asteraceae. They are late-flowering
annuals or tuberous perennials. Cosmos bipinattus (Cav.) and Cosmos sulphureus
(Cav.) are the most widely studied species of the genus, and are generally
* Corresponding author. Tel.: +44-118-9-316379; fax: +44-118-9-750630.
E-mail address: [email protected] (S. Pearson).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 8 1 - 3
266
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
considered to be short-day plants (Molder and Owens, 1985). The photoperiod for
optimal flowering in C. bipinnatus is less than 14 h per day. At longer daylengths
flower development is delayed and flower buds appear irregularly (Molder and
Owens, 1985). The response of C. bipinnatus is typical of a facultative SDP,
whereas C. sulphureus seems to be an obligate SDP.
C. atrosanguineus (Hook.) Voss has received little if any research attention. It is
a tuberous perennial, popular for its highly `chocolate' scented maroon-crimson
flowers, but is thought to be extinct in the wild. In nurseries, the tubers of C.
atrosanguineus are grown from micro-propagated mini-tubers. These can be
produced throughout the year, and in the UK generally between March and
August. Plants propagated in the first season are acclimated from the micropropagation environment and grown in plugs. Small quantities from the early
batches are sold in September but the majority of the crop suspends growth late in
autumn and overwinters in plugs as tubers. The following season the plants are
potted up and sold after the shoots have emerged and produced the first flower in
late spring. Chilling seems not to be involved in either the shoot emergence of
tubers or through vernalization, as tubers can be acclimated and grown to form
flowering plants in the late summer of the same season.
It would be highly advantageous if flowering could be advanced further, if only by
a few weeks, since it would allow the plants to be sold earlier than or during the time
of peak garden plant demand in the spring/early summer, when bedding plants are on
the market. This could be achieved either by bringing forward the emergence of the
plants from tubers or shortening the time from emergence to flowering.
The objectives of this study were therefore to investigate the role of
temperature and photoperiod on the emergence, subsequent growth and flowering
of overwintered C. atrosanguineus. Effects of photoperiod on emergence were
examined as it has been shown to influence the emergence of other subterranean
organs including the geophyte Colchicum tunicatum (Gutterman and Boeken,
1988). Temperature is also known to affect the rate of emergence of a number of
bulb, corm and tuber species (Rees, 1992).
2. Materials and methods
Two experiments were conducted to investigate the effects of temperature, as
well as photoperiod in one, on shoot emergence, growth and flowering of C.
atrosanguineus. A third experiment examined specific effects of photoperiod.
2.1. Plant material
For experiments 1 and 2, 300 C. atrosanguineus tubers in plug-trays (P80,
Plantpak, UK), raised from tissue culture, were received on the 15th January 1997
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
267
from Wyevale Nurseries (Herefordshire, UK), where they had been grown under
protection in a polyethylene tunnel. Plants were received on that date as they were
just beginning to senesce and, presumably, tubers were entering a temporary
suspension of growth. On average, plants had three to four stems, each 10±15 cm
in height with 15 leaves. Once received, and following commercial practice, all
the shoots were trimmed off. For experiment 3, similar plants were received on 12
February 1998. Plants were potted up in 2l pots with peat-bark (SHL, W. Sinclair
Horticulture Ltd., Lincoln, UK) compost. Three weeks after potting, a 6-monthrelease 5 g tablet of OsmocotePlus (15 N : 10 P : 12 K) controlled release
fertilizer was inserted into each pot. They were watered as required with tap
water.
The number of days to flowering (i.e. when the corolla of the first flower had
fully opened) were recorded for each plant. At flowering, the plant was harvested
to assess final height to the first flowering node.
2.1.1. Experiment 1: The effect of temperature and photoperiod on emergence,
subsequent growth and time to flower of C. atrosanguineus
The aim of the experiment was to assess the role of temperature and
photoperiod on shoot emergence and subsequent growth. The experiment was
carried out in the winter and spring of 1997. It started on 21st January (first batch)
and was repeated 6 weeks later starting on 11th of March (second batch). For the
6 weeks between the first and second batch, the plugs were maintained in a cold
store (58C).
At the start of each batch, the plugs were placed on movable trolleys in five, of
a linear array of eight, 2.7 m 7.2 m temperature controlled glasshouse
compartments. The experimental compartments used, had set-point heating
temperatures of 108C, 148C, 188C, 228C and 268C. The coolest compartment was
equipped with air conditioning units in order to maintain temperatures throughout
the experimental period. Ventilation occurred at temperatures 48C higher than the
set point. Mean diurnal temperatures within each compartment were calculated
from temperatures recorded with a data-logger (Datataker, DT500, Data
Electronics, Letchworth Garden City, UK) scanned every 15 s but recording
hourly averages, using aspirated PT100 temperature sensors. Each compartment
was equipped with four photoperiod controlled chambers, sealed from exterior
light sources. For this experiment, two of the chambers were used. Thus, the
plugs were kept on trolleys in the different glasshouse compartments from 08:00
hours to 16:00 hours receiving natural daylight. Thereafter, they were wheeled in
the photoperiod garages for the photoperiod treatment. In one compartment no
day extension lighting was used, giving a daylength of 8 h per day. In the second,
from 16:00 hours, the day was extended to 24:00 hours (16 h per day
photoperiod) by an irradiance of 11 mmol mÿ2 sÿ1 PAR, using a 40 W tungsten
and a 15 W compact fluorescent light bulb. Eight replicate plugs were maintained
268
E.A.G. Kanellos, S. Pearson / Scientia Horticulturae 83 (2000) 265±274
on each trolley within each compartment; in total 10 treatments. From the start of
the experiment, data on emergence were collected daily. Three weeks after
emergence plants were potted on and transferred to an unheated glasshouse
(15 38C) until flowering when they were recorded.
2.1.2. Experiment 2: The effect of different temperatures on the emergence,
subsequent growth and time to flower of C. atrosanguineus
The aim of the experiment was to assess the role of different forcing
temperatures on the growth of C. atrosanguineus. It began on 21st January 1997
and was repeated on a further occasion; with the repeat batch removed from a
cold store (58C) after 6 weeks.
The plugs were placed in four out of a 2 3 array of six, 7 m 7 m
temperature controlled glasshouse compartments. The experimental compartments used had set-point heating temperatures of 148C, 188C, 228C and 268C.
There were six replicate plants per batch per treatment. A 16 h per day daylength
(to avoid potential affects of changes in natural photoperiod between repeated
batches) was provided by SON-T lamps, with an irradiance of 120 mmol mÿ2 sÿ1
centred at mid-day.
At the start of the experiment, data on emergence were collected daily. The
plants were grown for four months in total. Data on the flower area of the fully
expanded corollas were also collected.
2.1.3. Experiment 3: The effects of photoperiod on the flowering and morphology
of C. atrosanguineus
The aim of this third experiment was to assess the role of photoperiod on
flowering and growth of C. atrosanguineus. The experiment was carried out in
the winter and spring of 1998, starting on 12th February. Plug plants were placed
on movable trolleys in a glasshouse compartment containing four photoperiodic
chambers, similar to experiment 1.
Inside the chambers, the extension of the daylength (from 16:00 hours
onwards) was provided by an irradiance of 10 mmol mÿ2 sÿ1 from a 60 : 40
(calculated on the basis of nominal wattage) mixture of tungsten lamps and warm
white fluorescent tubes. Treatments were arranged in a completely randomized
design of four photoperiods (8, 11, 14 and 17 h per day), with five replicate plants
monitored per treatment. To minimize positional effects, treatments were rerandomized between chambers twice during the experiment. Plants were grown
and recorded as in the first two experiments. The average daytime air temperature
(08:00 hours till 16:00 hours) for the duration of the experiment was 188C and the
chamber temperature was maintained via air conditioning units at 12 18C
throughout. Cultural treatments were as in experiments 1 and 2.
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269
3. Results
3.1. Experiment 1: The effect of temperature and photoperiod on emergence,
subsequent growth and time to flower of C. atrosanguineus
For the 21st January batch, time to shoot emergence was highly dependent
upon temperature, and not affected by photoperiod (Fig. 1). As temperature
increased, the time to emergence decreased curvi-linearly (r2 0.975), such that
at 11.58C on average 23.2 days were required for emergence, compared to 6.3
days at 27.28C. Similar responses were found for the second batch of plants (data
not shown).
The temperature and photoperiod treatments applied over the first three weeks
had no significant effects on the time from emergence to flowering (data not
shown) or the node of first flowering. However, effects on plant height were still
noted at flowering (Fig. 2), such that high emergence temperatures reduced final
quality by increasing height; plants which emerged at 11.58C were 10 cm at
flowering compared to 14.1 cm at 27.28C (P < 0.05). There was a trend towards
increasing plant height with long photoperiods (P < 0.01); however, it was not
clear whether this was a response to photoperiod per se or to the far-red light
provided by the tungsten lamps used to provide the daylength extension.
Fig. 1. The effect of mean temperature on the time to shoot emergence of tubers of C.
atrosanguineus forced on the 21st January 1997. Tubers forced under 8 h per day (*) and 17 h per
day (*). The relationship was fitted by regression analysis where, days to shoot
emergence 65.06(4.67) ÿ 4.8126(0.5)T 0.098(0.013)T2, r2 0.975, 9 d.f. where T,
represents the mean temperature from the start of the experiment to shoot emergence. Standard
errors of the mean are shown where bigger than the point.
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Fig. 2. The effect of temperature on plant height at first flowering of C. atrosanguineus. The
relationship was fitted by regression analysis where, plant height 6.19(1.59) 0.29(0.08)T,
r2 0.819, 4 d.f., T represents the mean temperature from the start of the experiment to first
flowering. Standard errors of the mean are shown where bigger than the point.
3.2. Experiment 2: The effect of different forcing temperatures and time of
forcing commencement on emergence, subsequent growth and time to flowering
of C. atrosanguineus
In general, there were relatively few differences between batches, notably there
were no significant differences between batches in terms of the mean time to
emergence, averaged over all treatments. Temperature had a small but significant
effect on the time to flowering from emergence, such that at 21.58C plants
flowered after 80 days, compared to a mean of 89 at 138C. Fig. 3 shows that for
the first batch, increasing temperatures above 21.28C led to slight delays in
flowering. The largest effects of temperature on flowering were in terms of flower
area, where high temperatures led to a substantial decrease in final flower size
(Fig. 3b). However, the most striking effects were in terms of plant height at first
flowering, which increased linearly with temperature (Fig. 4), increasing
temperature from 138C to 268C doubled plant height at flowering (P < 0.05).
However, all plants from all treatments flowered on the 7±8th internode.
3.3. Experiment 3: Effects of photoperiod on flowering and plant morphology
Time to flowering was significantly (P < 0.001) affected by photoperiod (see
Table 1), with plants grown at 17 h per day flowering 33 days earlier than those at
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271
Fig. 3. (a) The effect of mean temperature on the time to flowering of C. atrosanguineus. The days
to flower, as well as the mean temperature, were determined from shoot emergence. Plants of the
first batch ( ) forced on 21st January 1997, plants of the second batch (*) forced three weeks later.
The relationship was fitted by regression analysis where days to flower 122.8(13.95)
ÿ 3.86(1.49)T 0.089(0.038)T2, r2 0.747, 7 d.f., T represents the mean temperature from
shoot emergence to the first flower. (b) The effect of mean temperature on the flower size of
C. atrosanguineus. The relationship was fitted by regression analysis where flower area
26.496(2.97) ÿ 0.673(0.142)T, r2 0.789, 7 d.f. where T represents the mean temperature
from shoot emergence to the first flower. Standard errors of the mean are shown where bigger than
the point.
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Fig. 4. The effect of temperature on plant height at first flower of C. atrosanguineus. Plants of the
first batch (*) forced on 21st January 1997, plants of the second batch (*) forced three weeks
later. The relationship was fitted by regression analysis where plant height ÿ1.309(5.1)
1.062(0.243)T, r2 0.759, 7 d.f., where T represents the mean temperature from shoot emergence
to the first flower. Standard errors of the mean are shown where bigger than the point.
8 h per day. However, the node at which the first flower was formed was not
significantly affected by photoperiod. All plants flowered after the formation of
between 7 and 9 nodes. Plant height recorded at flowering was also significantly
reduced (P < 0.001), by twofold, under the short- (8 h per day) compared to the
long-day (>14 h per day) treatments. This reflected a dramatic change in plant
and leaf morphology with photoperiod (see Plate 1), which led to greater than
sevenfold reduction in plant fresh weight under short-day treatments.
Table 1
The effects of photoperiod on the days to flowering and plant morphology and growth of C.
atrosanguineus
Treatment
Days to
flowering
Plant
height (cm)
8 h per day
11 h per day
14 h per day
17 h per day
SED
157.2
150.2
130.8
124.0
2.4***
11.4
19.6
21.0
25.0
1.2***
***
Significantly different at the 0.001 level.
Branch
number
7
7.6
5.2
8
1.1 (NS)
Fresh
weight (g)
12.4
13.9
71.5
95.0
4.3***
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273
Plate 1. Effect of photoperiod on growth C. atrosanguineus. From left to right, plants grown under
17, 14, 11 and 8 h per day treatments.
4. Discussion
This study has shown that the time to emergence of C. atrosanguineus can be
advanced by using high temperature, and that cool temperatures considerably
delay time to emergence. The advance in time to emergence using warm
temperatures would be in the order of 17 days if 278C was used compared to
118C. This would give growers considerable opportunity to extend the current
season, especially as most C. atrosanguineus are grown in cold frost-protected
greenhouses. However, very high emergence temperatures may have a deleterious
effect on final plant height.
The time to flowering was relatively insensitive to temperature, an 88C increase
in temperature from 138C only shortened time to flowering by 9 days. This
insensitivity to temperature may explain why overwintered C. atrosanguineus
typically flowers at the same time between years (as noted by growers, A.
Johnson, personal communication), even though temperature may vary
substantially.
In this study, C. atrosanguineus were shown to be quantitative long-day plants.
This is in complete contrast to other Cosmos species studied, since C. bipinnatus
and C. sulphureus are SDP's (Molder and Owens, 1985). In this instance, the
effects of photoperiod were most likely to have occurred post flower initiation,
since node numbers below the flower were similar between all treatments. It is
not clear, however, whether the effects of the long-day treatments had a direct or
indirect effect on subsequent flowering, since photoperiod led to a dramatic
change in plant morphology and a seven-fold reduction in plant growth. Such
changes in plant morphology with photoperiod are not uncommon and have been
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widely reported in the literature (Thomas and Vince Prue, 1995). Here, it is not
unreasonable to assume that the changes in plant morphology with photoperiod
may be associated with the plant attempting to form tubers under short-days,
although final tuber weights were not measured.
Post emergence temperature also had a dramatic effect on final plant
appearance, with high temperatures leading to undesirable increases in plant
height and reductions in final flower size. Increasing plant height and reductions
in flower size with high temperature have been widely reported in other species
(see, for example, Langton and Cockshull, 1997; Pearson et al., 1995). Thus, in
terms of overall plant quality and flowering, there seems to be little advantage to
the grower to use high forcing temperatures. A two-phase production system,
therefore, seems to be the most efficient; during the first phase temperature
should be warm, until the shoots emerge, and then reduced in conjunction with
long-day treatments, to prevent subsequent final plant quality loss and the
maximum advancement of flowering.
Acknowledgements
We wish to thank Andy Johnson and Wyevale Nurseries for providing guidance
and the plant material. EAGK was funded by the University of Reading Research
Endowment Fund.
References
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