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 perennialCosmos 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.) andCosmos 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).
considered to be short-day plants (Molder and Owens, 1985). The photoperiod for optimal flowering inC. bipinnatusis 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, whereasC. 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 micro-propagation 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
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-month-release 5 g tablet of OsmocotePlus (15 N : 10 P : 12 K) controlled 6-month-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 m7.2 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 11mmol mÿ2sÿ1 PAR, using a 40 W tungsten
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 (1538C) 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 ofC. 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 23 array of six, 7 m7 m temperature controlled glasshouse compartments. The experimental compart-ments 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 120mmol mÿ2sÿ1
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 10mmol mÿ2sÿ1 from a 60 : 40
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 (r20.975), 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.
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
Fig. 2. The effect of temperature on plant height at first flowering of C. atrosanguineus. The relationship was fitted by regression analysis where, plant height6.19(1.59)0.29(0.08)T,
Fig. 3. (a) The effect of mean temperature on the time to flowering ofC. 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 flower122.8(13.95)
ÿ3.86(1.49)T0.089(0.038)T2, r20.747, 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
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.
Fig. 4. The effect of temperature on plant height at first flower ofC. 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,r20.759, 7 d.f., whereTrepresents the mean temperature from shoot emergence to the first flower. Standard errors of the mean are shown where bigger than the point.
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)
Branch number
Fresh weight (g)
8 h per day 157.2 11.4 7 12.4
11 h per day 150.2 19.6 7.6 13.9
14 h per day 130.8 21.0 5.2 71.5
17 h per day 124.0 25.0 8 95.0
SED 2.4*** 1.2*** 1.1 (NS) 4.3***
4. Discussion
This study has shown that the time to emergence ofC. atrosanguineuscan 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 mostC. 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. atrosanguineuswere shown to be quantitative long-day plants. This is in complete contrast to otherCosmosspecies 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
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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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 wittrockianaGams). J. Hortic. Sci. 70, 183±190.
