Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol85.Issue3.Agust2000:
Scientia Horticulturae 85 (2000) 231±241
Environmental regulation of ¯owering time in
heliotrope (Heliotropium arborescens L. cv. Marine)
Byong H. Park, Simon Pearson*
The Department of Horticulture, School of Plant Sciences, The University of Reading,
Reading RG6 6AS, UK
Accepted 11 November 1999
Abstract
The aim of this study was to examine the environmental regulation of ¯ower initiation and
subsequent development in heliotrope (Heliotropium aborescens L. cv. Marine). Five experiments
were conducted, two examined whether ¯owering could be advanced by cool temperatures. The
duration of cool temperature required to induce rapid ¯owering was also investigated. The ®nal
three experiments examined the effects of light integral, photoperiod and temperature on ¯ower
initiation and development.
It was found that plants grown for 9 days at 108C and than transferred to 208C ¯owered
signi®cantly earlier (®rst ¯owering recorded after 55 days) than plants held constantly at 208C (65.9
days to ¯owering). Plants grown at a constant temperature of 208C had signi®cantly more leaves
than all other treatments. This suggested that `cool' temperatures, prior to initiation, advanced
¯owering. In a transfer experiment, plants were moved from 10 to 208C at 3 days intervals postpinching. Earliest ¯owering (by 20 days compared to the 208C constant treatment) occurred when
plants were exposed to 108C for 9 days and then transferred to 208C.
Photoperiod was shown to have no effect on either ¯ower bud initiation or development (postinitiation). Both temperature and light integral strongly in¯uenced ¯ower development post-¯ower
bud initiation. However, the response to temperature plotted in terms of the reciprocal of days to
¯owering was non-linear. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Heliotrope; Scented plants; Flower development; Initiation; Temperature; Photoperiod;
Light integral
*
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 1 4 7 - 8
232
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
1. Introduction
Heliotrope (Heliotropium arborescens) was one of the most popular Victorian
(late 19th century) bedding plants. It was reported to ¯ower freely throughout the
year, and was widely used as a cut ¯ower for scented bouquets (Cooke, 1995).
Over the last century, its popularity declined, but it is still grown on a small scale
as a summer bedding plant. Due its strongly scented ¯owers, there is now
increasing commercial interest in heliotrope; scents range from vanilla, to
almonds, marzipan, cinnamon, marshmallow and honey (Cooke, 1995). However,
the environmental regulation of ¯owering in heliotrope is not understood, such
information would underpin the commercial production of this plant.
Thompson (1995) investigated the growth and ¯owering of heliotrope cv.
Marine in response to temperature and photoperiod, as well as the effect of
growth regulators. However, Thompson's study was limited. From his data, he
inferred cool temperatures may be required to induce ¯owers, but de®nitive
experimental evidence was not provided. A more in-depth study of ¯owering in
this species was therefore required.
The objectives of this study were therefore to establish whether a period of cool
temperature is required for ¯owering in heliotrope. To investigate the role of
temperature in both the initiation and subsequent development of ¯owers and to
establish the extent to which other environmental factors, such as photoperiod and
light integral, affect ¯owering in heliotrope.
2. Materials and methods
2.1. General plant culture
For all experiments, except the third, heliotrope plants were grown from 10 cm
tip cuttings taken from stock plants in 9 cm pots maintained at minimum
temperatures of 20 (experiment 1 and 2) and 258C (experiment 4 and 5). Cuttings
were struck into Plantpak P104 module trays containing SHL peat-based potting
compost with 20% perlite (v/v). The trays were placed on a bench supplied with
bottom heat at 218C and covered with white polyethylene. After propagation (10
days) the young plants were potted into 9 cm pots, containing the same peatbased substrate. For the third experiment, heliotrope cuttings (same cultivar as
above) were received on 20 April 1996 from a commercial supplier (Hollyacre
Plants) and immediately transplanted into 9 cm pots, containing the same
substrate as above.
For all experiments, plants were watered as necessary and fed twice weekly
with a solution of soluble fertilizer (Sangral 111), at 1500 ms (182 ppm N; 78 ppm
P; 150 ppm K) acidi®ed to pH 5.8 with a mixture of nitric and phosphoric acid.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
233
2.2. Experiment 1. Effects of temperature on ¯ower bud initiation and
development
This was a preliminary experiment to investigate the effects of temperature on
¯owering in heliotrope, in particular to establish whether cool temperatures
promote early ¯owering. Sixty young heliotrope plants were grown for 1 week in
a growth room at 208C. The light level inside the growth room was maintained at
90 mmol mÿ2 sÿ1 at the plant height supplied from warm white ¯uorescent tubes,
supplemented with 6.3% tungsten-®lament lamps (percentage calculated on the
basis of nominal wattage) for a 16 h per day photoperiod. The plants were then
soft-pinched on 11 August 1995 and immediately transferred to identical growth
rooms set at 10, 15 and 208C. Ten plants were grown on in each room until
¯owering. In addition, to examine the effects of cool temperatures on the
promotion of ¯owering, after 15 days 10 plants were transferred from 108C to
both 15 and 208C growth rooms and grown until ¯owering. At the same time, a
further 10 plants were transferred from 15 to 208C compartment. When the ®rst
¯ower bud opened (corolla fully re¯exed), the leaf number on the uppermost
lateral branch below the pinch to the ¯ower, and the ¯owering date were
recorded.
2.3. Experiment 2. Duration of low temperature required for advancement of
¯owering
This experiment was conducted to establish the optimum duration of low
temperature required for ¯owering in heliotrope. Rooted cuttings were grown in a
growth room (as above) at 208C for 1 week. The plants were then soft-pinched on
27 January 1998 and moved to either 10 or 208C growth room. After 3, 6, 9, 12,
15, 18 and 21 days, ®ve plants were transferred from 10 to 208C room, where
they remained until ¯owering. The plants were grown until ¯owering. Ten plants
were grown on as controls in 10 and 208C growth rooms. When the ®rst ¯ower
bud opened, the leaf number and the ¯owering date were recorded.
2.4. Experiment 3. Effects of temperature and photoperiod on time to ¯owering
The purpose of this experiment was to investigate the effect of photoperiod preinitiation and temperature post-initiation on time to ¯owering. From pinching on
21 April 1996, plants were grown at 118C for 15 days at either 8 or 17 h per day
and then transferred factorially to one of six identical glasshouse compartments
held at 11.5 0.15, 13.6 0.15, 17.7 0.28, 20.6 0.23, 23.9 0.08 or
27.7 0.28C. The duration of the photoperiod treatments and the temperature
applied for the `induction' treatments were determined following experiments 1
and 2. The photoperiods were imposed using light-tight chambers within a
234
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
glasshouse compartment. Plants were wheeled into the chambers at 16:00 hours
until 08:00 hours each day. For the 17 h per day treatment, the day-length was
extended by lighting the plants at 11 mmol mÿ2 sÿ1 at plant height from a 40 W
tungsten-®lament lamp and a 15 W compact ¯uorescent lamp. Five plants from
each photoperiod treatment were placed in each of the six temperature controlled
compartments, comprising the inner six of a linear array of eight compartments
(each having dimensions of 3.7 m 7 m). The two coolest compartments were
equipped with air conditioning units in order to maintain temperatures throughout
the experimental period. Temperatures within the compartments were recorded
with a Datataker 500 data logger (scanned every 15 s, recording hourly averages)
using aspirated PT100 temperature sensors. The plants were grown until ®rst
¯owering. When the ®rst ¯ower opened, leaf number and the ¯owering date were
recorded.
2.5. Experiment 4. The effect of temperature and photoperiod on ¯oral
development
The purpose of this experiment was to investigate the effect of photoperiod and
temperature on the rate of ¯ower development (post-initiation). Rooted heliotrope
were grown for 10 days at 108C in a growth room and transferred to a 208C
greenhouse. After ¯ower bud initiation (30 June 1998), checked by dissection
(>80% of plants), eight plants were grown at one of two different night
temperatures of 14 and 18.58C (day-time mean of 21.58C) combined factorially
with four photoperiods (8, 11, 14 and 17 h per day), respectively. The treatments
were imposed in a second suite of photoperiod garages, equipped with
refrigeration units to maintain night temperatures. Plants were wheeled into the
garages as in experiment 3, however, in this instance photoperiods were extended
using a 60:40 mixture of tungsten-®lament lamps and warm white ¯orescent
tubes (determined on the basis of nominal wattage) at 5 mmol mÿ2 sÿ1. The plants
were grown until ¯owering. When the ®rst ¯ower bud opened, leaf number and
the ¯owering date were recorded.
2.6. Experiment 5. The effects of temperature and light integral on time to
¯owering post-initiation
This experiment was conducted to establish the effects of temperature and light
integral on the duration of ¯ower development. Rooted cuttings were grown at
108C in a growth room for 15 days post-pinching and transferred to a greenhouse
at 208C. After ¯ower bud initiation (30 June 1998), checked by dissection (as
above), eight plants per treatment were grown in a factorial combination of six
temperatures (9.8 0.05, 13.1 0.03, 19.4 0.19, 21.6 0.19, 25.4 0.16 and
28.3 0.158C) and four light levels (full sunlight, 28, 50.0 and 77% shade),
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
235
respectively. Light transmission was measured using quantum sensors (400±
700 nm) prior to the start of the experiment. Daily light integral was recorded at a
weather station located 2 km from the greenhouse. The shade treatments were
imposed by placing the plants under Rokolene shade screens mounted within
each of the temperature controlled glasshouse compartments. When the ®rst
¯ower bud opened, leaf numbers and the ¯owering date were recorded.
3. Results
3.1. Experiment 1
Plants transferred from 10 to 208C after 15 days ¯owered signi®cantly
(P < 0.05) earlier than the other treatments; ¯owering 12.3 days earlier than
plants grown constantly at 208C. In addition, plants transferred from 10 to 208C
¯owered on average 11 days earlier than plants transferred from 15 to 208C
(Fig. 1A). Plants grown constantly at 108C throughout did not ¯ower by the end
of the experiment (120 days).
The leaf number of the plants grown constantly at 208C was 24.8 (Fig. 1B),
which was signi®cantly (P < 0.05) greater than that recorded for any of the other
treatments, which were not statistically different (Fig. 1B).
3.2. Experiment 2
Plants that were exposed to a low temperature (108C) from 3 to 15 days
¯owered earlier than the plants grown constantly at 208C (Fig. 2A). Plants
exposed to a low temperature for 9 days took 62.7 (/ÿ1.9) days to ¯ower,
whereas those grown constantly at 208C took 83.8 (/ÿ7.6) days to ¯ower
(P < 0.05). Extended durations of cold temperatures (>9 days) led to progressive
delays in time to ¯owering. Plants grown constantly at 108C took 183 (/ÿ 7.7)
days for ¯owering.
The leaf number of plants grown constantly at 208C was 27.2 (Fig. 2B), this
number was signi®cantly higher than all the transfers from low to warm
temperature. The leaf number of plants grown at 108C for 9 days followed by
growth at 208C was 17.6, subsequent transfers led to no signi®cant differences in
®nal leaf number. This suggests that >9 days of low temperature were suf®cient to
fully induce ¯owers.
3.3. Experiment 3
Photoperiod during chilling had no effect on subsequent time to ¯owering
(Figs. 3 and 4). Statistically, no difference was found in the leaf number below the
236
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
Fig. 1. (A) The effect of different temperatures post-pinching on days to ¯owering and (B) leaf
number in H. arborescens cv. Marine. Plants were grown at the initial temperature for 15 days and
subsequently transferred to the second temperature (15 or 208C) until ¯owering. Each bar is the
average of 10 plants.
¯ower between the long day and short day treatments (data not shown). Posttransfer, temperature signi®cantly (P < 0.001) advanced ¯ower development and
the optimum temperature for ¯ower development was 27.78C. At 11.58C plants
required 98 days until ¯owering compared to 40 days at 27.78C. Despite the high
temperature used, there was no evidence for a clear optimum temperature for
¯owering.
3.4. Experiment 4
The days to ¯owering were signi®cantly earlier (P < 0.05) at high night
temperatures (18.58C) compared to the lower temperature (148C), though in this
instance differences were small (4 days). No effects of photoperiod were found at
either temperature.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
237
Fig. 2. (A) The effect of transferring plants from 10 to 208C at various time post-pinching on time
to ¯owering in H. arborescens cv. Marine. (B) Leaf number from the plants reciprocally transferred
between 10 to 208C at various times post-pinching. Each bar (S.E.) is the average of seven plants.
Fig. 3. The effect of photoperiod (8, (*); 17 h per day, (*)), applied for 15 days post-pinching
during cold induction, and subsequent forcing temperature on time to ¯owering. The curve was
®tted by regression where 1/days to ¯owering ÿ0.00695 0.001817T ÿ 0.0000248T2, where T
is temperature (r2 0.99, 9d.f.). Each point (S.E.) is mean value of ®ve plants.
238
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
Fig. 4. The effect of photoperiod and night temperature (18.58C, (*); 148C, (*)) on ¯oral
development, post-initiation. Each point (S.E.) is mean value of eight plants.
3.5. Experiment 5
The effects of temperature and light integral, post-¯ower initiation were
examined in terms of the reciprocal of days to ¯owering (Fig. 5); data were
analyzed using multiple regression. A second order polynomial relationship ®tted
to the data accounted for 97% of the variance of reciprocal of time to ¯owering.
Reciprocal of days to ¯owering was curvi-linearly related to temperature, with a
signi®cant interaction with light integral. This suggests that at low temperatures
the rate of progress to ¯owering was less sensitive to light than warmer
Fig. 5. The effects of temperature and light integral in terms of level of shade imposed on duration
of ¯ower development. Shade was imposed to give light transmissions of 28% (*, ---), 50.0%
(*, ), 73% (&, ÐÐÐÐ) of full sun (&, ±±±). The curves were ®tted by regression where 1/days
to ¯owering ÿ0.022 0.00366T ÿ 0.00006T2 0.0000386 T M (r2 0.97, 20d.f.) where T is
temperature and M is light integral. Each point (S.E.) is mean value of eight plants.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
239
conditions. However, when data are considered in terms of days to ¯owering at
high temperature (28.38C), the difference in days to ¯owering between plants
grown under 100% sun light (15.1 mol mÿ2 per day) and those grown at the
highest level of shade (3.8 mol mÿ2 per day) was 9.5 days, compared to 28.7 days
at the lowest temperature (9.88C). Under the same light integral, there was a difference of 52.2 days between plants grown at 28.38C and plants grown at 9.88C.
4. Discussion
Flowering is a complex process, which can be subdivided into a number of
components. In simple terms, these include the processes leading up to ¯oral
initiation and subsequently to ¯ower opening (development). Rates of these
processes are related to environmental conditions; temperature, low temperature
(vernalization), photoperiod and light integral (see Thomas and Vince-Prue,
1997). In classical vernalization responses, when plants have received suf®cient
cold temperature, ¯ower bud primodia are not present and only differentiate when
the plants are returned usually to a higher temperature or long photoperiods
(Thomas and Vince-Prue, 1997). However, some plants can initiate directly in
low temperatures, and Bernier et al. (1981) distinguished this response from
classical vernalization. In this instance, ¯owers were fully induced after 9 days of
cool (108C) temperatures, and apical dissections of plants grown at 108C showed
that they had not initiated ¯ower buds until 39 days (unpublished data) even
though they were induced. However, data on leaf numbers below the ¯ower show
that after 9 days at 108C plants were committed to ¯ower at a particular node (see
experiment 2), irrespective of subsequent high temperatures. This indicates that
the response low temperature in heliotrope is not akin to a classical vernalization
response, as de®ned by Thomas and Vince-Prue (1997), but is a `direct' response
to low temperatures. Low temperatures were not an absolute requirement for
¯owering, as it occurred eventually at both high and low temperatures.
Interestingly, photoperiod had no effect on ¯ower initiation or development.
The most effective temperature range for vernalization is usually between 1 and
78C (Bernier et al., 1981). In the case of radish (Raphanus sativus) the optimum
vernalization temperature is 58C (Yoo, 1977) and its range for vernalization is
between 2 and 108C (Park et al., 1990). Some plants, however, have a higher
effective temperature range. The optimum temperature for ¯ower bud initiation in
Centradenia inaequilateralis is 158C, and the temperature range for rapid ¯oral
initiation is between 12 and 158C (Friis and Christensen, 1989). In this
investigation, the optimal temperature range for ¯oral induction, in heliotrope,
was demonstrated to be below 158C (Fig. 1). Although a very low temperature for
vernalization seems unlikely, since in a preliminary experiment heliotrope grown
at 58C died due to the cold (data not shown).
240
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
It seems clear that in heliotrope the process of ¯owering is accelerated by cool
temperature treatment. When plants were grown at 108C for 9 days, ¯owering
was signi®cantly earlier than under any other duration of low temperature
treatments. This is also supported by data on the leaf number below the ¯ower.
This is a very short duration of low temperature requirement, but it is not unusual.
Radish only requires 10 days of cool (58C) to satisfy this cold requirement (Yoo,
1977). Osteospermum required 11 days of chilling at 128C (Adams et al., 1998).
In terms of ¯ower development, plants grown under high light integrals and
high temperature ¯owered rapidly. The response of ¯ower development to
temperature was also similar between repeated experiments. This shows a rather
classical ¯owering response to temperature, with increasing temperature leading
to progressively earlier ¯owering. However, there was no evidence for a linear
relationship between the reciprocal of days to ¯owering and temperature, at any
of the light integrals examined. This is surprising, since in many species a linear
response has been demonstrated (see Hadley et al., 1983; Ellis et al., 1990). In
heliotrope, the response was asymptotic from 21.5 to 28.38C. The basis of the
shape of this response is important. One of the principle assumptions of the
concept of thermal time to predict time to ¯owering is the linearity of this
temperature response, which is not the case here. A linear phase only occurred
between 10 to 228C. Linearity is frequently assumed but infrequently tested. This
is not, however, the ®rst occasion when non-linear ¯owering responses have been
reported (see Larsen and Persson, 1999; Pearson et al., 1998). However, time to
¯owering can be simply predicted using the data shown here by integrating on a
daily basis the relationship between reciprocal of time to ¯owering against
temperature and light integral (see Pearson et al., 1993).
The information from these experiments has shown the principal factors
regulating ¯owering in heliotrope. This information should be of considerable
bene®t to underpin the commercial production of the crop.
Acknowledgements
We wish to thank Harry Kitchener for advice and encouragement throughout
and Dr. Steve Adams for comments on the manuscript.
References
Adams, S.R., Pearson, S., Hadley, P., 1998. In¯orescence commitment and subsequent development
differ in their responses to temperature and photoperiod in Osteospermum jucundum. Physiol.
Plant 104, 225±231.
Bernier, G., Kinet, J.M., Sachs, R.M., 1981. The Physiology of Flowering. CRC Press, Boca Raton,
FL, pp. 69±81.
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Cooke, I.K.S., 1995. Sweet scent of cherry pie. The Garden. August, pp. 495±497.
Ellis, R.H., Hadley, P., Roberts, E.H., Summer®eld, R.J., 1990. Quantitative relations between
temperature and crop development and growth. In: Jackson, M.T., Ford-Lloyd, B.V., Parry, M.L.
(Eds.), Climatic Change and Plant Genetic Resources. Belhaven Press, London, pp. 85±115.
Friis, K., Christensen, O.V., 1989. Flowering of Centradenia inaequilateralis `Cascade' as
in¯uenced by temperature and photoperiod. Sci. Hort. 41, 125±130.
Hadley, P., Roberts, E.H., Summer®eld, R.J., Minchin, F.R., 1983. A quantitative model of
reproductive development in cowpea (Vigna inguiculata L. cv. Walp) in relation to temperature
and photoperiod, and implications for screening germplasm. Ann. Bot. 51, 531±543.
Larsen, R.U., Persson, L., 1999. Modeling ¯ower development in greenhouse chrysanthemum
cultivars in relation to temperature and response group. Sci. Hort. 80, 73±89.
Park, B.H., Yoo, K.C., Lee, K.C., 1990. Studies on the physiology of bolting and ¯owering in
Raphanus sativus L. Ð Optimum vernalized temperature. J. Kor. Soc. Hort. Sci. 31, 334±339.
Pearson, S., Hadley, P., Wheldon, A.E., 1993. A re-analysis of the effects of temperature and
irradiance on the time to ¯owering in chrysanthemum (Dendranthema grandi¯ora). J. Hort. Sci.
68, 89±97.
Pearson, S., Hadley, P., LeMiere, P., 1998. Crop forecasting in horticulture. The Horticulturist 7,
2±8.
Thomas, B., Vince-Prue, D., 1997. Photoperiodism in Plants, 2nd Edition. Academic Press, London.
Thompson, I., 1995. The effect of photoperiod, temperature and cycocel on growth and
development of Heliotropium arborescens cv. `Marine' and cv. `Baby Marine'. B.Sc.
Dissertation, The University of Reading, UK.
Yoo, K.C., 1977. Studies on the physiology of bolting and ¯owering in Raphanus sativus L. J. Kor.
Soc. Hort. Sci. 18, 157±161.
Environmental regulation of ¯owering time in
heliotrope (Heliotropium arborescens L. cv. Marine)
Byong H. Park, Simon Pearson*
The Department of Horticulture, School of Plant Sciences, The University of Reading,
Reading RG6 6AS, UK
Accepted 11 November 1999
Abstract
The aim of this study was to examine the environmental regulation of ¯ower initiation and
subsequent development in heliotrope (Heliotropium aborescens L. cv. Marine). Five experiments
were conducted, two examined whether ¯owering could be advanced by cool temperatures. The
duration of cool temperature required to induce rapid ¯owering was also investigated. The ®nal
three experiments examined the effects of light integral, photoperiod and temperature on ¯ower
initiation and development.
It was found that plants grown for 9 days at 108C and than transferred to 208C ¯owered
signi®cantly earlier (®rst ¯owering recorded after 55 days) than plants held constantly at 208C (65.9
days to ¯owering). Plants grown at a constant temperature of 208C had signi®cantly more leaves
than all other treatments. This suggested that `cool' temperatures, prior to initiation, advanced
¯owering. In a transfer experiment, plants were moved from 10 to 208C at 3 days intervals postpinching. Earliest ¯owering (by 20 days compared to the 208C constant treatment) occurred when
plants were exposed to 108C for 9 days and then transferred to 208C.
Photoperiod was shown to have no effect on either ¯ower bud initiation or development (postinitiation). Both temperature and light integral strongly in¯uenced ¯ower development post-¯ower
bud initiation. However, the response to temperature plotted in terms of the reciprocal of days to
¯owering was non-linear. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Heliotrope; Scented plants; Flower development; Initiation; Temperature; Photoperiod;
Light integral
*
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 1 4 7 - 8
232
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
1. Introduction
Heliotrope (Heliotropium arborescens) was one of the most popular Victorian
(late 19th century) bedding plants. It was reported to ¯ower freely throughout the
year, and was widely used as a cut ¯ower for scented bouquets (Cooke, 1995).
Over the last century, its popularity declined, but it is still grown on a small scale
as a summer bedding plant. Due its strongly scented ¯owers, there is now
increasing commercial interest in heliotrope; scents range from vanilla, to
almonds, marzipan, cinnamon, marshmallow and honey (Cooke, 1995). However,
the environmental regulation of ¯owering in heliotrope is not understood, such
information would underpin the commercial production of this plant.
Thompson (1995) investigated the growth and ¯owering of heliotrope cv.
Marine in response to temperature and photoperiod, as well as the effect of
growth regulators. However, Thompson's study was limited. From his data, he
inferred cool temperatures may be required to induce ¯owers, but de®nitive
experimental evidence was not provided. A more in-depth study of ¯owering in
this species was therefore required.
The objectives of this study were therefore to establish whether a period of cool
temperature is required for ¯owering in heliotrope. To investigate the role of
temperature in both the initiation and subsequent development of ¯owers and to
establish the extent to which other environmental factors, such as photoperiod and
light integral, affect ¯owering in heliotrope.
2. Materials and methods
2.1. General plant culture
For all experiments, except the third, heliotrope plants were grown from 10 cm
tip cuttings taken from stock plants in 9 cm pots maintained at minimum
temperatures of 20 (experiment 1 and 2) and 258C (experiment 4 and 5). Cuttings
were struck into Plantpak P104 module trays containing SHL peat-based potting
compost with 20% perlite (v/v). The trays were placed on a bench supplied with
bottom heat at 218C and covered with white polyethylene. After propagation (10
days) the young plants were potted into 9 cm pots, containing the same peatbased substrate. For the third experiment, heliotrope cuttings (same cultivar as
above) were received on 20 April 1996 from a commercial supplier (Hollyacre
Plants) and immediately transplanted into 9 cm pots, containing the same
substrate as above.
For all experiments, plants were watered as necessary and fed twice weekly
with a solution of soluble fertilizer (Sangral 111), at 1500 ms (182 ppm N; 78 ppm
P; 150 ppm K) acidi®ed to pH 5.8 with a mixture of nitric and phosphoric acid.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
233
2.2. Experiment 1. Effects of temperature on ¯ower bud initiation and
development
This was a preliminary experiment to investigate the effects of temperature on
¯owering in heliotrope, in particular to establish whether cool temperatures
promote early ¯owering. Sixty young heliotrope plants were grown for 1 week in
a growth room at 208C. The light level inside the growth room was maintained at
90 mmol mÿ2 sÿ1 at the plant height supplied from warm white ¯uorescent tubes,
supplemented with 6.3% tungsten-®lament lamps (percentage calculated on the
basis of nominal wattage) for a 16 h per day photoperiod. The plants were then
soft-pinched on 11 August 1995 and immediately transferred to identical growth
rooms set at 10, 15 and 208C. Ten plants were grown on in each room until
¯owering. In addition, to examine the effects of cool temperatures on the
promotion of ¯owering, after 15 days 10 plants were transferred from 108C to
both 15 and 208C growth rooms and grown until ¯owering. At the same time, a
further 10 plants were transferred from 15 to 208C compartment. When the ®rst
¯ower bud opened (corolla fully re¯exed), the leaf number on the uppermost
lateral branch below the pinch to the ¯ower, and the ¯owering date were
recorded.
2.3. Experiment 2. Duration of low temperature required for advancement of
¯owering
This experiment was conducted to establish the optimum duration of low
temperature required for ¯owering in heliotrope. Rooted cuttings were grown in a
growth room (as above) at 208C for 1 week. The plants were then soft-pinched on
27 January 1998 and moved to either 10 or 208C growth room. After 3, 6, 9, 12,
15, 18 and 21 days, ®ve plants were transferred from 10 to 208C room, where
they remained until ¯owering. The plants were grown until ¯owering. Ten plants
were grown on as controls in 10 and 208C growth rooms. When the ®rst ¯ower
bud opened, the leaf number and the ¯owering date were recorded.
2.4. Experiment 3. Effects of temperature and photoperiod on time to ¯owering
The purpose of this experiment was to investigate the effect of photoperiod preinitiation and temperature post-initiation on time to ¯owering. From pinching on
21 April 1996, plants were grown at 118C for 15 days at either 8 or 17 h per day
and then transferred factorially to one of six identical glasshouse compartments
held at 11.5 0.15, 13.6 0.15, 17.7 0.28, 20.6 0.23, 23.9 0.08 or
27.7 0.28C. The duration of the photoperiod treatments and the temperature
applied for the `induction' treatments were determined following experiments 1
and 2. The photoperiods were imposed using light-tight chambers within a
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glasshouse compartment. Plants were wheeled into the chambers at 16:00 hours
until 08:00 hours each day. For the 17 h per day treatment, the day-length was
extended by lighting the plants at 11 mmol mÿ2 sÿ1 at plant height from a 40 W
tungsten-®lament lamp and a 15 W compact ¯uorescent lamp. Five plants from
each photoperiod treatment were placed in each of the six temperature controlled
compartments, comprising the inner six of a linear array of eight compartments
(each having dimensions of 3.7 m 7 m). The two coolest compartments were
equipped with air conditioning units in order to maintain temperatures throughout
the experimental period. Temperatures within the compartments were recorded
with a Datataker 500 data logger (scanned every 15 s, recording hourly averages)
using aspirated PT100 temperature sensors. The plants were grown until ®rst
¯owering. When the ®rst ¯ower opened, leaf number and the ¯owering date were
recorded.
2.5. Experiment 4. The effect of temperature and photoperiod on ¯oral
development
The purpose of this experiment was to investigate the effect of photoperiod and
temperature on the rate of ¯ower development (post-initiation). Rooted heliotrope
were grown for 10 days at 108C in a growth room and transferred to a 208C
greenhouse. After ¯ower bud initiation (30 June 1998), checked by dissection
(>80% of plants), eight plants were grown at one of two different night
temperatures of 14 and 18.58C (day-time mean of 21.58C) combined factorially
with four photoperiods (8, 11, 14 and 17 h per day), respectively. The treatments
were imposed in a second suite of photoperiod garages, equipped with
refrigeration units to maintain night temperatures. Plants were wheeled into the
garages as in experiment 3, however, in this instance photoperiods were extended
using a 60:40 mixture of tungsten-®lament lamps and warm white ¯orescent
tubes (determined on the basis of nominal wattage) at 5 mmol mÿ2 sÿ1. The plants
were grown until ¯owering. When the ®rst ¯ower bud opened, leaf number and
the ¯owering date were recorded.
2.6. Experiment 5. The effects of temperature and light integral on time to
¯owering post-initiation
This experiment was conducted to establish the effects of temperature and light
integral on the duration of ¯ower development. Rooted cuttings were grown at
108C in a growth room for 15 days post-pinching and transferred to a greenhouse
at 208C. After ¯ower bud initiation (30 June 1998), checked by dissection (as
above), eight plants per treatment were grown in a factorial combination of six
temperatures (9.8 0.05, 13.1 0.03, 19.4 0.19, 21.6 0.19, 25.4 0.16 and
28.3 0.158C) and four light levels (full sunlight, 28, 50.0 and 77% shade),
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
235
respectively. Light transmission was measured using quantum sensors (400±
700 nm) prior to the start of the experiment. Daily light integral was recorded at a
weather station located 2 km from the greenhouse. The shade treatments were
imposed by placing the plants under Rokolene shade screens mounted within
each of the temperature controlled glasshouse compartments. When the ®rst
¯ower bud opened, leaf numbers and the ¯owering date were recorded.
3. Results
3.1. Experiment 1
Plants transferred from 10 to 208C after 15 days ¯owered signi®cantly
(P < 0.05) earlier than the other treatments; ¯owering 12.3 days earlier than
plants grown constantly at 208C. In addition, plants transferred from 10 to 208C
¯owered on average 11 days earlier than plants transferred from 15 to 208C
(Fig. 1A). Plants grown constantly at 108C throughout did not ¯ower by the end
of the experiment (120 days).
The leaf number of the plants grown constantly at 208C was 24.8 (Fig. 1B),
which was signi®cantly (P < 0.05) greater than that recorded for any of the other
treatments, which were not statistically different (Fig. 1B).
3.2. Experiment 2
Plants that were exposed to a low temperature (108C) from 3 to 15 days
¯owered earlier than the plants grown constantly at 208C (Fig. 2A). Plants
exposed to a low temperature for 9 days took 62.7 (/ÿ1.9) days to ¯ower,
whereas those grown constantly at 208C took 83.8 (/ÿ7.6) days to ¯ower
(P < 0.05). Extended durations of cold temperatures (>9 days) led to progressive
delays in time to ¯owering. Plants grown constantly at 108C took 183 (/ÿ 7.7)
days for ¯owering.
The leaf number of plants grown constantly at 208C was 27.2 (Fig. 2B), this
number was signi®cantly higher than all the transfers from low to warm
temperature. The leaf number of plants grown at 108C for 9 days followed by
growth at 208C was 17.6, subsequent transfers led to no signi®cant differences in
®nal leaf number. This suggests that >9 days of low temperature were suf®cient to
fully induce ¯owers.
3.3. Experiment 3
Photoperiod during chilling had no effect on subsequent time to ¯owering
(Figs. 3 and 4). Statistically, no difference was found in the leaf number below the
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B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
Fig. 1. (A) The effect of different temperatures post-pinching on days to ¯owering and (B) leaf
number in H. arborescens cv. Marine. Plants were grown at the initial temperature for 15 days and
subsequently transferred to the second temperature (15 or 208C) until ¯owering. Each bar is the
average of 10 plants.
¯ower between the long day and short day treatments (data not shown). Posttransfer, temperature signi®cantly (P < 0.001) advanced ¯ower development and
the optimum temperature for ¯ower development was 27.78C. At 11.58C plants
required 98 days until ¯owering compared to 40 days at 27.78C. Despite the high
temperature used, there was no evidence for a clear optimum temperature for
¯owering.
3.4. Experiment 4
The days to ¯owering were signi®cantly earlier (P < 0.05) at high night
temperatures (18.58C) compared to the lower temperature (148C), though in this
instance differences were small (4 days). No effects of photoperiod were found at
either temperature.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
237
Fig. 2. (A) The effect of transferring plants from 10 to 208C at various time post-pinching on time
to ¯owering in H. arborescens cv. Marine. (B) Leaf number from the plants reciprocally transferred
between 10 to 208C at various times post-pinching. Each bar (S.E.) is the average of seven plants.
Fig. 3. The effect of photoperiod (8, (*); 17 h per day, (*)), applied for 15 days post-pinching
during cold induction, and subsequent forcing temperature on time to ¯owering. The curve was
®tted by regression where 1/days to ¯owering ÿ0.00695 0.001817T ÿ 0.0000248T2, where T
is temperature (r2 0.99, 9d.f.). Each point (S.E.) is mean value of ®ve plants.
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B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
Fig. 4. The effect of photoperiod and night temperature (18.58C, (*); 148C, (*)) on ¯oral
development, post-initiation. Each point (S.E.) is mean value of eight plants.
3.5. Experiment 5
The effects of temperature and light integral, post-¯ower initiation were
examined in terms of the reciprocal of days to ¯owering (Fig. 5); data were
analyzed using multiple regression. A second order polynomial relationship ®tted
to the data accounted for 97% of the variance of reciprocal of time to ¯owering.
Reciprocal of days to ¯owering was curvi-linearly related to temperature, with a
signi®cant interaction with light integral. This suggests that at low temperatures
the rate of progress to ¯owering was less sensitive to light than warmer
Fig. 5. The effects of temperature and light integral in terms of level of shade imposed on duration
of ¯ower development. Shade was imposed to give light transmissions of 28% (*, ---), 50.0%
(*, ), 73% (&, ÐÐÐÐ) of full sun (&, ±±±). The curves were ®tted by regression where 1/days
to ¯owering ÿ0.022 0.00366T ÿ 0.00006T2 0.0000386 T M (r2 0.97, 20d.f.) where T is
temperature and M is light integral. Each point (S.E.) is mean value of eight plants.
B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
239
conditions. However, when data are considered in terms of days to ¯owering at
high temperature (28.38C), the difference in days to ¯owering between plants
grown under 100% sun light (15.1 mol mÿ2 per day) and those grown at the
highest level of shade (3.8 mol mÿ2 per day) was 9.5 days, compared to 28.7 days
at the lowest temperature (9.88C). Under the same light integral, there was a difference of 52.2 days between plants grown at 28.38C and plants grown at 9.88C.
4. Discussion
Flowering is a complex process, which can be subdivided into a number of
components. In simple terms, these include the processes leading up to ¯oral
initiation and subsequently to ¯ower opening (development). Rates of these
processes are related to environmental conditions; temperature, low temperature
(vernalization), photoperiod and light integral (see Thomas and Vince-Prue,
1997). In classical vernalization responses, when plants have received suf®cient
cold temperature, ¯ower bud primodia are not present and only differentiate when
the plants are returned usually to a higher temperature or long photoperiods
(Thomas and Vince-Prue, 1997). However, some plants can initiate directly in
low temperatures, and Bernier et al. (1981) distinguished this response from
classical vernalization. In this instance, ¯owers were fully induced after 9 days of
cool (108C) temperatures, and apical dissections of plants grown at 108C showed
that they had not initiated ¯ower buds until 39 days (unpublished data) even
though they were induced. However, data on leaf numbers below the ¯ower show
that after 9 days at 108C plants were committed to ¯ower at a particular node (see
experiment 2), irrespective of subsequent high temperatures. This indicates that
the response low temperature in heliotrope is not akin to a classical vernalization
response, as de®ned by Thomas and Vince-Prue (1997), but is a `direct' response
to low temperatures. Low temperatures were not an absolute requirement for
¯owering, as it occurred eventually at both high and low temperatures.
Interestingly, photoperiod had no effect on ¯ower initiation or development.
The most effective temperature range for vernalization is usually between 1 and
78C (Bernier et al., 1981). In the case of radish (Raphanus sativus) the optimum
vernalization temperature is 58C (Yoo, 1977) and its range for vernalization is
between 2 and 108C (Park et al., 1990). Some plants, however, have a higher
effective temperature range. The optimum temperature for ¯ower bud initiation in
Centradenia inaequilateralis is 158C, and the temperature range for rapid ¯oral
initiation is between 12 and 158C (Friis and Christensen, 1989). In this
investigation, the optimal temperature range for ¯oral induction, in heliotrope,
was demonstrated to be below 158C (Fig. 1). Although a very low temperature for
vernalization seems unlikely, since in a preliminary experiment heliotrope grown
at 58C died due to the cold (data not shown).
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B.H. Park, S. Pearson / Scientia Horticulturae 85 (2000) 231±241
It seems clear that in heliotrope the process of ¯owering is accelerated by cool
temperature treatment. When plants were grown at 108C for 9 days, ¯owering
was signi®cantly earlier than under any other duration of low temperature
treatments. This is also supported by data on the leaf number below the ¯ower.
This is a very short duration of low temperature requirement, but it is not unusual.
Radish only requires 10 days of cool (58C) to satisfy this cold requirement (Yoo,
1977). Osteospermum required 11 days of chilling at 128C (Adams et al., 1998).
In terms of ¯ower development, plants grown under high light integrals and
high temperature ¯owered rapidly. The response of ¯ower development to
temperature was also similar between repeated experiments. This shows a rather
classical ¯owering response to temperature, with increasing temperature leading
to progressively earlier ¯owering. However, there was no evidence for a linear
relationship between the reciprocal of days to ¯owering and temperature, at any
of the light integrals examined. This is surprising, since in many species a linear
response has been demonstrated (see Hadley et al., 1983; Ellis et al., 1990). In
heliotrope, the response was asymptotic from 21.5 to 28.38C. The basis of the
shape of this response is important. One of the principle assumptions of the
concept of thermal time to predict time to ¯owering is the linearity of this
temperature response, which is not the case here. A linear phase only occurred
between 10 to 228C. Linearity is frequently assumed but infrequently tested. This
is not, however, the ®rst occasion when non-linear ¯owering responses have been
reported (see Larsen and Persson, 1999; Pearson et al., 1998). However, time to
¯owering can be simply predicted using the data shown here by integrating on a
daily basis the relationship between reciprocal of time to ¯owering against
temperature and light integral (see Pearson et al., 1993).
The information from these experiments has shown the principal factors
regulating ¯owering in heliotrope. This information should be of considerable
bene®t to underpin the commercial production of the crop.
Acknowledgements
We wish to thank Harry Kitchener for advice and encouragement throughout
and Dr. Steve Adams for comments on the manuscript.
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