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 aborescensL. 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 post-pinching. 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 (post-initiation). 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)
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 peat-based 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.
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 90mmol mÿ2sÿ1at 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
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 11mmol 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 m7 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 5mmol mÿ2sÿ1. 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
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 (/ÿ7.7) 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
¯ower between the long day and short day treatments (data not shown). Post-transfer, temperature signi®cantly (P< 0.001) advanced ¯ower development and the optimum temperature for ¯ower development was27.78C. 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.
Fig. 2. (A) The effect of transferring plants from 10 to 208C at various time post-pinching on time to ¯owering inH. arborescenscv. 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 ÿ0.006950.001817Tÿ0.0000248T2, whereT
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. 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.
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
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 mÿ2per day) was 9.5 days, compared to 28.7 days at the lowest temperature (9.88C). Under the same light integral, there was a dif-ference 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.
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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