The effects of temperature and daylength on
¯ower initiation and development in
Dianthus
allwoodii
and
Dianthus alpinus
D.C.E. Wurr
a,*, Jane R. Fellows
a, Lynn Andrews
ba
Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK b
Horticulture Research International, Efford, Lymington, Hampshire SO41 0LZ, UK
Accepted 21 December 1999
Abstract
In four experiments the major effect on ¯owering was that of temperature. Temperatures applied between 2 October and 20 February in¯uenced the time of ¯owering ofD. allwoodii, `Doris', while
inD. alpinus, `Pike's Pink', temperatures applied over a longer period between 2 October and 11
March in¯uenced the time of ¯owering. Keeping stock plants at low temperatures, before cuttings were taken, advanced ¯owering. The earliest ¯owering in `Doris' was achieved by storing stock plants at 3.98C and then rooting cuttings at 14.98C (rather than at 10.08C) suggesting that it was possible to satisfy their ¯ower induction requirement entirely during cold storage. In `Pike's Pink' earliest ¯owering was achieved by storing stock plants at 3.98C and then rooting cuttings at 10.08C, suggesting that the cold requirement for ¯ower induction was not entirely satis®ed by cold storing stock plants but in addition needed some time at low temperature during rooting. The cold requirement of `Pike's Pink' was greater than that of `Doris' and in several treatments was not satis®ed suf®ciently for ¯owering to occur.
The number of ¯owers in `Doris' was increased by low stock plant temperatures, low temperatures post-rooting, and higher temperatures during growing on. In `Pike's Pink' number of ¯owers were increased by low stock plant temperatures and low temperatures post-rooting.
#2000 Elsevier Science B.V. All rights reserved.
Keywords: Temperature; Light intensity; Daylength; Flower initiation
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*
Corresponding author. Tel.:44-1789-470382; fax:44-1789-470552.
E-mail address: [email protected] (D.C.E. Wurr)
1. Introduction
An increasingly competitive market for container nursery stock production wants plants in ¯ower, at point of sale, over an extended sales period. This requires the establishment of techniques to produce sequences of quality plants, at the optimum ¯owering stage, over an extended season. InDianthusthis involves understanding the roles of daylength, irradiance and temperature in ¯ower initiation and development.
Modern pinks are probably cultivars of D. allwoodii, a hybrid of D. caryophyllus and D. plumarius and possibly other species (Cockshull, 1985). However, knowledge of the physiology ofD. allwoodii is very limited and there appears to be no scienti®c literature concerning D. alpinus. There is some evidence of daylength effects in the garden pink `Doris'. Plants failed to produce ¯ower buds when grown for 6 months in short days, whereas plants given `dusk-to-dawn' lighting (long days) from tungsten lamps for 1 month initiated ¯owers which reached anthesis just over 1 month after the treatment ended (Harris, 1973).
It may be possible to infer some understanding of the physiology from that of
D. caryophyllus, the glasshouse carnation. Certainly inD. caryophyllusthere are effects of daylength, irradiance and temperature on ¯ower initiation (Bunt and Cockshull, 1985) but only effects of temperature on ¯ower development. According to Cockshull (1985) manyDianthus species require vernalization but there is a wide range of responses within the genusDianthusand sometimes even within a species.
2. Materials and methods
Four experiments were conducted, studying the effects of daylength, irradiance and temperature on the ¯owering ofD. allwoodii, `Doris' andD. alpinus, `Pike's Pink'. Temperature regimes are shown in Fig. 1, together with the timescales in days from 1 January in each year. Effects of treatments applied during three stages of growth: stock plant maintenance, rooting and post-rooting were studied. Where controlled environment cabinets were used to apply environmental treatments, plants were arranged within each cabinet in a blocked structure. This enabled true replication to be imposed once plants were removed from the cabinets.
2.1. Experiment 1 1995/1996 Ð effects of temperature post-rooting
Cuttings were taken from stock plants, maintained in pots under cold glass, on 22 June, rooted under mist and potted on into 9 cm pots and the terminal shoots of
Fig. 1. Temperature regimes for the four experiments showing time durations and the mean temperatures observed during each phase.
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`Doris' were pinched out. Forty-eight plants of each type were put, in four replicates, into each of four environments in controlled environment cabinets on 2 October for 8 weeks. The effect of continuous temperatures of 6.10.06, 10.10.06, 13.90.04 and 18.10.028C was studied with a daylength of 8 h given by 30 W mÿ2
PAR ¯uorescent lighting, together with tungsten lamps at 2 W mÿ2 PAR. This lighting regime was chosen because its daily light integral most closely represented that occurring during December and January in the UK. Samples of ®ve plants of each type were taken on 3 October, 30 October and 13 November and dissected under a binocular microscope. Plants were cut off at the lowest pair of leaves or leaf scars and the state of the apex was recorded. On 27 November the plants were moved onto ebb and ¯ow benches in a ventilated polyethylene tunnel with an average temperature of 9.20.398C until ¯owering was complete. Further samples were taken on 27 November, 19 December, 22 January, 4 March, 10 April, 26 April and 14 May. Flowers were tagged as they opened and the total number of ¯owers and the height of stems were recorded on all plants.
2.2. Experiment 2 1996 Ð effects of temperature during rooting and temperature and daylength post-rooting
This experiment and the two following imposed different temperature regimes using a combination of semi-commercial and experimental conditions. Cuttings were taken from stock plants on 29 October and rooted under mist at slightly different air temperatures: means of 9.80.27 and 11.30.298C. Resulting plants were then grown for 8 weeks in controlled environment cabinets from 15 January. There were four replicates of six plants receiving all combinations of two temperatures: 10.00.01 and 18.00.018C and two daylengths: 12 and 15 h, under the same irradiance conditions as in experiment 1. Plants were then moved to a ventilated polyethylene tunnel on 11 March. Flower development was recorded as previously and the average temperature over the whole period in the polyethylene tunnel was 12.60.468C.
2.3. Experiment 3 1996/1997 Ð effects of temperature during stock plant maintenance and rooting
The effects of temperature on one-year-old stock plants and the cuttings taken from them during rooting and post-rooting were explored further. In the 5 weeks up to 7 November stock plants were exposed to cold temperatures (3.90.018C) in an unlit cold store; ambient temperatures (13.40.288C) in an unheated clear polyethylene tunnel or warm temperatures (16.30.168C) in a heated glasshouse before cuttings were taken as previously described. Then mean continuous temperatures of 10.00.20 or 14.90.008C, combined with a 16 h daylength
given by high pressure sodium lamps at 16 W mÿ2 PAR, were applied during rooting from 7 November to 15 January. On 20 January 32 plants of each type and treatment were put in four replicate controlled environment cabinets at 10.00.018C under a 12 h daylength with irradiance as in experiments 1 and 2. Eight plants of each treatment went into each cabinet. Samples of three plants per treatment were taken for dissection on 20 January. Plants were moved to a ventilated clear polyethylene tunnel on 17 March. Flower development was recorded as previously and the average temperature in the polyethylene tunnel over the whole period was 15.60.338C.
2.4. Experiment 4 1997 Ð effects of temperature during stock plant maintenance, rooting and post-rooting
This was designed to extend the ideas developed from the previous three experiments. There were two replicates of four stock plant environments, two rooting temperatures and two temperatures post-rooting. Stock plants of both `Doris' and `Pike's Pink', which had previously been kept at the high temperature (16.30.168C) in experiment 3, were used. They were either kept warm for 36 days, starting from 15 January, or transferred to an unlit cold store at 4.80.078C for 36, 24 or 12 days, before cuttings were taken after 36 days. These were then rooted `warm' (14.90.158C) or `cool' (13.10.208C) from 21 February to 3 April and grown post-rooting at two temperatures from 25 April until the experiment was terminated on 31 August. A ventilated polyethylene tunnel (15.80.338C) and a cooled glasshouse compartment (13.00.198C) were used to give a mean temperature difference of 2.88C. Flower development was recorded as before.
2.5. Data analysis
Data were subjected to analysis of variance. For all experiments the two cultivars were analysed separately and the tables present results where there are signi®cant differences at p<0.05.
3. Results
3.1. Experiment 1
In `Doris' the earliest ¯ower initiation observed by apical dissection did not occur until 133 days after treatments ended. Lower temperatures resulted in signi®cantly earlier opening of the ®rst ¯ower, fewer ¯owers and longer stems (Table 1).
In `Pike's Pink' all plants from the 18.18C treatment died shortly after transfer to the polyethylene tunnel because they were unable to tolerate the freezing conditions that occurred. The earliest ¯ower initiation seen by apical dissection did not occur until nearly 3 months after treatments ended but there were signi®cant effects of temperature on ¯owering characters (Table 2). The day of ®rst ¯ower was signi®cantly earlier from lower temperatures. Thus in both species it seems likely that temperature affected ¯ower initiation rather than ¯ower development. The spread of ¯owering also increased with lower temperatures. There was no effect on the total number of ¯owers but stem length was reduced by lower temperatures.
3.2. Experiment 2
In `Doris' there were no effects of the daylengths employed on ¯owering and no interactions between rooting and post-rooting temperatures, but plants rooted at 11.38C ¯owered 19 days earlier (mean day 127) than those rooted at 9.88C (mean day 146) (Table 3). Additionally, `Doris' grown at 10.08C ¯owered 18 days later (mean day 146) than plants grown at 18.08C (mean day 128). The total number of ¯owers was higher from 10.08C than from 18.08C and there was an interaction between rooting and post-rooting temperature affecting stem length. Post-rooting temperature did not affect stem length in plants rooted at 9.88C,
Table 1
Effects of temperature applied post-rooting on ¯owering characters of `Doris' in experiment 1
Temperature (8C)
6.1 10.1 13.9 18.1 S.E.D., d.f. 9
Day of first flower (11 January) 160 161 165 166 1.0
Total number of flowers 12.1 13.1 22.3 20.0 2.53 Mean stem length (mm) 453 429 420 390 12.7
Table 2
Effects of temperature applied post-rooting on ¯owering characters of `Pike's Pink' in experiment 1
Temperature (8C)
6.1 10.1 13.9 18.1 S.E.D., d.f. 6 Day of first flower (11 January) 144 153 157 a 1.5
Spread of flowering (days) 27 23 17 a 1.7 Total number of flowers 26.7 33.3 29.7 a 3.26
Mean stem length (mm) 111 127 134 a 3.9
aAll plants died.
Table 3
Effects of rooting and post-rooting temperatures on ¯owering characters of `Doris' in experiment 2a
Temperatures (8C) S.E.D., d.f. 21
9.8 (rooting for 65 days) 11.3 (rooting for 65 days)
10.0 (post-rooting for 56 days)
18.0 (post-rooting for 56 days)
10.0 (post-rooting for 56 days)
18.0 (post-rooting for 56 days)
Day of first flower (11 January) 149 143 142 112 11.2
Total number of flowers 12.7 7.0 9.9 6.7 2.40
Mean stem length (mm) 327 336 301 362 15.1
aData values shown are means over daylength.
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Table 4
Effects of rooting and post-rooting temperatures on ¯owering characters of `Pike's Pink' in experiment 2a
Temperature (8C) S.E.D., d.f. 21
9.8 (rooting for 65 days) 11.3 (rooting for 65 days)
10.0 (post-rooting for 56 days)
18.0 (post-rooting for 56 days)
10.0 (post-rooting for 56 days)
18.0 (post-rooting for 56 days)
Day of first flower (11 January) 153 165 160 188 5.8
Total number of flowers 7.7 2.7 7.3 1.3 0.70
5.2 4.3 0.50
Mean stem length (mm) 109 168 126 137 9.2
139 131 6.5
aData values shown are means over daylength.
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whereas in plants rooted at 11.38C stems were longer from 18.08C than from 10.08C.
In `Pike's Pink' there were no effects of the daylengths employed on ¯owering but plants rooted at 9.88C (mean day 159) produced ¯owers 2 weeks earlier than those rooted at 11.38C (mean day 174) (Table 4). Many plants of `Pike's Pink', grown at 18.08C, failed to ¯ower. There were no effects on the spread of ¯owering but the total number of ¯owers was signi®cantly higher from plants rooted at 9.88C than from those rooted at 11.38C and from growing post-rooting at 10.08C compared to 18.08C. Stem length was not affected by rooting temperature but was increased by growing post-rooting at 18.08C (153 mm) compared with 10.08C (118 mm).
3.3. Experiment 3
In `Doris' there were signi®cant effects of stock plant environment and signi®cant interactions between rooting temperature and the stock plant environment, which affected every ¯owering character (Table 5). Plants produced from stock plants, cold stored at 3.98C, developed ¯ower buds during rooting at 10.0 and 14.98C, and when transferred to controlled environment cabinets on 20 January, dissections of samples showed that every plant from stock plants held at 3.98C had initiated a ¯ower bud with, on average, 14 leaf pairs. Plants from stock
Table 5
Effects of temperatures applied during stock plant maintenance and rooting on ¯owering characters of `Doris' in experiment 3
Stock plant temperature (8C) Rooting temperature (8C) Mean
10.0 14.9
Day of first flower(11 January)
3.9 100 71 86
13.4 138 146 142
16.3 136 147 141
S.E.D., d.f. 15 5.1 3.6
Spread of flowering(days)
3.9 60 117 89
13.4 49 27 38
16.3 23 11 17
S.E.D., d.f. 15 7.0 5.0
Total number of flowers
3.9 5.7 9.3 7.5
13.4 4.9 2.9 3.9
16.3 2.6 2.0 2.3
S.E.D., d.f. 15 0.72 0.51
plants held at ambient (13.48C) and at 16.38C had produced 22 leaf pairs without initiating a ¯ower. The day of ®rst ¯owering was earliest from stock plants stored at 3.98C and was particularly early in those plants stored at 3.98C and rooted at 14.98C. Similarly, the spread of ¯owering increased with cooler stock plants and particularly so in plants rooted at 14.98C. It ranged from 89 days for stock plants at 3.98C to 17 days for stock plants at 16.38C. The total number of ¯owers decreased with increasing stock plant temperature and this was particularly marked in plants rooted at 14.98C.
In `Pike's Pink' insuf®cient plants ¯owered to enable data to be subjected to analysis of variance but some trends were clear as can be seen in Table 6 which also showed the number of plants which ¯owered. Cold storing stock plants at 3.98C followed by rooting at 10.08C produced more ¯owers (8.7) earlier (day 137) than any other treatment. In contrast, cold storing stock plants followed by rooting at 14.98C gave fewer ¯owers (2.8) beginning later on day 174. Thus the timing of temperature treatment applied to stock plants and cuttings had an important effect on ¯owering.
3.4. Experiment 4
In `Doris' the rooting temperature had no effect on ¯owering, while both the stock plant temperature and the post-rooting temperatures had signi®cant effects (Table 7). Longer cold storage and a warmer post-rooting environment increased numbers of ¯owers and gave earlier ¯owering. Cold storage for 24 days gave the earliest ¯owering, while storage for 24 and 36 days both gave signi®cantly earlier ¯owering than 12 days cold storage and the control. The spread of ¯owering increased with a warmer post-rooting temperature but was not affected by cold storage of stock plants.
Table 6
Effects of temperatures applied during stock plant maintenance and rooting on ¯owering characters of `Pike's Pink' in experiment 3a
Stock plant temperature (8C) Rooting temperature (8C) Mean
10.0 14.9
Day of first flower(11 January)
3.9 137 (8) 174 (9) 156
13.4 176 (3) 167 (4) 172
16.3 174 (2) 170 (13) 172
Total number of flowers
3.9 8.7 (8) 2.8 (9) 5.8
13.4 3.8 (3) 4.8 (4) 4.3
16.3 4.5 (2) 4.7 (13) 4.6
a
In parentheses are the numbers of plants which ¯owered and contribute to the mean.
In `Pike's Pink' no plants ¯owered, showing that keeping the stock plants warm until 15 January meant that they and the cuttings did not accumulate suf®cient `cold' to satisfy the requirement for vernalization, irrespective of subsequent temperatures.
4. Discussion
The four experiments, each conducted at a different time under slightly different conditions (Fig. 1), showed con¯icting results which illustrate how complex the control of ¯owering is inDianthus. This work does not unravel these complexities but it does provide a novel insight of some key stages during crop production. The results show that ¯owering in `Doris' was affected by temperatures applied between 2 October and 20 February, while in `Pike's Pink' the important period was slightly longer, between 2 October and 11 March. Response to temperature over this period suggests that the time `window' for ¯ower induction in these species is wide. Since the experiments started at a range of times and growth stages speci®c dates are of limited value. However, we have shown that bothD. allwoodiiandD. alpinusrequire relatively low temperatures in order to produce ¯owers.
Table 7
Effect of cold storing stock plants and temperatures applied post-rooting on ¯owering characters of `Doris' in experiment 4a
Days cold storage at 4.88C Post-rooting temperature (8C)
Day of first flower(11 January)
0 189 13.0 194
12 190 15.8 174
24 174
36 184
S.E.D., d.f. 15 2.5 1.8
Spread of flowering(days)
0 24 13.0 28
12 24 15.8 22
24 27
36 24
S.E.D., d.f. 15 1.6 1.2
Total number of flowers
0 3.2 13.0 3.3
12 3.6 15.8 4.0
24 3.8
36 4.2
S.E.D., d.f. 15 0.30 0.21
a
Data values shown are means over rooting temperatures.
Experiment 1 showed that in `Doris' the earliest ¯ower initiation observed by apical dissection did not occur until 133 days after treatments ended, yet the day of ®rst ¯ower opening was signi®cantly earlier from lower temperatures. This shows effects of temperature during early growth on subsequent ¯owering and indicates that Dianthus sp. require vernalization Ð that is cool conditions allowing the development of the internal conditions leading to ripeness-to-¯ower (Napp-Zinn, 1987). In this case the plant is capable of producing ¯oral initials but has not necessarily done so and there is a delay between the completion of ¯ower induction and the initiation of ¯owers.
Perhaps the most interesting result was that in both species the time of ¯owering was affected by the temperature at which stock plants were kept before cuttings were taken suggesting an effect on ¯ower induction (experiments 3 and 4). While it is certainly known that the vernalization stimulus can be transmitted via cuttings in chrysanthemum (Chrysanthemum morifolium) (Schwabe, 1954) and the glasshouse carnation (D. caryophyllus L.) (Bunt and Cockshull, 1985) this was not known for the species studied here and should not be assumed to be true for all species, as illustrated here, by the incomplete response of `Pike's Pink'. Experiment 3 shows that in some treatments the vernalization requirement of `Doris' was completely satis®ed before cuttings were taken, the stimulus for ¯ower induction was transmitted in the cuttings and that visible ¯ower buds were produced during the rooting phase of propagation. Lower temperatures of the stock plants in October and early November resulted in earlier ¯owering.
In addition, when those stock plants of `Doris', previously kept at high temperature (16.38C), were cold stored at 4.88C for different periods in January (experiment 4) ¯ower appearance was earliest from 24 days cold storage. This suggests that further work might be needed to determine the relationship between cold requirement and time to ¯ower appearance. In contrast to `Doris', cuttings taken from `Pike's Pink' stock plants, which had been cold stored, produced attractive plants which, despite a range of subsequent temperatures, universally failed to ¯ower. This suggests that either the vernalization requirement was not satis®ed throughout the winter or that daylength after cold treatment is important. It is known that in D. caryophyllus ¯ower initiation is advanced by long days (Harris and Ashford, 1966) and so it seems unlikely that increasing daylength after cold storage would prevent ¯owering. It is far more likely that the high temperature of the stock plants until January was such that the limited period of cold was insuf®cient to satisfy the vernalization requirement.
Since stock plant shoots of `Pike's Pink' were not dissected before cuttings were taken there was no indication of the state of ¯ower induction before propagation but there was evidence that lower temperatures both before and after rooting of `Pike's Pink' advanced ¯owering. This suggests that the plants are responsive to appropriate temperatures over a long period and that the `cold' requirement is much greater than for `Doris'.
The effect of temperature on `Doris' in experiment 2 suggests that the plants had largely completed ¯ower induction when the cuttings were taken and that higher temperatures in controlled environment cabinets just accelerated ¯ower
development. This agrees with Harris and Scott (1969) who found in D.
caryophyllusthat the rate of ¯ower expression (development) was directly related to temperature. In experiment 4 ¯owering of `Doris' was earlier at higher temperatures post-rooting. This suggests that, following ¯ower induction at low temperature, ¯ower initiation and bud development to ¯owering occur more rapidly at a higher temperature post-rooting. This is in contrast to experiment 1, where plants had not completed ¯ower induction when temperature treatments were applied and higher temperatures delayed ¯owering. In `Pike's Pink' higher temperatures during rooting and post-rooting delayed ¯ower development suggesting that induction was incomplete when the higher temperatures were applied. In experiment 4, which had different temperatures through to ¯owering, `Pike's Pink' did not ¯ower because it had been unable to complete ¯ower induction. Thus it seems likely that plants can satisfy their cold requirement during different phases of growth (stock plant environment, rooting and post-rooting). Results from different experiments, which initially appear con¯icting, are almost certainly because the plants' state of ¯ower induction were not consistent between the growth stages at which treatments were applied. Ideally our work would have identi®ed exactly when ¯ower initiation occurred in each experiment but this was not logistically possible because of the numbers of plants required for repeat sampling.
In both species the temperature at which stock plants were kept before cuttings were taken and the temperature during rooting also affected the number of ¯owers. In `Pike's Pink' ¯ower numbers were increased by lower rooting and post-rooting temperatures, while in `Doris' there was an interaction between stock plant temperature and rooting temperature affecting ¯ower numbers. It appears that the in¯uence of temperature on ¯ower numbers is complex.
Variability in the time of ¯owering between individual plants within a treatment was considerable in `Doris'. It is likely that this variability is associated
with the management of the mother plant (Champeroux, 1983). In D.
caryophyllus axillary shoots from the upper nodes, immediately below the terminal ¯ower, form fewer leaf pairs before the ¯ower than cuttings taken from more basal nodes (Powell, 1979). Thus it is possible that the nodal origin of cuttings of garden pinks may affect their variability in time of ¯owering.
Acknowledgements
We thank the Ministry of Agriculture, Fisheries and Food for funding this work and Angela Hambidge, Jayne Akehurst, Carol Hannington and Andrew Cavill for technical assistance.
References
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