Annals of Botany 80 : 107–112, 1997
The Effects of Temperature, Photoperiod and Light Integral on the Time to
Flowering of Pansy cv. Universal Violet (Viola¬wittrockiana Gams.)
S. R. A D A M S, S. P E A R S ON and P. H A D L EY
The Department of Horticulture, School of Plant Sciences, The UniŠersity of Reading, Reading, RG6 6AS, UK
Received : 11 October 1996
Accepted : 24 February 1997
The effects of temperature, photoperiod and light integral on the time to first flowering of pansy (Viola¬wittrockiana
Gams) were investigated. Plants were grown at six temperatures (means between 14±8 and 26±1 °C), combined with
four photoperiods (8, 11, 14 and 17 h). The rate of progress to flowering increased linearly with temperature (up to
an optimum of 21±7 °C) and with increase in photoperiod (r# ¯ 0±91, 19 d.f.), the latter indicating that pansies are
quantitative long day plants (LDPs). In a second experiment, plants were sown on five dates between July and
December 1992 and grown in glasshouse compartments under natural day lengths at six temperatures (means between
9±4 and 26±3 °C). The optimum temperature for time to flowering decreased linearly (from 21±3 °C) with declining light
integral from 3±4 MJ m−# d−" (total solar radiation). Data from both experiments were used to construct a photothermal model of flowering in pansy. This assumed that the rate of progress to flowering increased as an additive
linear function of light integral, temperature and photoperiod. Independent data from plants sown on three dates, and
grown at five temperatures (means between 9±8 and 23±6 °C) were used to validate this model which gave a good fit
to the data (r# ¯ 0±88, 15 d.f.). Possible confounding of the effects of photoperiod and light integral are discussed.
# 1997 Annals of Botany Company
Key words : Pansy ; Viola¬wittrockiana, flowering, photo-thermal model, temperature, photoperiod, light integral.
Analytical approach : flowering responses to temperature,
photoperiod and irradiance
INTRODUCTION
Although pansies (Viola¬wittrockiana Gams.) are commercially-important bedding plants, little is known about their
flowering responses to temperature, photoperiod and light
integral. Considerable commercial benefits can be gained
from techniques which permit accurate scheduling of sowing
for predetermined dates of crop maturity. Previous studies
on pansies have concentrated on the close relative Viola
tricolor. Withrow and Benedict (1936) showed that a daylength extension with low irradiance light hastened the
flowering of Viola tricolor, indicating a long day flowering
response. Hughes and Cockshull (1966) also reported that
flowering of pansy was earlier when night-break lighting
was applied, but no quantitative data are available on the
photoperiod response of Viola¬wittrockiana, currently the
most commonly cultivated species. However, there is a
widely-held view among growers that it is day neutral, since
the plant commonly flowers during winter.
Similarly, very little is known about the flowering response
of pansy to temperature. Merritt and Kohl (1991) showed
that plants of cv. Universal Mix, grown at a mean diurnal
temperature of 21 °C, flowered earlier than those grown
under cooler conditions. Pearson et al. (1995 a) showed that
time from visible bud to flowering in pansy cv. Universal
Violet decreased linearly with increasing temperature from
10 to 25 °C, but the optimum temperature for time to
flowering has not been determined.
0305-7364}97}070107­06 $25.00}0
It has been shown for numerous plant species with both
short and long day responses to photoperiod that, under
inductive conditions, the rate of progress to first flowering
(i.e. the opening of the first bud) can be described as a simple
linear function of photoperiod and temperature :
1}f ¯ a­bT­cP,
(1)
where f is the number of days to first flowering, T is the
mean daily temperature ( °C), P is the mean photoperiod
(hd−"), and a, b and c are genotype-specific constants. For
long day plants (LDPs), lengthening of photoperiod causes
an increase in the rate of progress to flowering, so that c is
positive, and Šice Šersa for short day plants (SDPs) (Hadley
et al., 1983, 1984 ; Roberts, Hadley and Summerfield, 1985 ;
Ellis et al., 1990).
The temperature limits of the general thermal response
can be described by three cardinal temperatures : the base
(Tb), optimum (To) and ceiling (Tc) temperatures. At the
base and ceiling temperatures the rate of progress to
flowering is zero, whereas the highest rate of progress to
flowering occurs at the optimum temperature. The optimum
temperature can be estimated using the concept of effective
temperature (Pearson, Hadley and Wheldon, 1993). This
technique assumes that the response of the rate of
development to temperature is similar, but opposite, above
and below To ; if this assumption is not correct in practice,
the errors incurred will be small. Supra-optimal temperatures, for any estimated value of To, can then be converted
into effective temperatures (Te) which represent the sub-
bo970411
# 1997 Annals of Botany Company
108
Adams et al.—Time to Flowering in Pansy
optimal temperature equivalents of supra-optimal temperatures in relation to developmental rate :
Te ¯ To®r To®Ta r and Tb ! Ta ! Tc,
(2)
where Ta is the actual temperature. To select the optimum
temperature, Te is calculated for a range of optima. The
optimized value for To is the one which minimizes the
residual sum of squares of the regression between reciprocal
of days (rate of progress) to flowering and effective
temperature. By considering effective temperatures, eqn (1)
can be adapted to quantify the effects of both supra- and
sub-optimal temperatures on the rate of progress to
flowering :
(3)
1}f ¯ a­bTe­cP.
However, the general photo-thermal equation [eqn (3)] does
not take into account the influence of light integral per se on
the rate of progress to flowering.
To date, little quantitative information is available to
describe the effect of light integral on time to flowering.
There is evidence to suggest that, at least for chrysanthemum, the rate of progress to flowering (Cockshull,
1972 ; Pearson et al., 1993) and flower initiation (Langton,
1992) decrease rapidly at light integrals below approx.
3 MJ m−# d−" (total solar radiation). Increasing irradiance
above this threshold appears to have little additional affect
on the time to flowering.
The aims of this study were, therefore, to investigate the
flowering responses of Viola¬wittrockiana to temperature,
photoperiod and light integral, for plants grown under both
natural and controlled daylengths, and to investigate how
light integral can be accommodated within the general
photo-thermal flowering model.
MATERIALS AND METHODS
Experiment 1. The effect of temperature and photoperiod
on time to flowering : constant photoperiods
The objective of this experiment was to determine the
flowering response of pansy cv. Universal Violet to
photoperiod and temperature. Seeds were sown on 14 Feb.
1995 in a seed tray containing a peat-based seed and
modular compost (SHL ; William Sinclair Horticulture Ltd,
Lincoln, UK), germinated and grown on for 15 d at 20³1 °C
in a growth room providing 90 µmol m−# s−" (PAR) at plant
height from a mixture of white fluorescent and tungsten
bulbs (6±3 % tungsten calculated by nominal wattage), with
a 16 h photoperiod. In accordance with commercial practice,
these were then pricked out into plug-trays (Plantpak P84,
Plantpak Ltd, Maldon, UK ; volume of each cell, 40 ml)
containing the same peat-based compost. The plants were
placed on movable trolleys in the inner six, of a linear array
of eight temperature-controlled glasshouse compartments
(3±7 m¬7 m) set to provide minimum temperatures of 6, 10,
14, 18, 22 and 26 °C, with ventilation at temperatures 4 °C
above these set-points. Mean diurnal temperatures within
each compartment were calculated from temperatures
recorded on a data-logger (Datataker, DT500, Data Electronics, Letchworth Garden City, UK), linked to aspirated
PT100 temperature sensors (15 sec scans, logged hourly).
Each compartment was equipped with four photoperiodcontrolled chambers, sealed from exterior light sources.
Plants remained in the glasshouse for 8 h. At 1600 h each
day, they were wheeled into the photoperiod chambers where
they remained until 0800 h the following morning. Daylengths were extended inside each of the chambers by low
irradiance lighting (11µmol m−# s−", PAR) provided by a
40W tungsten and a 15W compact fluorescent light bulb.
In all treatments, the lamps were switched on automatically
at 1600 h for a duration dependent on the daylength length
required (8, 11, 14 and 17 h). The chambers were continuously ventilated, with an average air speed of 0±2 m s−"
over the plants when inside the chambers, to minimize any
temperature increase. This experimental design provided a
combination of six temperatures and four photoperiods.
When plants reached the five leaf stage, they were potted
on into 9 cm pots (volume 370 ml) containing a peat-based
potting compost (SHL) into which 25 % perlite had been
incorporated. An irrigation system provided Sangral 111
liquid feed (SHL, William Sinclair Horticulture Ltd,
Lincoln, UK) at each watering, at a conductivity of 1500 µS
(182 ppm N ; 78 ppm P ; 150 ppm K), acidified to pH 5±8. Six
replicate plants were grown in each treatment and the
number of days to flowering (i.e. when the corolla had fully
opened) were recorded for each plant.
Experiment 2. The effect of temperature and sowing date
on time to flowering : natural photoperiods
This experiment was carried out to establish the flowering
response of pansy to a wide range of temperatures, natural
photoperiods and light integrals. Seeds of cv. Universal
Violet were sown in a seed tray containing a peat-based
compost (Shamrock Special ; Shamrock Horticulture Ltd,
Bristol, UK) on five occasions ; 10 Jul. 1992, 4 Sep. 1992,
21 Oct. 1992, 11 Nov. 1992 and 2 Dec. 1992. These were
germinated and grown for 25 d in a growth cabinet
(Conviron S10H) at a temperature of 17³0±5 °C and an
irradiance of 90 µmol m−# s−" (PAR), provided by a 2 : 1
mixture of warm white fluorescent and tungsten bulbs
(determined on the basis of nominal wattage), with a 12 h
daylength. Plants were then pricked out into plug-trays, and
transferred to the same six temperature-controlled glasshouse compartments as used in expt 1 (set to provide
minimum temperatures of 4, 10, 14, 18, 22 and 26 °C). These
were grown under natural daylengths and potted up at the
five leaf stage as in expt 1, using the same peat-based
compost (Shamrock Special). Plants were irrigated by hand
and provided twice weekly with a solution of Sangral 101
soluble fertilizer (260 ppm N ; 0 ppm P ; 223 ppm K). The
pots were gradually re-spaced to avoid mutual shading,
until they reached a final plant density of 40 pots m−#.
Twenty replicate plants were used in each treatment and the
number of days for 50 % of the population to reach
flowering (corolla fully opened) were recorded.
Mean diurnal temperatures were calculated from temperatures recorded on a data-logger (Combine ; Murdoch,
1985) linked to aspirated thermistors (5 min intervals).
Mean daily light integrals were obtained from a meterological site located 300 m from the glasshouses. The
Adams et al.—Time to Flowering in Pansy
Experiment 3. Model Šalidation
In order to validate the model independently, a further
three crops were sown on 18 Oct. 1995, 29 Nov. 1995 and 10
Jan. 1996. Plants were raised as in expt 1 and grown on to
flowering in the same temperature-controlled glasshouse
compartments, set to provide minimum temperatures of 6,
10, 14, 18 and 22 °C. Plants were again potted on at the five
leaf stage and supplied with nutrients and water as described
for expt 2. For each sowing date, 20 replicate plants were
grown in each temperature compartment and the number of
plants flowering was recorded daily. Temperature data were
recorded using a DT500 data logger as in expt 1, light data
were measured as in expt 2, and photoperiods during the
course of the experiment were estimated using the method
of Sellers (1965).
RESULTS
Experiment 1. The effect of temperature and photoperiod
on time to flowering : constant photoperiods
By the end of the experiment, 143 d after sowing, all plants
had flowered except those grown at 26 °C and photoperiods
of 8 and 17 h. Rate of progress to flowering, estimated as the
reciprocal of time to flowering, increased linearly with
increasing photoperiod (Fig. 1), indicating a quantitative
200
175
Days to flower
glasshouse light transmission measured at the site (70 %)
was used to estimate the light integral transmitted into the
glasshouse. Daily photoperiods during the course of the
experiment were computed using the method of Sellers
(1965).
109
150
125
100
75
50
5
10
15
20
Temperature (°C)
25
30
F. 2. Relationships between time to flowering and mean temperature
from sowing for pansy cv. Universal Violet sown on 10 Jul. 1992 (E),
4 Sep. 1992 (D), 21 Oct. 1992 (_), 11 Nov. 1992 (^) and 2 Dec. 1992
(+). Each point is the time at which 50 % of the 20 replicate plants had
flowered.
long day response. There was no evidence of either a ceiling
or a critical photoperiod. In addition, flowering appeared to
be sensitive to photoperiod in all temperature}photoperiod
combinations. Rate of progress to flowering increased
linearly with temperature up to an optimum of 21±7 °C
[estimated using eqn (2)] and declined thereafter. Thus, for
example, plants grown at a mean temperature of 15 °C and
a 17 h photoperiod flowered after 99 d compared with 124 d
at 15 °C and a 8 h photoperiod. Multiple linear regression
showed that photoperiod and temperature affected the rate
of progress to flowering independently (r# ¯ 0±91, 19 d.f.),
indicating that the general photo-thermal model [eqn (3)]
was appropriate in describing the flowering response of
pansy to temperature and photoperiod.
1/Days to flower
0.013
0.012
Experiment 2. The effect of temperature and sowing date
on time to flowering : natural photoperiods
0.011
All crops flowered within the experimental period, except
those grown at the two highest temperatures in the 4
September sowing, and plants grown at the highest temperature in the 21 October and 11 November sowings. The
effect of temperature on time to flowering was relatively
small compared with the effect of sowing date (Fig. 2). The
earliest flowering occurred in crops sown during July,
whereas the slowest flowering occurred in crops sown
during September. Figure 2 also indicates that the optimum
temperature for earliest flowering decreased between the
July and September sowings. Thus, for plants sown in July,
which flowered rapidly, the optimum temperature appeared
to be in the region 19–23 °C, but for plants sown in
September, whose flowering was the slowest, the optimum
temperature appeared to be approx. 16 °C.
The optimum temperature for plants grown at each
sowing date was estimated using eqn (2) using the
FITNONLINEAR subroutine of GENSTAT 5 (GENSTAT
5, Release 3±1, 1993) and then plotted against the mean daily
light integral received for that particular sowing date (Fig.
0.010
0.009
0.008
17
14
Ph
ot
op
er
11
io
d
(h
d
–1
)
8 14
16
18
22
20
ure
rat
mpe
24
26
28
(°C)
Te
F. 1. The effects of mean temperature and photoperiod on the
reciprocal of days to flowering (1}f ) of pansy cv. Universal Violet. Each
point is the mean of six replicate plants. The mesh was fitted by
regression
analysis ;
1}f ¯ 0±002719(³0±000355)­0±000206
(³0±000037)Te­0±000294(³0±000023)P, r# ¯ 0±91, 19 d.f., where Te
and P represent effective temperature and photoperiod, respectively,
with an optimum temperature (To) of 21±7 °C.
Adams et al.—Time to Flowering in Pansy
24
180
22
160
Actual days to flower
Optimum temperature (°C)
110
20
18
16
14
3
2
4
6
5
–2 –1
Light integral (MJ m d )
7
F. 3. The relationship between estimated optimum temperature [as
described in eqn (2)] and light integral (MJ m−# d−" ; total solar
radiation) for crops sown on 10 Jul. 1992 (E), 4 Sep. 1992 (D), 21 Oct.
1992 (_), 11 Nov. 1992 (^), 2 Dec. 1992 (+) and 16 Feb. 1995 (*),
(r# ¯ 0±96, 4 d.f.). Vertical bars indicate s.e. of the estimates of
optimum temperature.
Predicted days to flower
120
100
80
100
120
140
160
Predicted days to flower
180
F. 5. Validation of the flowering model, comparing the time to
flowering of plants sown on three occasions ; 18 Oct. 1995 (E), 29 Nov.
1995 (D) and 10 Jan. 1996 (_), and grown at one of five different
temperatures between 9±8 and 23±6 °C, with those predicted by the
flowering model [eqn (5)]. The vertical bars indicate 95 % confidence
intervals for the mean flowering date of the 20 replicate plants
(r# ¯ 0±88, 15 d.f.).
The combined effects of temperature, photoperiod and light
integral on time to flowering
200
175
150
125
100
75
50
140
75
100
125
150
Actual days to flower
175
200
F. 4. The actual time to 50 % flowering of pansy plants grown at a
range of mean temperatures between 9±4 and 26±3 °C and sown on five
dates between 10 Jul. 1992 and 2 Dec. 1992, compared with those
predicted from the relationship between photoperiod and temperature
on the reciprocal of time to flowering [eqn (3) ; see legend to Fig. 1] from
expt 1 (D). (E), Predicted times to flowering, using the temperature,
light integral and photoperiod model [eqn (5)] compared with the actual
values of all the crops grown during expts 1 and 2. The solid line is
the line of identity (r# ¯ 0±94, 44 d.f.).
3). This shows that the optimum temperature decreased
linearly with decrease in light integral below 3±4 MJ m−# d−"
(total solar radiation). Above this value there was no
evidence of a change in optimum temperature, where To ¯
21±3 °C. A broken stick relationship fitted to the data
accounted for 96 % of the variance in the change in
optimum temperature with light integral :
Ta o ¯ 6±93 (³0±61)­4±25 (³0±72) Ma (r# ¯ 0±96, 4 d.f.),
(4)
where Ma ! 3±38 MJ m−# d−" (total solar radiation), if
Ma "3±38 MJ m−# d−" then Ta o ¯ 21±3 °C.
The application of the general photo-thermal flowering
model, using parameters derived from expt 1 [eqn (3)] was
then examined using data from expt 2. Figure 4 (open
symbols) shows the predicted times to flowering compared
with the observed flowering dates for all plants in expt 2.
This demonstrates that the model, taking into account only
temperature and photoperiod, did not predict time to
flowering accurately. However, the error was systematic,
such that the flowering times of the fastest-maturing plants
were overestimated, whereas those of the late-maturing
plants were underestimated.
Mean light integral was then incorporated into the
flowering model as a linear term, and the model fitted using
all the data from both experiments. The analysis, which
assumed that the optimum temperature for flowering varied
with light integral according to the relationship shown in
Fig. 3 [eqn (4)], described the data accurately :
1}f ¯®0±00155­0±000244Ta ­0±000291Pa ­0±000924Ma
e
(³0±00054) (³0±000031) (³0±000034) (³0±000080)
(r# ¯ 0±94, 44 d.f.),
(5)
a
where the optimum temperature to determine Te was derived
from eqn (4).
Therefore, all three environmental factors examined had
significant and independent linear effects on the reciprocal
of time to flowering. Further analysis revealed that there
were no interactions between any of the variables. Figure 4
shows the predicted Šs. actual flowering dates of the data
used to construct the relationship ; this indicates that there
was little evidence for any systematic deviation in predicted
compared with actual times to flowering.
Experiment 3. Model Šalidation
Independent data from the three crops sown on 18 Oct.
1995, 29 Nov. 1995 and 10 Jan. 1996, and grown at one of
Adams et al.—Time to Flowering in Pansy
five temperature regimes (minimum temperatures of 6, 10,
14, 18 and 22 °C) were used to test the validity of the
flowering model [eqn (5)]. For each data set, the model was
solved using an iterative procedure against running means
of average daily temperature, photoperiod and light integral,
up to the day on which the product of the average daily
contributions to flowering equalled one (determined as the
days from sowing multiplied by the average daily progress
to flowering, derived from eqn. (5) solved with running
means on each day). The accuracy of the predictions is
illustrated in Fig. 5, which indicates that the model gave a
good fit to the data (r# ¯ 0±88, 15 d.f.).
DISCUSSION
This study has shown that Viola¬wittrockiana is a
quantitative LDP, contradicting the widely-held view
amongst commercial producers that modern pansy varieties
are day neutral. The response to photoperiod was, however,
quite weak compared with that of the closely-related species
Viola tricolor ; Withrow and Benedict (1936) showed that
plants of the latter species grown in glasshouses with the
daylength extended to 21 h, flowered up to 72 d earlier than
control plants grown under natural short days. Here, the
difference between the 17 and 8 h treatments, at 15 °C, was
only 25 d. This weak response to photoperiod is probably
one of the reasons why Viola¬wittrockiana, often referred
to as ‘ winter flowering pansy ’ can flower and be produced
over the winter months.
This study has also indicated an important effect of light
integral on flowering, presumably as a consequence of
increased assimilate availability and growth. Previous
analyses using the general photo-thermal model have not
shown a significant effect of light integral on time to
flowering, with the exception of Pearson et al. (1993) in their
reanalysis of the effects of temperature and irradiance on
the time to flowering in chrysanthemum. However, the
present study indicates that such responses can be missed
particularly when daylengths are extended with lighting at
photosynthetic irradiances i.e. when light integral and
photoperiod are confounded.
To accommodate an influence of light integral, the general
photo-thermal equation was modified [eqn (5)] by assuming
that the rate of progress to flowering was a positive linear
function of light integral. This implies that the relationship
between days to flowering and light integral is hyperbolic
such that, at low light integral, small increases in light will
produce a substantial reduction in time to flowering.
Conversely, at high light integral, further increases in light
will produce little additional effect on time to flowering. A
similar response has been reported for flower commitment
of chrysanthemum (Langton, 1992). Information on the
effects of light integral on the time to flowering is of considerable value, since growers could control flowering, and
therefore grow to defined schedules, by manipulating the
light environment, either by shading or through the use of
supplementary lighting.
Temperature significantly affected the time to flowering,
111
as reported for many other species (see, for example, Ellis et
al., 1990). Other studies have shown that optimum temperature for certain species varies with ontogeny ; for
example, in Primula malacoides (Ru$ nger and Wehr, 1971),
and in Osteospermum jucundum (Pearson et al., 1995 b) the
optimum temperature for flower induction was lower than
for flower development following induction. However, the
present study has shown that, at low light integrals the
optimum temperature for flowering decreased. A similar
shift in optimum temperature was also suggested by an
analysis of the effects of light, photoperiod and temperature
on flowering of Senecio¬hybridus (Larsen, 1988). The
reason for this shift in optimum temperature with light
integral has not been established. It may be that the
optimum temperature declines as the assimilate availability
decreases as a result of low light integrals, but further
work is needed to establish the physiological basis of this
response.
The response to temperature was weak relative to the
effect of light integral such that the potential for manipulating maturity dates by adjusting the temperature is
relatively small. Furthermore, increases in temperature
during the later stages of plant development would have
deleterious effects on plant quality, since this would reduce
flower size (Pearson et al., 1995 a).
In addition to using the model to predict flowering
date, the model [eqn (5)] could also be used for the rapid
screening of new pansy germplasm, as has been proposed
with other species (Ellis et al., 1990). Cultivars with a
low value of b, the constant for the temperature response,
would flower adequately at a range of temperatures.
Varieties with a low value of c, the photoperiod response
constant, and the constant for the light integral response,
would have a greater propensity for flowering in the winter
months, where scheduling pansy production is difficult at
present.
The general photo-thermal model can be used to develop
improved sowing date schedules. The value of improved
scheduling techniques should not be underestimated, since,
in the UK alone, it has been estimated that up to 10 % of
bedding production is wasted as a result of inadequate
scheduling (S. Coutts, pers. com.). Conventionally, crop
schedules are developed by sowing a series of crops over a
range of dates and measuring their flowering dates, but such
schedules are notoriously inaccurate. The results achieved
are highly dependent not only on the environmental
conditions experienced during the development of the
programme, but also, as shown here, latitude, since
photoperiod and light integrals change with latitude.
However, by using the photo-thermal model, schedules can
be tailored for individual growers, since the model considers
environmental factors which vary between different locations ; i.e. photoperiod, irradiance and temperature. Furthermore the flowering model could be used ‘ on-line ’ by
growers to advise how maturity dates can be manipulated to
meet defined targets. Such an approach would be most
powerful if combined with models, such as those proposed
by Pearson et al. (1995 a) and Adams, Pearson and Hadley
(1997), for predicting the effects of environment on plant
quality.
112
Adams et al.—Time to Flowering in Pansy
A C K N O W L E D G E M E N TS
We thank the Horticultural Development Council and The
Ministry of Agriculture, Fisheries and Food who funded
this work. We would also like to thank Stuart Coutts and
Brian Crosby for their useful comments and encouragement.
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