BotantcalJournal ofthe Linnean Socieg (1998), 128: 369-384. With 4 figures
Article ID: bt980205
Effect of flower structure and flower colour on
intraflord warming and pollen germination and
pollen-tube growth in winter flowering Crocus L.
(Iridaceae)
JUNO MCKEE' AND A. J. RICHARDS'
Deflartment of Agricultural and Environmental Science, Rid19 Building,
Universip ofNewcastle, Newcastle NEl 7RU'
&c&d
March 1998; accepedfor publication 3 4 1998
The internal temperature of the flowers of three colour variants of winter flowering Crocus
chysanthus and of C. tommasinianus were compared with ambient in the dark, and when subject
to artificial horizontal illumination with daylight spectra. Illuminated flowers warmed up to
3OC above ambient. In the dark, flowers also showed slight warming. In all varieties, pollen
germinated more freely at 15°C compared to 6OC, and pollen tube growth also tended to
be faster at the higher temperature, although pollen growth was inhibited at 20°C. Closed
flowers began to open at between 10°C and 12"C, and tended to open more rapidly at
higher temperatures. We conclude that flower warming at low ambient temperatures may
mediate flower opening in Crocus and allow pollination, and may stimulate pollen germination
and the fertilization process. Crom has the typical attributes of 'microgreenhouse' flowers,
absorbing some spectra externally, transmitting other spectra internally, trapping heat energy
by reflecting inner tepal surfaces, and storing energy in large gynoecia. Of the varieties
tested, white and purple flowers showed the greatest flower warming, and yellow flowers the
least. Yellow flowers transmit no light of less than 500 nm, suggesting that the transmission
of short wavelength light may be important in flower warming.
0 1998 The Linnean Society of London
ADDITIONAL KEY WORDS:-flower opening - microgreenhouse flowers
absorption - spectral reflectance - spectral transmission - temperature.
-
spectral
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . .
. .
The genus Crocu . . . . . . . . . . . . . . . . . . . .
Material and methods
. . . . . . . . . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . . . .
.
Comparison of internal flower temperatures . . . . . . . . .
.
Comparison of pollen germination and pollen-tube growth at different
temperatures . . . . . . . . . . . . . . . . . . . .
Tepal and flower attributes in Crocus varieties . . . . . . . . . .
I
I
370
372
372
374
374
374
375
Present address: Official Seed Testing Station, NIAB, Huntingdon Road, Cambridge CB3 OLE, UK.
Corresponding author. E-mail a,[email protected]
002&4074/98/120369
+ 16 $30.00/0
369
0 1998 The Linnean Society of London
3 70
J. MCKEE AND A. J. RICHARDS
Discussion . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . .
377
383
383
INTRODUCTION
Many features of floral structure, colour and behaviour are apparently adaptive
with respect to the control of gynoecial temperature. In arctic species, elevations of
internal flower temperatures may be beneficial in countering very low ambient
temperatures by hastening reproductive development and maximizing reproductive
output, for instance in Papauer radicatum Rottb. (Corbett, Krannitz & Aarsen, 1992)
and B y a s octopatala L. (Kjellberg, Karlsson & Kertensson, 1982). Rates of pollen
development and germination, pollen-tube growth, and ovule development could
all potentially benefit in an arctic environment from floral warming (Kevan, 1989).
Higher temperature regimes within arctic flowers may also act as a mechanism for
attracting potential pollinators (Kevan, 1972). However, it is difficult to determine
whether floral structures have evolved to promote floral warming, or whether such
warming is merely an incidental property of flower shapes which serve different
functions (Knutson, 1981). A similar view can be adopted with regard to cool-season
(winter-flowering and vernal) temperature plants in which flower warming could
also be advantageous. For instance, Budel (1959)showed that flowers of the snowflake
( h c o j u m uernum L.), a cool-season temperate species, produce internal warming.
Kevan (1989) identified six categories of flower shape, found in arctic plants,
adapted to attaining elevated internal temperatures. Disc flowers (Asteraceae)absorb
insolation, resulting in floral warming in proportion to the cosine of the angle of
insolation. Flower temperatures between 2°C and 6°C above ambient are regularly
attained within flowers of Taraxacum arctogmum Dahlst., Arnica alpina (L.) Olin and
Erigeron cornpositus Pursh.
Papaver radicatum, Bym octopetula and D. intesrifolia Vahl have bowl-shaped flowers,
which are diaheliotropic for at least part of the day and act as solar furnaces. Flowers
of I? radicatum track the sun 24 hours a day, and can attain floral warming up to
10°C above ambient (Kevan, 1975). In all bowl-shaped flowers, the maintenance
of temperature elevations is thought to be a combined effect of the shape of the
corolla, its reflective properties, and the mass of stamens and pistils (Kevan, 1975).
In some species, including Papauer radicatum (Mralgaard, 1989) and Crocus spp. (Kay,
Daoud & Stirton, 198I), the petal epidermis shows multiple reversed-papillate cell
structure. The inner tangential surface of the epidermal cell is papillate and traps
light reflected from mesophyll cells and from below (Kay et al., 1981). Such an
internal cellular arrangement of the epidermis in combination with a smooth outer
wall optimizes reflection of light when radiation is parallel to the floral axis. Striation
of the epidermal surface is poorly developed, which also increases the efficiency of
light and heat reflection (Kay et al., 1981 ; Mralgaard, 1989).
Heat gains within flowers cause increased number and weight of mature seeds in
€? radicatum (Corbett et al., 1992)and increased seed weight in &as octopetala (Kjellberg
et al., 1982) dependent on the intensity of the sun and time of day, ambient air
temperature and wind (Kevan, 1975). Trillium niuale is also diaheliotropic with bowlshaped flowers which can be 4°C to 5°C warmer than ambient (Knutson, 1981).
Hepatica spp. and Anemone spp. are less accurate sun trackers but are extremely
FLOWER WARMING IN CROCUS
371
efficient collectors of solar heat. For example, Pulsatilla patens (L.) Mill. can achieve
floral elevations of 14°C to 18°C warmer than ambient in bright sunshine (Knutson,
1981). Other bowl-shaped arctic flowers such as Potentilla spp. are not diaheliotropic,
but show some floral warming.
A third category of arctic thermoregulatory flowers are ‘inverted bells’, which are
not diaheliotrophic,but can attain floral temperature increases. Flowers of the purple
Sm@zgia opposit@liu L. are often 6°C warmer than ambient in sunshine, but are no
warmer than ambient in shade (Kevan, 1989).
‘Hanging bell’ flowers (Ericaceae)act as traps for rising warm air from the ground
and possibly also as microgreenhouses. Temperature excesses of between 3°C and
4°C occur in the flowers of Cassiope tetragonu (L.) D. Don (Kevan, 1989). Hanging
flowers of the spring flowering snowflake (Leucojum vernum) of temperate regions have
been shown to develop temperature differentials of up to 11°C (Budel, 1959).
‘Microgreenhouse’ flowers (Pedicularis, Silene, Fabaceae) and ‘hairy heat trap’
flowers (Salk spp.) are thought to be similar in their heat-trapping mechanisms. The
relatively large pistil absorbs short-waveradiation and emits this at longer wavelengths
which are trapped within the flower (Kevan, 1989). Buds of Salix polark Wahlenb.
can show internal teperatures 15°C to 25°C above ambient when insolation is high
but air temperature is below zero centrigrade (Krog, 1955). Temperature increases
up to 10°C have been shown within catkins of Salk arctica Pall. In addition, Kevan
(1990) showed that pistillate catkins were significantly warmer than staminate ones,
although the differential was small. Female catkins are more highly pubescent than
males and presumably benefit from the heat gain which promotes fruit maturation
(Kevan, 1990).
Flower colour in Papauer radicatum has a significant effect on internal flower
warming. The internal temperatures of white flowers off?radicatum are approximately
1.5”C cooler than the more common yellow flowers when measured simultaneously
and this difference has been related to habitat. White flowered plants are found on
the north Greenland coast where sunshine is limited, while yellow pigmented flowers
are more common in plants inland and at higher elevations (Merlgaard, 1989). It
was suggested that in habitats with reliably high levels of sunshine, it is cost-effective
for plants to invest in expensive petal pigmentation, thereby increasing intrafloral
temperatures, and benefiting from increased reproductive efficiency. In general,
dark coloured arctic flowers are more efficient at attaining intrafloral temperature
elevation than are light coloured ones (Budel, 1959; Tikhomirov, Shamurin &
Shtepa, 1960).
Striking flower colour polymorphisms are more typical of winter and vernal
flowering geophytes of the Mediterranean, for instance in Anemone (purple, red, pink
and white), Ranunculus asiaticus L. (red, pink, yellow and white), Fritillaria (brown or
purple, green or white), Primula vukaris Huds. (pink, white or yellow), and Cmcus,
such as C. uernus All. and C. bgorus Mill. (purple or white). Previous attempts to
account for the common occurrence of such polymorphisms in this biome have
shown that pollinators discriminate differentially between colour morphs, so that
disruptive selection may maintain the polymorphism in early flowering polyphilic
species (Kay, 1978, 1982).
In this paper we report experiments designed to test the hypothesis that floral
warming, flower opening behaviour and reproductive efficiency differ between
related flower colour variants in Crocus. If variants differ in fitness in the absence of
372
.J. h1CKEE AND A. J. RICHARDS
pollinators, the occurrence of colour polymorphism amongst vernal flowering species
might have an additional explanation.
The genus Crocus
Crocus (Iridaceae) contains about 100 species and is widely distributed from
Portugal to central Asia (Mathew, 1982). The majority of species, including C.
chTsanthus (Herbert) Herbert occur in the Mediterranean basin and most have some
flower colour variation, especially in external tepal coloration (‘feathering’).All are
vernal or autumnal geophytes. Most species have the capability to open and close
flowers in response to ambient temperature.
We have found no information on Crocus breeding systems. We suggest that some
species are potentially autogamous, being homogamous, non-herkogamous and
capable of setting fertile seed without the flower opening. Others exhibit approach
herkogamy and/or partial protogyny so that selfing within closed buds would be
less likely. Self-incompatibility has not been recorded but may occur. We suggest
that some species may not set seed unless the flowers open and are visited by insects,
while in other species, flower opening is not vital to seed-set, but should result in
more outcrossed, fitter, offspring.
Crocus spp. flower either in the autumn or spring months when ambient temperatures may be low. Crocus flowers presumably close to protect the erect flowers
from inclement weather. Nevertheless, flowers have the capability of opening and
reproducing in cold but sunny weather as they act as a microgreenhouse.
hL4TERIAL AND METHODS
Three varieties of Crocus chlysanthus and one of C. tommasinianus Herbert were
selected on the basis of flower colour. C. chrysanthus ‘Snow Bunting’ has white tepals,
with green feathering externally in the basal flower portion; ‘E.A. Bowles’ has yellow
flowers, with brown basal feathering; and ‘Blue Pearl’ has blue tepals and no
feathering. This variety probably resulted from hybridization between C. chlysanthus
and C. 62$om (Mathew, 1982). C. tommasinianus ‘Whitewell Purple’ has purple tepals.
Bulbs were planted separately in 9 c m pots in John Innes No. 3 compost. Crocus
lrarieties have been derived vegetatively from single seedlings and can be regarded
as single clones, so repeat readings within varieties are true replicates. Single Aowers
per bulb were investigated on each occasion.
Internal flower and ambient temperatures were simultaneously measured using
copper/constantan bare-wire micro-thermocouples (0.25 mm diameter) linked to a
portable Omega datalogger. Thermocouple probes were inserted into closed flowers,
so that the bulb was central within the closed flower. Normal erect flower posture
for Crocus was maintained by careful manipulation of the micro-thermocouples. All
datalogging runs were conducted in a constant temperature growth room. Ambient
temperatures ranged from 5OC to 9°C and relative humidity was continuously
monitored using a portable thermohygrograph and remained at approximately 50%
during runs. Each datalogging period lasted approximately one hour. For each
datalogging run, four micro-thermocouples measured internal temperatures within
FLOWER WARMING IN CROCUS
373
different flowers and a fifth simultaneously measured ambient temperature between
flowers at the same distance from the light source. Thermocouples were not disturbed
during each individual datalogging run. Thermocouples were arbitrarily chosen to
be inserted into each flower and to record ambient temperature so that between
runs probes were interchanged. The datalogger was calibrated by Omega Engineering
during manufacture and the thermocouples were purchased as batch calibrated
probes. The datalogger was not re-calibrated over the two years of use in this study.
Where readings from a thermocouple seemed to be erratic it was discarded and a
new one used. Comparative checks were made at six monthly intervals between the
datalogger probes and an ice-point calibrated British standard glass thermometer.
Results of comparative checks were not recorded but no discrepancies were found.
Direction light was provided at a distance of 20cm and at 90" on a horizontal
plane relative to flowers. The light source consisted of two desk-lamps in parallel,
fitted with 100 W daylight bulbs, providing approximately 50 pmol m-' sec-' at
that distance. Comparisons of the experimental light provided and typical solar
irradiance in March and May (solar composition data from 1993) made by Dr
Richard Dunne (University of Newcastle) revealed that the compositions of the
spectra were similar. The intensity of sunlight was however, far greater than that
of the artificial light source. Between 340nm and 400nm, solar intensity at that
time of year was approximately 10 000 times stronger than the artificial light source.
Between 400 nm and 800 nm, solar intensity was approximately 1000 times stronger
than the artificial light source.
Five runs (approximately 5 hours in total) were conducted with directional light
for each Crocus variety and four (each approximately 4 hours) for each variety with
no light source.
For each Crocus variety, 45 flowers were self-pollinated, using a fine paint brush
to distribute pollen over clean stigmas (checked as pollen-free x 20). For each Crocus
variety, 15 flowers were placed in each of three constant temperature regimes; 6"C,
15"C, or 20°C. Four days after pollination, flowers were cut at the base of the
flower stem and tepal tissue removed from the style and stigma. Gynoecia were
fixed and stained according to Wedderburn & Richards (1990) and McKee &
Richards (1998). Preparations were viewed x 400 with a Nikon SF u-v transmission
microscope and the effect of different temperature treatments at pollination on
pollen germination and pollen-tube growth through the style was assessed.
Data concerning pollen grain germination and pollen-tube growth were not
subjected to normalizing transformation due to high variance within samples and
unequal variance between treatments. Descriptions of all percentage data are
presented as medians with the range described by the 25th and 75th interquartile
ranges as high levels of zero percentages were frequent in some treatments. The
results presented alongside the percentage data are based on total numbers of pollen
grains germinating/pollen-tubes growing per treatment, i.e. results for all replicates
within a treatment were bulked. In some instances the results appear to contradict
the median and range values. In some cases
analysis of data was not possible
due to low expected values, nor were the data suitable for the alternative Fisher's
Exact test.
A Macam Photometrics spectroradiometerwas used to produce reflectance spectra
for each variety of Crocus. The inner and outer surfaces of five tepals per variety
were scanned between 280 nm and 800 nm at 1 nm intervals. Tepal reflectivity was
divided by that of a PTFE standard to obtain reflectance in W/m2/nm.
x2
x'
x2
374
J. MCKEE AND A. J. RICHARDS
Absorption and transmission of light through each surface of tepals was assessed
using a Unicam 8700 series UV/vis scanning spectrometer. Five tepal samples per
variety were placed in glass cuvettes and inner and outer surfaces scanned between
190 nm and 90 nm at 10 nm intervals.
Flower opening as a response to temperature was assessed by placing closed
flowers (taken from an ambient temperature of less than 10°C) in a dark growth
room, measuring ambient temperature, and timing flower opening responses. Flowers
were briefly exposed to low light as the growth room door was opened at approximately five minute intervals to check the status of flowers. Four replicate flowers
were used at each temperature and after a flower had opened once it was discarded.
This procedure was repeated at a number of different ambient temperatures.
Twenty closed flowers of each colour variety except yellow were cut at the base
of the flower and weighed to obtain fresh flower weights. The cut flower surface
was subsequently sealed with rubber glue and the flower reweighed. Flowers were
then placed upright in a dark growth room at approximately 6°C and 5O0/o relative
humidity. After approximately 20 minutes flowers were removed and reweighed.
Weight loss in mg m - ' was calculated as a measure of energy loss from the closed
flower by evaporation.
The same twenty closed flowers per variety were then used to calculate the
absorptive area (the tepal area receptive to directional light used in datalogging
experiments), exposed surface area (the area available for interception of nondirectional light), and the total tepal surface. All area measurements were made
using a Delta-T area meter.
RESULTS
Comparison Of intenzalJower hperatures
For every experimental period, apart from two on yellow flowers, there was a
significant increase of temperature above ambient within flowers (Table 1, Fig, 1).
Significant temperature differentials ranged from 0.28"C to 2.98"C, Internal flower
temperatures for all flower colours averaged over all runs were significantly greater
= 695.48, P<O.OOI; yellow, F,1,5901
= 79.03, P<O.OO 1;
than mean ambient (white, 4,,3j01
blue, I$,,,,, =311.6, P<O.OOI; purple, F'1.2781
=451.97, P<O.OOI).
Control datalogging runs performed under identical conditions but with no
illumination indicated that significant internal flower warming also occurred in the
absence of light (F,1.3031
= 19.29, BO.00 1) (Table 2). However, temperature differentials were very low, not exceeding 0.26"C above ambient in any case. Small
amounts of floral heating in the absence of a radiation source probably occurred as
a result of respiratory activity.
Comparison of pollen gemination and pollen-tube growth at dferent temperatures
For white, yellow and purple flowers, overall proportions of pollen grains germinating were greatest when the temperature at pollination was 15OC. For blue
flowers pollen germination was greatest at 20°C. Pollen germination was significantly
FLOWER WARMING IN CROCUS
375
TABLE
1. Mean differences between internal Crocus flower temperatures and ambient temperature
under artificial illumination (approximately 50 pmol m-? sec-’) for five experimental runs per variety
Tepal
colour
Mean ambient
temp. (“C)
(kSD)
Mean flower
temp. (“C)
( k SD)
Mean flower/
ambient difference
(“C) (fSD)
t-value
(two-sample t-test)
flower v ambient
White
5.98 (f1.09)
6.73 (f 1.OO)
6.92 (+ 1.48)
7.21 ( f 1.00)
7.91 (f0.79)
8.85 (k1.06)
9.71 (f0.93)
8.92 ( f 1.30)
8.05 (k0.80)
9.07 ( f0.63)
2.87 (k0.89)
2.98 (f0.80)
2.00 (f0.43)
0.83 (f0.55)
1.16 (f0.52)
22.86 (***)
24.90 (***)
4.53 (***)
6.93 (***)
15.11 (***)
Yellow
6.27 (51.11)
6.41 ( f 0.90)
7.54 (f0.88)
7.80 ( f 1.52)
8.57 (f1.57)
7.25 (f1.05)
8.09 ( f0.80)
8.69 (k0.86)
7.88 (f1.51)
8.63 (f1.40)
0.98 (+0.43)
1.68 (k0.33)
1.15 (k0.21)
0.08 (k0.16)
0.05 (kO.29)
10.19 (***)
19.89 (***)
13.57 (***)
0.67 (n.s.)
0.35 (n.s.)
Blue
7.03 (f0.88)
7.84 ( f 1.27)
7.95 (f0.95)
9.00 ( f l . l O )
8.65 (f0.87)
8.11 (f0.69)
9.73 (k0.98)
10.71 (fO.90)
1.62 (k0.72)
0.28 (k0.96)
1.79 (f0.63)
1.71 (f0.73)
15.60 (***)
2.12 (*)
15.36 (***)
12.54 (***)
Purple
7.83 (k1.04)
7.87 (f1.23)
8.21 (fO.90)
8.27 (k1.40)
9.42 (f1.36)
10.30 (f1.23)
9.22 (f0.75)
9.78 (50.81)
9.78 ( f 0.69)
11.54 (f1.25)
2.47 (f0.69)
1.35 (f0.96)
1.56 (k0.66)
1.51 ( f 1.25)
2.12 (f0.83)
18.12 (***)
10.63 (***)
14.19 (***)
11.30 (***)
12.40 (***)
greater at 15°C compared to 6°C in all colour varieties (Table 3). Pollen-tube
growth to all stylar positions measured at 6°C either grew well, or not at all. Pollentube growth in flowers of the white ‘Snow Bunting’ and the purple C. tommasinianus
showed a similar response to pollination temperature as did pollen germination,
being significantly greater at 15°C than at 6°C at the style top and base. ‘Blue
Pearl’ showed a different response, a pollination temperature of 6°C resulting in
significantly greater pollen-tube growth than either 15°C and 20°C although the
response at the lowest temperature was unpredictable.
Tepal andjower attributes in Crocus uarieties
The inner and outer tepal surfaces of each colour variety of Crocus showed similar
reflective properties, but over the whole spectrum inner surfaces were significantly
more reflective (Fig. 2A) than the outer surface (Fig. 2B) (white, t=3.64, P<O.OOl;
yellow, t=6.56, P<O.OOl; blue, t=5.42, P<O.OOl; purple, t=2.51, P<0.05). All
colours of tepal reflected UV light (280-360 nm) poorly but were highly reflective
of the highest wavelengths (720-800 nm). White tepals showed the greatest overall
reflectance. Yellow tepals were highly reflective from 520 nm upwards, while blue
and purple tepal reflectance peaked at between 441 and 480 nm and again at
72 1-760 nm.
White tepals showed low transmission and high absorbance of low wavelength
light (390nm). Between 420 and 890nm, percentage transmission was high and
absorbance low. Yellow tepals showed extremely high absorbance and low percentage
transmission of light between 390 and 500 nm. Both white and yellow tepals showed
,J. XICKEE .LVD A. J. RICHARDS
376
11
12
Ic
I
5‘
’
Time (30 second intervals)
5’
I
Time (30 second intervals)
Figure I. Comparisons of internal flower temperatures (---) in illuminated flowers (average of four
flowers measured simultaneously) with ambient temperatures (--)
at 30s intervals over 1 h in four
varieties of Cmcus. A, White ‘Snow Bunting’; B, yellow ‘E.A. Bowles’; C, blue ‘Blue Pearl’; D, purple
CL tornmarinianus.
T.mx 2. Comparison of mean internal temperatures of Cmcus flowers and mean ambient temperature
in the dark
~~~~~~
Tepal
CokJUr
Total
number
of runs
Mean ambient
temp. ( T I
4
5.41 (klJ.53)
LVhite
Yellow
Blur
Purple
~~
+ SD)
~
1
5
5.85 (k0.52)
6.42 (kO.51)
Mean flower
temp. (“C)
(
+s q
5.67 ( f0.5)
~~~
6.04 (k0.42)
6.57 (kO.51)
Mean flower/
ambient difference
(“C)(+SD)
(two-sample t-test)
flower v ambient
0.26 (k0.15)
4.39 (***)
~~
~
0.18 (+0.17)
0.15 (f0.16)
t-value
~~~
3.45 (***)
2.43 (*)
an increased in percentage transmission of light and a decrease in absorbance at
890 nm. Blue and purple tepals showed similar absorption and transmission spectra
(Fig. 3). Transmission of light through the spectra remained fairly constant, with a
FLOWER WARMING IN CROCUS
377
TABLE
3. Median pollen grain germination and pollen-tube growth (expressed as a percentage of
pollen-tubes at the stigma) of self-pollinated Croczls gynoecia exposed to different temperature regimes
at pollination (pollen germination determined from examination of all pollen grains present) (medians
with range expressed as 25", 75Ih interquartiles).
shows order of significant differences between
different temperature treatments within cells using data bulked between replicates. ***P<O.OO 1,
**P<O.Ol, *P<0.05
x2
Flower
colour
White
Temp. at
pollination
("C)
germination
Style top
Style middle
Style base
6
15
0 (0,50)
0
15>6*
(om
80 (50,100)
100 (67,100)
15>6***
80 (50,100)
89 (67,100)
15=6
50 (31,100)
72 (50,100)
15>6*
6
15
20
0 (0,47)
0 (0,50)
0 (0,O)
15>20=6**
0 (0,100)
100 (100,100)
100 (100,100)
n.p.
0 (0,100)
100 (100,100)
91 (57,100)
n.p.
6
15
20
36 (5,64)
50 (43,76)
78 (0,93)
20>15>6***
100 (0,100)
59 (44,90)
66 (52,100)
6>20> 15***
100 (0,100)
6
15
20
70 (42,82)
70 (64,87)
47 (37,67)
15>6>20***
62 (46,75)
68 (56,80)
33 (25,64)
15>6>20***
x1
Yellow
x2
Blue
xz
Purple
x2
Yo pollen-tube growth expressed as a Yo of pollen-tubes at the stigma
% pollen
34 (32,50)
41 (33,70)
6>15 =20***
42 (17,50)
62 (42,69)
25 (10,33)
15>6>20***
0 (0,O)
100 (82,100)
80 (57,100)
n.p.
100 (0,100)
39 (28,50)
38 (26,43)
6>15=20***
24 (12,42)
29 (16,50)
23 (7,33)
15% = 20***
n.p. = test not possible due to low expected values.
large increase between 850 and 890 nm. Absorbance decreased at higher wavelengths.
The absorption and transmission spectra did not significantly differ between inner
and outer surfaces of all colours of tepals.
Flower opening was strongly related to ambient temperature in all flower colours
tested (Fig. 4). No opening reponse was recorded for flowers at ambient temperatures
less than 9°C. In a dark growth room, flowers generally began to open at ambient
temperatures of 10°C to 12°C. Between ambient temperatures of 11°C and 14°C
there was a rapid decrease in the time taken to opening of all flower colours.
Ambient temperatures of 14-2 1"C allowed an opening response in approximately
10 minutes for all flower colours.
White and blue flowers had on average the greatest mass, while yellow and purple
were smaller, but comparable. However, there was high variation for individual
flower weight, particularly for white flowers. Mean weight loss due to water
evaporation was very low and was comparable in all flower colours. Surface areas
of flowers tended to correspond to flower weights so that, as would be expected,
the heaviest blue flowers had the greatest surface areas (Table 4).
DISCUSSION
Closed Crocus flowers act as microgreenhouses (Kevan, 1989) at low ambient
temperatures when provided with horizontally aligned daylight sources equivalent
to approximately 50 pmol m-' sec-', when average flower warming above ambient
can be as high as 3°C above ambient (Table 1). Respiratory flower warming in the
J. ILfCKEE AND A. J. RICHARDS
378
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601-640
681-720
761-800
321-360
401440
481-520
561-600
641-680
721-760
70 -
B
Figure 2. Reflectance spectra of inner (A) and outer (B) surfaces of C r o w tepals from 280 to 800 nm.
White ‘Snow Bunting’, yellow ‘ E . k Bowles’, blue ‘Blue Pearl’, purple C. tommasiniunus measured by
spectroradiometer.
dark is responsible for less than 10% of this warming (Table 2). Unlike Kevan’s
arctic examples, Crocus are mostly of Mediterranean distribution, flowering between
October and April when ambient temperatures are low. Both the intensity and the
azimuth of incident sunlight experienced by crocus flowers in the wild, particularly
in montane regions when plants flower later in the season, will be much higher
than our experimental irradiation, and will also usually exceed those experienced
379
FLOWER WARMING IN CROCUS
2.9
- 1.3
- 1.2
- 1.1
-A
3 ;:;1
h
3.0
2.9
1.0
1.4
1.3
- 1.2
-B
s ;:;1
-
- 1.1
- 1.0
- 0.9
,-0.8
-_--’0.7
------___
->-L--T-=-----~
0.6
- 0.5
0.4
-‘0.3
--
8 2.6
‘i: 2.5 a
8
2
4
_ ___
__--
2.4
2.3 2.2 2.1 2.0 1.9 -
I
I
I
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
2.1
3.0
2.9
s ;:;
h
.*
2.6
2.5
-k ;:;
2
4
2.2
2.1
2.0
. n
l.3
390
- 0.2
0.1
490
590
690
Wavelength (nm)
790
896
Absorption: inner surface
Absorption: outer surface
-- - - - - - -
% Transmission: inner surface
----
% Transmission: outer surface
Figure 3. Averages of five readings of absorption (solid lines) and ‘Yo transmission (broken lines) spectra
of inner (narrow line) and outer (bold line) tepal surfaces in (A) white (‘Snow Bunting’), (B) yellow
(‘E.A. Bowles’), (C)blue (‘Blue Pearl) and (D) purple (C. tummasiniunus) tepals in glass cuvettes measured
by a scanning spectrophotometer.
J. MCKEE AND A. J. RICHARDS
380
i
i
l
l
l
l
l
l
l
l
l
i
l
l
" 5 6 7 8 9 10111213141516171819202122
- 5 6 7 8 9 10111213141516171819202122
60
D
-
50
40
30
20
10
0
5 6 7 8 9 101112131415161718192021 2
Mean ambient temperature ("C)
Mean ambient temperature ("C)
Figure 4. Average time (n=4)in minutes (y axis) taken for closed Crocus flowers (<lO"C) to initiate
opening (0)
and complete opening (*) in the dark when transferred to one of a range of ambient
temperatures between 9°C and 21°C for (A) white ('Snow Bunting'), (B) yellow ('E.A. Bowles'), (C)
blue ('Blue Pearl') and (D) purple (C. tommasinionl~s)varieties.
TABLE
4. Summav of mean values of flower weight, weight loss due to evaporation and tepal area
( 3k SD)
'I'epal
Lolour
Mean floster
weight (mg)
i\'hite
Yello\+
Rlue
Purplr
28 I . 1 ( + 103)
138.3(*18)
340.5 (+67)
195.6 ( k 6 2 )
Alean weight
loss due to
evaporation
(mg/minute)
0.008 ( +0.006)
0.011 (k0.006)
0.008 (fO.010)
0.019 (k0.032)
Mean total
surface area
(mm7
1568.0 (f 309)
~
2042.0 ( f 388)
I 123.9 (_+325)
Mean exposed
surface area
(mm7
594.6 (k1 19)
465.5 ( k 9 2 )
733.6 (+ 121)
542.8 (f187)
Mean
absorptive area
(mm')
247.4 (f 76)
143.95 ( f 2 1 )
274.2 (f57)
159.1 (_+50)
FLOWER WARMING IN CROCUS
38 1
by cultivated material flowering under winter conditions in the UK. Even in the
UK during Crocus flowering, the natural light flux we have recorded ranges from
100 to 700pmol m-* sec-I. Under natural conditions, internal flower warming
could therefore be much greater than has been found in this study.
Kevan (1989) describes a microgreenhouse flower as having translucent floral
parts and relatively massive gynoecia which absorb short-wave insolation and reradiate it at longer wavelengths which are trapped within the flower as heat. Between
400 and 800 nm wavelength light, white flowers (variety ‘Snow Bunting’) transmitted
light more efficiently than did the other colours tested: purple C. tommasinianus
showed the poorest transmission, particularly for wavelengths below 700 nm (Fig.
3). Gynoecia within C. chysanthus flowers are large, and are even larger in C.
tommasinianus. Absorption spectra of all colours of Crocus tepals demonstrated a
capacity for high absorbency of short-wavelength light, below 400 nm (Fig. 3). Also,
the ‘feathering’ of dark, purple or brown, pigmentation which is typical of the
proximal regions of the outside surface of Crocus tepals will mediate energy absorption.
The inner surface of tepals in all colour varieties was significantly more reflective
than the outer surface (Fig. 2) so that the inner surface may help to retain energy
within the closed flower. When ambient temperatures are low and the flower is
closed, energy is trapped within the tepals and internal flower temperatures are
raised above ambient. The highly reflective inner surface of the Crocus tepal
probably contributes to this effect. Thus, Crocus flowers possess all the attributes of
microgreenhouse flowers.
However, when ambient temperatures are higher and the flowers open, the highly
reflective inner surface should allow the flower to act as a parabolic mirror, so
reflecting heat energy within the open flower (Kevan, 1975).
Flower size may also contribute to flower warming. Amongst the experimental
material, blue flowers (‘Blue Pearl’) had the heaviest flowers which provided the
largest surface area, and the largest areas which could absorb energy (Table 4),
while the yellow variety ‘E.A. Bowles’ had the least heavy flowers with the smallest
absorptive surface. Nevertheless the white variety ‘Snow Bunting’ showed a greater
capacity for intrafloral warming than did the larger blue flowers (Table l), suggesting
that the effect of relative flower size on floral warming was not large.
Corcus may be advantaged by flower warming at low ambient temperatures
through enhanced rates of pollen development, increases in pollen germination and
in rates of pollen tube growth. For self-pollinations onto four colour varieties of
Crocus, significantly more pollen germinated at 15°C compared with 6°C (Table 3).
Pollen-tube growth four days after pollination also tended to be enhanced at 15°C
compared to 6”C, although in the blue flowered ‘Blue Pearl’ the greatest pollentube growth was recorded at 6°C. Pollen of ‘Blue Pearl’ showed an all or nothing
response at 6”C, with 40% of the treatments showing zero pollen-tube growth, and
60% 100 percent pollen-tube growth. Where low or high temperature affected
pollen germination and pollen-tube growth adversely, it was not possible to conclude
whether total inhibition of pollen tube growth had occurred or whether growth
processes had merely slowed down. However, later stages in the reproductive process
are unlikely to be affected by flower warming as the ovaries in Crocus spp. are held
at the apex of the corm and are well insulated underground, so temperature effects
on seed-set were not studied.
Crocus species are adapted to attract insects, chiefly intermittently active overwintering bees, by conspicuous flowers and the production of nectar. Closed (erect)
J. MCKEE AND A. J. RICHARDS
382
TABLE
5. Summary table of main attributes of different coloured Cmcus flowers
'White
Yellow
Mean 0.65"C
(7.38OC)
Blue
Purple
Mean 1.4%
(7.85"C)
Mean 1.78OC
(8.2.3" c)
Mean 0.19OC
(5.85"C)
Mean 0.15"C
(6.42"C)
15"C>6"C= 20°C
20°C> 15">6OC
15°C>60C>200C
15°C 2 6°C
all or nothing
response at 6OC
15°C 2 2OoC>6"C
all or nothing
response at 6OC
6OC> 15OC 2 20°C
I 5°C>60C>200C
inner>outer surface
2 8 0 4 0 nm LOW
40 1--800nm HIGH
inner>outer surface
280-520 nm LOW
520-800 nm HIGH
inner>outer surface
2 8 W 0 nm LOW
441-480; 721-760nm
inner>outer surface
280-400 nm LOW
441--480; 721-760 nm
PEAKS
PEAKS
1
Mean 1.89OC
(7.13'q
2
Mean 0.26 "C
(5.41%)
3
15"C>6°C
4
3
6
inner =outer surface
1390 nm HIGH
400-800 nm MED.
800-890 nm HIGHER
inner=outer surface
<500 nm HIGH
500-890 MED.
inner =outer surface
<400 nm HIGH
580 nm PFAK
670-890 nm LOW
inner =outer surface
<400 nm HIGH
5 10-600 nm PEAK
600-890 nm LOW
7
inner =outer surface
<390 nm LOW
400-890 nm HIGH
inner = outer surface
<500 nm LOW
500- 890 nm MED
inner = outer surface
<700 nm LOW
790-890 nm HIGH
inner = outer surface
<690 nm LOW
690-890 nm HIGH
8
i9'C none
10°C initial-30 min
16"C+full-lO min
19°C none
10°C initial40 min
19°C initial- I 2 min
<9"C none
lO0C initial-55 min
14°C+full-10 min
<9"C none
10°C initial--55 min
1 3.5'C full--10 min
Large flowers; high
absorptive area
Smallest flowers by
weight; low absorptive
area
Largest flowers by
weight; high absorptive
area
Small flowers with low
absorptive area
Minimal
h4inimal
Minimal
Minimal
9
1 (1
+
+
I. Average flower/ambient difference in the light (at mean ambimt hpmatuz).
2. Average flower/arnbient difference in the dark (at mean ambimt temperature).
3. Pollen germination at different ambient temperatures.
4. Pollen-tube growth at different ambient temperatures.
5. Reflectance spectra of inner and outer tepals.
6. Absorption spectra of inner and outer tepals.
7. Percentage transmission spectra of inner and outer tepals.
8. Flower opening response to temperature.
9. Flower size and absorptive area.
10. Water evaporation from flower surface at low ambient temperature and 50% R.H.
flowers protect anthers, stigmas and nectaries from damage by rain and frost. We
do not know whether flowers which remain closed can self-pollinate and self-fertilize
autogamously. However, cross-pollination will only occur when flowers open, either
when ambient temperatures rise above about 11°C (Fig. 4),or when flowers are
illuminated by sunlight and warm beyond internal opening temperature thresholds
at lower ambient temperatures. Flower warming in bright sunshine may promote
flower opening at ambient temperatures below those at which flower opening would
occur in overcast light, which may form a useful adaptation for vernal flowers.
We conclude that different colour varieties of Crocus flower vary in their potential
for flower warming. Both the proportion and spectral range of light energy transmitted
through tepals, and the proportion and spectral range of light energy absorbed by
FLOM‘EK WARhllNG IN CROCCS
383
tepals may have influenced the temperature to which flowers warmed above ambient
(Fig. 3). Thus, the amount of flower warming in the white ‘Snow Bunting’, the
tepals of which transmit light well over a wide spectral range but absorb less well,
was almost equalled by that in C. tommasinianus, the tepals of which absorb light well
over a similar range but transmit less light (Table 5). Consequently, we believe it
unlikely that white and purple variants of Crocus with a comparable size and
structure would show differential fitness with respect to flower warming and ambient
temperatures at which flowers opened. However, the yellow variant ‘E.A. Bowles’
differed markedly from the other varieties by transmitting no short-wave light of
less than SOOnm through tepals (Fig. 3) while absorbing very strongly throughout
the visible spectrum less than 500nm (Figs 2, 3, Table 5). This variant showed
significantly less flower warming than the others, which suggests that transmitted
light below 500 nm may play a major role in flower warming. As yet it is unclear
whether such a reduction in flower warming potential might be of adaptive
importance, so the role of flower warming, if any, in the maintenance of flower
colour polymorphisms remains unclear.
ACKNOWLEDGEMENTS
This work was supported by the UK National Environmental Research Council
GR3/8467 ‘Effects of temperature on pollen germination and pollen tube growth’.
We should like to thank Dr Lyn Woods and Dr Richard Dunne of the University
of Newcastle for information and advice.
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