Journal of Experimental Botany, Vol. 55, No. 402, pp. 1509–1517, July 2004
DOI: 10.1093/jxb/erh164 Advance Access publication 18 June, 2004
RESEARCH PAPER
Inflorescence structure and control of flowering time and
duration by light in buckwheat (Fagopyrum esculentum
Moench)
Muriel Quinet1, Valérie Cawoy1, Isabelle Lefèvre1,2, Francxoise Van Miegroet1,
Anne-Laure Jacquemart2 and Jean-Marie Kinet1,*
1
Unité de Biologie végétale, Institut des Sciences de la Vie et Département de Biologie,
Université catholique de Louvain, Croix du Sud, 5 (bte 13), B-1348 Louvain-la-Neuve, Belgium
2
Unité d’Ecologie et de Biogéographie, Centre de Recherches sur la Biodiversité et Département de Biologie,
Université catholique de Louvain, Croix du Sud, 5, B-1348 Louvain-la-Neuve, Belgium
Received 11 December 2003; Accepted 26 March 2004
Abstract
Morphogenesis of the reproductive structure of buckwheat and the impact of light conditions on flowering
time and duration have been investigated using the
variety ‘La Harpe’. Inflorescences were initiated acropetally, in leaf axils, by the shoot apical meristem until
its arrest of functioning which was accompanied by the
abortion of the last inflorescence produced. The buckwheat inflorescence is a compound raceme that produces laterally flowered cymose clusters, the number
of which was affected by the position of the inflorescence along the main stem. Similarly, the number of
flowers in a lateral cluster was dependent on the
inflorescence’s position on the stem. The development
of each inflorescence was stopped as its meristem
stopped functioning and, in a situation reminiscent of
the shoot apical meristem, the latest initiated cyme
aborted. The development of each cyme was also
terminated with the abortion of a few young flowers.
The variety ‘La Harpe’ is a facultative short-day plant:
the number of nodes generated before the initiation of
the first inflorescence and the number of days from
sowing to macroscopic appearance of this inflorescence were reduced in 8 h days as compared with 16 h
days. The number of inflorescences, and thus flowering duration, was also strongly reduced by short days.
It was unaffected by light irradiance in 8 h days while,
in 16 h days, it was prolonged when light intensity was
increased, suggesting the interaction of two different
mechanisms for its regulation. Buckwheat is a distylous species, but inflorescence structure and flowering
behaviour were not affected by floral morph.
Key words: Buckwheat, Fagopyrum esculentum, flowering
duration, light irradiance, floral morph, photoperiod, reproductive structures.
Introduction
Buckwheat, Fagopyrum esculentum Moench, one of the
few non-Poaceae cereals, has been cultivated for a long time
in several countries of Asia, Europe, and North America for
human and cattle consumption. Its production has, however,
strongly declined for decades and has almost disappeared in
many western countries, despite several obvious appealing
properties. Buckwheat has a short vegetative period and is
not susceptible to most cereal diseases, its seeds contain
proteins rich in lysine (Feldheim and Wisker, 1997), honey
derived from nectar is appreciated (Dalby, 2000), and the
plant produces rutin, a secondary metabolite of medicinal
use (Oomah and Mazza, 1996; Campbell, 1997). Buckwheat has fallen out of favour for various reasons including
its failure to benefit from the improvement arising from
modern agriculture and also because its flowering is profuse
and long lasting (Tahir and Farooq, 1991) making it difficult
to determine the optimum harvest time. Seeds at different
* To whom correspondence should be addressed. Fax: +32 10 47 34 35. E-mail: [email protected]
Abbreviations: SAM, shoot apical meristem; SD, short days; LD, long days; LI, low irradiance (75 lmol mÿ2 sÿ1); HI, high irradiance (150 lmol mÿ2 sÿ1);
SDP, short-day plants; LDP, long-day plants; Sn, sterile nodes; Dne, day neutral; Ppd, photoperiod response.
Journal of Experimental Botany, Vol. 55, No. 402, ª Society for Experimental Biology 2004; all rights reserved
1510 Quinet et al.
developmental stages coexist at the same time on the plant,
and even in a single inflorescence (Santo-Tomas and
Ledent, 1998; Funatsuki et al., 2000).
Increasing productivity and yield stability, as well as
improving the efficiency of breeding programmes, requires
a better understanding of the morphological and physiological parameters that affect the duration of the flowering
period of buckwheat. Flowering duration is regularly
recorded for flowering evaluation of crop plants, but studies
investigating the subtending mechanisms are lacking.
Flowering duration appears to be related to the number of
reproductive structures produced by the plant (Seddigh and
Jolliff, 1994; Saitoh et al., 1998) and is affected by plant
genotype and environmental conditions (Asumadu et al.,
1998; Ru and Fortune, 1999; Iliadis, 2001; Upadhyaya
et al., 2002). Studies on the genesis of the reproductive
structures in buckwheat reported that flower initiation was
most usually advanced by short days (SD) compared with
long days (LD) (Minami and Namai, 1986; Hagiwara et al.,
1998; Lachman and Adachi, 1990). Michiyama et al.
(2003) and Lachmann and Adachi (1990) also reported
that, with prolonged photoperiod, more inflorescences on
the main stem and more flowers per inflorescence were
initiated in various varieties, including ‘La Harpe’. However, their studies did not discriminate between a daylength
effect per se and a role for the daily light energy integral.
Thus, the mechanisms responsible for the protracted
flowering period of the species remain to be elucidated, as
well as the way environmental parameters and the morphogenetic programme interact in its regulation. Finally,
buckwheat is a hermaphroditic self-incompatible species
producing distylous flowers (Nagatomo and Adachi, 1985;
Campbell, 1997) and information concerning a possible
relationship between floral morph and reproductive structure edification is also lacking.
In this study, the initiation process of the reproductive
structures of a European buckwheat variety, ‘La Harpe’, is
described and, under controlled conditions, the influence
of daylength and light intensity on the initiation, the early
development, and the number of inflorescences and flowers
produced per plant are investigated. The aim was to define
whether the light effects could be related either to photosynthate availability or to photoperiodically regulated
messages or to both. How morph type and inflorescence
position on the shoot affect the flowering parameters is also
reported.
Materials and methods
Plant material and growing conditions
‘La Harpe’ is a French diploid variety of buckwheat (Fagopyrum
esculentum Moench), developed by INRA (Institut National de la
Recherche Agronomique, France), which is well adapted to various
regions of Western Europe. Seeds were obtained from ‘Agri
Obtentions’ (France).
All the experiments were performed in growth chambers of
the Department of Biology of the Catholic University of Louvain
(Louvain-la-Neuve, Belgium), except for those intended to investigate the influence of daylength and light irradiance upon flowering
which were carried out at the Department of Life Sciences of the
University of Liège (Belgium).
In Louvain-la-Neuve, plants were grown either in soil or in
a hydroponic system. For soil cultivation, two seeds were sown in
0.5 l pots in a superficial layer of peat compost covering a silt:clay:compost:sand (1:1:2:2, by vol.) mixture. Germination occurred within
3–4 d and, 7 d after sowing, the less vigorous plant in each pot was
removed. A 2 cm layer of substrate was then added to prevent plant
logging. For hydroponic culture, seeds were germinated on rock wool.
Eight days after sowing, seedlings of similar size were transferred
into 1.8 l plastic containers (6 seedlings per container) filled with
a modified Yoshida’s nutrient solution made of (mM) 1.4 NH4NO3,
0.3 NaH2PO4.2H2O, 0.7 CaCl2.2H2O, 1.6 MgSO4.7H2O and 0.5
K2SO4. Micronutrients consisted of (lM) 43 Fe-EDTA, 58 H3BO3,
0.1 Na2MoO4.2H2O, 0.4 CuSO4.5H2O, 0.4 ZnSO4.7H2O, and 11
MnSO4.H2O. The pH of the medium (about 5.5) was not buffered and
the volume in each container was maintained by regularly adding
fresh nutrient solution to compensate for plant consumption and
evaporation. Light was provided by Philips HPIT 400 W lamps. The
day/night cycle was of 16/8 h, the light irradiance of 120 lmol mÿ2
sÿ1 and temperature was kept at 21/18 8C (day/night).
In Liège, seeds were allowed to germinate in peat compost. Tenday-old seedlings were planted singly into 7 cm (diameter) pots
and, after 35 d, they were transferred into 12 cm (diameter) pots and
fertilized weekly with a solution containing 15 g lÿ1 of a 16:18:21
N:P:K fertilizer. Light was provided by Osram Cool white 40W
fluorescent lamps. Temperature was kept at 21/17 8C (day/night).
Flowering response assessment
Flowering time was assessed by two different measurements: (1) the
number of days from sowing to macroscopic appearance of the first
inflorescence and (2) the position of the node where the first
inflorescence appeared (the nodes were counted acropetally, the
cotyledonary node being disregarded). The macroscopic appearance
of the inflorescence was recorded when the tepals of the first flower
buds were turning white.
The number of inflorescences on the main stem and the number
of lateral flower clusters per inflorescence of the main stem were
recorded just after the occurrence of the first anthesis on the last
inflorescence. The date of the first anthesis and the total number of
flowers were also recorded on each inflorescence of the main stem. In
one experiment in soil conditions, the kinetics of flower production
was monitored by a daily record of flowers reaching anthesis on the
main stem.
Histological studies
Up to 22 d after sowing, shoot apices were harvested three times
a week and fixed in FAA (70% ethanol:acetic acid:formaldehyde,
18:1:1, by vol.). During development, inflorescences and lateral
flower clusters were also collected and fixed. The samples were then
dehydrated in a graded ethanol series, embedded in paraffin, and
5 lm sections taken. Serial longitudinal sections were stained with
haematoxylin–fast green and observed with a light microscope.
Statistical analysis
Normality tests were performed and no further transformation of the
raw data was required. The effects of daylength and light irradiance
were evaluated using ANOVA II and the influence of flower morph
using ANOVA I (SAS System for Windows V8). Differences
between means were scored for significance according to the Scheffe
Flowering in buckwheat
F-test. The frequency of both floral morphs in plant population was
evaluated using 2 test.
Results
Morphological and histological study of reproductive
structure initiation and development
Depending on the environmental conditions, flowering of
the buckwheat variety ‘La Harpe’ started after the production of a variable number of leaves (3–7 in these
conditions), with the shoot apical meristem (SAM) initiating on its flanks the first inflorescence primordium (Fig.
1A). The SAM continued to produce leaf primordia and
inflorescences acropetally, with one inflorescence per node
at each leaf axil (Fig. 1B). Leaves became progressively
sessile and reduced in size, becoming invisible to the naked
eye at the uppermost nodes.
At the same time, in axils below the level where the first
inflorescence was produced by the SAM, buds resumed
growth basipetally and developed in lateral shoots in the
same way as the main shoot. Their terminal meristem first
initiated leaves and frequently afterwards produced inflorescences. Basipetal flowering of the lateral shoots started
when acropetal flowering on the main stem was still going
on. Thus, many inflorescences, at different developmental
stages, grew concomitantly on the plant.
On both the main stem axis and lateral shoots, flowering
ceased with the production of inflorescences of decreasing
size (Fig. 1C) followed by the arrest of the morphogenetic
activity of their apical meristem along with the early
abortion of the last formed inflorescence.
Buckwheat produces a compound inflorescence that is
a raceme initiating, acropetally on its flanks, 7–16 uniparous cymes (Fig. 1B). The arrest of the morphogenetic
activity of the raceme meristem and the abortion of the last
formed cyme (Fig. 1E) terminated the construction of the
inflorescence.
Cymes developed in the axil of a bract wrapping the
young flower buds (Fig. 2A) that were initiated sequentially
at the base of the pedicel of the preceding flower (Fig. 2B).
Morphogenesis of the cymes stopped with the abortion of
some of the latest formed flowers.
Each flower, surrounded by a thin membraneous sheath,
topped a short pedicel that elongated during the last stages
of flower development up to anthesis (Fig. 1D). Flowers
were either of the pin or of the thrum type. Both floral
morphs were equally distributed in the plant population
(n=208, 2=0, P=1).
In each individual inflorescence, anthesis started on the
first cyme produced at the base and, at any time, there was
only one opened flower per cyme, the life span of which was
one day. Anthesis progressed acropetally from cyme to
cyme on the raceme and two to four successive cymes were
able to open a flower at the same time. As the first anthesis of
cymes was progressing toward the distal end of the in-
1511
florescence, a second flower opened on the basal cymes of
the raceme and the opening pattern for these second flowers
of the cymes was similar to that described for the first
flowers and so on for flowers of higher orders on the cymes.
One to eight flowers reached anthesis daily on individual
inflorescences and the number of flowers opening daily on
the main stem increased gradually with the number of
inflorescences bearing open flowers, to reach a maximum
between the 40th and 50th day after sowing (Fig. 3), when
the ten first inflorescences had flowers at anthesis. When all
inflorescences were bearing open flowers, the number of
flowers at anthesis on the main stem started to decrease
slowly from the 70th day after sowing, partly because of the
reduced number of flowers on the last inflorescences (see
Fig. 5C).
Effects of light conditions upon flowering time and
duration
To allow discrimination between photoperiodic effects and
daily light energy integral effects, plants were cultured under
four different light regimes consisting of a combination of
two daylengths (8 h short days (SD) and 16 h long days
(LD)) with two light irradiances (75 (LI) or 150 (HI) lmol
mÿ2 sÿ1 over the waveband 400–700 nm, at the top of the
canopy). SD as compared with LD advanced flowering in
buckwheat (Table 1). Both the position of the node where the
first inflorescence was initiated on the main stem (F=161.34,
P<0.0001) and the number of days from sowing to
macroscopic appearance of this inflorescence (F=25.90,
P<0.0001) were significantly lowered in 8 h days. SD also
reduced the duration of the flowering period, the development of the main stem being terminated earlier than in LD as
indicated by the reduced number of nodes with an inflorescence, due to the earlier arrest of meristem functioning
(F=455.53, P <0.0001) (Fig. 4). As a consequence, the total
number of inflorescences initiated per plant was also less in
SD (LD: 16.563.1; SD: 7.864.9; F=390.52, P<0.0001).
Obviously, the recorded effects were truly photoperiodic as
revealed by the comparison of the flowering behaviour of
plants that received the same daily light energy integral
regardless of the length of the light period (compare SD-HI
and LD-LI treatments in Table 1). Light irradiance did not
affect the position of the node where the first inflorescence
was formed, but slightly affected the macroscopic appearance of this structure, which was earlier under HI (F=18.75,
P<0.0001), especially in SD (Table 1). Duration of the
flowering period was unaffected by light irradiance in SD
but, under LD conditions, it was lengthened when light
irradiance was higher (F=11.32, P=0.0016) (Fig. 4).
Inflorescence development as a function of its position
on main stem and of flower morph type of the plant
The development of the successive inflorescences has been
investigated in LD, separately for plants of each of the two
1512 Quinet et al.
Fig. 1. (A) Histological longitudinal section of a shoot apex of buckwheat aged 11 d. The SAM is initiating a first inflorescence primordium (I). Five
leaves (L1 to L5) and two axillary meristems (A) are visible. (B) Histological longitudinal section of a shoot apex of buckwheat aged 18 d. The SAM is
initiating the fifth inflorescence primordium. Four leaves (L6 to L10) and five inflorescences (I1 to I5) are visible. Lateral flowered clusters (C1 to C8) are
initiated on the two first inflorescences. (C) The last inflorescences visible on the main stem of buckwheat. The arrow shows the last initiated
inflorescence. (D) A lateral flowered cluster of a buckwheat inflorescence showing four flowers (F1 to F4) at different stages of development. (E)
Histological longitudinal section of the last lateral flowered cluster of a buckwheat inflorescence. Three flowers (F1 to F3) aborted.
Flowering in buckwheat
1513
Fig. 2. Histological longitudinal section of a lateral flowered cluster of a buckwheat inflorescence. (A) The three first initiated flowers are visible (F1 to
F3). (B) The three last initiated flowers are visible (F1 to F3).
Number of open flowers on main
stem
floral morphs. Inflorescences produced by lateral branches
developing basipetally below the node of the first inflorescence of the main stem were not considered in this study,
these inflorescences were indeed rather small in these
growth conditions.
The number of inflorescences per plant was not affected
by floral morph (F=1.70, P=0.2010). It averaged 9.9
(60.2), with some plants producing up to 13 inflorescences. Similarly, floral morphs did not exhibit any significant
differences in time of anthesis of the first flower (Fig. 5A)
(F=0.14, P=0.7051), number of cymes (Fig. 5B) (F=0.22,
P=0.6360), and number of flowers (Fig. 5C) (F=0.49,
P=0.4848) in inflorescences of the same ontogenetic rank.
By contrast, the number of cymes (F=33.17, P <0.0001)
and flowers (F=75.64, P<0.0001) per inflorescence was
strongly affected by inflorescence ranking on the main
stem, decreasing acropetally and progressively from the
4th to the 5th inflorescence (Fig. 5B, C). The reduction in
flower number was particularly impressive, from approximately 100 in each of the first five inflorescences to four in
the uppermost ones.
Discussion
The reproductive structure of buckwheat
The buckwheat inflorescence was usually considered to
be a compound raceme (Nagatomo and Adachi, 1985;
Marshall and Pomeranz, 1982) although precise descriptions were missing. This study of the ‘La Harpe’ variety
Table 1. The effect of photoperiod and light irradiance on
flowering time in buckwheat (n=24)
30
15
Flowering time was measured by the position of the node where the first
inflorescence was initiated on the main axis and by the number of days
from sowing to macroscopic appearance of this reproductive structure.
Node number was counted acropetally, the cotyledonary node being
disregarded. LD, 16 h long days; SD, 8 h short days; HI, 150 lmol mÿ2
sÿ1; LI, 75 lmol mÿ2 sÿ1.
10
Photoperiod
Light
irradiance
Node of the
first initiated
inflorescence
(6SE)
LD
LD
SD
SD
HI
LI
HI
LI
5.960.1
5.660.2
3.560.1
4.060.1
25
20
5
0
20
30
40
50
60
Number of days from sowing
70
80
Fig. 3. Number of flowers reaching anthesis daily on the main stem of
buckwheat. Means 6SE (n=29). Plants were cultivated in soil conditions.
y=0.0007x3ÿ0.141x2+8.7461xÿ152.52. R2=0.9403.
aa
a
b
b
Days before macroscopic
appeareance of the first
initiated inflorescence
(6SE)
32.361.0
35.661.2
27.460.4
31.760.6
ab
a
c
b
a
Mean separation within columns by Scheffe F-test, P=0.05. Values
followed by the same letter are not statistically different.
Number of days from sowing
LDHI
LDLI
SDHI
SDLI
3
5
7
9
11 13 15 17
Node number
19
21
23
25
Fig. 4. Effect of daylength and light irradiance on the percentage of
buckwheat plants producing an inflorescence at different node positions
on the main stem (n=24). SDHI=8 h SD at 150 lmol mÿ2 sÿ1; SDLI 8 h
SD at 75 lmol mÿ2 sÿ1; LDHI=16 h LD at 150 lmol mÿ2 sÿ1; LDLI=16
h LD at 75 lmol mÿ2 sÿ1. Node number was counted acropetally, the
cotyledonary node excluded. Plants were cultivated in soil conditions.
clearly demonstrates the complexity of reproductive morphogenesis in buckwheat which went through successive
phases implicating first the SAM and afterwards the inflorescence and floral meristems.
In the course of a plant’s life, the SAM produced two
types of metamers. First, during the vegetative phase, type 1
metamers consisted of a node with a leaf developing an
ochrea at the base and of an axillary meristem whose
growth was delayed until after floral transition occurred at
higher nodes. Second, during the reproductive phase, the
SAM shifted to the production of metamers each consisting
of a node with a leaf or a bract, the size of which was
progressively reduced, and of a precocious axillary meristem which developed into an inflorescence. The inflorescence was a compound raceme that produced, acropetally,
uniparous cymes on its flanks.
The morphogenetic processes implicated in the establishment of the reproductive structures of buckwheat are
potentially endless. They are dependent on the activity of
meristems, the SAM and raceme meristems, with an
indeterminate mode of functioning which account for the
intrinsic potentialities of buckwheat to extend its flowering
phase for protracted time periods.
The temporal sequence of flower openings followed
roughly that of flower initiations, but it is worth noticing
that the estimation of the number of flowers reaching
anthesis daily reported here may not be representative of
what happens in the field because the experimental plants
were cultivated in growth chambers therefore preventing
pollination by insects and, as shown in other studies, flower
numbers per plant of buckwheat increase when seed-setting
is low (Michiyama et al., 1998, 1999, 2003).
Control of flower initiation
Buckwheat has been reported to initiate flowers over a
wide range of daylengths (Nagatomo and Adachi, 1985;
80
70
A
60
50
40
30
20
10
0
20
Number of cymes per
inf lorescence
100
90
80
70
60
50
40
30
20
10
0
B
15
10
5
ND ND
0
140
Number of flowers per
inf lorescence
Frequency of inf lorescences
(% of plants)
1514 Quinet et al.
120
C
P
T
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10 11 12 13
Inf lorescence rank
Fig. 5. Flowering behaviour of buckwheat as a function of flower morph
type: T, thrum flower-type; P, pin flower-type. (A) Time from sowing to
first anthesis in successive inflorescences of the main stem. Means 6SE
(n=15). Plants were cultivated in soil conditions. (B) Number of lateral
cymes in the first 11 inflorescences of the main stem. Means 6SE (n=20).
Plants were cultivated in hydroponic conditions. ND: not determined
because the number of plants producing an inflorescence at these nodes
was very low. (C) Number of flowers in the successive inflorescences of
the main stem. Means 6SE (n=15). Plants were cultivated in soil
conditions.
Lee et al., 2001), and even under continuous illumination
(Hao et al., 1995), so that it has sometimes been classified
as a day neutral plant. In fact, available evidence indicates
that all buckwheat varieties do not react similarly to
daylength. Studies conducted in Japan suggested that
summer cultivars were usually non-sensitive to daylength
while autumn cultivars were short day plants (SDP)
(Minami and Namai, 1986; Michiyama and Hayashi,
1998; Michiyama et al., 1998). Comparing three Japanese
Flowering in buckwheat
varieties and ‘La Harpe’, Lachmann and Adachi (1990)
concluded that flowering of buckwheat is delayed under
LD, ‘La Harpe’ being the less affected by daylength. Results
in this study corroborate that ‘La Harpe’ is a quantitative
SDP. The node where the first inflorescence was formed
was indeed significantly lowered under SD conditions, regardless of the light intensity. Other works have also
reported that flower initiation in buckwheat is advanced
by SD, 8–10 h photoperiods being the most effective
(Hagiwara et al., 1998; Kachonpadungkitti et al., 2001).
Light intensity is apparently without any effect upon floral
transition of the ‘La Harpe’ variety since the node rank
where the first inflorescence was initiated was unaffected by
this parameter whatever the daylength. By contrast, irradiance appeared to influence the early stages of inflorescence
development as revealed by the number of days to macroscopic appearance of the first initiated inflorescence, which
was reduced under HI, in both SD and LD.
Control of flowering duration
If the production of reproductive structures by buckwheat
is potentially endless, thanks to the activity of indeterminate
meristems, results in this study also clearly show that
abortion processes, affecting meristems and young reproductive structures, are the mechanisms contributing to the
arrest of the morphogenesis. Despite the fact that meristem
senescence and abortion during the development of reproductive structures are regular events affecting inflorescence architecture and size (Kinet et al., 1985; Greyson,
1994), they have been poorly studied and are thus poorly
understood. There is no doubt that these processes are
genetically determined and are also under the influence of
environmental factors. In Pisum sativum L., a quantitative
long-day plant (LDP), apical senescence is regulated by
several genes (Reid, 1980; Murfet, 1985; Singer et al.,
1990; Reid et al., 1996; Li et al., 1998; Zhu et al., 1998)
including the three complementary dominant genes, Sn
(Sterile nodes), Dne (Day neutral), and Ppd (Photoperiod
response) which confer to the plant a photoperiod response
with their activity enhanced under SD. In the ‘La Harpe’
buckwheat, correlative influences were implicated as well,
since the size of the inflorescence and of the cymose
clusters was strongly dependent on inflorescence position
on the stem. Nevertheless, a major role appears to be
attributable to light conditions. LD markedly delayed SAM
abortion as revealed by the high number of inflorescences
produced under these conditions. Interestingly, two treatments providing the plants with the same daily light energy
integrals (that is LD at LI and SD at HI) unambiguously
indicated that there was a truly photoperiodic effect of light
upon meristem abortion. Interestingly, Michiyama et al.
(1998, 2003) reported that while LD increased the number
of flowers compared with SD, they also decreased the seedsetting ratio and the number of seeds. The question arises
then as to whether SD can provide the necessary ‘semi-
1515
dwarf’ structure ensuring expected yields despite limited
flower production. If so, it would be advantageous to
cultivate buckwheat in SD. This should be possible in some
countries but not in Western Europe where other methods
to reduce the number of inflorescences per plant under field
conditions remain to be developed.
Usually, the nature of the global signal that triggers the
arrest of meristem activity observed in many species is still
unclear. Two non-mutually-exclusive mechanisms are most
often suggested to account for this phenomenon: (i) depletion of a key nutrient, (ii) production of an inhibitory
signal. In Pisum, the Sn, Dne, Ppd system acts through the
production of a graft-transmissible inhibitor whose primary
role could be to direct assimilate flow (Taylor and Murfet,
1994). In buckwheat, a role for photosynthetic assimilate, in
sustaining SAM activity, could be suggested by the observation that, under LD, inflorescence production was significantly increased when light intensity was higher. However,
the fact that under conditions providing equal daily light
energy integrals, SD caused early apical senescence suggests that the production of an inhibitor may be photoperiodically regulated by SD. Experiments are in progress to
test these hypotheses which are not mutually exclusive.
Flowering behaviour and floral morph
Finally, it is worth emphasizing that both morphs behaved
in the same way as far as time to flowering and flowering
duration were concerned. In fact, in most distylous species,
the action of the genes which control heterostyly is confined
to the floral parts of the plants (Lewis and Jones, 1992),
affecting, in addition to style and stamen filament length
(and therefore stigma and anther presentation), the size of
the stigmatic papillae and of the pollen grains (Stevens,
1912; Barrett, 1992). In a minority of heterostylous plants,
additional floral differences between the two morphs were
found in corolla shape or size, the colour of pollen grains,
and ovule size (Lloyd and Webb, 1992). In buckwheat, as
in other distylous species, pin and thrum-type plants differ
only in floral traits, including variations in pollen production (Ganders, 1979), size (Marshall and Pomeranz,
1982), and exine sculpturing (Bahadur et al., 1984). Thus,
both morphs may be gathered together for physiological
studies as clearly demonstrated in the present work.
Acknowledgements
The authors are grateful to Dr H Batoko (Plant Biology Unit, Université
catholique de Louvain) for comments on the manuscript. This work was
supported by the Fonds National de la Recherche Scientifique (FNRS)
of Belgium (FRFC contract no. 2.4562.99) and by the Université
catholique de Louvain (Fonds Spécial de Recherche).
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