Annals of Botany 79 : 687–693, 1997
Effects of Seed Number on Competition and Dominance among Fruits in
Capsicum annuum L.
L. F. M. M A R C E L I S and L. R. B A A N H O F M AN-E I J E R
Research Institute for Agrobiology and Soil Fertility (AB-DLO), P.O. Box 14, 6700 AA Wageningen,
The Netherlands
Received : 30 September 1996
Accepted : 24 January 1997
The effects of seed number on set, development and growth of a fruit, and on inhibition of later-developed fruits were
studied by varying the pollen load on the stigma of sweet pepper flowers (Capsicum annuum L.). Despite much
variation, a linear increase in individual fruit weight with seed number could be observed. Seed number affected the
growth rate rather than the growing period of fruit. When seed numbers were low, the probability of fruit setting
was positively related to seed number. However, a relatively low seed number (50–100 seeds}fruit : 20–30 % of the
maximum seed number) was sufficient for maximal fruit set.
An increase in seed number increased the inhibitory effect of a fruit on set and growth of later-developing fruits.
As a result, when pollination treatments were applied to all the flowers of a plant, results could be quite different to
those obtained when only a limited number of flowers were treated. Fruit set of the second fruit was reduced by the
application of a high pollen load to the first flower, even when the first fruit aborted before it had accumulated much
dry matter. Our results suggest that growth inhibition of the second fruit by seed number of the first fruit is controlled
both by competition for limited assimilates, as well as by dominance due to the production of plant growth regulators
by the developing fruit.
# 1997 Annals of Botany Company
Key words : Sweet pepper, Capsicum annuum L., pollination, fruit set, abortion, abscission, fruit growth, first-fruit
dominance, sink strength.
INTRODUCTION
In indeterminate glasshouse vegetable crops such as sweet
pepper, cucumber and tomato, the fruits on a plant compete
strongly with each other and with the vegetative plant parts
for the available assimilates (Hall, 1977 ; Ali and Kelly,
1992 ; Marcelis and De Koning, 1995). The sink strength of
the fruits (competitive ability to attract assimilates) is an
important factor in the regulation of assimilate partitioning
in these crops (Marcelis, 1996). It has been shown that the
total generative sink strength of cucumber and tomato
primarily depends on the number of fruits set, their
developmental stage and the temperature (Marcelis and De
Koning, 1995).
Sweet pepper is essentially a self-pollinating crop (Rylski,
1986). Under normal growing conditions the amount of
seed per fruit is highly variable : seed dry weight as a
fraction of the fruit dry weight may vary from 0 to 18 %
(Marcelis and Baan Hofman-Eijer, 1995). This variation is
likely to affect the sink strength of a fruit and hence the dry
matter partitioning. In many species, including pepper, fruit
size and fruit set have been reported to be positively
correlated with seed number (Rylski, 1973 ; Varga and
Bruinsma, 1976 ; Stephenson, 1981 ; Picken, 1984 ; Stephenson, Devlin and Horton, 1988 ; De Ruijter, Van den Eijnde
and Van der Steen, 1991 ; Shipp, Whitfield and Papadopoulos, 1994). However, Bae$ r and Smeets (1978) and
Bakker (1989) did not find a correlation between seed
number and fruit size in sweet pepper. Moreover, Bisaria
0305-7364}97}060687­07 $25.00}0
and Prakash (1978) reduced seed set in sweet pepper with
the morphactin chlorfurenol, and observed that both the
number of fruits set, and the fruit size, increased. In
addition, in Vicia plants which were topped, preventing
pollination of the flowers increased the number of pods set
(Chapman, Fagg and Peat, 1979). As seed number may
affect fruit set, and as fruits compete with each other for the
available assimilates, seed number may also affect growth of
individual fruits indirectly via an effect on the number of
competing fruits.
The presence of a developing fruit can inhibit subsequent
set and growth of a young fruit (Stephenson et al., 1988 ;
Bangerth, 1989). This inhibition may be caused by competition for available assimilates, by dominance due to
production of plant growth regulators from the developing
fruit, or by a combination of competition and dominance
(Tamas et al., 1979 ; Stephenson et al., 1988 ; Bangerth,
1989). A clear distinction between dominance and competition is usually difficult to make. Bangerth (1989)
hypothesized that auxin export from the earlier-developed
fruit inhibits auxin export of the later-developed fruits. This
depression of auxin export acts as the signal leading to
inhibited growth of the later-developed fruit. Auxin production and export by a fruit is predominantly confined to
the seeds (Varga and Bruinsma, 1976 ; Sjut and Bangerth,
1982, 1984). Stephenson et al. (1988) showed that in
Cucurbita pepo the seeds of the first fruit may indeed affect
the growth of the second fruit, but little is known about the
bo970398
# 1997 Annals of Botany Company
688
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
nature of the inhibitory effect of seed number on laterdeveloped fruits.
The aim of this paper is to quantify the effect of seed
number on set, growth and development of the sweet pepper
fruit. Moreover, the question is addressed to what extent the
seed number of a fruit affects the set and growth of laterdeveloped fruits and whether this can be attributed to
competition for assimilates or dominance due to production
of plant growth regulators.
MATERIALS AND METHODS
Sweet pepper plants (Capsium annuum L. cv. Mazurka) were
grown in glasshouses, in an aerated nutrient solution (expts
1, 2, 4, 5 and 6) or on rockwool (expt 3). Plants were pruned
to one main branch per plant, with side shoots pruned
above their first leaf. Four pollination treatments were
applied : (1) low pollen load : 2–3 d before anthesis the
stigma was covered with lanolin to minimize pollination ; (2)
medium pollen load : on the morning of the first day of
anthesis, the stigma was covered with lanolin, after which
pollination was minimized ; (3) normal pollen load : natural
self-pollination ; no treatment applied ; (4) high pollen load :
at anthesis flowers were hand-pollinated with their own
pollen. In the low pollen load treatment the flower buds had
to be slightly opened (bending of petals) to apply lanolin ; in
all treatments flower buds were opened before anthesis. The
treatments did not visibly damage the flowers. Emasculation
in order to completely prevent pollination was not considered in these experiments because the wounding may
induce ethylene production, and because anthers with pollen
are a rich source of gibberellins and auxins (Hedden and
Hoad, 1985). Both factors may affect abscission. As
pollinating insects were not present, cross-pollination can
be excluded.
Experiment 1
Two flowers were retained per plant. The four pollination
treatments were applied to the first flower, while the second
flower was hand-pollinated (high pollen load). Flower 1 was
retained at the third node of the main branch ; flower 2 was
located two nodes above flower 1. All fruits were harvested
26 d after anthesis (DAA) of flower 1. Each treatment was
applied to 24 replicate plants, distributed over six blocks.
Experiment 2
Two weeks after the end of expt 1 the same plants were
used for expt 2 (new randomization of treatments). As in
expt 1, two flowers were retained per plant. Four pollination
treatments were applied to the first flower, while the second
flower was hand-pollinated (high pollen load). Flower 1 was
retained at the twelfth node of the main branch ; flower 2
was located one node above flower 1. All fruits were
harvested when they had turned red (on average 65 DAA).
Each treatment was applied to 24 replicate plants, distributed over six blocks.
Experiment 3
One, 2, 4 or 6 untreated flowers were retained between
nodes nine and 15 of the main branch, and at node 17 four
pollination treatments were applied to the flower. As there
was no significant interaction between the effects of the
number of untreated fruits and pollination treatments, only
the means for pollination treatments are shown. Fruits were
harvested when they had turned red (on average 55 DAA).
Each treatment was applied to 56 replicate plants, distributed over 14 blocks.
Experiments 4 and 5
Two flowers were retained per plant. A low, medium and
high pollen load was applied to flower 1, and each of these
pollen loads for flower 1 was combined with a low, medium
and high pollen load for flower 2, resulting in nine
treatments. Flower 2 was located two nodes above flower 1.
The same treatments were applied in expts 4 and 5 using the
same plants (different randomization of treatments). Experiment 5 started 2 weeks after end of experiment 4. Flower
1 was located at node 3 (expt 4) or node 11 (expt 5) and all
fruits were harvested 28 (expt 4) or 33 (expt 5) DAA of
flower 1 (flower 2 : 17 or 22 DAA, respectively). Results of
both experiments were comparable and, therefore, they
were pooled. Each pollination treatment of flower 1 or
flower 2 was applied to 72 replicate plants (total of expts 4
and 5), but each treatment combination of flower 1 and 2
was applied to 24 replicate plants, distributed over 12 blocks
(two experiments with six blocks).
Experiment 6
Five flowers (on five successive nodes) were retained on
the main stem. They all received either a high, or a normal,
pollen load. All fruits were harvested 46 DAA of flower 1
(30 DAA of flower 5). Each treatment was applied to 12
replicate plants, distributed over 12 blocks.
Average temperatures were 22, 20, 24, 20, 20 and 21 °C
for expts 1, 2, 3, 4, 5 and 6, respectively. Solar radiation,
measured outside the glasshouse, was 9±3, 4±0, 17±3, 3±8, 1±4
and 14±8 MJ m−# d−" for expts 1, 2, 3, 4, 5 and 6, respectively
and in expts 2, 4 and 5 supplementary photosynthetically
active radiation was provided by high pressure sodium
lamps at a rate of 0±1, 0±2 and 0±3 MJ m−# d−", respectively.
All flowers or fruits that had abscised or small fruits that
had ceased to grow and were less than 5 g fresh weight at
harvest were considered to have aborted ; all other fruits
were considered to be set. No flower bud abortion occurred.
At harvest, seed number was measured and the dry weight
of the fruit (excluding seeds and pedicel) and seeds were
determined after drying at 100 °C. In expt 2, fresh weight of
fruit 1 was determined twice a week by measuring the length
and circumference as described by Marcelis and Baan
Hofman-Eijer (1995).
Statistical analysis
In all experiments treatments were arranged in a randomized block design. In most experiments the abortion of
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
Seed number
300
200
0
100
Fruit set (%)
The number of seeds and total seed weight per fruit
increased with pollen load (Fig. 1 A and C). At a low pollen
load seed formation was completely prevented in about
60 % of the fruits (data not shown). A low pollen load
reduced fruit set in most experiments, but in expt 2 fruit set
was 100 % even at the lowest pollination level (Fig. 1 B).
Increasing the pollen load above the medium level had no,
or only a small effect on fruit set, although seed number
increased 2–4 times, or 3–5 times at normal or high pollen
load, respectively (Fig. 1 A and B).
Fruit dry weight increased with increasing pollen load
(Fig. 1 C), which was due to an increase in growth rate
rather than growing period (Figs 1 D and 2). However, in
expt 2, but not in expt 3, the period from anthesis until the
fruit turned red increased slightly at low pollen loads (Fig.
1 D). The effects of pollination on fruit growth rate were
most obvious during the period of maximum growth (Fig.
2). Despite a large variation in the relationship between fruit
dry weight and seed number (Fig. 3), regression analysis
showed a significant linear relationship between dry weight
(y) and seed number (x) of individual fruits (y ¯
10±9­0±016x ; r# ¯ 0±55). A polynomial function did not
give any better fit than a linear function (P " 0±05). Of the
average increase in fruit dry weight per extra seed,
0±006–0±008 g was due to increased weight of the seeds, and
the remaining 0±008–0±010 g to the increase in pericarp and
placental tissues. Experiment 3 yielded a similar relationship
between dry weight and seed number (y ¯ 10±0­0±018x ;
r# ¯ 0±75).
A higher pollen load on the first flower of a plant resulted
in an increase in its percentage fruit set and fruit size (Table
1), but decreased fruit set and size of the second fruit. The
effect on size of the second fruit was not statistically
significant in expts 4 and 5 (Table 1), but in expt 2, where
almost all first and second fruits had set, an increase in
pollen load of the first fruit significantly reduced the final
fruit size of the second fruit which was hand-pollinated.
Final dry weights of the second fruits in expt 2 were 16±8,
15±1, 15±2 and 14±7 g at low, medium, normal and high
pollen loads, respectively. The reduced fruit set of the
second fruit when the first fruit had a high pollen load (expts
4 and 5), was partly due to increased set of the first fruit,
because with the same pollen load fruit set of the second
fruit was lower when the first one had set (Table 1).
A
100
B
75
50
25
0
16
Fruit dry weight (g)
RESULTS
400
C
12
8
4
0
60
Growing period (d)
fruits resulted in an unbalanced design for treatment effects
on fruit characteristics such as fruit size, amount of seed and
growing period. Analysis of variance was carried out using
generalized linear models, with binomial distribution for
fruit set and normal distribution for other fruit characteristics. Differences between treatments were tested by the
Students’ t-test (P% 0±05). This type of analysis yields a
different standard error of the mean for each treatment, but
as differences were small, only the average standard errors
of the means are presented in the tables. In expt 3 a very
high percentage (50 %) of the fruit was affected by blossomend rot ; these fruits were not included in the analysis of
treatment effects on fruit characteristics other than fruit set.
689
D
40
20
0
Low
Medium
Normal
Pollen load
High
F. 1. Effect of pollen load on (A) seed number per fruit, (B) fruit set,
(C) dry weight of the total fruit and seeds (in expts 1 and 2, the upper
part of the columns indicate seed dry weight) and (D) growing period
from anthesis until the fruit had turned red in expt 1 (*), expt 2 (+)
and expt 3 (8). Treatments were applied to one flower per plant.
Vertical bars indicate s.e.m.
However, if only the plants in which the first fruit set are
considered, then set of the second fruit also decreased with
pollen load of the first flower (Table 1). When the first fruit
had aborted, a high pollen load of the first flower still had
a negative effect on fruit set of the second flower (Table 1).
In this experiment abscission of the first fruit occurred at
about 2 weeks after anthesis, which was a few days after
anthesis of the second flower, and dry weights were less than
690
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
1 g per fruit. However, in another experiment the first fruit
did not induce abortion of the second fruit before 10 d from
anthesis of the first fruit, as removal of the first fruit at the
flower bud stage or 5–10 d after pollination (high pollen
load) resulted in 90 % set of the second fruit (anthesis about
7 d after that of first fruit), while 30 % aborted when the first
fruit was not removed (data not shown).
An increase in pollen load of the second flower increased
fruit set and fruit size of this fruit, but had no significant
effect on fruit set or size of the first fruit (Table 2). Although
an increase in the pollen load of an individual fruit increased
its probability of setting, the total number of fruits set in
plants with two fruits was lower when the first flower had a
high pollen load than an intermediate pollen load (Table 3).
In addition, in expt 6, where five successive flowers all
received either a high (hand-pollination) or a normal pollen
load, fruit set of the first four fruits was high in both
treatments, but the set of the fifth fruit was considerably
decreased by hand-pollination (Table 4). On each node fruit
dry weight and seed number were considerably increased by
hand-pollination (Table 4).
10
Rate of fresh weight increase (g d–1)
8
6
4
2
DISCUSSION
0
20
40
Days after anthesis
60
Seed number
F. 2. Effect of pollen load on the time course of fruit growth rate (expt
2). A low (_), medium (^), normal (E) or high (D) pollen load was
applied to one flower per plant.
25
The number of seeds per fruit increased with increasing
pollen load indicating that in sweet pepper seed number is
limited by pollen load. This is in agreement with observations
in tomato, where seed and fruit formation is limited by
failure of pollen production or pollination, rather than by
impaired pollen germination, pollen tube growth, ovule
production or fertilization (Picken, 1984). Therefore, the
reported variability in amount of seed per pepper fruit
(Marcelis and Baan Hofman-Eijer, 1995) may be related to
a great extent to the variability in the amount of pollen
reaching the stigma.
20
Fruit dry weight (g)
Fruit size and growing period
15
10
5
0
200
400
600
Seed number
F. 3. Relationship between final fruit dry weight and number of seeds
per fruit. Each symbol represents one fruit (expt 2). y ¯ 10±9­0±016x ;
r# ¯ 0.55.
Size of sweet pepper fruits showed a positive relationship
with seed number (as manipulated by pollen load), as has
been found before in sweet pepper (Rylski, 1973 ; De Ruijter
et al., 1991 ; Shipp et al., 1994) and other species (Varga and
Bruinsma, 1976 ; Stephenson, 1981 ; Stephenson et al.,
1988). Picken (1984) concluded that the precise relationship
between fruit weight and seed number in tomato may vary
because several factors may affect fruit weight (e.g. number
of competing fruits) without affecting seed number. We
observed that, on average, fruit weight increased linearly
with seed number in sweet pepper, which agrees with the
results of Rylski (1973). However, for individual fruits this
relationship varied considerably, which corroborates the
suggestions of Picken (1984) that seed number is only one of
many factors affecting fruit weight. This may also explain
why Bakker (1989) did not observe a significant correlation
between weight and seed number of individual pepper fruits.
If there was a strict linear relationship between fruit weight
and seed number, supplementary hand-pollination (high
pollen load) is expected to increase fruit weight compared to
the normal pollination. However, supplementary hand-
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
691
T     1 . Effect of pollen load of the first flower on fruit set and dry weight of the first and the second fruit, for all plants
or for plants where fruit 1 had aborted and set separately (expts 4 and 5)
Fruit set (%)
Fruit dry weight (g)
Fruit 2
Pollen load
Fruit 1
All
All
Fr1 aborted
Fr1 set
Fruit 1
All
Fruit 2
All
23a
68b
71b
6
69a
51b
28c
6
73a
72ab
43b
9
65a
42ab
20b
8
3±42a
5±04b
6±55c
0±24
1±64a
1±51a
1±43a
0±16
Low
Medium
High
s.e.m.
Means within a column, followed by the same superscript were not significantly different (P % 0±05).
T     2 . Effect of pollen load of the second flower on fruit
set and dry weight of the first and the second fruit (expts 4 and
5 : all plants included in the analysis)
Fruit set (%)
Pollen load
Low
Medium
High
s.e.m.
Fruit dry weight (g)
Fruit 1
Fruit 2
Fruit 1
Fruit 2
58a
51a
54a
6
31a
52b
64b
6
5±15a
4±72a
5±14a
0±21
1±02a
1±43b
2±14c
0±15
Means within a column, followed by the same superscript were not
significantly different (P % 0±05).
T     3 . Total set of fruit 1 and fruit 2 (number of fruits per
plant) as affected by pollen load of the first and the second
flower (expts 4 and 5)
Flower 2
Flower 1
Low
Medium
High
Low
Medium
High
s.e.m.
0±82a
0±96a
0±92a
0±14
0±88a
1±16ab
1±04a
0±14
1±06a
1±46b
1±00a
0±14
weight, seed number was already quite high (" 200 seeds)
when the pollen load was normal and irradiance was rather
low (expts 1 and 2). Other authors (Rylski, 1973 ; Shipp et
al., 1994) have reported smaller amounts of seed per fruit
when no supplementary pollination was applied, and they
observed an increase in fruit weight by supplementary
pollination. Hence, the effects of seed number on fruit
weight may saturate at high seed numbers and as a result the
effects of supplementary pollination on fruit size may
depend on the amount of seed already formed in the absence
of supplementary pollination.
In several crops the growing period from anthesis until
fruit maturity increased when seed number was reduced
(Varga and Bruinsma, 1976 ; Lyrene, 1989 ; Shipp et al.,
1994 ; Woodburn and Andersen, 1996). Varga and Bruinsma
(1976) showed that the increase in growing period was
mainly because the start of growth was delayed shortly after
anthesis. In contrast, Staudt, Schneider and Leidel (1986)
observed a more rapid fruit development in grape when seed
number was reduced. However, in our experiments seed
number had little effect on the growing period.
Fruit set
Means followed by the same superscript were not significantly
different (P % 0±05).
pollination only increased fruit weight in half of the
experiments (expts 3 and 6). In the experiments where
supplementary pollination had no significant effect on fruit
At low seed numbers the probability of fruit setting was
positively related to seed number (as manipulated by
pollen load). However, a relatively low seed number (50–100
seeds}fruit : 20–30 % of the maximum seed number) was
sufficient for maximal fruit set. In pecan (Carya illinoinensis (Wangenh.) K. Koch) a small amount of vital pollen
(5 %) was also sufficient for maximum fruit set (Marquard,
1992). Although in our experiments supplementary pol-
T     4 . Effect of pollen load on aŠerage fruit set, fruit dry weight and seed number of all fruits when all fiŠe fruits receiŠed
either a normal, or a high pollen load
Fruit set (%)*
Pollen load
Normal
High
s.e.m.
All fruits
Fruit 1–4
Fruit 5
Fruit d.wt (g)
All fruits
Seed number
All fruits
93a
81b
3
93a
91a
3
87a
43b
9
7±63a
8±95b
0±26
73a
212b
19
* The average fruit set for the first four fruits and the fifth fruit is shown (expt 6).
Means within a column, followed by the same superscript were not significantly different (P % 0±05).
692
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
lination did not increase fruit set compared to normal
pollination, it may enhance fruit set under conditions of
poor pollination, e.g. under poor light conditions (Picken,
1984). Under certain conditions (expt 2) fruit set may be
100 % even when flowers are not pollinated. The high
percentage fruit set of unpollinated flowers was probably
possible because of a high source}sink ratio (many leaves,
few competing fruits, low temperature and an intermediate
radiation of 9 MJ m−# d−" during 14 d from anthesis). In
some species parthenocarpic fruit set can be obtained only
when competition with other sinks is minimal (Chapman et
al., 1979 ; Carbonell and Garcı! a-Martı! nez, 1980). The effects
of seeds on fruit set are probably additive to other factors
affecting fruit set such as the sink}source ratio or environmental factors. The effects of pollination on fruit set
and growth are likely to result from an increased number of
developing seeds, but there are also indications that physical
contact between pollen tube and ovule, without fertilization
of the ovule, may be sufficient to stimulate fruit development
(Varga and Bruinsma, 1990).
Inhibition of later-deŠeloped fruits
Besides stimulating set and growth of a fruit, the seed
number also increased the inhibitory effect of the fruit on set
and growth of later-developing fruits. As a result, our data
show that when pollination treatments are applied to all the
flowers of a plant, results may be quite different to those
obtained when only a limited number of flowers are treated.
This, together with the fact that fruit set is already maximal
at a relatively low seed number, may explain why some
authors (Bae$ r and Smeets, 1978 ; Bisaria and Prakash, 1978 ;
Chapman et al., 1979) found promoting seed set had no
effect on fruit set, whilst some (Picken, 1984 ; Stephenson et
al., 1988) found an increase and others (De Ruijter et al.,
1991) observed a decrease in fruit set. In addition, it may
explain why effects of plant exposure to pollinating insects
have little effect on total fruit fresh weight production per
annum in sweet pepper (De Ruijter et al., 1991 ; Kristjansson
and Rasmussen, 1991), although during a short period
insect pollination may have a strong positive effect on fruit
production (Kristjansson and Rasmussen, 1991 ; Shipp et
al., 1994). Young and Young (1992) reviewed 99 experiments
in which hand-pollination was compared to normal pollination. In 42 % of the experiments hand-pollination had a
positive effect on either seed or fruit set, in 40 % it had no
effect and in 17 % it had a negative effect. They proposed
several mechanisms for the reduced seed or fruit set at high
pollen load, including pollen tube crowding, damage during
hand-pollination, missed stigma receptivity and the application of inviable pollen. However, these authors did not
take into account the fact that increased inhibitory effects of
the earlier fruits on later-developed fruits might have been
the primary cause of the decreased fruit and seed set.
production seeds of a fruit may affect competition between
fruits, either by increasing the sink strength (competitive
ability to attract assimilates) of the fruit, or by suppressing
the sink strength of other fruits (Varga and Bruinsma, 1976 ;
Bangerth, 1989). In the first situation an increase in seed
number of the first fruit may reduce growth of the second
fruit because of competition for limited assimilate supply,
while in the second situation growth reduction is due to
hormones produced by older fruits (dominance). Generally
a clear distinction between competition and dominance is
difficult to make. The positive effects of pollination on
growth of the treated fruit (first fruit) were stronger than the
negative effects on growth of the untreated fruit (laterdeveloped fruit). This suggests that the effects on sink
strength of the treated fruit are larger than on the sink
strength of the second fruit. Marcelis (1996) proposed that
the potential fruit growth rate is a quantitative measure of
sink strength and that dry matter partitioning into an organ
is proportional to its sink strength relative to that of the
other organs. Accordingly, we calculated that a reduction in
sink strength of fruit 1 of about 40 % with an unaltered sink
strength of fruit 2 could result in the observed dry weight
reduction of about 5 g in fruit 1, while the dry weight of the
second fruit increased by about 2 g by reducing the pollen
load on fruit 1 (data not shown). Our results showed that
fruit set of the second fruit was reduced by a high pollen
load of the first flower even when fruit 1 aborted before it
had accumulated much dry matter. This indicates that the
negative effect of seed number of the first fruit on fruit set
of later-developed fruits is not likely to be fully regulated by
competition for assimilates. Hence, it becomes likely that
growth inhibition of a second fruit is controlled both by
competition for limited assimilates as well as dominance due
to the production of plant growth regulators by the earlierdeveloped fruit.
Fruit production
In several crops, including sweet pepper, fruit production
shows great cyclic fluctuations during a growing season
(Hall, 1977 ; Marcelis and De Koning, 1995). The increase in
both growth of the treated fruits and inhibition of laterdeveloped fruits with increasing pollen load indicates that
cycles of fruit production may be more pronounced when
pollen loads are high, as also suggested by Stephenson et al.
(1988). From our results it can be speculated that reducing
pollination during peaks in fruit set and promoting
pollination during periods of low rates of fruit set is an
effective way of creating a regular production of fruits.
Moreover, breeding strategies to reduce seed numbers in
fruits with acceptable weight might offer a method of
producing more consistent or stable yields.
A C K N O W L E D G E M E N TS
Competition vs. dominance
Seeds are well known to be a rich source of plant growth
regulators (Hedden and Hoad, 1985). As a result of auxin
We thank J. C. M. Withagen for his statistical help and
advice, R. van der Valk for carrying out expt 3 and J. P.
F. G Helsper and B. W. Veen for critically reading the
manuscript.
Marcelis and Baan Hofman-Eijer—Seed Number and Competition among Fruits in Capsicum
LITERATURE CITED
Ali AM, Kelly WC. 1992. The effects of interfruit competition on the
size of sweet pepper (Capsicum annuum L.) fruits. Scientia
Horticulturae 52 : 69–76.
Bae$ r J, Smeets J. 1978. Effect of relative humidity on fruit set and seed
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