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Production of fruits with attached flowers in zucchini squash is
correlated with the arrest of maturation of female flowers
By A. PEÑARANDA1,4,M.C.PAYAN
1,4, D. GARRIDO2, P. GÓMEZ1and M. JAMILENA3*
1Departamento de Biotecnología. IFAPA de Almería. Autovía del mediterráneo, La Mojonera,
Almería, Spain
2Departamento de Fisiología Vegetal, Facultad de Ciencias, Universidad de Granada, 18071 Granada,
Spain
3Departamento de Biología Aplicada. Escuela Politécnica Superior, Universidad de Almería, 04120
Almería, Spain
(e-mail address: mjamille@ual.es) (Accepted 4 March 2007)
SUMMARY
Results from two trials carried out under different environmental conditions indicated that the high temperatures
reached in greenhouses during the Spring-Summer growing season are the main environmental factor inducing
production of zucchini fruits with attached flowers. The fact that the incidence of this characteristic is genotype-
dependent, with the percentage of fruits with attached flowers varying from 1.4 – 73% among the different cultivars
grown during the Spring-Summer season, provides an opportunity for direct counter-selection for this trait in current
zucchini squash breeding programmes. High temperatures in the Spring-Summer growing season also induced male-
ness in all the cultivars analysed, delaying the production of female flowers, and increasing the number of male flowers
on the main stem. Indeed, those flowers that remained attached to harvested zucchini fruits were transformed into
bisexual flowers, exhibiting different degrees of stamen development, and were arrested as immature, closed floral
buds. A detailed analysis of the maturation and abscission times in female and male flowers of different zucchini
cultivars indicated that, although abscission time did not differ, maturation time was longer in male than in female
flowers. By comparing sexual expression in different cultivars in different environments, we concluded that inhibition
of female flower bud maturation occurs concomitantly with a delay in flower abscission, a process accompanied by the
conversion of the female bud into a bisexual bud. Given that Spring-Summer conditions promote the maturation of
both female and male flowers,and that the arrest of female flower maturation and abscission are associated with floral
sex determination, it is possible that the arrest of female flower maturation is not directly induced by high
temperatures, but is mediated instead by hormones such as ethylene or gibberellins.
The production of zucchini squash in greenhouses,
especially under environmental conditions that
favour a high growth rate, requires cultivars in which
abscission of the flower occurs during the first stages of
fruit development, and always before harvesting.In fact,
delay in the abscission of zucchini flowers is now a major
problem for greenhouse cultivation of zucchini squash in
southeast Spain. When the large flowers of zucchini
remain attached to fruits after harvesting, they must be
removed manually, thereby wounding the fruit and
making it more susceptible to infection and rotting
during storage and transport, thus diminishing its shelf-
life, commercial quality, and value.
It is well-documented that abscission of floral organs is
a programmed developmental process that occurs after
pollination and is mainly under the control of the
hormone ethylene in a number of species (Brown, 1997;
Van Doorn and Stead, 1997). Treatments with ethylene,
or ethylene inhibitors can accelerate or delay the
abscission of floral organs, respectively (González-
Carranza et al., 1998). In addition, ethylene-insensitive
mutants, such as etr1 and ein1 in Arabidopsis (Bleecker
et al., 1988;Grbic and Bleecker, 1995),and the Never ripe
(nr) mutant of tomato (Lanahan et al., 1994; Yen et al.,
1995) are characterised by a delay in flower abscission.
In the family Cucurbitaceae, sex expression is
regulated not only by environmental conditions such as
temperature and photoperiod, but also by different
hormones, including ethylene, auxins and gibberellins
(Rudich 1990; Yin and Quin, 1995; Wien, 1997). The
application of ethylene or ethylene-releasing agents
increases the production of female flowers, while
treatment with inhibitors of ethylene biosynthesis, or
ethylene perception, induce the production of a higher
number of male flowers (Nitsch et al., 1952; Robinson et
al., 1969; Rudich et al., 1969; Owens et al., 1980; Hume
and Lovell, 1983a,b). Accordingly, the levels of
endogenous ethylene in gynoecious cultivars of
cucumber were higher than in monoecious and
andromonoecious genotypes (Trebitsh et al., 1987;
Yamasaki et al., 2001). Auxins also promote the
production of female flowers, although this effect was
indirect and appeared to be mediated by ethylene
*Author for correspondence.
4These authors contributed equally to this work.
Journal of Horticultural Science & Biotechnology (2007) 82 (4) 579–584
Fruits with attached flowers in zucchini squash
(Trebitsh et al., 1987; Rudich, 1990). In contrast, the
application of gibberellins (GA) promotes the
production of male flowers, while treatment with
gibberellin-inhibitors induces female-ness (Rudich et al.,
1972; Wien, 1997).
Given the major influence of flower abscission on
post-harvest conservation and the commercialisation of
zucchini fruits produced under greenhouse conditions,
we analysed the genetic and environmental factors that
may be involved in the induction of fruits with attached
flowers. To this end, we evaluated the maturation and
abscission times of female and male flowers in a number
of zucchini cultivars grown under different greenhouse
conditions. By comparing sexual expression in different
cultivars, in different environments, we conclude that
inhibition of female flower bud maturation occurs
concomitantly with a delay in flower abscission, a process
accompanied by the conversion of the female bud into a
bisexual bud.
MATERIALS AND METHODS
Plant materials and growing conditions
Plant materials were derived from commercial hybrids
provided by different seed companies. A total of nine
commercial hybrid cultivars were used: ‘Elite’ and
‘Storr’s Green’ (Seminis), ‘Baccara’, ‘Cora’, and ‘Tosca’
(Clause-Tezier), ‘Balboa’ (Ramiro Arnedo), ‘Cavili’
(Nunhems), and Mastil and ‘Xsara’ (Syngenta). Seeds
were germinated on wet filter paper in 9 cm Petri dishes
at room temperature for 2 – 3 d in the dark, after which,
seedlings were transplanted into rockwool cubes
(Grodan BV, Roermond, The Netherlands) in a
greenhouse. When transplants developed three-to-four
leaves, they were transferred to 1 m rockwool slabs at a
density of two plants per slab. Plants were grown in a
greenhouse in La Mojonera, Almería, Spain, following
standard local commercial practices for plant nutrition
and for insect pest and disease control.
Plants were grown during Winter 2003 and early
Spring 2004 (Trial 1; average temperature 14.7ºC; 12 d
with maximum temperatures over 30ºC); or during
Spring-Summer 2004 (Trial 2; average temperature of
20.8ºC; 64 d with maximum temperatures over 30ºC).
Both Trials were carried out in the absence of pollinating
insects. In each Trial, the field plan was a randomised
complete block design, with three blocks and six
replicates per block, for each cultivar. Marketable fruits
of approx. 20 cm in length were harvested two-to-four-
times per week, depending on the growing season.
Evaluation of flower abscission and sex expression
Flower abscission in each cultivar, under each
environmental condition, was evaluated by determining
the percentage of fruits with an attached flower.
Harvested fruits of a marketable length and quality, but
which retained their flowers, were scored as fruits with
attached flowers. Sex expression in each plant was
determined as the number of initial nodes with male
flowers before the production of the first female flower,
and as the percentage of female flowers per plant in the
first 30 nodes of the main stem (“female-ness”).
The maturation and abscission times in male and
female flowers of each cultivar were estimated as the
number of days that a flower bud 2 cm in length needed
to reach anthesis, and as the number of days from flower
anthesis to abscission, respectively. For this, we recorded
the date on which floral buds were approx. 2 cm, then
observed them daily until they reached anthesis and
abscission. In squash flowers, anthesis normally occurs
when the ovary is 5 – 7 cm in length. Therefore, non-
abscising flowers that remained green and closed in
marketable fruits approx. 20 cm length were defined as
arrested in their development and maturation. To
determine the sex phenotype of these flowers, they were
examined after the fruits had been harvested.
Statistical analysis
For each variable analysed, statistical differences
among cultivars in each Trial (Winter-Spring or Spring-
Summer), and between plants of the same cultivars
grown in the different seasons, were determined by
analysis of variance (ANOVA), followed by a Tukey’s
multiple comparison test by using the STATISTIX 8.0
software package (Statistix, Tallahassee, FL, USA).
RESULTS
Floral organ abscission in zucchini
To analyse the effects of genotype and environmental
conditions on the incidence of zucchini fruits with an
attached flower, we carried out varietal Trials with nine
hybrid cultivars of zucchini squash grown in a
greenhouse in two different seasons. Figure 1
summarises the data obtained in the two greenhouse
Trials carried out during the Winter-Spring 2003 – 2004
and Spring-Summer of 2004, respectively. The
percentage of marketable fruits with attached flowers
differed significantly among cultivars and between
growing seasons. During the Winter, the floral organs
abscised normally from the fruit, and the percentage of
marketable fruits with an attached flower was low.
Nevertheless, during the Spring-Summer, flower
abscission was delayed significantly, increasing the
number of fruits with an attached flower in five out of
the nine cultivars analysed (Figure 1).With the exception
580
Percentage of fruits with attached flowers
100
80
60
40
20
0
Winter-Spring
Spring-Summer
aaa
a
b
‘Storr’s Green’
‘Cora’
Zucchini cultivar name
‘Tosca’
‘Balboa’
‘Elite’
‘Baccara’
‘Cavill’
‘Xsara’
‘Mastil’
b
c
cc
FIG.1
The percentages of fruits with an attached flower in nine different
cultivars of zucchini squash grown under Winter-Spring or Spring-
Summer conditions. Under Spring-Summer conditions, different lower-
case letters on the bars indicate statistical differences among cultivars by
Tukey’s test at P≤0.05. No statistical differences were detected among
cultivars grown under Winter-Spring conditions.
A. PEÑARANDA,M.C.PAYAN,D.GARRIDO,P.GÓMEZ and M. JAMILENA
of ‘Storr’s Green’,‘Cora’ and ‘Balboa’, the percentage of
fruits with an attached flower between the Winter-Spring
and the Spring-Summer conditions for each cultivar
were statistically different. Three groups of cultivars
could be distinguished under Spring-Summer conditions.
‘Storr’s Green’, ‘Cora’, ‘Tosca’, and ‘Balboa’ in the first
group, and ‘Elite’ and ‘Baccara’ in the second, represent
cultivars that showed low or intermediate percentages of
fruits with attached flowers, respectively (Figure 1). The
highest delay in floral organ abscission was observed in
the third group represented by the cultivars ‘Cavili’,
‘Mastil’, and ‘Xsara’ (Figure 1).
Flower development and sex expression in fruits with
attached flowers
Flowers that remained attached to harvested fruits
were still green and completely closed once the fruit had
acquired marketable weight (Figure 2A,B), suggesting
that the delay in flower abscission was caused by an
inhibition of flower bud maturation, rather than by an
inhibition of senescence and abscission processes after
pollination. Even in fruits with advanced development,
these flowers remained as green immature buds, and
most of them never reached anthesis (Figure 2A,B).
All flowers that remained attached to fruits were
bisexual, and exhibited different degrees of abnormal
stamen development. Some were even able to produce
mature pollen (Figure 2C,D).We also determined sexual
expression in zucchini cultivars grown under two
different environmental conditions. Two developmental
phases could be distinguished in a typical zucchini plant
(Figure 3): an initial phase in which the plant produced
only male flowers in the axil of each leaf, followed by a
second phase in which the production of female flowers
alternated with male flowers. To determine sex
expression, we measured the number of male flowers in
the initial phase of development of the plant, as well as
the number of male and female flowers per plant in a
constant number of nodes.The data indicated that Spring-
Summer conditions promoted “male-ness” in all the
cultivars analysed, as the mean number of female flowers
per plant decreased (Table I). In certain cultivars such as
‘Elite’, ‘Cora’, ‘Tosca’, and ‘Mastil’, the number of male
nodes in the initial segment of the plant also increased
significantly under Spring-Summer conditions.The higher
incidence of fruits with attached flowers observed in
zucchini cultivars grown under Spring-Summer
conditions (Figure 1) is therefore related not only to the
conversion of female into bisexual flowers, but also with
an increase in the number of male flowers per plant.
Maturation and abscission times of male and female
flowers
To analyse whether the arrest in flower bud
development observed in female flowers with delayed
abscission could be associated with their conversion into
bisexual flower buds, we compared maturation and
abscission times in female and male flowers of zucchini
squash. Maturation and abscission times were measured
in female and male flowers from different cultivars of
zucchini grown under two different environmental
conditions. Bisexual flowers with delayed abscission
581
FIG.2
Fruits of zucchini with attached flowers. Panel A, zucchini squash plant
showing fruits at different stages of development. Note that the
development and maturation of the flower attached to fruit (1) is
arrested and, although older than the flowers on (2),(3) and (4), it is still
green and closed. Such green flowers do not abscise, and remain
attached to the harvested fruits. Panel B, development of zucchini fruits
with attached flowers. Flower development and maturation is arrested
in such a way that even fruits of marketable size (the two fruits on the
left) retain flowers that are still green and closed. Panels C–D, sex
phenotypes of flowers that remained attached to the fruits.All attached
flowers were bisexual, although the degree of development of the
stamens differed between flowers. In some, anther development is
arrested at an early stage (Panel C); while in others, anthers developed
normally (Panel D).
Female
Male
Mixed phase
Male phase
FIG.3
Diagram of a monoecious zucchini squash plant showing the disposition
of male and female flowers on the main stem.
Fruits with attached flowers in zucchini squash
were not used as they did not normally reach maturation
and therefore remained green and closed in harvested
fruit. The maturation time of male flowers was similar
under both environmental conditions (Figure 4).
However, Spring-Summer conditions significantly
accelerated the time of anthesis of female flowers in all
the cultivars analysed, except ‘Cavili’ (Figure 4). This
suggests that the delay in maturation and abscission
observed in female flowers of these cultivars was not
caused directly by the Spring-Summer environmental
conditions. No significant differences were detected
among cultivars in maturation times or abscission times
(Figure 4). Nevertheless, the data demonstrated that the
maturation of male flowers was much slower than that of
female flowers, taking twice or even more-than-three-
times as long (Figure 4). Once opened, the time that a
male flower took to reach abscission was similar to that
of a female flower (Figure 4).
DISCUSSION
Delayed flower abscission in zucchini squash causes a
considerable reduction in greenhouse production and
quality in southeast Spain. Harvested fruits with non-
abscised flowers have a reduced shelf-life, thereby
reducing their quality and commercial value.
Greenhouse trials have demonstrated that, compared
with Winter-Spring conditions, Spring-Summer growing
conditions promote a substantial increase in the
percentage of harvested zucchini fruits with attached
flowers.Considerable genetic variability for this trait was
observed among the cultivars analysed, especially when
grown during Spring-Summer. Indeed, while the
percentage of fruits with attached flowers under Winter
conditions varied between 0 – 12% in ‘Xsara’ and
‘Cavili’, respectively; the equivalent variation under
Spring-Summer conditions was between 0 – 73% in
‘Balboa’ and ‘Xsara’,respectively.Therefore, selection of
zucchini genotypes with reduced numbers of fruits with
attached flowers requires breeding and selection
programmes to be carried out under the extreme
Summer conditions of this region. Certain cultivars,
including ‘Xsara’, ‘Cavili’ and ‘Mastil’, were more
susceptible to Spring-Summer environmental conditions,
showing the highest percentage of fruits with non-
abscised flowers, while others such as ‘Balboa’, ‘Cora’
and ‘Storr’s Green’ were less susceptible. Genotypic
differences in susceptibility to adverse environmental
conditions such as high temperatures have been found
in other Cucurbita species, including C. pepo (NeSmith
et al., 1994;Wien et al., 2004).
Although it is difficult to determine the
environmental factor(s) that influence the development
of zucchini flowers and fruits under Spring-Summer
conditions, high temperatures are most likely to affect
the production of fruits with non-abscised flowers. The
fact that zucchini fruit with delayed flower abscission
appeared not only during the Summer season, but also
during the Winter-Spring season, especially after sunny
days on which the maximum temperature in the
greenhouse reached over 30ºC, may exclude other
possible factors such as irradiance level and daylength.
High temperature also appeared to be the main
environmental factor that delayed the formation of
female flowers and fruits in pumpkin during the
Summer growing season (Wien et al., 2004).
The delay in female flower abscission observed in
some zucchini squash cultivars under high temperatures
582
Flower maturation time (d)
Flower abscission time (d)
Winter-Spring Spring-Summer
FIG.4
Maturation and abscission times of female () and male () flowers of
four cultivars of zucchini squash grown under Winter-Spring or Spring-
Summer greenhouse conditions. Maturation (Panel A) and abscission
(Panel B) times were estimated as the number of d that a floral bud,
2 cm in length, took to reach anthesis, and the number of d from anthesis
to abscission, respectively.
Winter-Spring Spr ing-Summer
‘Cora’ ‘Cavili’ ‘Xsara’ ‘Elite’
‘Cora’ ‘Cavili’ ‘Xsara’ ‘Elite’
A
B
35
30
25
20
15
10
5
0
7
6
5
4
3
2
1
0
TABLE I
Sex expression in different cultivars of zucchini squash grown under different environmental conditions
Number of initial male flowers per plant Number of female flowers per plant
Cultivar Winter-Spring Spring-Summer PWinter-Spring Spring-Summer P
‘Elite’ 6.0 ± 0.9 b†7.7 ± 0.8 b ** 19.5 ± 2.4 a 11.67 ± 1.5 a **
‘Storr’s Green’ 7.0 ± 1.5 a 6.8 ± 0.4 b ns 16.5 ± 2.1 c 11.33 ± 1.5 b **
‘Baccara’ 8.0 ± 1.2 a 8.8 ± 1.3 a ns 16.3 ± 2.1 c 10.80 ± 0.8 b **
‘Cora’ 5.7 ± 0.8 c 7.2 ± 1.2 b * 20.5 ± 0.8 a 11.00 ± 1.1 b **
‘Tosca’ 5.7 ± 1.2 c 8.0 ± 1.9 a * 21.0 ± 1.3 a 11.00 ± 1.2 b **
‘Balboa’ 5.8 ± 1.8 c 6.0 ± 1.2 c ns 18.7 ± 0.8 b 14.80 ± 0.8 a **
‘Cavili’ 6.2 ± 1.2 b 7.2 ± 0.8 b ns 22.3 ± 1.2 a 13.60 ± 3.1 a **
‘Mastil’ 8.7 ± 0.5 a 10.3 ± 1.2 a * 14.8 ± 1.6 d 10.17 ± 1.9 c **
‘Xsara’ 7.3 ± 0.8 a 7.8 ± 1.5 b ns 19.0 ± 2.0 a 13.17 ± 1.6 a **
The number of male and female flowers was scored in the first 30 nodes of the main stem of each plant.
†Values are means (± SD), and those followed by a different lower-case letter in each column are statistically different by Tukey’s test at P≤0.05.
For each cultivar, the Pcolumn indicates non-significant (ns) or significant differences at P≤0.05 (*) or at P≤0.01 (**) between the Winter-Spring
and Spring-Summer conditions.
A. PEÑARANDA,M.C.PAYAN,D.GARRIDO,P.GÓMEZ and M. JAMILENA
is caused by the arrest of female flower maturation. The
fact that many of the flowers that remained attached to
harvested fruits did not reach anthesis, but arrested their
development as immature closed green buds, suggests
that the maturation of these flowers was inhibited. This
developmental arrest does not appear to be caused
directly by environmental conditions such as high
temperatures, since we determined that Spring-Summer
growing season conditions accelerated the maturation of
female flowers in zucchini cultivars.Given that the arrest
of maturation of female flowers was correlated with an
increase in the “male-ness” of the plant, and that the
flowers that remained attached to harvested fruits were
bisexual, the observed delay in female flower abscission
during the Spring-Summer growing season is more likely
to be caused by partial conversion into a bisexual flower.
Our data indicating that the maturation time of male
flowers is higher than that of female flowers also support
this hypothesis.
Several studies have shown that sexual expression in
squash flowers is regulated by environmental conditions
such as temperature and/or photoperiod (Rylski and
Aloni, 1990; Stapleton et al., 2000; Wien et al., 2004), as
well as by hormones such as ethylene, auxins and
giberellins (Nitch et al., 1952; Rudich et al., 1970; Rudich,
1972; 1990; Robinson et al., 1970; Shannon and Robinson,
1979; Hume and Lovell, 1983a,b; Trebitsh et al., 1987).
High temperature conditions, as well as treatments with
gibberellins or ethylene inhibitors, favour the production
of male flowers in a number of cucurbit species (Rudich
et al., 1972; Hume and Lovell; 1983a,b; Wien, 1997). It is
therefore likely that high temperatures change the sex
determination of female flower buds into bisexual flower-
buds and consequently alter their maturation and
abscission times, by increasing the level of gibberellins
and/or by reducing the production of ethylene. It is
documented that giberellins inhibit the opening of female
flowers in Luffa acutangula (Krishnamoorthy, 1972). On
the other hand, ethylene has recently been reported to be
involved in the development and maturation of flowers in
Arabidopsis (Hall and Bleecker, 2003) and melon
(Papadopoulou et al., 2005).
This work was supported by Grants C03-180 and
AGL2005-06677-CO2, awarded by the Consejería de
Agricultura y Pesca de la Junta de Andalucía, Andalucía,
Spain, and by the Ministerio de Educación y Ciencia,
Spain, respectively. We are indebted to the seed
companies who kindly provided seeds of the hybrid
cultivars analysed in this paper.
583
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