II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES) – 101
(e.g. the receptacle is always much more reduced). After pollination, fruit develops from
the pre-formed ovary at the base of the female flower. The shape of the ovary prior to
pollination is indicative of the mature fruit shape.
Cross-pollination is favoured by the monoecious nature of the plants, and in some
cases the male flowers are slightly larger than the female ones on the same plant.
Cucurbita can exhibit wide variation in the proportion of male to female flowers on a
plant (Janick and Paull, 2007). Zomlefer (1994) reported that production of female
flowers is frequently less than that of male flowers. In C. pepo, Nepi and Pacini (1993)
found a 16.5:1 relation between the number of male and female flowers. Temperature and
light influence the production of male and female flowers in several of the species
(Whitaker and Davis, 1962). More male flowers are produced on long and very hot days,
whereas short and cold days induce the development of more female flowers (Robinson
and Decker-Walters, 1997). The first flowers on the vine are male, after which three or
four female flowers appear. Although female flowers differentiate later in plant
development, females develop faster than the males, resulting in near synchronization at
anthesis of the flowers of both sexes (Janick and Paull, 2007). Flowers open early in the
morning and close around noon of the same day, never to reopen (Nepi and Pacini, 1993).
Flowering time both in male and female flowers of C. pepo varies depending on the time
of year in which the plants develop: male flowers begin to open 15 minutes before the
female flowers when the days are longer. On days further along in the year this difference
is not significant (Nepi and Pacini, 1993). Scheerens et al. (1987) found similarities on
the daily opening time of flowers of C. foetidissima of both sexes. Whitaker and
Robinson (1986) observed that in some genotypes, a short photoperiod is needed for
flowering to begin, and flowers develop only when the days are short.
Flower development in Cucurbitaceae, apart from being regulated by genetic and
environmental mechanisms such as temperature and the duration of days, can be
modulated by chemical regulators – substances such as gibberellins and ethylene (Rudich,
1990). For example, ethylene is involved in the regulation of fruit ripening and sex
expression and in the plant’s response to herbivore damage.
Pollen
Cucurbita pollen grains are large and sticky, and well suited to transport by insects.
Wind does not pollinate Cucurbita species. Ovules are fertile only during the period of
flowering, or the day prior. Good fruit and set development requires 500-1 000 live pollen
grains on the stigma of the female flower (Stephenson, Devlin and Horton, 1988;
Vidal et al., 2010). Pollen viability in a newly opened male flower is about 92%, but by
the time it closes that same morning the viability will have dropped to 75%, and by the
next day will be only 10% (Nepi and Pacini, 1993). Environmental conditions at the time
of anthesis are important. High or low temperature can result in a more rapid decrease in
pollen viability. In addition, in windy, dry conditions, pollen can lose viability rapidly.
Fruit
Fruits of Cucurbita are of the pepo type: a berry with numerous seeds surrounded by
a fleshy wall that does not open at maturity. Fruits have a thin and soft, or rigid and
woody, shell that emerges from the outer layer of the ovary (exocarp), whereas the pulp
around the seed is derived from the ovarian internal layers (mesocarp and endocarp).
In cultivated plants, the fruits are produced in a great variety of shapes, sizes, colours and
types of surface, whereas in wild plants they are all relatively small and relatively