104 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
Few studies deal with seed banks, germination and establishment of seedlings;
nevertheless, in general terms, germination is high and rapid, and there is no seed bank or
it is limited for most species. Cucurbita such as C. argyrosperma ssp. sororia can be
opportunists, adapted to rapidly colonise available open spaces at the beginning of the
rainy season. The colonisation strategy seems to be based more on the germination speed
of the seeds produced during the last season than on the development of a seed bank in
the soil. Under experimental conditions, 86-100% of the seeds of C. argyrosperma
ssp. sororia germinate two to six days after they begin to hydrate. This germination is
synchronic and begins with the first main rains that can keep the soil damp for more than
one day (Mariano, 2001).
Sexual reproduction
Pollination
Among the Cucurbita, some agent – usually bees – is necessary to transfer pollen
from the male to female flower; as the pollen is large (80 to 150 μm diameter) and sticky,
the species of the genera are not wind pollinated. In the Americas, the solitary bees of the
genera Peponapis and Xenoglossa (Hurd, Linsley and Whitaker, 1971; Canto-Aguilar and
Parra-Tabla, 2000) have developed a close relationship with wild and cultivated
Cucurbita plants – both adults and larvae feed almost exclusively on the nectar and pollen
of the plants. Indeed, the bees are dependent on pollen and nectar produced by Cucurbita
flowers for their survival (Hurd, Linsley and Whitaker, 1971), and appear to have co-
evolved with the Cucurbita (Hurd, Linsley and Whitaker, 1971). These bees display some
behaviours that appear to be adaptations to their interaction with the Cucurbita,
e.g. an ability to fly at low temperatures, with low light intensity and certain
modifications that allow for an adequate extraction and transportation of pollen.
These bees often fly from flower to flower while still dark to see which flowers are open,
apparently oriented by olfactory cues emitted by some of the species, and probably also
with the help of visual and/or hearing sensors. Both the Peponapis and Xenoglossa are
very efficient pollinators of the Cucurbita (Hurd, Linsley and Whitaker, 1971).
The efficiency and specificity of these bees makes them responsible for moving larger
amounts of pollen between wild and cultivated Cucurbita than any other group of
pollinators.
Pollinators collect large amounts of nectar from the female flowers, and pollen and
nectar from the male flowers. Nectar is secreted from a ring of tissue surrounding the
style and just inside the perianth tube. When a bee forages in a masculine flower in search
of nectar, the pollen adheres to the bee’s body and will then be transferred to the stigmas
when it visits female flowers (Zomlefer, 1994). The most active period for the bees
coincides with the beginning of the plant’s floral opening, just before daybreak, and this
high level of activity is maintained for several hours (Mariano and Dirzo, 2002).
The hind legs of the bees of the genera Peponapis and Xenoglossa are adapted to the
collection and manipulation of the pollen grains of this genus. However, the pollen grains
vary in size and structure between the species and the pollen-collecting devices of the
bees vary also (Hurd, Linsley and Whitaker, 1971). This variation between the bees has
been shown to be species specific (Hurd and Lindsley, 1970), and apparently has
profoundly influenced the ability of the different species of bees to collect and utilise
pollens of the various Cucurbita, both wild and domestic (Hurd, Linsley and Whitaker,
1971; see Table 2.5).
