Agriculture Reference
In-Depth Information
fi rst fl owers during spring and have a second fl owering during the summer, the
latter being the more productive (Sanewski, 1991).
Pollination and fruit set
Natural pollination
The fl owers exhibit both dichogamy and a protogynous nature. This poses
a serious problem in obtaining high yields. The fl owering seasons of A.
squamosa and A. cherimola coincide. When sweetsop pollen is shed at about 2
a.m., cherimoya fl owers are receptive, opening around 7-9 a.m. and, when
cherimoya pollen is shed at 3-6 p.m., sweetsop fl owers are receptive. This
fl ower synchrony, together with complementary functional sexes, favours
cross-pollination, leading to natural hybridization. This is attested to by the
frequent appearance of hybrid seedlings under the trees of sweetsop and
cherimoya when grown in close proximity. Sweetsop is a more ef ective pollen
source than cherimoya in cross-combination studies.
The atemoya female parts are receptive between 4 p.m. and 8 a.m.
and appear moist and sticky (Thakur and Singh, 1964). Atemoya pollen
is discharged in the afternoon of the same day from 3 to 6 p.m., if the RH is
above 80% and the temperature >22°C. At lower temperatures, pollen is
released on the afternoon of the second day. The pollen sacs turn a greyish
colour as pollen is discharged. Pollen grains from fl owers that appear early in
a fl owering season have thick walls, are high in starch, germinate poorly and
give poor fruit set. Pollen of later fl owers shows a high proportion of individual
pollen grains without starch grains, which germinate well. Upon opening,
fl owers are receptive for about 24 h. It has also been shown that spraying the
fl owers with water or putting a drop into the fl ower can signifi cantly increase
fruit set, probably because of the increased humidity slowing the drying of the
stigma and pollen grains. Night-time irrigation with micro-sprinklers or late
afternoon rains also increases fruit set.
Nitidulid beetles ( Carpophilus and Uroporus spp.) are the important
pollinators of Annona fl owers, with wind- and self-pollination being low
(1.5%). Fruit set of 'African Pride' atemoya increases linearly with increasing
numbers of nitidulid beetles per fl ower. Three or more beetles per fl ower
increased fruit set to nearly 25%. Studies also show that these beetles breed
rapidly in rotting fruit media and that their populations can be increased by
maintaining a rotting-fruit attractant.
Hand-pollination is frequently practised to ensure pollination and good
fruit shape. Pollen must be collected in the evening from fully open fl owers,
when the sacs have turned from white to cream. The fl owers are held in a
paper bag, not a closed container, and should discharge that afternoon. The
fl owers are shaken over a shallow tray or paper to collect the pollen, which is
transferred to a small container and held in the refrigerator for use the next
 
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