Agriculture Reference
In-Depth Information
Pollination and Fruit Set
The cycle of the cherimoya flower is completed in two days. In the first day of the
cycle the flower opens in the female stage about midday under the environmental
conditions of southern Spain. This phase lasts for about 30 h with the flower switch-
ing to the male stage on the second day of the flower cycle, followed by anther
dehiscence around 17:00-18:00 h, depending on environmental conditions, mainly
temperature (Lora et al.
2009b
). After anther dehiscence, the receptive stigma rap-
idly dries up losing receptivity. The length of the stigmatic receptive phase varies
depending on environmental conditions with high humidity and low temperatures
extending stigmatic receptivity whereas stigmatic receptivity is shortened with low
humidity and high temperatures. Thus, under some conditions (mainly low tem-
peratures and high humidity), the stigmas can be receptive even when the flowers
have already switched to the male state (Lora et al.
2011a
; George et al.
1989
).
Recent studies on the final stages of pollen development in cherimoya have
shown a rapid and active pollen development before pollen release, resulting in the
production of both bi and tricellular pollen at anther dehiscence, a very uncommon
fact in flowering plants. The ratio between these two different types of pollen is
greatly influenced by temperature (Lora et al.
2009a
) and the pollen is released in
a partially hydrated stage (Lora et al.
2012
). Individual pollen grains are released
from the tetrads after contact to the stigmas or to an artificial germination medium.
Similarly to other tricellular pollen (Brewbaker
1967
), cherimoya pollen is difficult
to germinate in vitro (Rosell et al.
1999
). This is due to a short viability (Rosell et al.
2006
) and high sensitivity to desiccation and, consequently, pollen storage is diffi-
cult (Lora et al.
2006
). Recent work (Lora et al.
2012
) has shown a significant influ-
ence of environmental conditions (mainly temperature and humidity) on different
pollen characteristics during the final stages of pollen development modifying pol-
len behaviour and explaining the variability observed in pollen tube performance.
Cherimoya shows a simple primitive pistil with a short style and a semi-open con-
tinuous secretory carpel that provides substrate for pollen tube growth (Lora et al.
2010
). Pollen tube growth is fast and the pollen tubes arrive to the ovule 1 day after
pollination and only a single tube is able to penetrate the ovule and achieve fertiliza-
tion (Lora et al.
2010
).
Nitidulid beetles that breed and feed in decaying fruits or sap flows are the main
pollinators of most
Annona
species including cherimoya (Peña et al.
2002
). Work in
Spain has shown that hemiptera insects of the genus
Orius
could also be effective
as pollinating vectors in cherimoya; in fact, the presence of high densities of
Orius
in flowers and leaves of maize have led to the planting maize plants among cheri-
moya trees to increase natural pollination (Guirado et al.
2001
). However, natural
pollination often results in the production of asymmetric fruits with low commer-
cial value. In fact, inadequate pollination is one of the most important factors that
limit commercial production of
Annona
species in most locations. Since the crop
was propagated from its areas of origin without its natural pollinator agent, hand
pollination with pollen and stamens together is a common practice for commercial
production in all countries where the crop has been introduced (Schroeder
1971
).
Moreover, hand-pollinated flowers result in fruits that show better symmetry
