Agriculture Reference
In-Depth Information
Although the avocado flower is bisexual with functional male and female or-
gans, outcrossing is promoted through a synchronous protogynous dichogamous
breeding system also present in other early-divergent angiosperm species, such as
cherimoya. In the case of avocado, each flower opens twice, the first as a female
flower with a white receptive stigma; the flowers close overnight and the follow-
ing day the flowers reopen functionally as male flowers, the anthers dehisce and
the stigmas loose receptivity (Davenport
1986
). Based on their flowering cycle, the
different avocado cultivars are classified in two groups (A or B) (Nirody
1922
). In
type A cultivars, the flowers open in the female stage in the morning, close at mid-
day and reopen the afternoon of the following day at the male stage. In the type B
cultivars, the flowers open in the afternoon at the female stage, close in the evening
and reopen the following morning in the male stage (Stout
1923
). However, this flo-
ral behavior is only observed under optimum climatic conditions because the cycle
can vary depending on environmental conditions, particularly cold temperatures
being especially the case for B types (Sedgley
1977
; Sedgley and Annells
1981
;
Sedgley and Grant
1983
; Alcaraz and Hormaza
2009a
).
Pollination and Fruit Set
The avocado is mainly pollinated by insects. Different species (mainly Hymenop-
tera) act as natural pollinating vectors in Central America but in most avocado pro-
ducing countries the European honeybee (
Apis mellifera
) is the main pollinator of
avocado (Ish-Am and Eisikowitch
1993
; Gazit and Degani
2002
) even if avocado
speciation took place before honey bees were present on earth. As a consequence,
the avocado flower is not well-adapted to honey bee pollination and honey bees
prefer flowers of other species that coevolved with them (Ish-Am and Eisikowitch
1998
). The success of a good pollination in avocado requires the presence of a suf-
ficient amount of beehives just before flowering.
Three different pollination systems can be observed in avocado (Gazit and
Degani
2002
): (1) autogamy, when the pollination takes place in the same flower;
(2) geitonogamy or close pollination, when the pollen of one flower is deposited
on the stigma of another flower of the same tree or of a different tree of the same
cultivar; or (3) allogamy, when the pollen belongs to a different cultivar. Although
several works have reported that stigmas are no longer receptive in flowers in the
male stage, other results suggest that pollination and fertilization can occur in this
stage allowing autogamy (Davenport
1986
). The differences are probably due to
the temperature and humidity conditions at flowering with an extended receptivity
under lower temperatures and higher humidity situations that can prevent the stigma
from desiccation during the male stage allowing pollen germination (Alcaraz and
Hormaza
2009b
). Geitonogamy is possible because there is a variable time of over-
lap between closing female flowers and opening male flowers that can allow trans-
fer of pollen between flowers of the same cultivar in different developmental stages
(Alcaraz and Hormaza
2009a
).
