Biology Reference
In-Depth Information
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The mature A. cherimola flower is a syncarpous gynoecium with a conic shape
composed of about 100 fused carpels surrounded at its base by several rows of
anthers with up to 200 stamens, encircled by two whorls of three petals. The
flower cycle from opening to anther dehiscence lasts two days: the flower opens
on the morning of the first day in the female stage and remains in this stage until
the afternoon of the following day when the flower enters the male stage. Anther
dehiscence occurs concomitantly in all stamens of a flower and, as the anthers
dehisce, they detach from the flower and fall over the open petals.
Flower buds of A. cherimola develop in the leaf axes following leaf expansion;
the basal nodes are differentiated in the year preceding anthesis. The uppermost
distal buds differentiate in synchrony with shoot growth [25]. Flower bud growth
begins 39 days prior to anthesis. Anther differentiation proceeded centripetally,
with the most developmentally advanced anthers placed in the outermost rows
and the different stages of anther and pollen development present within the same
flower. This fact was helpful for establishing successive stages of anther develop-
ment. The anther becomes septate with pollen mother cells positioned between
rows of interstitial tapetum similar to the anthers described in a sister species,
Annona squamosa [26].
To determine if pollen development followed a standard pattern and whether
pollen tetrads at anther dehiscence corresponds with the cytological and morpho-
logical features of mature pollen, anther and pollen development were examined
from microsporogenesis to maturity. Special attention was given to the events
responsible for pollen cohesion. Microgametogenesis and tapetum degeneration
were also examined sequentially.
Microsporogenesis
Initial hypodermal archesporial cells were apparent 24 days before anthesis (Fig-
ure 1A). From them, anther septa initials and pollen mother cells (PMC) devel-
oped in 9 cm long flower buds 19 days before anthesis (Figure 1B). Each anther
contained a uniseriate row of pollen mother cells with a conspicuous common
wall. The PMC began to accumulate starch grains (Figure 1C) and increased in
size (Figure 1C, 1D). Starch grains vanished concomitantly with the beginning of
meiosis, some 15 days before anthesis, as a translucent cell wall layer was apparent
surrounding the PMC (Figure 1D). Meiosis proceeded rapidly and was followed
by a new accumulation of starch grains in the young microspores (Figure 1E) 14
days before anthesis. The microspore tetrads remained together in isolated tapetal
