Biomedical Engineering Reference
In-Depth Information
every spikelet enclose two florets: the lower one is sterile and is represented by a
lemma and the upper fertile floret has a lemma and palea. Two lodicules are placed
on either side of the ovary at its base. Androecium consists of one whorl of three
stamens. The anthers are attached at the base of the ovule by a very fine filament and
are versatile and yellowish. Gynoecium is centrally placed and consists of two
pistils with one ovule from which two feathery stigmas protrude. The sessile
spikelet contains a perfect flower. It varies in shape from lanceolate to almost
rotund and ovate and is sometimes depressed in the middle. The pediceled spikelets,
usually lanceolate in shape and possess only anthers, occasionally have a rudimen-
tary ovary and empty glumes [
9
].
Anthesis and Pollination
Anthesis starts after panicle emergence from the boot leaf. Flowers begin to open
2 days after full emergence of the panicle. Floret opening or anthesis is achieved by
swelling of the lodicules and is followed by the exertion of anthers on long
filaments and of stigmas between the lemma and palea. Sorghum head begins to
flower at its tip and flowers successively downward over a 4- or 5-day period.
Flowering takes place first in the sessile spikelets from top to bottom of the
inflorescence. It takes about 6 days for completion of anthesis in the panicle with
maximum flowering at 3 or 4 days after anthesis begins. Flowering proceeds
downwards to the base in a horizontal plane on the panicle. When flowering of
the sessile spikelets is halfway down the panicle, pedicellate spikelets start to open
at the top of the panicle and proceed downwards [
22
]. Anthesis takes place during
the morning hours and frequently occurs just before or just after sunrise, but may be
delayed on cloudy damp mornings. It normally starts around midnight and proceeds
up to 10:00 AM depending on the cultivar, location, and weather. Maximum
flowering is observed between 6:00 and 8:00 AM. The anthers dehisce when they
are dry and pollen is blown into air. The pollen remains viable several hours after
shedding. The flowers remain open for 30-90 min. Dehiscence of the anthers for
pollen diffusion takes place through the apical pore. The pollen drifts to the stigma,
where it germinates; the pollen tube, with two nuclei, grows down the style, to
fertilize the egg and form a
2n
nucleus [
2
,
3
,
19
].
Cytoplasmic male sterility has been found in sorghum (A
1
-A
4
systems) and has
made possible the development of a hybrid seed industry. A good male-sterile plant
will not develop anthers, but in some instances dark-colored shriveled anthers with
no viable pollen will appear. Partially fertile heads are also observed, and although
the anthers frequently have viable pollen, the quantity is less than in normal plants.
There are two types of male sterility, viz., (a) genetic male sterility (GMS) and
(b) cytoplasmic nuclear male sterility (CMS), both widely used in sorghum
improvement programs [
4
].
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