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X. Floral Biology and Hybridization

X. Floral Biology and Hybridization

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FIG. 11. A spike of pearl millet showing protogyny. Note stigmas have emerged all over the

earhead and anthers are emerging toward the tip.



FIG. 12. A scanning electron micrograph of a pearl millet spikelet with two florets; the larger one

with protruding feathery stigma is hermaphrodite and the smaller one is staminate. [ ~ 2 4 1





An important feature of Pennisetum species is their protogynous (profos =

before or ahead) nature, which means that the carpels emerge and mature before

the stamens. Consequently, anthesis starts only after most or all stigmas have

emerged. Protogyny is particularly conspicuous in pearl millet (Fig. 11). It is, of

course, a welcome feature from the evolutionary and breeding standpoints. It

facilitates the introgression of characters from wild or semiwild, annual penicillarias into pearl millet and, thus, has helped in the genetic enrichment of this


The emergence of stigmas generally starts near the tip of the partially exserted

spikes and proceeds downward. Sometimes the spikes still inside the boot leaf

have fully exserted stigmas.

Pearl millet, like most other species of Pennisktum, has bifid, feathery stigmas (Fig. 13a). The two stigmatic branches provide enough surface for effective

pollinations. The fully emerged stigmatic branches of pearl millet are glistening

white with a bluish tinge. They usually remain receptive for three days. Since

pollination is accomplished by wind (anemophily), the stigmatic surface-a reticulum of feathery hairs (Fig. 13b)-plays an important role in bringing about

effective trapping of pollen.

Generally, one day after the process of the emergence of stigmas is completed

on a particular head, the anthers start emerging toward the tip (Fig. 11) and

work their way down the head. The time lag between the two processes depends

primarily on the temperature conditions. Warmer temperatures are conducive to

earlier emergence of anthers and their dehiscence.

The species of the section Penicillaria (all cultivated and semiwild or wild

pearl millets, and napier grass) are characterized by the presence of conspicuously penicillate anther tips, i.e., they end in a tuft of fine hairs (Fig. 14),

whereas most other species of Pennisetum have glabrous anther tips. However,

the function of these cilia or trichomes is not known to date.


Although anemophily-or wind pollination-is the rule in pearl millet, insects

are reported to effect occasional cross-pollination (Leuck and Burton, 1966).

Using a marker gene, Rao ef al. (1949) estimated 77.8% natural crossing in pearl

millet. Of course, the extent of cross-pollination varies with different factors,

such as the time of flowering of the parental plants, spacing between plants, and

wind velocity and direction. In a uniformly flowering germplasm pool, Burton

(1974) estimated natural crossing of 69 and 82% in two consecutive years. In

controlled matings, glassine bags must be used.



FIG. 13. Scanning electron micrographs of feathery stigmas of pearl millet. (a) A bifid stigma;

note numerous stigmatic hairs. (b) A portion of stigma enlarged to show a reticulum of feathery hairs

that help in effectively trapping wind-borne pollen. [(a) ~ 2 1 (b)

; ~451



The pearl millet pollen grains are generally spheroidal (Fig. 15a). They

are uniaperturate (monoporate) with nearly isodiametric pore (porus) (Fig. 15b),

2.5-4 k m in diameter. The nexine (the inner unsculptured layer of exine) is

thickened around the porus to form a costa. There is slight variation in the costae

of different cultivars; in some it is more pronounced than in others.

Under humid conditions, the fresh pollen grains are generally inflated to give a

spheroidal appearance. Under dry and hot conditions, however, they shrink to

varying degrees. Shrinkage probably makes them lighter and more buoyant so

that they can be carried long distances. Thus, this feature seems to have an

adaptive value in wind-pollinated species. With the moisture from the stigmatic

surface, the shrunken pollen grains swell up and this is probably the first step

toward germination.

The pearl millet pollen normally remains viable for a few hours, although it



FIG. 14. Scanning electron micrographs of pearl millet anthers showing penicillate anther tips.

(a) Complete anther. (b) Anther tip magnified to show the tuft o f hairs. [(a) X28; (b) X2401

can be preserved under suitable conditions for future crossing. Cooper and Burton (1965) reported that pollen stored at 4.5"C for 3 weeks gave 80% as good

seed set as fresh pollen, whereas that stored for 4 weeks was inviable. Cryopreservation of pollen should be tried with a view to preserving pearl millet germplasm for future use.

The hybridization work is best carried out in the mornings between 7 A . M . and

9 A . M . under the Indian conditions. However, Cooper and Burton (1965) have

found that hybrids may be made at any time of the day, but those made at midday

generally set the least amount of seed per centimeter of spike.



Although mutations have played a significant role in bringing about diversity

in the biological world and, hence, are a major force in organic evolution, the

catalytic effect of hybridization upon evolution should not be underestimated.

FIG. 15. Scanning electron micrographs of a pollen grain of pearl millet. (a) A spheroidal grain;

note a single germ pore (potus) with cmfa around it. (b) A portion of pollen grain magnified to show

the porus with costa (C)around i t . [(a) X2600; (b) x5400l

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X. Floral Biology and Hybridization

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