MODES OF POLLINATION
Pollination refers to the transfer of pollen grains from anthers to stigmas. Pollen from an anther may fall on the same flowers leading to ” Self pollination ” or ” autogamy”.
When pollen from flowers of one plant are transmitted to the stigmas of flowers of another plant, it is known as “cross pollination” or allogamy.
A third situation “Geitonogamy” results when pollen from a flower of one plant galls on the stigmas of other flower of the same plant e.g. maize. The genetic consequences of geitonogamy are the same as those of quitogamy.
Many cultivated plant species reproduce by self-pollination.
These species as a rule must have hermaphrodite flowers. But in most of this sp., self-pollination is not complete and cross-pollination may occur upto 5%. Several factors like variety,
environmental conditions like temperature humidity and location affect the degree of cross-pollination. There are various mechanisms that promote self-pollination.
CLEISTOGAMY – In this case, flowers do not open at all. This ensures complete self-pollination. Since, foreign pollen cannot reach the stigma of closed flowers. It occurs in some varieties of wheat (Triticm sp. ) Oats (Avena sp.), Barley (Hordaum vulgare) and in a number of other grasses.
CHASMOGAMY – In some sp. the flower open, only after pollination has taken place. This occur in many cereals, such as wheat, barley, rice and oats.
- In crops like tomato (Lycopersicum esculentum) and brinjal ( Solanum melongena), the stigma are closely surrounded by anthers. Pollination generally occurs after the flowers open. But the position of anthers in relation to stigma ensures self-pollination.
- In some species, flowers open but the stamens and the stigma are hidden by other floral organs. In several legumes e.g. pea, mung bean, urd bean, soybean, bengalgram. Two petals forming a keel enclose the stamens and the stigma.
- In a few sp. stigmas become receptive and elongate through stamina columns. This ensures pre-dominant self-pollination.
GENETIC CONSEQUENCES OF SELF POLLINATION
Self-pollination leads to a very rapid increase in homozygosity. Therefore, populations of self-pollinated species are highly homozygous. Self-pollinated species do not show in breeding depression, but may exhibit considerable heterosis. Therefore the aim of breeding methods generally is to develop homozygous varieties.