In cross-pollination species, the transfer of pollen from a flower the stigmas of the other may be brought about the wind (anemophilic) water (hydrophilic) or insects (entomophilic).
Many of the crop plants are naturally cross-pollinated. In many sp. a small amount (upto 5-10 per cent) of selfing may occur. There are several mechanisms that facilitate cross-pollination.
DICLINY – or unisexuality -is a condition in which flowers are either staminate (mala) or pistillate (female).
A. Monecy –
Staminate and pistillate flowers occur in the same plant, either in the same inflorescence e.g. castor, mango, banana, coconut or in separate inflorescence e.g maize. Other monoecious species cucurbits, walnut, chestnut, straw berry, rubber, grapes and cossava.
The male and female flowers are present on different plants. E.g. Papaya, date, hemp, asparagus and spinach.
Stamens and pistils of hermaphrodite flowers may mature at different times facilitating cross pollination.
Protogzymy – In crop sp. like cumbu, gynoecium matures fruits.
Protandry – In maize and sugar beets androecium mature first.
- A combination of two or more of the above mechanisms may occur in some species. This improves the efficiency of the system in promoting cross pollination. EX. Maize exhibits bothmonecy and protandry.
- Self incompatibility – It refers to the failure of pollen from a flower to fertilize the some flowers or other flowers on the same plant. It is highly effective in preventing self pollination EX. self icompatibility in common in Brassica, Nicotiana, Radish, Rye and Many grasses.
- Male sterility – It refers to the absence of functional pollen grains. But it is of great value for the production of hybrid seeds.
Genetic consequences of cross-pollination
Cross-pollination preserves and promotes heterozygosity in a population.
Cross-pollinated sp. are highly heterozygous and show mild to severe inbreeding depression and a considerable amount of heterosis.
The breeding methods in such species aim at improving the crop species with out reducing heterozygosity to an appreciable degree.