Modes of Pollination – Self Pollination
Many cultivated plant species reproduce by self pollination. Self pollinated species are believed to have originated from cross- pollinated ancestors in response to the environmental conditions to which such species are typically subjected. Often in breeders are found at the extremes of the species distribution in marginal habitats and particularly in stress environments. Plant species showing self- pollination, i.e. inbreeding, are generally annuals or in any case, short –lived. These species, as a rule , must have hermaphrodite flowers. But in most of these species, self- pollination is not complete and cross-pollination may occur upto 5%. The degree of cross-pollination in self- pollinated species is affected by several factors, E.g. variety, environmental conditions like temperatures and humidity, and location.
Mechanism Promoting Self Pollination:
The various mechanisms that promote self- pollination are generally more efficient than those promoting cross pollination. These mechanisms are briefly summarised below.
1. Cleistogamy:
In this case, flowers do not open at all. This ensures complete self pollination since foreign pollen cannot reach the stigma of a closed flower. Cleistogamy occurs in some varieties of wheat (Triticum sp), oats (Avena sp), barley ( H.vulgare) and in a number of other grasses.
2. Chasmogamy:
In some species, the flowers open, but only after pollination has taken place. This occurs in many cereals, such as wheat, barley, rice and oats. Since the flowers do open, some cross-pollination may occur.
3. In crops like tomato (L.esculentum) and brinjal (S. melongena), the stigmas are closely surrounded by anthers. Pollination generally occurs after the flowers open. But the position of anthers in relation to stigmas ensures self-pollination.
4. In some species, flowers open but the stamens and the stigma are hidden by other floral organs. In several legumes, e.g. pea (P.sativum), Mung ( V. radiate) , Urd ( V. mungo), soybean ( G.max) and gram ( C. arientinum), the stamens and the stigma are enclosed by two petals forming a keel.
5. In a few species, stigmas become receptive and elongate through the staminal columns. This ensures predominant 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 inbreeding depression, but may exhibit considerable heterosis. Therefore, the aim of breeding method generally is to develop homozygous varieties.