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There are three basic or natural systems of pollination which determine the nature of breeding system of a plant: A. Natural self-pollination; B. Natural crosses pollination; C. Occasional cross pollination.
A. Self-Pollinated Crops:
Self-pollination is the process of transfer of pollen grains from the anther of a flower to the stigma of the same flower. In the self-pollinated crop plants the self-pollination is not complete, 5% cross pollination may occur which is affected by several factors – variety, environmental condition and location.
Contrivances of Self-Pollination are:
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1. Cleistogamy: The flowers do not open at all, e.g., wheat, barley, oat, etc.
2. Chasmogamy: The flowers open but after pollination as in some varieties of rice, wheat, barley, oat, etc.
3. Homogamy: The stamens and carpels of a flower mature at the same time, so there is greater chance for self-pollination.
4. Position of anther lobes and stigma are very much close to promote self-pollination in crops like tomato and brinjal.
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5. The flower opens but the androecium and gynoecium are enclosed within two petals as in legumes.
6. The receptive stigma elongates through the fertile staminal column to get pollinated.
Consequences of Self-Pollination are:
(a) Continuous inbreeding results in progressive loss of genetic variability, i.e., leads to fixation of genotypes.
(b) Intra-varietal genetic variation is absent.
(c) Inter-varietal genetic variation is large due to random fixation of different alleles in different lines.
(d) Prolonged inbreeding promotes discrete grouping within population.
(e) Inbreeding favours only the homozygotes not the heterozygotes.
B. Cross Pollinated Crops:
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Transfer of pollen grains from one flower to the stigma of another flower borne in different plants is called cross pollination. The flowers may be bisexual or unisexual; the pollination needs some agencies like wind, water or animals, e.g., insects, birds, snails, etc.
Contrivances for Cross Pollination are:
1. In unisexual flower cross pollination is obligatory.
2. In bisexual flower there are some adaptive features for cross pollination:
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(a) Self-Sterility:
The self-sterility or self-incompatibility may be due to many factors:
(i) Incapability of pollen germination,
(ii) Failure of pollen tube growth,
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(iii) Production of sterile pollen by the plant.
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(b) Dichogamy:
The phenomenon of maturation of male and female sex organs at different times; anthers mature before stigma (protandry); stigma matures before anther lobe (protogyny).
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(c) Herkogamy:
When the structure of male and female sex organs become barrier to self-pollination, stigma projects out of the stamen.
(d) Heteromorphism:
The flowers are of two or three morphological types may be dimorphic or trimorphic, which usually involves heterostyly (styles of different lengths) and heteroanthy (different position of anthers).
Consequences of Cross Pollination are:
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(a) Cross pollination always creates and maintains genetic variability and high degree of heterozygosity in the population.
(b) The population becomes fairly dynamic because allogamy prevents genetic stagnation.
(c) Dominant genotypes become prevalent in a population.
(d) Random pollination leads to mixing of out-breeding population which creates the loss of genetic identity.
(e) Genetic correlation among the progenies tend to get reduced as there is mating between phenotypically dissimilar individuals.
(f) Hybrid vigour is lost when there is inbreeding within cross pollinated crops; the inbred lines may be developed and hybrid vigour can be recovered by hybridization between two inbred lines.
C. Occasional Cross Pollinated Crops:
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Some of the crops like cotton, sorghum are exposed to both cross and self-pollination, self-pollination is predominant but cross pollination occurs by windblown pollen grains occasionally. So in these types of crops the advantages of both self and cross pollination can be obtained; but stability achievement in these crops is problematic.
