Pollination, which results in the transfer of pollen to the stigma, is of two types: self-pollination and cross-pollination. Continuous self-pollination or inbreeding throughout successive generations, however, is not desirable as it leads to inbreeding depression, a condition characterised by a decrease in genetic vigour and vitality which results in reduced fitness in offspring. In fact, when genetically-similar parents self-pollinate, the offspring are more likely to have genes in homozygous condition.
Dichogamy is of two types: protandry and protogyny. In protandry, which is observed in plants such as maize, pollen is released before the stigma becomes receptive, that is, the androecium of the flower matures earlier than the gynoecium whereas in protogyny, which is observed in millet or bajra, the gynoecium matures prior to the androecium, that is, the stigma becomes receptive long before the pollen is released.
In some plant species such as Hibiscus, the anther and stigma are placed at different heights, which disallows self-pollination by preventing the pollen from falling on to the stigma. This condition is called herkogamy. However, other plant species such as Passiflora have developed a genetic mechanism called self-incompatibility or self- sterility. Adept at preventing both autogamy and geitonogamy, self-incompatibility, prevents self-pollen, that is, pollen from the same flower or different flowers borne on the same plant, from germinating or developing a pollen tube, a move that prevents fertilisation of the egg. Pollen pre-potency is another mechanism that prevents self-pollination in certain plants such as Dolichos. This mechanism allows the pollen from another plant of the same species to germinate earlier than the pollen of the same flower. The production of unisexual flowers is also a mechanism developed to thwart self-pollination.
In the case of monoecious plants such as coconut, unisexual flowers prevent autogamy but not geitonogamy. However, in the case of dioecious plants such as papaya, unisexual flowers prevent both autogamy and geitonogamy.
Interestingly, post-pollination, the pollen and pistil start interacting with each other by releasing a series of chemicals. This interaction allows the pistil to recognise whether the pollen is the right or wrong type. The right type of pollen implies that the pollen belongs to the same species as that of the stigma. In the case of female plant-bearing unisexual flowers, emasculation is not needed. Instead, the female flower is bagged before it blooms. After the flower has bloomed and the stigma has become receptive, the desired pollen is dusted over the stigma before re-bagging the flower.
In this way, pollen-pistil interaction allows the plant to recognise the pollen and promote its germination for fertilisation to take place. Moreover, gaining knowledge in this field enables the growth of a superior variety of crops.