Read the passage and answer the following questions.
Flowering plants employ various mechanisms to discourage self-pollination and promote cross-pollination. These mechanisms include temporal separation of pollen release and stigma receptivity, spatial separation of anther and stigma, self-incompatibility, and the production of unisexual flowers. In some species, the timing of pollen release and stigma receptivity is not synchronised. This prevents self-pollination as either the pollen is released before the stigma becomes receptive or the stigma becomes receptive before the pollen is released. Similarly, in other species, the anther and stigma are positioned in a way that prevents self-pollination by avoiding contact between the pollen and stigma of the same flower. Self-incompatibility is a genetic mechanism that prevents self-pollen, whether from the same flower or other flowers of the same plant, from fertilising the ovules. It achieves this by inhibiting pollen germination or pollen tube growth in the pistil. The production of unisexual flowers is another device to prevent self-pollination. For example, in plants like castor and maize, which are monoecious, having both male and female flowers on the same plant prevents self-pollination (autogamy) but not pollination between flowers on the same plant (geitonogamy). On the other hand, in species like papaya, male and female flowers are present on different plants (dioecy), effectively preventing both autogamy and geitonogamy. During pollination, the pistil may encounter pollen of the wrong type, either from other species or from the same plant if it is self-incompatible. The pistil has the ability to recognise whether the pollen is of the right type (compatible) or the wrong type (incompatible). If the pollen is compatible, the pistil accepts it and promotes post-pollination events leading to fertilisation. However, if the pollen is incompatible, the pistil rejects it by preventing pollen germination on the stigma or pollen tube growth in the style. The recognition and acceptance or rejection of pollen by the pistil involve a continuous dialogue between the pollen grain and the pistil. This dialogue is facilitated by chemical components present in both the pollen and pistil. In recent years, scientists have made progress in identifying some of these components and understanding the interactions that lead to the recognition, acceptance, or rejection of pollen by the pistil.
What is the purpose of self-incompatibility in flowering plants?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → To prevent inbreeding depression
Self-incompatibility is a genetic mechanism. It stops pollen from the same flower, or from other flowers of the same plant, from fertilising the ovules. It does this by blocking pollen germination or pollen tube growth in the pistil. Continued self-pollination leads to inbreeding depression, a fall in vigour and fitness of the offspring.
Option (1) To promote self-pollination — Incorrect. Self-incompatibility does the opposite. It blocks self-pollen, so self-pollination cannot end in fertilisation.
Option (2) To prevent inbreeding depression — Correct. By rejecting self-pollen, the plant is forced to depend on pollen from other plants. This prevents inbreeding, and so prevents inbreeding depression. The book calls it the third device to prevent inbreeding.
Option (3) To encourage autogamy — Incorrect. Autogamy is the transfer of pollen to the stigma of the same flower. Self-incompatibility stops such pollen from fertilising the ovules, so it works against autogamy.
Option (4) To synchronise pollen release and stigma receptivity — Incorrect. Timing of pollen release and stigma receptivity is a different outbreeding device. There, the two are kept out of step, not synchronised. Self-incompatibility acts inside the pistil and has nothing to do with timing.
NCERT Extract:
Outbreeding Devices – “Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination result in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. … The third device to prevent inbreeding is self-incompatibility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.”