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 happens when incompatible pollen lands on the stigma of a flowering plant?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The stigma rejects the pollen and prevents fertilisation.
The passage states that “if the pollen is incompatible, the pistil rejects it by preventing pollen germination on the stigma or pollen tube growth in the style”. Pollen of another species, or pollen of the same plant in a self-incompatible species, can land on the stigma. The pistil (stigma, style and ovary) can recognise whether the pollen is compatible or incompatible, and then accepts or rejects it.
Option (1) The stigma accepts the pollen and promotes fertilisation. — Incorrect. Acceptance happens only with compatible pollen. Then the pistil allows the post-pollination events that lead to fertilisation.
Option (2) The stigma rejects the pollen and prevents fertilisation. — Correct. Incompatible pollen is rejected. Its germination or pollen tube growth is stopped, so the pollen tube never reaches the ovule and fertilisation cannot take place.
Option (3) The pollen germinates and grows a pollen tube. — Incorrect. This is what compatible pollen does. With incompatible pollen, germination or pollen tube growth is blocked.
Option (4) The pollen undergoes self-pollination. — Incorrect. Self-pollination is the transfer of pollen to the stigma of the same flower or plant. It is a way the pollen arrives, not something that happens after incompatible pollen has landed.