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.
Which mechanism is NOT used by flowering plants to discourage self-pollination?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Synchronised timing of pollen release and stigma receptivity
The passage states that in some species, “the timing of pollen release and stigma receptivity is not synchronised”. Such features are called outbreeding devices; continued self-pollination leads to inbreeding depression.
Option (1) Different positions of anther and stigma — A device that discourages self-pollination. The anther and the stigma sit at different places in the flower. So the pollen cannot touch the stigma of the same flower, and autogamy is prevented.
Option (2) Production of unisexual flowers — A device that discourages self-pollination. A unisexual flower has either stamens or a pistil, not both. So pollen cannot reach a stigma inside the same flower.
Option (3) Self-incompatibility — A device that discourages self-pollination. It is a genetic mechanism. The pistil blocks pollen germination or pollen tube growth when the pollen comes from the same flower or the same plant.
Option (4) Synchronised timing of pollen release and stigma receptivity — Not a device that discourages self-pollination. The real device is the opposite: the two are kept out of step. If both happened at the same time, self-pollination would become easier, not harder.