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 synchronized. 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 fertilizing 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 recognize 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 fertilization. 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 of the following plant species exhibit dioecy?

Papaya.

The plant species that exhibit dioecy (having separate male and female plants) among the options provided are: Papaya (Option 2).

Unisexual flowers, also known as imperfect flowers, are those that contain either male reproductive structures (stamens) or female reproductive structures (pistils), but not both. The presence of unisexual flowers on a plant is another device that helps prevent self-pollination.

In monoecious plants, such as castor and maize, both male and female flowers are present on the same plant. This arrangement prevents autogamy, which is the self-fertilization of flowers on the same plant, because the male and female flowers are physically separated. However, it does not prevent geitonogamy, which is the transfer of pollen between flowers of the same plant. Geitonogamy can still occur in monoecious plants if pollen from a male flower on one part of the plant reaches the female flowers on another part of the same plant.

On the other hand, in dioecious plants, such as papaya, male and female flowers are present on different plants. Each plant is either exclusively male or exclusively female. This condition prevents both autogamy and geitonogamy because there is no possibility for pollen transfer between flowers on the same plant. For successful fertilization to occur in dioecious plants, pollination must happen between separate male and female plants.

The production of unisexual flowers and the presence of monoecious or dioecious plant types ensure cross-pollination, where pollen is transferred between different plants of the same species. Cross-pollination promotes genetic diversity within a population and reduces the risk of inbreeding depression, as it introduces new genetic combinations and prevents mating between closely related individuals.

Overall, the presence of unisexual flowers and the arrangement of male and female flowers on different plants (dioecy) or separate parts of the same plant (monoecious) are strategies employed by plants to prevent self-pollination and encourage outcrossing, thereby maintaining genetic diversity and improving overall reproductive success.