Explain Mechanisms Preventing Self-pollination In Flowers

To ensure cross-pollination and maintain genetic diversity, plants have evolved several mechanisms, known as outbreeding devices, to discourage or prevent self-pollination.

• Dichogamy: This is the temporal (time-based) separation of pollen release and stigma receptivity in the same flower.
  • Protandry: The anthers (male parts) mature and release pollen *before* the stigma (female part) of the same flower is receptive. (e.g., Sunflowers, Salvia)
  • Protogyny: The stigma becomes receptive *before* the pollen in the same flower is mature. (e.g., Avocado, Magnolia)
• Herkogamy: This is the spatial (space-based) separation of anthers and stigma. They are positioned in a way that makes self-pollination physically difficult. (e.g., In a Hibiscus flower, the stigma is located far above the anthers).
• Self-Incompatibility (SI): This is a genetic mechanism that allows the stigma to recognize and reject its own pollen. If pollen from the same plant (carrying the same S-genes) lands on the stigma, its germination or pollen tube growth is blocked, preventing fertilization. (e.g., Tomatoes, Petunias, many fruit trees).
• Dioecy: The most extreme mechanism. The species has entirely separate male and female plants. Male plants (staminate) only have male flowers, and female plants (pistillate) only have female flowers. This makes self-pollination physically impossible. (e.g., Kiwi, Holly, Papaya).
• Monoecy: The plant bears separate male and female flowers on the *same* plant. While this can lead to self-pollination (geitonogamy), it often encourages cross-pollination, especially if the male and female flowers mature at different times (dichogamy). (e.g., Corn, Squash).