How do plants prevent self-fertilization?

Plants prevent self-fertilisation through mechanisms such as self-incompatibility, dichogamy, and herkogamy.

Self-fertilisation, or selfing, is a process where pollen from a plant fertilises the same plant's ovules. While this can be beneficial in certain circumstances, it can also lead to inbreeding depression, a reduction in fitness due to increased homozygosity. To avoid this, many plants have evolved mechanisms to prevent self-fertilisation.

One such mechanism is self-incompatibility. This is a genetic mechanism that prevents pollen from fertilising ovules of the same plant. The pollen and the stigma (the part of the flower that receives the pollen) both contain proteins that interact with each other. If the proteins are too similar, which would be the case in self-fertilisation, the stigma can recognise this and will not allow the pollen to germinate.

Another mechanism is dichogamy, where the timing of male and female flower maturity is staggered. In protandry, the male parts of the flower mature before the female parts, while in protogyny, the female parts mature first. This temporal separation of male and female flower parts reduces the chance of self-fertilisation.

Herkogamy is a physical mechanism that separates the male and female parts of the flower. This can be achieved through different lengths of stamens and pistils, or through the positioning of these parts in different locations within the flower. This physical separation makes it more difficult for pollen to reach the stigma of the same flower.

In addition to these mechanisms, some plants also have a mechanism called heterostyly, where different individuals of the same species have different lengths of stamens and pistils. This encourages cross-fertilisation between different individuals, as the pollen from one flower is more likely to reach the stigma of a flower with a different style length.

These mechanisms all work to increase genetic diversity within a plant population, which can increase the population's resilience to environmental changes and disease.