SEXUAL REPRODUCTION IN FLOWERING PLANTS
Outbreeding devices:
Outbreeding devices, also known as mechanisms for promoting outcrossing, are strategies employed by flowering plants (angiosperms) to enhance genetic diversity by promoting cross-pollination between different individuals rather than self-pollination. These devices are adaptations that reduce the likelihood of self-fertilization and ensure that pollen from one plant is transferred to another plant of the same species. Here are some common outbreeding devices in flowering plants:
Dioecy:
- Dioecious plants have separate male and female individuals within the same species.
- Each plant produces only one type of reproductive organ (either male or female), preventing self-fertilization.
- Examples include some species of holly and asparagus.
Self-Incompatibility Mechanisms:
- Many plants have evolved self-incompatibility systems that prevent the pollen from the same plant from fertilizing its own ovules.
- These mechanisms involve genetic and biochemical processes that recognize and reject self-pollen.
- Self-incompatibility can be sporophytic (controlled by genes in the parent plant) or gametophytic (controlled by genes in the pollen or ovule).
- Examples include various species in the Brassicaceae family (e.g., broccoli, cabbage, and radish).
Heterostyly:
- Heterostylous plants have two or more distinct flower morphs, each with different styles and stamen lengths.
- This arrangement promotes cross-pollination because pollen from one flower morph is more likely to reach the stigma of a flower with a different morph.
- Examples include some primroses (e.g., Primula spp.).
Temporal Separation of Male and Female Functions:
- In some plant species, the male and female reproductive organs mature at different times, reducing the chances of self-pollination.
- For example, a flower may produce pollen before its stigma becomes receptive, or vice versa.
Herkogamy:
- Herkogamy refers to physical barriers within the flower that prevent self-pollination.
- Examples include the presence of a long style and short stamens or vice versa, ensuring that the stigma and anthers are at different heights within the flower.
Heterodichogamy:
- Heterodichogamous plants have individuals with different temporal patterns of sexual function.
- Some individuals function as male during one part of the day or season and as female during another part, while others have the opposite pattern.
- This temporal separation encourages cross-pollination.
- Examples include some species of avocado and persimmon.
Blossom Color and Scent Variation:
- Plants may exhibit variations in blossom color and scent that attract different pollinators, reducing self-pollination.
- For instance, a plant may have two color morphs that appeal to different types of pollinators, such as bees and butterflies.
These outbreeding devices have evolved in different plant species to increase genetic diversity, enhance adaptation to changing environments, and ensure the long-term survival of the species through cross-pollination and the avoidance of self-fertilization.
Pollen-pistil Interaction in Flowering Plants:
Pollen-pistil interaction is a critical step in the reproductive process of flowering plants (angiosperms). It involves the interaction between pollen grains (carrying male gametes) and the pistil (the female reproductive structure), ultimately leading to fertilization and seed production. Here’s an overview of the pollen-pistil interaction in flowering plants:
Pollen Deposition on the Stigma:
- The pollen-pistil interaction begins when a pollen grain lands on the stigma, the receptive tip of the pistil.
- Pollen deposition can occur through various means, including wind, insects, birds, or other pollinators.
Recognition and Compatibility:
- After pollen deposition, the stigma may recognize the pollen as compatible (from the same species) or incompatible (from a different species or genetically similar individual).
- In the case of self-incompatible species, the stigma may also detect self-pollen and prevent self-fertilization.
Pollen Germination:
- If the pollen is recognized as compatible, it undergoes germination on the stigma.
- Pollen germination involves the formation of a pollen tube, which grows down through the style (a structure connecting the stigma to the ovary) toward the ovule within the ovary.
Pollen Tube Growth:
- The pollen tube extends down the style by digesting its way through the transmitting tissues.
- It is through this tube that the male gametes (sperm cells) will travel to reach the ovule.
Chemical Signaling:
- During pollen tube growth, chemical signaling between the stigma, style, and the pollen tube occurs. These signals help guide the pollen tube toward the ovule.
Pollen-pistil interaction is a complex and highly regulated process that ensures the successful transfer of genetic material from the male parent to the female parent, leading to the formation of seeds and the continuation of plant species. It involves intricate biochemical and structural adaptations in both pollen and the pistil to facilitate fertilization.
Artificial hybridisation in plants:
Artificial hybridization in plants, also known as controlled or artificial pollination, is a technique used by plant breeders and horticulturists to deliberately cross-pollinate two different plant varieties or species with desired traits to create new hybrids. This process allows for the development of plants with specific characteristics, such as improved disease resistance, higher yield, unique colors, or other desirable traits. Here’s an overview of the steps and considerations involved in artificial hybridization:
Steps in Artificial Hybridization:
Selection of Parent Plants:
- The first step is to carefully select two parent plants with desirable traits that the breeder wants to combine in the resulting hybrid.
- The parent plants can be from the same species or different species if they are genetically compatible.
Emasculation:
- In many cases, it is essential to prevent self-pollination or unwanted cross-pollination by emasculating (removing or preventing the development of) the anthers (male reproductive organs) of the chosen female parent.
- Emasculation ensures that only the desired pollen will be used for fertilization.
Pollination:
- After emasculation, the breeder can then apply pollen from the selected male parent to the stigma (female reproductive organ) of the female parent.
- Pollination can be achieved by using a paintbrush, forceps, or other tools to transfer pollen carefully.
Isolation and Protection:
- To prevent any contamination from external pollen sources, the hybridization process often involves isolating the female parent using bags or other protective coverings.
- These protective barriers prevent the entry of pollen from unwanted sources.
