Sexual Reproduction in Flowering Plants — NCERT Solutions

Bagging Technique:

Definition: Bagging is a technique used in plant breeding in which the emasculated (or un-emasculated female) flowers are covered with a bag (usually made of butter paper or polythene) to prevent contamination of the stigma with unwanted pollen.

Procedure:

1. In cross-pollination experiments, the flower selected as the female parent is first emasculated (anthers are removed before they dehisce) if it is a bisexual flower.
2. The emasculated flower (or a female flower in case of unisexual plants) is immediately covered with a bag to prevent any foreign/unwanted pollen from reaching the stigma.
3. When the stigma becomes receptive, the bag is carefully removed.
4. Desired pollen (from the selected male parent) is dusted on the receptive stigma.
5. The flower is re-bagged to prevent further contamination until the fruit/seed develops.

Usefulness in Plant Breeding Programme:

1. Prevents unwanted pollination: The bag prevents contamination of the stigma by undesired pollen carried by wind, insects, or other agents, ensuring that only the desired cross is made.

2. Ensures controlled pollination: The breeder can precisely control which pollen fertilises the egg, enabling the production of desired hybrids with specific traits.

3. Maintains purity of cross: It ensures that the seeds produced are truly from the intended cross and not from accidental self-pollination or cross-pollination.

4. Used in hybrid seed production: It is an essential technique in producing F₁ hybrid seeds used in agriculture and horticulture.

5. Useful in studying inheritance: It allows breeders and geneticists to study the inheritance of specific traits in a controlled manner.

Triple Fusion:

Definition: Triple fusion is the fusion of one male gamete with the two polar nuclei (or the secondary nucleus/diploid nucleus formed by their fusion) present in the central cell of the embryo sac, resulting in the formation of a triploid primary endosperm nucleus (PEN).

Where does it take place?
Triple fusion takes place inside the embryo sac (female gametophyte), specifically in the central cell of the embryo sac, which is located within the ovule in the ovary of the flower.

How does it take place?

1. After compatible pollination, the pollen grain germinates on the stigma and the pollen tube grows through the style.
2. The pollen tube enters the embryo sac through the micropyle and discharges two male gametes into one of the synergids.
3. One male gamete moves towards the egg cell and fuses with it — this is called syngamy, forming the diploid zygote (2n).
4. The second male gamete moves towards the central cell and fuses with the two polar nuclei (2 × n = 2n).
5. This fusion of one male gamete (n) + two polar nuclei (n + n) results in a triploid (3n) primary endosperm nucleus (PEN).

$$\text{Male gamete } (n) + \text{Two polar nuclei } (n + n) \rightarrow \text{Primary Endosperm Nucleus (PEN) } (3n)$$

Nuclei involved in triple fusion:

• One male gamete nucleus (haploid, n)
• Two polar nuclei (each haploid, n) — present in the central cell

Significance: The PEN (3n) develops into the endosperm, which provides nutrition to the developing embryo.

Note: Since two fusion events occur in the embryo sac (syngamy + triple fusion), the process is collectively called double fertilisation, a unique feature of angiosperms.

Reason for Dormancy of Zygote in a Fertilised Ovule:

The zygote remains dormant for some time after fertilisation in a fertilised ovule due to the following reasons:

1. Endosperm development must precede embryo development: The primary endosperm nucleus (PEN) divides rapidly to form the endosperm tissue before the zygote starts dividing. The endosperm provides essential nutrients, hormones, and growth factors required for the proper development of the embryo. The zygote waits until sufficient endosperm is available to support embryo growth.

2. Preparation of nutritive environment: The zygote remains dormant until the surrounding tissues (endosperm, nucellus) are adequately developed to create a suitable nutritive and hormonal environment for embryo development.

3. Physiological readiness: The zygote needs to undergo certain metabolic and physiological changes before it can begin active cell division and differentiation.

4. Coordination of development: Dormancy ensures that embryo development is coordinated with the development of the seed coat (from integuments) and other seed structures, so that all parts of the seed develop in a synchronised manner.

In summary: The dormancy of the zygote ensures that the endosperm is formed first to provide the necessary nutrition and conditions for the subsequent development of the embryo, thereby ensuring successful seed formation.

(a) Hypocotyl and Epicotyl:

(b) Coleoptile and Coleorrhiza:

(c) Integument and Testa:

(d) Perisperm and Pericarp:

Apple as a False Fruit:

True fruit is defined as a fruit that develops only from the ovary of the flower after fertilisation.

False fruit (Pseudocarp): A fruit in which parts other than the ovary also contribute to the formation of the fruit is called a false fruit or pseudocarp.

In apple:

• The fleshy, edible part of the apple that we eat is not derived from the ovary.
• It develops from the thalamus (receptacle) of the flower, which enlarges, becomes fleshy and forms the major edible portion.
• The true fruit (developed from the ovary) forms only the core of the apple — the hard, papery inner part containing the seeds.

Therefore, apple is called a false fruit because the major edible portion is formed by the thalamus (a non-ovary floral part) and not by the ovary alone.

Parts of the flower that form the fruit in apple:

1. Thalamus (receptacle) — forms the major fleshy, edible part of the apple.
2. Ovary — forms the core (true fruit) containing the seeds.

Other examples of false fruits: Strawberry (thalamus), cashew (peduncle), pear.

Emasculation:

Definition: Emasculation is the process of removal of anthers (male reproductive organs) from the flower bud of the plant selected as the female parent before the anthers dehisce (before pollen is shed), to prevent self-pollination.

When is emasculation employed?
Emasculation is carried out before the anthers mature and dehisce (i.e., at the bud stage), so that the anthers are removed before they can shed pollen on the stigma of the same flower. This is done when the flower is still in the bud stage.

Why does a plant breeder employ emasculation?

1. To prevent self-pollination: In bisexual flowers, if the anthers are not removed, self-pollen may land on the stigma and cause self-fertilisation, which would defeat the purpose of making a controlled cross.

2. To ensure cross-pollination: By removing the anthers, the breeder ensures that only the desired pollen (from the selected male parent) will fertilise the egg of the female parent.

3. To produce desired hybrids: Plant breeders use emasculation to make specific crosses between two plants with desired traits to produce F₁ hybrids with improved characteristics such as higher yield, disease resistance, better quality, etc.

Procedure after emasculation:

• After emasculation, the flower is bagged to prevent contamination by unwanted pollen.
• When the stigma becomes receptive, the bag is removed and desired pollen from the selected male parent is dusted on the stigma.
• The flower is then re-bagged until seeds develop.

Parthenocarpy and Selection of Fruits:

Parthenocarpy is the development of fruit without fertilisation, resulting in seedless fruits. It can be induced by the application of growth hormones such as auxins (IAA), gibberellins (GA), or cytokinins.

Fruits selected for inducing parthenocarpy:

I would select fruits that are:

1. Consumed for their fleshy part (not for seeds)
2. Commercially important and widely consumed
3. Fruits where seeds are undesirable from the consumer's perspective

Examples and Reasons:

1. **Banana (Musa):** Already naturally parthenocarpic. Seedless bananas are preferred worldwide as seeds make them unpalatable. Inducing parthenocarpy ensures seedless, edible fruits.

2. **Grapes (Vitis vinifera):** Seedless grapes are highly preferred by consumers for eating and for making raisins. Gibberellin treatment is already used commercially to produce seedless grapes.

3. **Watermelon (Citrullus lanatus):** Seedless watermelons are preferred as seeds are inconvenient while eating.

4. **Tomato (Solanum lycopersicum):** Seedless tomatoes would be preferred in food processing and cooking.

5. Orange/Lemon (Citrus): Seedless citrus fruits are highly preferred by consumers.

Reason for selection: These fruits are consumed directly and the presence of seeds is undesirable from the consumer's point of view. Seedless fruits are:

• More convenient to eat
• Have better texture and taste
• Have longer shelf life in some cases
• Are commercially more valuable

Note: Parthenocarpy would not be desirable in fruits grown for their seeds (e.g., sunflower, groundnut, cereals).

Role of Tapetum in the Formation of Pollen-Grain Wall:

Tapetum is the innermost layer of the microsporangium wall that directly surrounds the sporogenous tissue. It is a nutritive layer that plays a crucial role in the development of pollen grains, especially in the formation of the pollen wall.

Structure of Pollen Wall:
The pollen grain wall has two layers:

1. Exine — the outer, hard, resistant layer made of sporopollenin.
2. Intine — the inner, thin, delicate layer made of cellulose and pectin.

Role of Tapetum:

1. Provides sporopollenin for exine formation:

• The tapetal cells synthesise and secrete sporopollenin — one of the most resistant biological substances known.
• Sporopollenin is deposited on the outer surface of the microspore to form the exine of the pollen wall.
• Sporopollenin makes the exine extremely resistant to physical, chemical, and biological degradation.

2. Provides precursors for sporopollenin:

• The tapetum provides the precursor molecules (lipids, carotenoids) that are used by the developing microspores to synthesise sporopollenin.

3. Pollen kitt formation:

• Tapetal cells produce pollen kitt (a sticky, oily coating) that covers the exine surface.
• Pollen kitt helps in adhesion of pollen to pollinators (especially in insect-pollinated flowers) and protects pollen from UV radiation.

4. Provides tryphine:

• Tapetum contributes tryphine (a mixture of proteins, lipids, and pigments) to the pollen surface, which plays a role in pollen-pistil recognition (compatibility reactions).

5. Nutrition:

• Tapetum provides nutrients and enzymes to the developing microspores/pollen grains during their maturation.

Summary: The tapetum is essential for the formation of the exine (through sporopollenin synthesis), the pollen kitt, and provides nutrition for pollen development. Without a functional tapetum, viable pollen grains cannot be formed.

Apomixis:

Definition: Apomixis is a form of asexual reproduction in plants that mimics sexual reproduction — it is the production of seeds without fertilisation (i.e., without the fusion of male and female gametes). The seeds produced by apomixis are genetically identical to the mother plant.

Mechanism of Apomixis:
Apomixis can occur by different mechanisms:

1. Diploid egg cell development: The diploid cells of the nucellus or integuments (instead of the haploid egg cell) develop directly into the embryo without fertilisation.
2. Parthenogenesis: The egg cell develops into an embryo without fertilisation.
3. Adventive embryony: Nucellar cells surrounding the embryo sac start dividing and develop into embryos (e.g., in Citrus, mango).

Importance of Apomixis:

1. Maintenance of hybrid vigour:

• In hybrid varieties, if apomixis is introduced, the hybrid characters are maintained in the progeny without segregation.
• Farmers can use seeds from hybrid plants year after year without loss of hybrid vigour.

2. Economic benefit to farmers:

• Hybrid seeds are very expensive to produce every year.
• If hybrids are made apomictic, farmers do not need to buy hybrid seeds every year — they can save seeds from their own crop and replant them, reducing the cost of cultivation.

3. Preservation of superior genotypes:

• Apomixis allows the perpetuation of superior genotypes without the risk of genetic segregation that occurs in sexual reproduction.

4. Polyembryony:

• Apomixis leads to polyembryony (more than one embryo per seed), which can be exploited in horticulture for producing multiple plants from a single seed.

5. Clonal propagation through seeds:

• Apomictic seeds produce plants that are clones of the mother plant, combining the advantages of seed propagation (ease of storage, transport) with clonal uniformity.

6. Research importance:

• Understanding the genetics of apomixis and transferring apomictic genes into hybrid varieties is an active area of research that could revolutionise hybrid seed industry worldwide.

Conclusion: Apomixis is of great significance in agriculture and horticulture as it can help in reducing the cost of hybrid seed production and ensuring that hybrid vigour is maintained across generations.