Chapter 1: Sexual Reproduction in Flowering Plants Application Questions | biology Class 12 CBSE

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Chapter 1: Sexual Reproduction in Flowering Plants Application Questions | biology Class 12 CBSE | ByteNotes.in

Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 1

Applied Concept

A plant breeder wants to develop a new hybrid variety by crossing two different species. She notices that despite placing pollen from species A on the stigma of species B, no seed formation occurs. With reference to pollen-pistil interaction, explain why this might be happening and suggest how she could overcome this problem.

The failure of seed formation is most likely due to incompatibility between the pollen of species A and the pistil of species B. The pistil recognises the foreign pollen as incompatible through chemical interactions between pollen and pistil components, and inhibits pollen germination on the stigma or arrests pollen tube growth in the style.
This recognition-rejection mechanism (pollen-pistil interaction) is a protective device against interspecific hybridisation under normal conditions. The molecular dialogue between pollen and pistil components determines whether pollination leads to fertilisation.
To overcome this, the breeder could use techniques that bypass the pistil's incompatibility response, such as using growth regulators or chemical agents that suppress the rejection mechanism, or applying a mentor pollen technique where compatible pollen mixed with incompatible pollen can facilitate tube growth — all approaches explored in modern plant breeding.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

A farmer observes that the maize plants in his field produce male and female flowers on the same plant (monoecious), yet the seeds produced have greater genetic variation than in nearby self-pollinating crops. Explain this observation with reference to the type of pollination and genetic outcome.

Although maize is monoecious (male and female flowers on the same plant), it prevents autogamy (self-pollination of the same flower). However, geitonogamy (transfer between flowers of the same plant) can still occur, which is genetically equivalent to autogamy.
The greater genetic variation observed is because maize also undergoes significant xenogamy (cross-pollination) — pollen from one maize plant is carried by wind to the stigma of a different plant. Since maize is wind-pollinated and produces light, non-sticky pollen in large quantities, cross-pollination between different plants is common.
Xenogamy introduces new gene combinations into the progeny through recombination, leading to greater genetic diversity compared to obligate self-pollinators, which explains the higher variation observed in maize seeds compared to self-pollinating crops.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A student observes that pollen grains collected from a flower remain viable for only 20 minutes when left at room temperature. However, the same pollen stored in liquid nitrogen remains viable for years. Explain this observation and its significance in crop breeding.

Pollen viability refers to the period for which pollen grains remain capable of fertilisation. At room temperature, metabolic activity continues, and enzymatic degradation and desiccation cause rapid loss of viability — especially in cereals like rice and wheat where pollen can lose viability within 30 minutes of release.
Storage in liquid nitrogen (-196°C) drastically reduces metabolic activity by bringing it to near zero, preventing enzymatic degradation, dehydration, and damage. This preserves the structural and functional integrity of the pollen for years.
Pollen banks (similar to seed banks) created by storing pollen in liquid nitrogen are highly significant in crop breeding programmes, as they allow breeders to use pollen from different seasons, locations, or even extinct varieties, enabling hybridisation across time and space to produce improved crop varieties.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

With reference to the floral rewards system, explain how a bee and a flower maintain a mutualistic relationship during pollination. What happens when a visitor acts only as a nectar robber?

In mutualistic pollination, the flower offers rewards — nectar and pollen — to the bee. To access these rewards, the bee must enter the flower and come into contact with the anthers (getting pollen dusted on its body) and the stigma (depositing pollen from a previously visited flower), thereby facilitating cross-pollination.
The bee benefits by obtaining food (nectar rich in carbohydrates; pollen rich in proteins), and the plant benefits by achieving cross-pollination, which promotes genetic diversity.
A nectar robber (e.g., some insects that bite through the base of the flower to access nectar) obtains the reward without contacting the reproductive structures (anthers or stigma). This means pollen is neither collected nor deposited — no pollination occurs — making the visit detrimental to the plant as resources are consumed without any reproductive benefit.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 5

Applied Concept

A plant scientist discovers a species in which all four megaspores produced by the MMC are functional, and each develops into a separate embryo sac. How does this differ from the typical angiosperm pattern, and what would be the consequences for seed development in this species?

In a typical angiosperm (monosporic development), only one of the four megaspores produced by meiosis is functional, while the other three degenerate. The single functional megaspore develops into the female gametophyte (embryo sac).
In the hypothetical species where all four megaspores are functional, each could theoretically develop into an embryo sac. This would mean that a single ovule could potentially contain more than one embryo sac, a condition far removed from the standard angiosperm pattern.
The consequence for seed development would likely be competition between embryo sacs for space and nutrients within the nucellus, potentially leading to polyembryony — more than one embryo in a single seed — similar to what is observed in Citrus and Mango through nucellar embryony. This would have significant implications for genetic diversity and agricultural exploitation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 6

Applied Concept

In an experiment, a researcher cuts the style of a flower just after compatible pollen lands on the stigma. After a week, no seeds are formed. Explain why, with reference to pollen tube growth and fertilisation.

After landing on the stigma, a compatible pollen grain germinates and produces a pollen tube through one of the germ pores. The pollen tube must grow through the entire length of the style to reach the ovary and then enter the ovule through the micropyle.
By cutting the style, the researcher interrupts the path of pollen tube growth — the tube cannot reach the ovary without a continuous style tissue to grow through. The male gametes inside the pollen tube therefore cannot be delivered to the embryo sac.
Without delivery of male gametes into the embryo sac, neither syngamy nor triple fusion can occur, so no zygote or primary endosperm nucleus is formed. Consequently, no embryo or endosperm develops, and no seeds are produced, confirming that the intact style is essential for pollen tube growth and successful fertilisation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

A student cracks open an orange and finds seeds of different sizes inside. Upon squeezing one large seed, multiple embryos emerge. Explain this phenomenon with reference to apomixis and its genetic consequences.

The presence of multiple embryos in a single seed is the phenomenon of polyembryony, commonly observed in Citrus (including orange) and Mango. In these species, nucellar cells surrounding the embryo sac begin to divide, protrude into the embryo sac, and develop into additional embryos alongside the sexually produced zygotic embryo.
The nucellar embryos arise by apomixis — they develop from somatic (nucellar) cells without fertilisation. Since these cells are part of the maternal (sporophyte) tissue with the same diploid genotype as the mother plant, the nucellar embryos are genetically identical to the mother plant (clones).
The sexually produced (zygotic) embryo is genetically unique due to recombination, while the nucellar embryos are clonal. This mixture in a single seed has practical significance in horticulture: nucellar seedlings in Citrus are used to maintain true-to-type varieties free from virus infections.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

A student reads that coconut water is free-nuclear endosperm. Explain how free-nuclear endosperm forms and how it becomes the solid coconut kernel. What is the ploidy of the coconut water?

After triple fusion, the triploid primary endosperm nucleus (PEN, 3n) undergoes repeated mitotic divisions without immediate cell wall formation, producing a large number of free nuclei floating in a common cytoplasm — this stage is called free-nuclear endosperm. In a tender coconut, the liquid (coconut water) represents this free-nuclear endosperm stage, containing thousands of free nuclei.
As the coconut matures, cell wall formation occurs around the free nuclei — a process called cellularisation — converting the free-nuclear endosperm into cellular endosperm. This white, solid, mature endosperm forms the coconut kernel.
Since the PEN is formed by the fusion of three haploid nuclei (one male gamete + two polar nuclei), the ploidy of coconut water (free-nuclear endosperm) is triploid (3n).

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 9

Applied Concept

Why do hybrid seed companies charge high prices for seeds, and how would introducing apomixis into hybrid crops change the seed industry? Discuss from the perspective of farmers in developing countries.

Hybrid seeds are produced by controlled cross-pollination between two specific inbred parent lines. This labour-intensive process requires emasculation, bagging, and careful pollination, making hybrid seed production expensive. Moreover, seeds saved from hybrids show genetic segregation (F2 breakdown), losing hybrid vigour, so farmers must buy fresh hybrid seeds every year.
Introducing apomixis into hybrid crops would allow seeds to be produced without fertilisation while maintaining the exact hybrid genotype. Since apomictic progeny are genetically identical to the parent (no segregation), farmers could save and reuse seeds year after year without loss of hybrid characteristics, eliminating the need to repurchase seeds annually.
For farmers in developing countries, who often cannot afford the high cost of hybrid seeds, apomictic hybrids would be transformative: they would gain access to high-yielding varieties at minimal recurring cost, improving food security, reducing dependence on seed companies, and lowering agricultural input costs.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 10

Applied Concept

With reference to the bagging technique, explain why a plant breeder bags the flower of the female parent both before and after pollination. What would happen if the flower were left unbagged after the first bagging?

Bagging before pollination: after emasculation, the flower bud is covered with a butter-paper bag to protect the stigma from contamination by unwanted pollen (from other plants or wind) until the stigma becomes receptive. This ensures only the desired pollen is used for pollination.
After pollination with desired pollen, the flower is re-bagged to prevent any additional pollen (cross-contaminating pollen) from landing on the stigma and causing unintended fertilisation, which would compromise the purity of the hybrid.
If left unbagged after the first bagging was removed for pollination, unwanted pollen from other plants in the vicinity could land on the receptive stigma and fertilise the ovules, producing seeds that are not the intended hybrid — making the entire experiment invalid and the seeds commercially worthless.

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Chapter 1: Sexual Reproduction in Flowering Plants | Application Questions

A farmer observes that the maize plants in his field produce male and female flowers on the same plant (monoecious), yet the seeds produced have greater genetic variation than in nearby self-pollinating crops. Explain this observation with reference to the type of pollination and genetic outcome.

A student observes that pollen grains collected from a flower remain viable for only 20 minutes when left at room temperature. However, the same pollen stored in liquid nitrogen remains viable for years. Explain this observation and its significance in crop breeding.

With reference to the floral rewards system, explain how a bee and a flower maintain a mutualistic relationship during pollination. What happens when a visitor acts only as a nectar robber?

A plant scientist discovers a species in which all four megaspores produced by the MMC are functional, and each develops into a separate embryo sac. How does this differ from the typical angiosperm pattern, and what would be the consequences for seed development in this species?

In an experiment, a researcher cuts the style of a flower just after compatible pollen lands on the stigma. After a week, no seeds are formed. Explain why, with reference to pollen tube growth and fertilisation.

A student cracks open an orange and finds seeds of different sizes inside. Upon squeezing one large seed, multiple embryos emerge. Explain this phenomenon with reference to apomixis and its genetic consequences.

A student reads that coconut water is free-nuclear endosperm. Explain how free-nuclear endosperm forms and how it becomes the solid coconut kernel. What is the ploidy of the coconut water?

Why do hybrid seed companies charge high prices for seeds, and how would introducing apomixis into hybrid crops change the seed industry? Discuss from the perspective of farmers in developing countries.

With reference to the bagging technique, explain why a plant breeder bags the flower of the female parent both before and after pollination. What would happen if the flower were left unbagged after the first bagging?

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