Chapter 13: Plant Growth and Development Application Questions | biology Class 11 CBSE

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Chapter 13: Plant Growth and Development Application Questions | biology Class 11 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A farmer notices that after trimming the tops of his tea bushes regularly, the plants become bushier with more lateral branches. With reference to the concept of apical dominance, explain the mechanism behind this observation.

The growing apical bud produces auxin, which moves down the stem and inhibits the growth of lateral (axillary) buds — a phenomenon called apical dominance.
When the shoot tip is removed (decapitation/trimming), auxin production at the apex is eliminated, and its inhibitory influence on lateral buds is lost.
Lateral buds are then released from inhibition and begin to grow into lateral branches, making the plant bushier.
This principle is the scientific basis for regular pruning in tea plantations and hedge-making to encourage branching and a bushy growth habit.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

A gardener wants to prepare a weed-free lawn by applying a herbicide that kills broad-leaved weeds but does not harm the grass. Which plant growth regulator would you recommend and why?

The recommended herbicide is 2,4-D (2,4-dichlorophenoxyacetic acid), a synthetic auxin.
2,4-D selectively kills dicotyledonous (broad-leaved) weeds without affecting mature monocotyledonous plants such as grass.
This selectivity makes 2,4-D ideal for weed-free lawn preparation, as the grass remains unaffected while the weeds are eliminated.
Auxins at higher concentrations act as herbicides by disrupting normal growth regulation in dicots, leading to abnormal growth and eventual death of the weed plant.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A fruit vendor stores unripe bananas along with overripe oranges and notices that the bananas ripen faster than usual. Explain this observation with reference to the relevant plant growth regulator and its mode of action.

The overripe oranges release ethylene, a gaseous plant growth regulator produced in large amounts by ripening and senescing fruits.
Ethylene diffuses through the air to the stored unripe bananas and stimulates their ripening by enhancing the rate of respiration (respiratory climacteric).
This observation was first reported by H.H. Cousins (1910), who confirmed that a volatile substance from ripened oranges hastened ripening of unripened bananas; the substance was later identified as ethylene.
The practical implication is that ethylene (commercially supplied via ethephon) is widely used in agriculture to synchronise and accelerate fruit ripening in tomatoes, apples, and other crops.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

During a drought, stomata in a plant close rapidly to prevent water loss. Identify the plant growth regulator responsible for this response and explain how it helps the plant survive stress conditions.

Abscisic acid (ABA) is the plant growth regulator responsible for stimulating the closure of stomata in response to water stress.
ABA is called the stress hormone because it mediates a range of plant responses to abiotic stresses (drought, salinity, cold) and biotic stresses.
By inducing stomatal closure, ABA reduces transpirational water loss, conserving water in the plant during drought conditions.
ABA also increases the overall stress tolerance of the plant and induces seed and bud dormancy to ensure survival during prolonged periods of unfavourable conditions.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

A horticulturist wants to root new plants from stem cuttings of a prized ornamental plant. Which plant growth regulator would help, and how does it work?

Auxin (specifically IAA, IBA, or synthetic NAA) is the plant growth regulator used to initiate rooting in stem cuttings.
Auxins are produced at the growing apices and promote root initiation by stimulating cell division and differentiation at the cut surface of the stem, leading to the formation of adventitious roots.
Treating stem cuttings with auxin solutions or powder significantly enhances rooting success, making it a widely used technique in plant propagation.
This application is particularly important in horticulture for propagating plants that are difficult to root naturally, allowing rapid multiplication of desirable varieties.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

With reference to the 'bakanae' disease of rice, explain how this agricultural problem accidentally led to the discovery of an important class of plant growth regulators.

The 'bakanae' (foolish seedling) disease of rice caused abnormal elongation of infected seedlings, making them taller and spindly compared to healthy plants; the causal organism was the fungal pathogen Gibberella fujikuroi.
E. Kurosawa (1926) experimentally reproduced the symptoms of the disease in healthy rice seedlings by treating them with sterile filtrates of the fungus, demonstrating that a chemical from the fungus was responsible.
The active substances in the filtrates were later isolated, purified, and chemically characterised as gibberellic acids — a new class of plant growth promoters.
This accidental discovery revealed that gibberellins are natural regulators of stem elongation in plants, leading to their widespread use in agriculture for increasing crop yield and improving fruit quality.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 7

Applied Concept

A sugarcane farmer wants to increase the yield of his crop without increasing the area of cultivation. How can plant growth regulators help, and what is the expected benefit?

Spraying the sugarcane crop with gibberellins (plant growth promoters) is the recommended approach to increase yield.
Gibberellins promote elongation of the stem internodes in sugarcane, increasing the length of the stem where carbohydrate (sugar) is stored.
Longer stems mean a greater volume of sugar-storing tissue per plant, which can increase the overall carbohydrate yield by as much as 20 tonnes per acre.
This application demonstrates how understanding plant growth regulation can translate directly into agricultural productivity gains without requiring additional land.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 8

Applied Concept

A researcher observes that deep water rice plants rapidly elongate their internodes when submerged, keeping their leaves above water. Identify the PGR responsible and explain the mechanism.

Ethylene is the plant growth regulator responsible for promoting rapid internode and petiole elongation in deep water rice plants.
When rice plants are submerged, the reduced gas exchange and mechanical stress trigger ethylene synthesis in the submerged tissues.
Ethylene promotes rapid internode and petiole elongation, allowing leaves and upper parts of the shoot to remain above the water surface and continue photosynthesis and gas exchange.
This adaptive response mediated by ethylene is an important survival mechanism for deep water rice cultivars that grow in flood-prone regions.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

A grower of pineapples wants to synchronise flowering so that all fruits ripen at the same time for a uniform harvest. Which PGR would you recommend, and why?

Ethylene (applied via ethephon) is recommended for initiating and synchronising flowering and fruit-set in pineapples.
Ethylene has the ability to induce flowering in pineapples; when applied to a field of pineapple plants, it triggers synchronised flowering across all plants.
Synchronised flowering leads to uniform fruit development and simultaneous ripening, enabling a single coordinated harvest that saves labour and improves marketability.
Auxins can also promote flowering in pineapples; both ethylene and auxin have been used in pineapple cultivation for this purpose, demonstrating how PGRs can be harnessed for commercial agricultural management.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

During plant tissue culture experiments, a researcher forgets to add cytokinin to the culture medium. What consequences would be expected, and why?

Without cytokinin, the callus (mass of undifferentiated cells) would not undergo normal cytokinesis (cell division) or differentiate into shoot buds, as cytokinins are essential for promoting cell division in tissue culture.
Skoog's experiments showed that callus from tobacco stem internodes proliferates only when both auxins and cytokinin-like substances are present in the medium.
The absence of cytokinin would shift the auxin-to-cytokinin ratio, likely leading to root differentiation rather than shoot formation (high auxin-to-cytokinin ratio favours rooting), or to poorly differentiated callus.
This demonstrates that cytokinins are indispensable for normal organogenesis in tissue culture and that the balance between auxin and cytokinin concentrations is critical for directing the developmental fate of cultured plant cells.

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Chapter 13: Plant Growth and Development | Application Questions

A farmer notices that after trimming the tops of his tea bushes regularly, the plants become bushier with more lateral branches. With reference to the concept of apical dominance, explain the mechanism behind this observation.

A gardener wants to prepare a weed-free lawn by applying a herbicide that kills broad-leaved weeds but does not harm the grass. Which plant growth regulator would you recommend and why?

A fruit vendor stores unripe bananas along with overripe oranges and notices that the bananas ripen faster than usual. Explain this observation with reference to the relevant plant growth regulator and its mode of action.

During a drought, stomata in a plant close rapidly to prevent water loss. Identify the plant growth regulator responsible for this response and explain how it helps the plant survive stress conditions.

A horticulturist wants to root new plants from stem cuttings of a prized ornamental plant. Which plant growth regulator would help, and how does it work?

With reference to the 'bakanae' disease of rice, explain how this agricultural problem accidentally led to the discovery of an important class of plant growth regulators.

A sugarcane farmer wants to increase the yield of his crop without increasing the area of cultivation. How can plant growth regulators help, and what is the expected benefit?

A researcher observes that deep water rice plants rapidly elongate their internodes when submerged, keeping their leaves above water. Identify the PGR responsible and explain the mechanism.

A grower of pineapples wants to synchronise flowering so that all fruits ripen at the same time for a uniform harvest. Which PGR would you recommend, and why?

During plant tissue culture experiments, a researcher forgets to add cytokinin to the culture medium. What consequences would be expected, and why?

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