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

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

Case Study

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Case Set 1

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A biology teacher demonstrated plant growth using two pots of maize seedlings. In the first pot, she marked equal intervals on the root with a marker pen and observed the positions after 24 hours. She noted that the marks near the root tip had moved apart the most, while marks further from the tip had barely shifted. She explained that the root tip contains a region of actively dividing cells, followed by a zone where cells enlarge, and finally a zone where cells mature and perform specific functions. She asked students to identify the three zones and explain why the marks near the tip shifted the most. She also pointed out that one maize root apical meristem alone can produce more than 17,500 new cells per hour.

Question 1: Name the three phases of growth visible in the root tip from the apex towards the base.

The three phases of growth in the root tip are: meristematic phase (at the apex, cells actively divide), elongation phase (cells proximal to meristematic zone enlarge and vacuolate), and maturation phase (cells furthest from apex attain maximum size and differentiate).
Cells in the meristematic phase are rich in protoplasm with thin cellulosic primary cell walls and abundant plasmodesmatal connections.
The maturation phase represents fully differentiated cells with thickened walls that perform specialised functions.

Question 2: Why did the marks placed near the root tip shift apart more than the marks placed further from the tip?

The region immediately behind the apex is in the elongation phase, where cells show increased vacuolation, cell enlargement, and new cell wall deposition — resulting in maximum elongation and displacement of marks.
Marks further from the tip are in the maturation zone where cells have already attained their maximal size and wall thickening is complete, so negligible further elongation occurs.
This differential elongation along the root axis explains why marks closest to the apex move the most during a fixed time period.

Question 3: The teacher mentioned that growth in the maize root is measured as increase in cell number, while growth of a watermelon cell is measured as increase in cell size. Analyse what this tells us about the different ways growth can be expressed, and state the mathematical formula for the growth rate in geometric growth.

Growth can be expressed in multiple ways depending on the organism or organ: as increase in cell number (as in maize root apical meristem producing 17,500 new cells/hour), or as increase in cell size (watermelon cells increasing up to 3,50,000 times in volume).
Other parameters include increase in fresh weight, dry weight, length, area, and volume — each appropriate for different contexts (e.g., length for pollen tube, area for a dorsiventral leaf).
The formula for geometric (exponential) growth is W1 = W0e^rt, where W1 is final size, W0 is initial size, r is the relative growth rate (efficiency index), t is time, and e is the base of natural logarithms.

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Case Set 2

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In a sugarcane research station, scientists wanted to increase the annual yield of sugarcane without expanding the area of cultivation. They decided to spray experimental plots with a well-known plant growth regulator. After the treatment, the stems of the sugarcane plants in the treated plots were significantly longer than those in the control plots. The yield from treated plots increased by approximately 20 tonnes per acre compared to the untreated control. In a parallel experiment, the same regulator was sprayed on juvenile conifers, which matured much earlier than normal. The scientists also noted that in beet plants, the same treatment caused rapid internode elongation just prior to flowering — a phenomenon known as bolting.

Question 1: Identify the plant growth regulator used in this experiment and state one other crop or plant mentioned in the chapter where this regulator produces a notable effect.

The plant growth regulator used is gibberellin (gibberellic acid, GA3).
Gibberellins cause elongation of grape stalks and improve the shape of apple fruits, making them commercially important in fruit production.
Gibberellins also delay senescence in fruits, allowing them to remain on trees longer and extending the market period.

Question 2: Explain the term 'bolting' and state the types of plants in which it is promoted by gibberellins.

Bolting refers to rapid internode elongation that occurs just prior to flowering in certain plants.
Gibberellins promote bolting in plants with a rosette habit — plants in which internodes remain compressed under normal conditions, giving a flat, low-growing appearance.
Examples of rosette plants where gibberellins promote bolting include beet, cabbages, and many other plants with rosette habit.

Question 3: The scientists observed that the same gibberellin treatment hastened maturity in juvenile conifers. Analyse how this observation, along with the sugarcane yield increase and bolting in beet, demonstrates the diverse physiological roles of gibberellins in plants.

Gibberellins demonstrate diverse physiological roles: in sugarcane they promote stem elongation (increase in internode length), directly increasing the volume of sugar-storing tissue and yield by up to 20 tonnes per acre.
In juvenile conifers, GAs hasten the maturity period, leading to early seed production — demonstrating a role in accelerating the transition from juvenile to adult phase of plant development.
Bolting in beet and cabbage shows that GAs control developmental transitions tied to reproduction, triggering internode elongation prior to flowering in rosette habit plants.
Taken together, these effects show that gibberellins act at multiple developmental stages — vegetative growth, phase transition, and reproductive development — illustrating that a single class of PGR can regulate diverse processes across different plant species and organs.

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Case Set 3

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A student observed that when she placed ripe oranges alongside unripe bananas in a closed box overnight, the bananas ripened faster than those stored separately. Her biology teacher explained that this phenomenon was first reported by H.H. Cousins in 1910, who confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. The teacher further explained that this gaseous substance promotes fruit ripening by enhancing the rate of respiration during the ripening process. The teacher also mentioned that the same substance, when applied to cucumbers, increases the proportion of female flowers, thereby increasing yield, and is commercially supplied through a compound called ethephon.

Question 1: Name the gaseous plant growth regulator described in the passage and state one structural feature that distinguishes it from other PGRs.

The gaseous plant growth regulator is ethylene (chemical formula C2H4).
Unlike other PGRs (which are liquids or solids), ethylene is a gas at room temperature, making it unique among plant growth regulators in its mode of dispersal.
It is a simple molecule and is synthesised in large amounts by tissues undergoing senescence and ripening fruits.

Question 2: What is the 'respiratory climacteric' and how is it related to ethylene's role in fruit ripening?

The respiratory climacteric is the rise in the rate of respiration that occurs during the ripening of fruits.
Ethylene enhances this respiration rate during ripening, accelerating the biochemical processes that lead to softening, colour change, and sugar accumulation in fruits.
This ethylene-triggered rise in respiration is a key mechanism by which ethylene promotes and coordinates the ripening process in climacteric fruits.

Question 3: Analyse the commercial significance of ethephon as a source of ethylene, describing how it works in the plant and listing at least three agricultural uses mentioned in the chapter.

Ethephon is the most widely used commercial compound as a source of ethylene; in an aqueous solution it is readily absorbed and transported within the plant, where it slowly releases ethylene, allowing controlled and sustained delivery of the hormone.
Agricultural use 1: Ethephon hastens fruit ripening in tomatoes and apples, enabling uniform ripening for harvest and reducing post-harvest losses.
Agricultural use 2: Ethephon accelerates abscission in flowers and fruits, enabling thinning of crops such as cotton, cherry, and walnut to improve fruit quality and facilitate mechanical harvesting.
Agricultural use 3: Ethephon promotes female flower formation in cucumbers, increasing the proportion of fruit-bearing flowers and thereby raising yield per plant.

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Case Set 4

Case Analysis

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During a practical class, a botany professor brought two sets of plants: Set A consisted of rosette-forming cabbages, and Set B consisted of mature tea bushes. She sprayed Set A with a specific plant growth regulator that caused rapid internode elongation and induced flowering. For Set B, she demonstrated that trimming the shoot tips regularly over several growing seasons had resulted in extremely bushy plants with dense lateral branches — a practice fundamental to commercial tea production. She asked students to identify the PGRs involved in each case and explain the mechanisms underlying the observed growth responses. She emphasised that both observations are directly applicable in agricultural and horticultural practices.

Question 1: Identify the PGR applied to the cabbage plants in Set A and name the growth response it induced.

The PGR applied to the cabbage plants is gibberellin (GA).
Gibberellins induced bolting — rapid internode elongation just prior to flowering — in the rosette-habit cabbage plants.
Under normal conditions, cabbage plants maintain a compact rosette form with compressed internodes; gibberellin application overcomes this and triggers the elongation associated with the reproductive phase.

Question 2: Explain the mechanism by which regular trimming of shoot tips in tea plants leads to increased lateral branching.

The growing apical bud of tea plants produces auxin, which moves downward and inhibits the growth of lateral (axillary) buds — this is the phenomenon of apical dominance.
When the shoot tip is removed by trimming (decapitation), auxin production at the apex ceases and its inhibitory influence on lateral buds is removed.
Released from inhibition, the lateral buds resume growth and develop into lateral branches, resulting in the bushy growth form desirable in tea plantations.

Question 3: Compare the roles of auxin and gibberellin in controlling stem growth, and explain how their effects demonstrate that PGRs can have complementary or distinct physiological roles depending on the context.

Auxin promotes elongation of cells behind the apical meristem and is responsible for tropic responses and apical dominance; at the cellular level, it stimulates cell elongation by loosening cell walls, but its primary effect at the organ level is suppression of lateral growth through apical dominance.
Gibberellins promote internode elongation by stimulating both cell division and cell elongation throughout the internode, resulting in overall stem length increase; they are responsible for bolting in rosette plants and increase crop yield in sugarcane.
The two PGRs are complementary in promoting stem elongation but have distinct targets: auxin acts primarily on cells immediately behind the apex, while gibberellins act throughout the internode and also affect developmental phase transitions.
This demonstrates that PGRs do not have a single universal role but act in context-dependent ways — sometimes synergistically (both promoting elongation), sometimes with distinct effects (auxin causing apical dominance, gibberellin promoting bolting), reflecting the complexity of plant hormonal regulation.

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Case Set 5

Case Analysis

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A plant physiologist was studying the stress responses of plants during a prolonged drought. She observed that within hours of water deficit, the stomata of test plants closed almost completely, significantly reducing water loss through transpiration. She identified a specific plant hormone as the key mediator of this response. She further noted that seeds of the same plant species remained dormant for extended periods in dry soil, germinating only after adequate rainfall. She explained that the same hormone that closed the stomata was also responsible for maintaining seed dormancy and that it acts as an antagonist to another class of PGRs that promotes germination. This hormone is produced in chloroplasts, plastids, and other parts of the plant.

Question 1: Identify the hormone described in the passage and state why it is called the 'stress hormone'.

The hormone described is abscisic acid (ABA).
ABA is called the stress hormone because it stimulates the closure of stomata and increases the tolerance of plants to various kinds of abiotic and biotic stresses, helping the plant conserve resources under adverse conditions.
ABA mediates a broad range of stress responses, making it the primary chemical signal that prepares plants to survive unfavourable environmental conditions.

Question 2: Explain how ABA helps seeds survive prolonged periods of unfavourable conditions such as drought.

ABA plays an important role in seed development, maturation, and dormancy; by inducing dormancy, ABA suppresses seed germination even when seeds are present in the soil.
ABA helps seeds withstand desiccation (extreme drying) and other factors unfavourable for growth, ensuring the seed's viability is preserved until conditions improve.
When favourable conditions (adequate water, temperature) return, ABA levels decline and allow germination to proceed, ensuring seeds germinate only when survival of the seedling is likely.

Question 3: The passage mentions that ABA acts as an antagonist to another class of PGRs that promotes germination. Identify this class and analyse how the balance between ABA and these PGRs determines whether a seed germinates or remains dormant.

The PGR class that ABA antagonises is gibberellins (GAs); while ABA promotes and maintains seed dormancy and inhibits germination, gibberellins actively promote germination and seedling growth.
In a dormant seed, ABA levels are high relative to gibberellins; ABA inhibits the synthesis of hydrolytic enzymes (such as amylases) needed to mobilise stored nutrients for germination, keeping the seed in a metabolically suppressed state.
When environmental conditions become favourable, gibberellin levels rise and ABA levels fall; gibberellins promote the synthesis of enzymes that break down seed reserves, initiating germination.
The ABA-to-gibberellin ratio thus acts as a molecular switch for dormancy versus germination — a classic example of antagonistic PGR interactions where the balance between an inhibitor and a promoter determines the developmental outcome of the seed.

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Chapter sections

Chapter 13: Plant Growth and Development | Case Study Questions

Passage

Question 1: Name the three phases of growth visible in the root tip from the apex towards the base.

Question 2: Why did the marks placed near the root tip shift apart more than the marks placed further from the tip?

Passage

Question 1: Identify the plant growth regulator used in this experiment and state one other crop or plant mentioned in the chapter where this regulator produces a notable effect.

Question 2: Explain the term 'bolting' and state the types of plants in which it is promoted by gibberellins.

Question 3: The scientists observed that the same gibberellin treatment hastened maturity in juvenile conifers. Analyse how this observation, along with the sugarcane yield increase and bolting in beet, demonstrates the diverse physiological roles of gibberellins in plants.

Passage

Question 1: Name the gaseous plant growth regulator described in the passage and state one structural feature that distinguishes it from other PGRs.

Question 2: What is the 'respiratory climacteric' and how is it related to ethylene's role in fruit ripening?

Question 3: Analyse the commercial significance of ethephon as a source of ethylene, describing how it works in the plant and listing at least three agricultural uses mentioned in the chapter.

Passage

Question 1: Identify the PGR applied to the cabbage plants in Set A and name the growth response it induced.

Question 2: Explain the mechanism by which regular trimming of shoot tips in tea plants leads to increased lateral branching.

Question 3: Compare the roles of auxin and gibberellin in controlling stem growth, and explain how their effects demonstrate that PGRs can have complementary or distinct physiological roles depending on the context.

Passage

Question 1: Identify the hormone described in the passage and state why it is called the 'stress hormone'.

Question 2: Explain how ABA helps seeds survive prolonged periods of unfavourable conditions such as drought.

Question 3: The passage mentions that ABA acts as an antagonist to another class of PGRs that promotes germination. Identify this class and analyse how the balance between ABA and these PGRs determines whether a seed germinates or remains dormant.

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