401. Which of the following statements about modes of plant growth regulator interaction are valid?
I. Complementary regulators may contribute different parts of one developmental outcome.
II. Synergistic regulators may produce an enhanced combined effect.
III. Antagonistic regulators exert opposing influences on a response.
IV. One regulator must use the same interaction mode in every tissue.
ⓐ. I and IV only
ⓑ. II, III and IV only
ⓒ. I, II and III only
ⓓ. I, II, III and IV
Correct Answer: I, II and III only
Explanation: Complementarity describes different regulatory contributions that fit together within one developmental process. Synergy describes reinforcement in which the combined response is greater than expected from the separate effects. Antagonism describes opposing influences on a shared event. For the described process, statements I, II and III represent distinct recognised interaction modes. Statement IV is incorrect given that regulator behaviour is context-dependent. The same PGR may act independently in one tissue, cooperate with another regulator in a second event and oppose a different regulator elsewhere. Statement IV is invalid since the mode of regulator interaction can change with tissue, developmental event and environmental context. Developmental stage, organ identity, environment and tissue responsiveness all influence the interaction observed.
402. Assertion: A plant growth regulator may act individually in one developmental event and interact with another regulator in a different event.
Reason: Plant growth regulators are generally small molecules.
ⓐ. Both are true, and the Reason correctly explains the Assertion
ⓑ. Both are true, but the Reason does not explain the Assertion
ⓒ. Only the Assertion is true; the Reason is a false statement
ⓓ. Only the Reason is true; the Assertion is a false statement
Correct Answer: Both are true, but the Reason does not explain the Assertion
Explanation: The assertion is true; regulatory action depends on biological context. A PGR may be sufficient to produce one response by itself, while another process may require complementary, synergistic or antagonistic interaction with additional regulators. In biological terms, the reason is generally true: PGRs are relatively small regulatory molecules. Their small size, however, does not explain why interaction modes change among tissues or developmental events. Context dependence arises from the responding tissue, developmental state, environmental conditions and presence of other regulatory influences. The two propositions can coexist without one explaining the other, so truth and explanatory relevance must remain separate. The reason provides a chemical characteristic but not the causal account required for the assertion.
403. Seed responses under different regulator conditions are shown below.
| Condition | ABA influence | Ethylene influence | Developmental outcome |
|---|
| P | High | Low | Dormancy maintained |
| Q | Reduced | High | Germination begins |
| R | High | High | Intermediate response |
Which conclusion best fits the table?
ⓐ. ABA–ethylene interaction can shift a seed between dormancy and germination
ⓑ. Ethylene reinforces ABA-mediated dormancy under the three conditions
ⓒ. ABA and ethylene regulate dormancy through independent, non-interacting effects
ⓓ. External conditions determine germination without contribution from internal regulators
Correct Answer: ABA–ethylene interaction can shift a seed between dormancy and germination
Explanation: Condition P associates strong ABA influence with maintenance of dormancy. Q shifts the balance toward ethylene and shows renewed embryo development. R produces an intermediate outcome when both influences remain high. The pattern supports regulation of dormancy through more than one PGR rather than through a single-hormone switch. ABA contributes to maintenance of the resting state, while ethylene can help break dormancy and initiate germination. Environmental conditions remain necessary, but they do not eliminate internal regulation. The contrasting seed states identify regulator balance, rather than one isolated treatment value, as the decisive variable. The intermediate response in R prevents either regulator from being treated as an all-or-none switch under every combination. The table also demonstrates that opposing regulators can coexist, with the observed response depending on their relative influence and tissue sensitivity.
404. Young fruits and old leaves are exposed to comparable auxin and ethylene conditions. Young fruits are retained, while old leaves senesce and detach more rapidly. Which interpretation best explains the contrast?
ⓐ. Regulator effects on abscission are independent of organ identity and age
ⓑ. Organ age alone determines abscission, making regulator action irrelevant
ⓒ. Fruit retention proves that ethylene is absent from every young tissue
ⓓ. Organ identity and developmental stage modify regulator effects on abscission
Correct Answer: Organ identity and developmental stage modify regulator effects on abscission
Explanation: The two observations concern different organs at different developmental stages. Auxin can help prevent premature drop of young fruits, whereas ethylene is associated with senescence and abscission, particularly in ageing organs. Young fruits being retained while old leaves detach more rapidly is consistent with those stage-dependent responses. The outcome cannot be predicted from the name of a regulator alone: the identity and developmental condition of the responding organ also matter. Comparable exposure conditions do not make a young fruit and an old leaf biologically equivalent. This example supports an interpretation in which regulator effects on abscission depend on the target tissue and its stage. It does not establish that age acts without regulators or that every organ given the same treatment must detach at the same rate.
405. Cytokinin treatment delays senescence in one group of leaves, while ethylene exposure accelerates senescence in another group. If both regulators are supplied together, the final timing of senescence will most reasonably depend on:
ⓐ. the assumption that both regulators become chemically identical
ⓑ. their relative effects and the responsiveness of the leaf
ⓒ. permanent elimination of ageing from the treated leaves
ⓓ. conversion of the leaves into dormant seeds
Correct Answer: their relative effects and the responsiveness of the leaf
Explanation: Cytokinins can delay leaf senescence, partly through nutrient mobilisation and maintenance of functional activity. Ethylene commonly promotes senescence. When both regulators act on the same tissue, the outcome cannot be predicted by assuming that either effect is automatically absolute. The timing will depend on their relative influence, concentration, developmental stage and the ability of the leaf cells to respond. The combined treatment does not make the regulators chemically identical or permanently abolish ageing. The timing of senescence depends on regulator balance and on whether the leaf can express the cytokinin and ethylene responses. This case demonstrates antagonistic regulation of a complex developmental event rather than a simple one-hormone command. No fixed outcome follows from naming both regulators alone. The leaf's developmental stage, sensitivity and the relative strengths of cytokinin-supported maintenance and ethylene-promoted decline determine which influence predominates.
406. Identical shoots retain their apical buds. In Shoot P, auxin-related apical influence remains strong and cytokinin activity in lateral buds is low. In Shoot Q, cytokinin activity in lateral buds is increased. Q develops more branches. Which conclusion is supported?
ⓐ. Lateral buds develop independently of internal regulator activity
ⓑ. Cytokinin increases branching by weakening the apical bud
ⓒ. Auxin and cytokinin act in separate shoots rather than one regulatory system
ⓓ. apical auxin and cytokinin in lateral buds jointly regulate shoot branching
Correct Answer: apical auxin and cytokinin in lateral buds jointly regulate shoot branching
Explanation: Shoot P displays the weak lateral growth expected under strong apical dominance. In Q, the apex remains present, but increased cytokinin activity promotes outgrowth of lateral buds. The comparison shows that branching depends on interacting influences rather than only on whether the apical bud exists. Auxin associated with the apex supports dominance, while cytokinin increases the developmental activity of axillary buds. Cytokinin does not need to kill the apex to produce this effect. The specified tissue, stage and environmental condition are consistent with: Apical auxin and bud cytokinin jointly shape branching. The result illustrates multi-regulator control of architecture, with the final form determined by the balance between main-axis dominance and lateral-shoot promotion.
407. Place the following events in a multi-regulator explanation of leaf ageing and loss.
P. Ethylene-related promotion of senescence becomes strong.
Q. Cytokinin-supported maintenance of leaf function declines.
R. The mature leaf begins losing physiological efficiency.
S. Abscission and leaf separation occur.
ⓐ. P → S → Q → R
ⓑ. Q → R → S → P
ⓒ. R → Q → P → S
ⓓ. Q → P → R → S
Correct Answer: Q → P → R → S
Explanation: Cytokinin can help maintain leaf function and delay senescence. As this protective influence declines, ethylene-related promotion of ageing can become more prominent. The mature leaf then shows loss of physiological efficiency and other senescence features. Abscission and separation occur later as the developmental process advances. The sequence is simplified and does not claim that each regulator changes in a perfectly isolated step. It organises the transition from reduced maintenance through promoted ageing to visible decline and final organ loss. Loss of cytokinin-supported maintenance precedes stronger ethylene-related ageing, visible functional decline and final separation, fixing Q → P → R → S. The event is better understood through interacting regulators than through one PGR acting alone. Loss of cytokinin-supported maintenance permits stronger ageing signals, after which functional decline precedes activation of the separation zone.
408. Which statement best corrects the misconception that one plant growth regulator completely controls each developmental event?
ⓐ. The regulator at the highest concentration alone determines each event
ⓑ. External conditions matter only when no internal regulator is active
ⓒ. Developmental events reflect interacting internal and external controls
ⓓ. Each developmental event is controlled by one regulator acting independently
Correct Answer: Developmental events reflect interacting internal and external controls
Explanation: Major developmental events arise from coordinated regulation rather than isolated commands from one hormone. Dormancy can involve ABA-supported maintenance and ethylene- or gibberellin-related release. Senescence may be delayed by cytokinin and promoted by ethylene. Apical dominance reflects auxin-related apical influence together with cytokinin effects in lateral buds, while abscission depends on organ age and several PGRs. External conditions and genomic control also contribute. The highest concentration of one regulator does not automatically determine the complete outcome, since tissue sensitivity and interaction type matter. The treatment-control difference supports this inference: Many developmental events result from interacting internal and external controls. The corrected view treats development as an integrated process. Genomic capacity, regulator interactions and environmental conditions together shape the final response, so assigning exclusive control to one molecule oversimplifies development. Plant development emerges from the combined state of the tissue, its genome, environmental inputs and interacting growth regulators.
409. Genetically similar plants receive the same cytokinin treatment. One group grows with adequate water and suitable temperature, whereas another experiences water shortage and unfavourable temperature. The first group shows much stronger growth and branching. The evidence most directly shows that:
ⓐ. External conditions can limit a cytokinin response despite similar genotype and treatment
ⓑ. Cytokinin fixes the same developmental outcome under all water and temperature conditions
ⓒ. Environmental stress permanently replaces genomic control whenever plant growth becomes restricted
ⓓ. Genetic similarity prevents water status and temperature from modifying regulator responsiveness
Correct Answer: External conditions can limit a cytokinin response despite similar genotype and treatment
Explanation: The plants have similar inherited capacity and receive the same named cytokinin treatment, so those two factors are held broadly constant. Their response differs when water and temperature conditions differ. Adequate water supports enlargement and metabolism, while a suitable temperature permits the biochemical processes needed for growth. Water shortage and an unfavourable temperature can therefore restrict the visible response even when cytokinin is supplied and the genotype is comparable. The evidence specifically demonstrates environmental modification of regulator action; it does not show that the environment erases genomic control or that cytokinin becomes irrelevant. Nor can cytokinin guarantee one fixed outcome independently of basic growth conditions. The comparison isolates an important qualifier: internal regulatory signals operate within an environmental context, and poor external conditions may limit how fully a tissue expresses its potential response.
410. Two plants receive equal gibberellin treatment and identical environmental conditions. Plant P has a normal gene required for gibberellin responsiveness, while Plant Q carries a non-functional version of that gene. P elongates strongly, but Q does not. The result demonstrates that:
ⓐ. Environmental conditions determine the response despite the genetic difference
ⓑ. the plant's genomic capacity determines responsiveness to applied gibberellin
ⓒ. Gibberellin failed to reach Plant Q despite equal treatment
ⓓ. External gibberellin bypasses the need for a functional response gene
Correct Answer: the plant's genomic capacity determines responsiveness to applied gibberellin
Explanation: Plants P and Q receive equal gibberellin treatment under identical environmental conditions, so unequal treatment and environment do not explain their different elongation responses. The stated genetic difference is a functional gene needed for gibberellin responsiveness in P and a non-functional version in Q. P elongates strongly, while Q does not, linking the visible response to the plant's inherited capacity to respond to the applied regulator. Supplying a hormone is insufficient when the plant cannot carry out the response that normally follows. The observations do not establish that gibberellin failed to reach Q, nor do they identify every intermediate step between hormone exposure and growth. They support the narrower conclusion that genomic capacity affects responsiveness under the tested conditions.
411. Growth responses are recorded under four combinations.
| Condition | Genomic response system | PGR supply | Environment | Growth |
|---|
| P | Functional | Adequate | Favourable | High |
| Q | Functional | Adequate | Severely dry | Low |
| R | Functional | Deficient | Favourable | Reduced |
| S | Non-functional | Adequate | Favourable | Reduced |
Which conclusion best integrates the results?
ⓐ. A favourable environment can replace genomic and PGR control
ⓑ. Adequate PGR supply guarantees high growth even under severe environmental drought
ⓒ. growth integrates genomic capacity, regulator activity and environment
ⓓ. Genomic response systems matter only when the plant is dead
Correct Answer: growth integrates genomic capacity, regulator activity and environment
Explanation: Condition P combines a functional response system, adequate regulatory supply and a favourable environment and shows the highest growth. Q demonstrates that severe environmental limitation can reduce growth even when intrinsic controls are intact. R shows that a regulator deficiency can restrict development under favourable surroundings. S shows that hormone supply cannot produce a full response when the genomic system required for responsiveness is defective. No single factor replaces all others. Interpreting condition together with growth is essential; their combined pattern supports: Growth integrates genomic capacity, regulator activity and environment. The four conditions show that complete growth requires compatible genetic capacity, internal regulation and a supportive environment. This table supports an integrated model in which inherited capacity, internal chemical regulation and external conditions jointly determine the observed developmental outcome. Only P combines all three favourable components, while each remaining condition removes one component and reduces growth, demonstrating joint necessity rather than a single sufficient cause.
412. Assertion: Plant growth regulators are the only intrinsic controls of plant development.
Reason: Genomic regulation also contributes to developmental capacity and tissue responsiveness.
ⓐ. Both are true, and the Reason correctly explains the Assertion
ⓑ. Both are true, but the Reason does not explain the Assertion
ⓒ. Only the Assertion is true; the Reason is a false statement
ⓓ. Only the Reason is true; the Assertion is a false statement
Correct Answer: Only the Reason is true; the Assertion is a false statement
Explanation: The assertion is false because plant growth regulators are not the only intrinsic controls of development. Genomic information determines developmental capacity, tissue identity and the ability to perceive or execute regulator responses. The stated reason remains therefore true: genomic regulation contributes alongside plant growth regulators to the internal control of development. Environmental conditions then interact with these intrinsic systems and influence which developmental pathway is expressed. A plant may receive a regulator yet fail to respond normally if the relevant gene products or competent tissues are absent. Conversely, genomic potential does not remove the need for appropriate regulatory and environmental signals. The integrated view retains the reason while rejecting the assertion's exclusive claim.
413. Identical plants are divided into two groups. Group P receives a light condition that promotes flowering, while Group Q remains under a non-promotory light condition. A change in internal regulator activity is detected in P before flowering begins. Which inference is most appropriate?
ⓐ. Light promotes flowering directly without changing internal regulation
ⓑ. Light may influence flowering through changes in internal regulator activity
ⓒ. The regulator change follows flowering and cannot contribute to its onset
ⓓ. The observations identify the complete molecular pathway from light to flowering
Correct Answer: Light may influence flowering through changes in internal regulator activity
Explanation: The light treatment precedes the change in internal regulator activity, and flowering follows later. This temporal sequence supports an inference that an external light condition can influence development partly through changes in internal regulation. The finding does not show that light is converted into floral tissue, nor does it make plant growth regulators unnecessary. It also provides no evidence for a particular receptor, signalling molecule or complete molecular pathway. The flowering difference under altered light, together with changed regulator activity, supports an indirect environmental effect mediated through internal control. The defensible conclusion is limited to the demonstrated relation among the environmental condition, the internal regulatory change and the later flowering response. Whole-plant interpretation connects the local treatment with the developmental stage and the process measured. Integrated reasoning avoids assigning every response to one regulator when genomic and environmental controls remain active.
414. Seeds remain dormant at one temperature but germinate after transfer to a suitable temperature. The transfer is accompanied by reduced inhibitory regulator influence and increased promotory influence. Which interpretation is best supported?
ⓐ. Temperature may shift internal regulator balance and promote germination
ⓑ. Suitable temperature promotes germination without altering internal regulation
ⓒ. Germination begins after promotory and inhibitory regulators disappear
ⓓ. Temperature alters regulator balance after germination is complete
Correct Answer: Temperature may shift internal regulator balance and promote germination
Explanation: The environmental change and the regulator shift occur together before germination. This supports a relationship in which temperature affects the developmental state partly through altered internal regulatory influence. Reduced inhibition and increased promotion favour the transition from dormancy to active embryo growth when water and oxygen are also available. The seed retains its genome throughout the treatment, and the response does not require disappearance of every PGR. Temperature also does not transform one seed structure directly into another. The conclusion integrates an external physical factor with an internal developmental balance without importing unsupported molecular detail. The paired change in temperature and internal balance supports an integrated interpretation rather than direct proof that temperature alone acts as a hormone. Suitable temperature permits a regulatory shift that releases the embryo from dormancy.
415. Genetically similar young plants receive equal water, nutrients and suitable temperature. Their internode lengths are measured after the same growth period.
| Group | Light regime | Treatment | Mean internode length |
|---|
| P | Normal | No applied gibberellin | \(8\,\text{mm}\) |
| Q | Normal | Applied gibberellin | \(12\,\text{mm}\) |
| R | Reduced | No applied gibberellin | \(6\,\text{mm}\) |
| S | Reduced | Applied gibberellin | \(10\,\text{mm}\) |
Which conclusion follows from these observations?
ⓐ. Light affects elongation, but gibberellin does not in either regime
ⓑ. Gibberellin affects elongation, but light does not in either treatment
ⓒ. Reduced light raises elongation, whereas gibberellin lowers it throughout
ⓓ. Light regime and gibberellin each affect elongation in the tested groups
Correct Answer: Light regime and gibberellin each affect elongation in the tested groups
Explanation: Compare groups that differ in only one listed condition. Under normal light, applied gibberellin is associated with a change from \(8\,\text{mm}\) in P to \(12\,\text{mm}\) in Q. Under reduced light, the corresponding comparison is \(6\,\text{mm}\) in R against \(10\,\text{mm}\) in S. Gibberellin treatment is associated with greater internode length in both light regimes. To assess light, compare P with R without applied gibberellin, and Q with S when gibberellin is applied. Reduced light is associated with a shorter internode in both comparisons. The matched conditions let the table test an external growth condition and a plant growth regulator at the level of observed growth. The data do not identify a detailed light-direction mechanism or show that every species must respond by these exact amounts.
416. Two groups of plants experience the same temperature shift. Group P shows the usual change in internal regulator activity and enters a new developmental phase. Group Q senses the temperature shift but cannot alter regulator activity and remains in its earlier phase. The strongest inference is:
ⓐ. Temperature influences development without involving internal regulators
ⓑ. Regulator activity changes as a consequence of the completed phase transition
ⓒ. PGR changes may mediate developmental transitions triggered by temperature
ⓓ. Group Q remained in the earlier phase because it did not sense the temperature shift
Correct Answer: PGR changes may mediate developmental transitions triggered by temperature
Explanation: Both groups receive the same external temperature change, so the difference lies in their internal regulatory response. P changes regulator activity and proceeds to a new developmental phase, while Q detects the environment but cannot complete the internal adjustment and fails to transition. This pattern supports mediation: the environmental signal influences development partly through a PGR-related change. Such evidence does not show that temperature is irrelevant or that the groups experienced different conditions. The recorded observation justifies the following conclusion within the tested conditions: PGR changes may mediate developmental transitions triggered by temperature. The conclusion remains limited to the supported level and does not claim that the regulator is the only pathway connecting temperature with development. Plant development emerges from the combined state of the tissue, its genome, environmental inputs and interacting growth regulators.
417. Assess the following statements concerning environmental effects mediated through plant growth regulators.
I. Light and temperature can influence developmental events partly through changes in PGR activity.
II. Environmental influence does not eliminate genomic control or tissue responsiveness.
III. Every light- or temperature-controlled event is explained completely by one PGR.
IV. An environmental cue may alter PGR activity without itself becoming a plant growth regulator.
ⓐ. I and III only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Light and temperature can modify developmental events partly through changes in plant growth regulator activity, so statement I is valid. Genomic control and the responsiveness of the target tissue remain necessary for expression of the response, which supports statement II. No single regulator explains every event influenced by light or temperature; several internal and external controls may act together, making statement III too absolute. An environmental cue can change regulator production, distribution or action without being chemically classified as a PGR, so statement IV is also valid. In these statements, I, II and IV preserve environmental mediation without removing genomic competence; III makes an unsupported one-regulator claim. The supported set is I, II and IV.
418. Identical viable seeds are exposed to a favourable temperature, adequate water and sufficient oxygen. Seed P has normal genomic responsiveness, a declining ABA influence and rising gibberellin-related promotion. Seed Q has the same environment and regulator changes but carries a defect preventing embryo growth. Which conclusion best integrates the case?
ⓐ. germination needs environment, regulator balance and genomic competence
ⓑ. A favourable environment guarantees germination regardless of embryo condition
ⓒ. Regulator balance alone can restore growth in non-responsive tissue
ⓓ. Genomic control becomes irrelevant after water enters the seed
Correct Answer: germination needs environment, regulator balance and genomic competence
Explanation: Seed P combines favourable external conditions, a regulator shift away from dormancy and an embryo capable of responding, so germination can proceed. Seed Q receives the same water, oxygen, temperature and internal regulator changes but cannot express embryo growth because its genomic response system is defective. This comparison demonstrates that no single component is sufficient. Environmental conditions provide necessary resources and signals, PGR balance influences the developmental state, and genomic capacity allows living tissues to execute the response. Q fails despite favourable conditions and regulator balance, showing that genomic competence remains essential for germination. Matching the environment and regulator state isolates embryo competence as the variable that determines whether renewed growth can be executed. The case integrates the major control levels without assuming that one regulator or one environmental factor completely determines development.
419. A population of plant cells begins with a small number of actively dividing cells. During early growth, both daughter cells of each division continue dividing and resources remain abundant. Later, many cells differentiate and nutrients become limiting. Which growth pattern is most likely across the entire period?
ⓐ. A constant linear decline followed by unlimited exponential growth
ⓑ. Arithmetic growth throughout because one daughter cell always differentiates
ⓒ. A stationary phase from the beginning followed by a sudden linear rise
ⓓ. An initially exponential increase followed by slowing and a sigmoid pattern
Correct Answer: An initially exponential increase followed by slowing and a sigmoid pattern
Explanation: At first, both daughter cells continue dividing and resources are abundant. The population of dividing cells can then increase multiplicatively, producing a period of rapid, approximately exponential growth. Later, many cells differentiate and leave the actively dividing population. At the same time, nutrients become limiting, so the earlier high rate cannot continue indefinitely. Growth consequently slows and may approach a stationary phase. Viewed across the whole period, an early slower part, a rapid middle rise and a later slowdown form a sigmoid pattern. An arithmetic pattern throughout would require a different continuing division arrangement, such as one daughter remaining active while the other leaves the cycle at each step. The changed cell states and resources explain the transition in the observed growth course.
420. Two cultures begin with equal cell numbers.
| Culture | Cell behaviour | Resource condition | Observed pattern |
|---|
| P | Both daughter cells continue dividing | Continuously replenished | Rapidly accelerating increase |
| Q | Both daughter cells initially divide | Not replenished | Increase followed by a plateau |
Which conclusion best explains the difference?
ⓐ. Cell division contributes to growth in P but not in Q
ⓑ. Culture Q plateaus because daughter cells stop dividing after the first division
ⓒ. Replenished resources force Culture P into an early permanent stationary phase
ⓓ. Resource limitation turns potentially exponential growth into a sigmoid pattern
Correct Answer: Resource limitation turns potentially exponential growth into a sigmoid pattern
Explanation: Both cultures initially possess cells capable of geometric increase, so their early behaviour can be similar. Culture P continues receiving resources, allowing accelerating growth to persist for longer. In Q, nutrients and space become progressively limiting. The growth rate therefore declines even though the cells were initially capable of repeated division, and the record approaches a plateau. This produces the later portion of a sigmoid curve. The difference is not evidence that division never occurred in Q or that its cells were dead. The divergence begins when Q becomes resource-limited, showing how environmental restriction converts potential exponential growth into a sigmoid course. The later plateau in Q follows resource exhaustion rather than a programmed change in the identity of every cell. It shows that cellular potential and environmental support jointly determine the visible growth pattern over time.
