301. Review the following statements about cytokinin action.
I. Cytokinins can promote the growth of lateral buds.
II. Cytokinins may help overcome apical dominance.
III. Cytokinins can delay leaf senescence by promoting nutrient mobilisation.
IV. Cytokinins maintain apical dominance by permanently suppressing every axillary bud.
ⓐ. I and IV only
ⓑ. I, II and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and III only
Explanation: Cytokinins can stimulate lateral-shoot development and help axillary buds overcome the suppressive condition associated with apical dominance. They also delay leaf senescence, partly by promoting movement of nutrients toward the treated or cytokinin-rich tissue. This nutrient mobilisation helps maintain chlorophyll, metabolic activity and functional capacity for a longer period. Statement IV reverses the recognised effect. Cytokinins do not preserve apical dominance by permanently suppressing every lateral bud; their action can favour release and growth of those buds. Read together, I, II and III describe cytokinin effects on branching and leaf longevity; IV states the opposite. When read with their qualifiers, statements I, II and III describe two connected cytokinin roles: modification of shoot architecture and extension of functional leaf life.
302. Shoot P retains an active apical bud and shows weak axillary-bud growth. Shoot Q also retains its apex, but cytokinin is applied directly to several axillary buds, which then begin growing. Which interpretation best fits the result?
ⓐ. Cytokinin has strengthened suppression by the apical bud
ⓑ. Cytokinin promotes lateral-bud growth only after the apex becomes inactive
ⓒ. The apical bud is no longer producing any internal regulator
ⓓ. Cytokinin can help lateral buds overcome apical dominance
Correct Answer: Cytokinin can help lateral buds overcome apical dominance
Explanation: The apical bud remains intact in both shoots, so the experiment does not remove the source associated with apical dominance. The changed condition is direct cytokinin application to the axillary buds of Q. Their subsequent growth shows that cytokinin can increase the ability of lateral buds to develop despite the continuing presence of the apex. This result does not establish complete loss of every apical influence, nor does it transform the buds into roots. Instead, it demonstrates that shoot architecture reflects interacting regulatory effects. Auxin associated with the apex supports dominance, whereas cytokinin in responsive lateral buds can favour their outgrowth and partially overcome that suppression. Since both shoots retain their apices, apex removal cannot explain the difference. Local treatment changes cytokinin influence at the buds themselves, demonstrating that bud responsiveness can modify the strength of apical dominance without eliminating the apex.
303. Equal detached leaves are kept in darkness under identical conditions. Leaf P receives water, while Leaf Q receives cytokinin. Q remains green longer and retains a higher concentration of mobile nutrients. The strongest conclusion is:
ⓐ. Cytokinin mobilises nutrients toward the treated leaf and delays senescence
ⓑ. Cytokinin prevents treated leaves from progressing beyond maturity
ⓒ. Cytokinin delays senescence only by increasing the light available to the leaf
ⓓ. Cytokinin prevents nutrient movement while independently maintaining greenness
Correct Answer: Cytokinin mobilises nutrients toward the treated leaf and delays senescence
Explanation: The leaves experience the same darkness and general environment, while cytokinin treatment is the principal difference. Leaf Q remains green for longer and retains more mobile nutrients, linking cytokinin with delayed senescence and nutrient mobilisation. The response does not mean that ageing has been abolished permanently or that cytokinin itself becomes a pigment. Rather, the regulator helps maintain the functional state of the leaf by directing or retaining resources that support cellular activity. Leaf P provides the comparison needed to distinguish this effect from hydration alone. The treated leaf's stronger nutrient retention and delayed yellowing connect cytokinin action with prolonged functional life. The water-treated leaf provides the baseline, so the linked nutrient and colour differences can be attributed to cytokinin under these conditions. The observations connect a biochemical resource pattern with a visible developmental outcome: continued greenness and delayed decline.
304. A leaf is supplied with cytokinin, but its cells are experimentally unable to respond by attracting or retaining mobile nutrients. Which result is most likely?
ⓐ. Senescence is delayed normally because nutrient movement is unnecessary
ⓑ. Senescence is delayed through direct chlorophyll synthesis independent of nutrients
ⓒ. The cytokinin-related delay of senescence is reduced
ⓓ. The leaf remains green permanently despite failed nutrient mobilisation
Correct Answer: The cytokinin-related delay of senescence is reduced
Explanation: Cytokinin can delay leaf senescence in part by promoting nutrient mobilisation toward the responding tissue. If the leaf cannot carry out that response, the regulator loses an important route through which it maintains metabolism and functional integrity. Senescence may therefore proceed sooner than in a normally responsive cytokinin-treated leaf. The prediction does not require immediate root formation or unrestricted division of mature cells. This observation does not claim that nutrient mobilisation is the only influence on senescence. The changed condition specifically weakens one supported cytokinin effect, so a reduced delay is the most defensible outcome. Failure to mobilise nutrients removes an important route by which cytokinin prolongs leaf function, reducing the delay of senescence. The case separates regulator supply from the tissue processes needed to express its action. Regulator presence and target response are therefore separate requirements; supplying cytokinin cannot preserve the full effect when a downstream nutrient-mobilisation response has been disabled.
305. Assertion: Cytokinin treatment may keep a detached leaf green for longer.
Reason: Cytokinin can promote nutrient mobilisation toward the treated tissue and delay senescence.
ⓐ. 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, and the Reason correctly explains the Assertion
Explanation: The assertion describes a recognised cytokinin effect: treated leaves may retain chlorophyll and remain functionally green for a longer period than comparable untreated leaves. The reason provides the biological connection. Cytokinin promotes mobilisation of nutrients toward responsive tissue, helping maintain metabolism and delaying the age-related decline characteristic of senescence. The regulator does not make the leaf permanently young, but it can extend the period before visible and functional deterioration becomes pronounced. The assertion-reason relation is explanatory rather than merely associative, since the reason accounts for the precise outcome asserted. Cytokinin-supported nutrient mobilisation and delayed senescence directly account for the prolonged greenness of the treated leaf. Both propositions are true, and the nutrient-mobilisation effect directly explains the prolonged greenness described in the assertion.
306. Cytokinin responses are compared in two tissues.
| Record | Tissue condition | Cytokinin treatment | Response |
|---|
| P | Responsive axillary bud | Applied locally | Bud outgrowth increases |
| Q | Detached responsive leaf | Applied to leaf | Yellowing is delayed |
| R | Cytokinin-insensitive axillary bud | Applied locally | Little outgrowth |
| S | Leaf unable to mobilise nutrients | Applied to leaf | Senescence delay is reduced |
Which conclusion best integrates the records?
ⓐ. Cytokinin produces tissue-specific responses that depend on cellular responsiveness and the supported process
ⓑ. Cytokinin delays senescence only by converting leaves into axillary buds
ⓒ. Every cytokinin-treated tissue must show identical cell-division and ageing responses
ⓓ. Local cytokinin application acts independently of tissue sensitivity and nutrient movement
Correct Answer: Cytokinin produces tissue-specific responses that depend on cellular responsiveness and the supported process
Explanation: Records P and Q show two established cytokinin outcomes in different tissues. A responsive axillary bud increases its outgrowth, whereas a detached leaf remains green longer. Records R and S reveal that application alone is not sufficient. The insensitive bud shows little response, and the leaf that cannot mobilise nutrients shows a reduced delay of senescence. Cytokinin therefore acts through the competence and physiological machinery of the target tissue. Bud growth and leaf maintenance are not one identical process disguised in two organs: one concerns activation of lateral growth, while the other involves nutrient mobilisation and delayed decline. The table also rules out the idea that a regulator mechanically forces the same response everywhere. Cytokinin has multiple effects, but each depends on the developmental state, sensitivity and functional pathway available in the treated tissue.
307. Ethylene is most accurately described as:
ⓐ. an acidic regulator associated mainly with stem elongation
ⓑ. an adenine-related regulator associated with cell division
ⓒ. a synthetic auxin associated with selective weed control
ⓓ. a gaseous regulator associated with ripening and senescence
Correct Answer: a gaseous regulator associated with ripening and senescence
Explanation: Ethylene is chemically distinct among the principal plant growth regulators since it is a simple gaseous hydrocarbon. Its molecular formula is \(\mathrm{C_2H_4}\). The gaseous state allows it to move through air spaces and influence nearby responsive tissues or fruits. It is not a gibberellin, cytokinin or synthetic auxin, although its developmental effects may overlap with those of other regulators in particular situations. Correct identification requires combining physical state, chemical formula and regulatory function. Ethylene participates in ripening, senescence, abscission, dormancy breaking and several specialised growth responses rather than being restricted to one organ or application.
308. Ethylene production is compared in four plant materials.
| Material | Developmental state | Relative ethylene production |
|---|
| P | Young actively expanding leaf | Low |
| Q | Senescing leaf | High |
| R | Unripe fruit before rapid ripening | Low |
| S | Ripening fruit | High |
Which interpretation is supported?
ⓐ. Young expanding tissues produce more ethylene than senescing tissues
ⓑ. Ripening is accompanied by a decline in ethylene production
ⓒ. Ethylene production is enhanced during senescence and fruit ripening
ⓓ. Ethylene production is similar in young, senescing and ripening tissues
Correct Answer: Ethylene production is enhanced during senescence and fruit ripening
Explanation: The table shows high ethylene production in the senescing leaf and ripening fruit, while the younger leaf and pre-ripening fruit show lower values. This pattern agrees with the recognised association of abundant ethylene production with ageing tissues and fruits undergoing ripening. This finding does not mean that ethylene is absent from all other tissues, only that production is especially prominent in the listed developmental states. The comparison also shows why developmental stage must be specified when discussing regulator production. A leaf or fruit cannot be assigned one fixed ethylene output throughout its life. The two high-production rows correspond to senescence and ripening, while younger tissues remain lower. The strongest conclusion integrates the two high-producing categories rather than generalising from one organ alone. Comparing P with Q and R with S shows the same stage-related shift in two organs: production rises during decline or ripening rather than simply with tissue presence.
309. Ripe fruits are placed in a chamber connected by an open air passage to a second chamber containing responsive unripe fruits. The second group begins changing sooner than an isolated control. Which property of ethylene most directly permits this influence?
ⓐ. Its gaseous nature allows movement through the shared atmosphere
ⓑ. It is transported from ripe fruits to unripe fruits through xylem
ⓒ. It remains within ripe-fruit tissues and triggers a separate signal
ⓓ. It requires direct contact between ripe and unripe fruit surfaces
Correct Answer: Its gaseous nature allows movement through the shared atmosphere
Explanation: Ethylene is a gaseous plant growth regulator. Ripe fruits can release it into the surrounding air, allowing it to travel through the open passage and reach responsive unripe fruits in the second chamber. Those fruits begin changing sooner than the isolated control, which supports an influence carried through the shared atmosphere. Direct contact between fruit surfaces is unnecessary, and separate fruits do not have a shared xylem connection through which the regulator must travel. The isolated control matters because it helps distinguish the open-air arrangement from an ordinary change occurring at the same time in all fruits. The observations support airborne ethylene influence under these conditions without specifying an exact gas concentration or proving that every unripe fruit would respond identically.
310. A young expanding leaf produces little ethylene but still shows a clear growth response when exposed to the gas from a nearby ripening fruit. Which conclusion follows?
ⓐ. Low endogenous production means the young leaf cannot respond to external ethylene
ⓑ. Ethylene production by a tissue and its responsiveness to ethylene are distinct properties
ⓒ. Only senescing tissues possess the cellular machinery needed for ethylene responses
ⓓ. The young leaf must first become a ripening fruit before responding to the gas
Correct Answer: Ethylene production by a tissue and its responsiveness to ethylene are distinct properties
Explanation: Endogenous production describes how much ethylene a tissue releases, whereas responsiveness describes whether its cells can perceive and react to the regulator. The young leaf produces little ethylene under the stated condition, yet an external source supplies the gas and a response occurs. This demonstrates that low production does not equal absence of sensitivity. Ripening fruits and senescing organs often produce ethylene abundantly, but other responsive tissues can still react when exposed to it. The gaseous nature of ethylene permits movement through the surrounding atmosphere from the fruit to the leaf. The comparison separates source strength from target competence: one tissue can be a strong producer, while another can be a responsive target despite weak endogenous production. Developmental state influences both properties, but it does not make them identical. Ethylene-related changes should be assigned from the treated tissue and observed response rather than from the gas’s presence alone.
311. Consider the following effects of ethylene on an ordinary dicot seedling.
I. Promotion of horizontal growth
II. Swelling of the seedling axis
III. Formation of an apical hook
IV. Unrestricted vertical elongation of every internode
ⓐ. I, II and IV only
ⓑ. I, III and IV only
ⓒ. I, II and III only
ⓓ. II, III and IV only
Correct Answer: I, II and III only
Explanation: Ethylene produces a characteristic set of responses in ordinary dicot seedlings. The seedling shows reduced simple axial extension, increased horizontal growth, swelling of the axis and a pronounced apical hook. These features alter the direction and proportions of growth rather than merely accelerating vertical elongation. Statement IV is therefore inconsistent with the described response. The three valid statements should be interpreted together, since the ethylene phenotype includes directional change, thickening and hook formation. Read together, I, II and III together define the ethylene seedling response; IV reverses the axial effect. The effect also illustrates why ethylene cannot be described as universally promotory or inhibitory: it suppresses one dimension of growth while promoting other structural responses in the same seedling.
312. Examine the arrangement described below. Seedling P grows as a relatively straight, slender axis. Seedling Q receives ethylene and develops a shorter swollen axis, more horizontal growth and a curved apical region. Which feature of Q is the apical hook?
ⓐ. The increase in root-hair number
ⓑ. The widening of the lower axis
ⓒ. The horizontal orientation of the entire seedling
ⓓ. The curved structure protecting the shoot apex
Correct Answer: The curved structure protecting the shoot apex
Explanation: The apical hook is the curved upper region surrounding and protecting the delicate shoot apex of the young dicot seedling. It must be distinguished from the swollen axis and the broader change toward horizontal growth, which are separate ethylene-associated features. In Seedling Q, all three responses occur together, but the hook refers specifically to curvature near the apex. Root hairs belong to another ethylene response studied in roots and do not identify the described shoot structure. The hook's position above the cotyledons and around the growing shoot tip identifies its protective role during emergence. Recognising the hook requires using position and form rather than treating every visible change in the seedling as the same effect. The apical hook belongs to the protective curvature of the emerging shoot, whereas horizontal growth and swelling describe the altered axis.
313. Identical dicot seedlings are grown in darkness. Group P receives air, while Group Q receives ethylene. Q becomes shorter, thicker and more horizontally oriented, with a stronger apical hook. The strongest inference is:
ⓐ. Ethylene redirects seedling growth rather than stopping it completely
ⓑ. Ethylene uniformly accelerates vertical elongation throughout the seedling
ⓒ. Ethylene increases radial swelling through vascular-cambium activity
ⓓ. Ethylene suppresses hook formation while enhancing straight axial growth
Correct Answer: Ethylene redirects seedling growth rather than stopping it completely
Explanation: Group Q continues developing but distributes growth differently from the control. Its axis elongates less, becomes thicker, shifts toward horizontal growth and forms a stronger apical hook. These coordinated changes show that ethylene modifies direction and proportion rather than eliminating all biological activity. The thickening described in a young seedling is not automatically secondary growth, which requires lateral-meristem activity and organised radial tissue production. Hook formation also protects a living apex rather than indicating its death. Holding the remaining variables constant isolates the evidence for: Ethylene redirects seedling growth rather than stopping it completely. The comparison demonstrates a context-specific ethylene response in which some growth dimensions are inhibited while others are promoted. Production of ethylene and responsiveness to it are separate properties; failure of either can alter the final phenotype.
314. A mutant dicot seedling is unable to respond to ethylene but otherwise grows normally. When exposed to the regulator, it is most likely to show:
ⓐ. a stronger triple response than a normal ethylene-responsive seedling
ⓑ. normal horizontal growth but no swelling of the seedling axis
ⓒ. little horizontal growth, axis swelling or apical-hook enhancement
ⓓ. enhanced apical-hook formation without swelling or horizontal growth
Correct Answer: little horizontal growth, axis swelling or apical-hook enhancement
Explanation: The regulator is present, but the seedling lacks the capacity to express the recognised ethylene response. Hormone availability alone cannot produce the expected phenotype when the target tissue is insensitive. Compared with a normal responsive seedling, the mutant should show reduced or absent horizontal growth, axis swelling and apical-hook enhancement after treatment. The case does not concern fruit ripening or secondary xylem formation. It illustrates a general principle of developmental regulation: an observed response depends on both the regulator and a competent living tissue. Target insensitivity therefore predicts little horizontal growth, swelling or enhancement of the apical hook despite ethylene exposure. Removing either side of that relation weakens the outcome even when environmental conditions remain suitable. The same regulator can inhibit elongation in ordinary seedlings yet promote elongation in deep-water rice under a different adaptive context.
315. Arrange the events in a logical sequence after an ordinary dicot seedling is exposed to ethylene.
P. Ethylene reaches responsive seedling tissues.
Q. Longitudinal elongation of the axis is inhibited.
R. Radial expansion and horizontal growth become prominent.
S. A short, thick seedling with a pronounced apical hook is expressed.
ⓐ. Q → P → S → R
ⓑ. P → Q → R → S
ⓒ. R → P → Q → S
ⓓ. P → R → S → Q
Correct Answer: P → Q → R → S
Explanation: Ethylene must first reach tissues capable of responding, so P is the starting event. In an ordinary dicot seedling, ethylene then suppresses normal longitudinal extension of the axis. Reduced elongation is accompanied by increased radial expansion and a shift toward horizontal growth, placing R after Q in the logical progression. The visible outcome is a short, thick seedling with a pronounced apical hook, which is the integrated phenotype represented by S. The dependency between ethylene reaches responsive seedling tissues and longitudinal elongation of the axis is inhibited fixes the beginning of P → Q → R → S. The order describes a dependency from regulator contact through altered growth processes to the final observable form; it does not require every component of the phenotype to begin at a completely separate instant. Perception of the gas precedes altered elongation and swelling, and these cellular responses produce the visible seedling form.
316. Assertion: Ethylene-treated dicot seedlings may become shorter and thicker than untreated seedlings.
Reason: Ethylene can reduce ordinary axial elongation while promoting axis swelling and horizontal growth.
ⓐ. 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, and the Reason correctly explains the Assertion
Explanation: The assertion accurately describes the altered proportions of an ethylene-treated dicot seedling. Reduced extension along the principal axis produces a shorter form, while promoted radial swelling makes the axis thicker. Increased horizontal growth further changes the seedling's orientation. That reason directly identifies the combination of growth changes responsible for the visible phenotype. Ethylene has not stopped all growth; it has redistributed it among different directions and structures. Reduced axial extension combined with increased swelling directly produces the shorter, thicker dicot-seedling form. Each proposition is true, and the change from axial elongation toward swelling and horizontal growth explains why the treated seedling appears shorter and broader. The reason states the altered growth pattern that produces the shorter, thicker seedling described in the assertion.
317. Evaluate the following statements about ethylene.
I. It promotes senescence in plant organs.
II. It promotes abscission, especially of leaves and flowers.
III. It permanently prevents separation of every mature organ.
IV. It may shorten the functional life of responsive tissues.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Ethylene is strongly associated with acceleration of ageing and organ separation. In responsive tissues, it can promote senescence, reducing the period of mature function, and can hasten abscission of organs such as leaves and flowers. These effects do not mean that ethylene causes every organ to fall under all conditions, since tissue age, regulator sensitivity and interactions with other controls remain important. Statement III reverses the recognised relation by claiming universal prevention of separation. When read with their qualifiers, statements I, II and IV correctly connect ethylene with both the physiological decline of an organ and its eventual detachment. Accordingly, I, II and IV fit ethylene's influence on ageing and separation; III states the opposite universal effect. Senescence and abscission are related but distinct events, and ethylene can influence both.
318. Identical flowering shoots are placed in separate chambers. Shoot P receives normal air, while Shoot Q receives ethylene. Flowers on Q age and detach earlier. The strongest conclusion is:
ⓐ. Ethylene accelerates both floral senescence and subsequent abscission
ⓑ. Ethylene delays floral abscission while accelerating senescence
ⓒ. Ethylene accelerates floral senescence without affecting separation
ⓓ. Earlier flower detachment indicates reduced sensitivity to ethylene
Correct Answer: Ethylene accelerates both floral senescence and subsequent abscission
Explanation: The two shoots differ mainly in ethylene exposure. Earlier ageing and detachment of flowers on Q link the regulator with both senescence and abscission. That observation does not show that the flowers lacked maturity; senescence commonly follows a period of mature function. That observation does not represent organ conversion or delayed ageing. The paired design is especially informative because it records two stages of the same developmental progression: decline in floral function followed by separation from the plant. Tissue competence and developmental context are both retained in this conclusion: Ethylene accelerates floral senescence and abscission. Because ethylene exposure is the principal changed variable, the linked acceleration of ageing and detachment is the justified inference. The conclusion remains specific to acceleration under the tested conditions and does not require every flower species or developmental stage to respond identically.
319. A plant carries mature leaves that normally abscise after an ethylene rise. A mutation prevents those leaves from responding to ethylene while leaving their attachment tissues otherwise functional. What is the most likely effect?
ⓐ. faster leaf abscission than in normal ethylene-responsive tissue
ⓑ. immediate reversion of mature leaves to a juvenile developmental state
ⓒ. complete loss of ethylene production in every other plant organ
ⓓ. reduced ethylene-dependent acceleration of mature-leaf abscission
Correct Answer: reduced ethylene-dependent acceleration of mature-leaf abscission
Explanation: The question separates ethylene availability from a leaf's ability to respond to it. These mature leaves normally abscise more quickly after an ethylene rise, but the mutation prevents them from responding to that signal. Even if ethylene is still produced, its usual contribution to accelerating separation is weakened in those leaves. Their attachment tissues remain functional, so the result should be stated as reduced ethylene-dependent acceleration, not as proof that abscission can never occur. Ageing and other regulatory influences may still affect eventual leaf loss. The mutation is specified for the leaves' response and provides no basis for claiming that ethylene production stops in every other organ. The prediction follows from the missing response in the target tissue.
320. Match each observation with the ethylene-related process it most directly represents. A Column II entry may be reused.
| Column I entries | Column II entries |
|---|
| P. Progressive decline in an old leaf; Q. Detachment of a flower from its stalk; R. Loss of metabolic efficiency before leaf fall; S. Separation of a mature leaf | 1. Senescence; 2. Abscission |
ⓐ. P-2, Q-1, R-2, S-1
ⓑ. P-1, Q-2, R-1, S-2
ⓒ. P-1, Q-1, R-2, S-2
ⓓ. P-2, Q-2, R-1, S-1
Correct Answer: P-1, Q-2, R-1, S-2
Explanation: Senescence refers to the progressive age-related decline in structure and function, so the old leaf in P and the loss of metabolic efficiency in R belong to that process. Abscission is the separation of an organ from the plant, so detachment of the flower in Q and loss of the mature leaf in S fit the second category. Ethylene can promote both processes, but the terms are not synonyms. An organ may begin senescing before it separates, and the developmental sequence can involve both stages. Progressive functional decline in P and R is senescence, whereas physical separation in Q and S is abscission. Reuse of the entries is necessary because two observations illustrate each process, within the supplied observations.
