201. Cytokinins are synthesized mainly in:
ⓐ. Mature bark and old xylem vessels
ⓑ. Root apices and other actively growing regions
ⓒ. Dry seed coat after dormancy is complete
ⓓ. Dead sclerenchyma cells of stems
Correct Answer: Root apices and other actively growing regions
Explanation: Cytokinins are produced mainly in actively dividing or actively growing tissues, especially root apices. From these regions, they may be transported to other parts of the plant where they influence growth and development. Their production in dynamic tissues fits well with their role in stimulating cell division and growth. Dead or highly mature tissues are not major sites of cytokinin synthesis. Thus, root tips and similar growing regions are important sources of cytokinins.
202. Which statement best describes the role of cytokinins in plant growth?
ⓐ. They suppress mitosis in meristematic tissues
ⓑ. They act mainly as reserve food materials in young organs
ⓒ. They promote cell division and support growth in suitable tissues
ⓓ. They prevent all enlargement in developing leaves
Correct Answer: They promote cell division and support growth in suitable tissues
Explanation: Cytokinins are growth-promoting regulators that stimulate cell division and help maintain active growth in many tissues. Their effect is especially important in regions where cells are capable of responding by entering or continuing mitosis. In addition to division, they also support developmental processes related to tissue activity and longevity. They do not function as food reserves, nor do they generally suppress growth. Therefore, promotion of cell division and support of growth best summarize their role.
203. The delay of senescence by cytokinins means that cytokinins help:
ⓐ. Speed up drying and death of leaves
ⓑ. Keep tissues functionally active for a longer time
ⓒ. Induce immediate abscission of green leaves
ⓓ. Stop all metabolic activity in old organs
Correct Answer: Keep tissues functionally active for a longer time
Explanation: Senescence is the aging process of plant tissues, especially leaves, in which metabolic activity gradually declines. Cytokinins can delay this process and help tissues remain green and active for a longer period. This effect is agriculturally and physiologically important because it can prolong photosynthetic activity and general tissue function. Delaying senescence does not mean making tissues permanently young, but it does slow their aging. Thus, cytokinins help maintain functional activity for a longer time.
204. Which effect of cytokinins is most directly related to their use in keeping detached leaves green for longer?
ⓐ. Promotion of senescence
ⓑ. Delay of senescence
ⓒ. Induction of dormancy
ⓓ. Stimulation of abscission
Correct Answer: Delay of senescence
Explanation: Detached leaves normally begin to age and lose chlorophyll after some time, leading to yellowing. Cytokinins can delay this process by slowing senescence and helping maintain metabolic activity in the leaf. As a result, the leaf remains green for a longer period. This is one of the classic visible effects of cytokinins and is often used to demonstrate their action. Therefore, the relevant cytokinin effect here is delay of senescence.
205. Which of the following is the best non-example of a principal cytokinin function?
ⓐ. Promotion of cell division in actively growing tissues
ⓑ. Retardation of senescence in leaves
ⓒ. Support of growth in responsive plant organs
ⓓ. Induction of fruit ripening in many climacteric fruits
Correct Answer: Induction of fruit ripening in many climacteric fruits
Explanation: Cytokinins are mainly associated with cell division, growth promotion, and delay of senescence. Fruit ripening in many climacteric fruits is more closely linked with ethylene rather than cytokinins. Therefore, fruit ripening is the best non-example among the options given. The other choices match the major known roles of cytokinins. This question helps distinguish cytokinin function from that of other plant growth regulators.
206. Which statement about cytokinins and leaf senescence is most accurate?
ⓐ. Cytokinins generally accelerate chlorophyll loss in leaves
ⓑ. Cytokinins have no relation to aging of plant organs
ⓒ. Cytokinins can delay aging and help preserve greenness in leaves
ⓓ. Cytokinins cause immediate drying of detached foliage
Correct Answer: Cytokinins can delay aging and help preserve greenness in leaves
Explanation: One of the important functions of cytokinins is the delay of senescence, especially in leaves. When senescence is slowed, chlorophyll breakdown is delayed and the leaf remains green for a longer time. This also helps preserve metabolic function and usefulness of the tissue. Such an effect is opposite to rapid aging or drying. Therefore, preservation of greenness and delay of aging are accurate descriptions of cytokinin action.
207. Which statement best explains why cytokinins are considered growth-promoting regulators?
ⓐ. They only stop development and maintain permanent dormancy
ⓑ. They promote cell division and support active tissue development
ⓒ. They act only in fully dead tissues after maturation
ⓓ. They always inhibit metabolic activity in leaves
Correct Answer: They promote cell division and support active tissue development
Explanation: Cytokinins are called growth-promoting regulators because they stimulate important positive developmental processes. Their best-known effect is promotion of cell division, and they also help support continued activity in growing tissues. In addition, by delaying senescence, they prolong the useful functioning of organs such as leaves. These actions together contribute to growth and development rather than suppressing them. Hence, cytokinins are grouped among the major growth-promoting plant hormones.
208. In a plant tissue culture experiment, two callus samples are grown on different media. One medium has a relatively higher cytokinin level than auxin, while the other has a higher auxin level than cytokinin. The first sample is most likely to show:
ⓐ. Greater shoot bud formation
ⓑ. Immediate leaf abscission
ⓒ. Permanent seed dormancy
ⓓ. Selective weed killing
Correct Answer: Greater shoot bud formation
Explanation: In plant tissue culture, the balance between cytokinin and auxin strongly influences organ formation. When cytokinin is present in relatively higher proportion than auxin, shoot formation is generally promoted. In contrast, a higher auxin level usually favors root initiation. This makes the auxin-cytokinin ratio an important practical tool in organogenesis. The question is therefore testing the application of cytokinin in directing developmental pathways. A higher cytokinin level is especially associated with shoot bud development.
209. A gardener applies cytokinin paste to axillary buds of a plant, and the side branches begin to develop more actively. This most directly shows that cytokinins can:
ⓐ. Convert lateral buds into roots
ⓑ. Promote lateral bud outgrowth and branching
ⓒ. Induce abscission of terminal buds
ⓓ. Stop all elongation in the main shoot
Correct Answer: Promote lateral bud outgrowth and branching
Explanation: Cytokinins are known to encourage the growth of lateral or axillary buds, which increases branching in plants. This effect is important because it can partially counter the suppressive influence of the shoot apex. The hormone does not convert buds into roots or stop all growth in the main axis. Instead, it helps dormant or less active side buds begin active development. In practical horticulture, this property is useful for promoting bushier plant form. Thus, increased branching after cytokinin application shows stimulation of lateral bud growth.
210. Which of the following is the best non-example of a principal cytokinin function or use?
ⓐ. Promotion of cell division in dividing tissues
ⓑ. Delay of senescence in detached leaves
ⓒ. Induction of adventitious roots in stem cuttings
ⓓ. Promotion of shoot formation in suitable tissue culture conditions
Correct Answer: Induction of adventitious roots in stem cuttings
Explanation: Adventitious root induction in stem cuttings is mainly associated with auxins rather than cytokinins. Cytokinins are better known for promoting cell division, encouraging shoot formation, delaying senescence, and supporting active tissue development. Therefore, rooting of cuttings is the best non-example among the options given. This distinction is important because questions often compare the characteristic uses of different plant growth regulators. Recognizing which hormone is mainly linked with which response helps avoid common confusion.
211. Assertion (A): A detached leaf treated with cytokinin may remain green longer than an untreated detached leaf. Reason (R): Cytokinins can promote nutrient mobilization and delay senescence in leaf tissues.
ⓐ. Both A and R are true, and R is the correct explanation of A
ⓑ. Both A and R are true, but R is not the correct explanation of A
ⓒ. A is true, but R is false
ⓓ. A is false, but R is true
Correct Answer: Both A and R are true, and R is the correct explanation of A
Explanation: Cytokinins help delay senescence, so treated leaves often remain green and physiologically active for a longer time. One reason is that they support metabolic activity and help in nutrient mobilization toward the tissue. This slows the usual aging changes such as chlorophyll loss and decline in function. Therefore, the assertion is correct. The reason is also correct and directly explains why cytokinin-treated leaves stay green longer. This is a classic physiological effect of cytokinins.
212. A researcher observes that pale developing leaf tissue becomes greener and shows improved chloroplast development after treatment with a plant hormone. The hormone most likely involved is:
ⓐ. Abscisic acid
ⓑ. Ethylene
ⓒ. Cytokinin
ⓓ. 2,4-D
Correct Answer: Cytokinin
Explanation: Cytokinins are known not only for cell division but also for promoting chloroplast development in leaves. As chloroplast development improves, the tissue can appear greener and become more functionally active in photosynthesis. This role connects cytokinin with leaf vitality and maintenance of active metabolism. The other substances listed are associated with different functions and are not the best fit for this observation. Therefore, improved greening and chloroplast development point most strongly to cytokinin action.
213. Which statement best compares the developmental roles of auxin and cytokinin in organogenesis under tissue culture conditions?
ⓐ. Auxin and cytokinin always produce identical organs in the same concentration range
ⓑ. Auxin generally favors root initiation, whereas cytokinin generally favors shoot initiation
ⓒ. Cytokinin always inhibits organ formation, whereas auxin alone forms both roots and shoots
ⓓ. Auxin promotes senescence, whereas cytokinin induces fruit ripening
Correct Answer: Auxin generally favors root initiation, whereas cytokinin generally favors shoot initiation
Explanation: In plant tissue culture, auxin and cytokinin often act together, but their relative balance affects what organ develops. Higher auxin generally promotes root initiation, while higher cytokinin generally promotes shoot initiation. This relationship is one of the most important practical principles of plant organogenesis in vitro. The two hormones do not have identical roles, and their effects depend on proportion rather than simple presence alone. Understanding this comparison is essential in tissue culture applications.
214. A student says, “Cytokinins are only anti-aging substances and have no real role in active growth.” Which response is most accurate?
ⓐ. The statement is correct because cytokinins work only in old detached leaves
ⓑ. The statement is incorrect because cytokinins also promote cell division and shoot development
ⓒ. The statement is correct because cytokinins only destroy apical tissues slowly
ⓓ. The statement is incorrect only in seeds but not in vegetative organs
Correct Answer: The statement is incorrect because cytokinins also promote cell division and shoot development
Explanation: Cytokinins do delay senescence, but their function is much broader than simply slowing aging. They are important promoters of cell division and also contribute to shoot formation and active tissue development. This makes them central to both growth and maintenance processes. Limiting them only to anti-aging action gives an incomplete picture of their biological role. Therefore, the student’s statement is incorrect because cytokinins participate directly in major growth-related processes as well.
215. Ethylene is best described as a plant growth regulator that is:
ⓐ. A gaseous hydrocarbon effective in very small amounts
ⓑ. A mineral ion absorbed directly from the soil
ⓒ. A protein hormone stored in large quantities
ⓓ. A carbohydrate used mainly for respiration
Correct Answer: A gaseous hydrocarbon effective in very small amounts
Explanation: Ethylene is unusual among plant growth regulators because it is a gaseous substance. It is a simple hydrocarbon and can influence plant processes even when present in very small amounts. Unlike nutrients, it does not serve as a structural material or energy source. Instead, it acts as a regulatory signal in plant development. Its gaseous nature allows it to diffuse easily through plant tissues and surrounding air. This makes ethylene a distinctive and important plant hormone.
216. The plant growth regulator most directly associated with fruit ripening is:
ⓐ. Cytokinin
ⓑ. Ethylene
ⓒ. Auxin
ⓓ. Gibberellin
Correct Answer: Ethylene
Explanation: Ethylene is the hormone most closely linked with the ripening of many fruits. It promotes changes such as softening, colour development, conversion of starch into sugars, and characteristic aroma formation. These changes make the fruit more mature and attractive for seed dispersal. Other plant growth regulators have their own major roles, but fruit ripening is most strongly associated with ethylene. This is why ethylene is widely used in horticulture to regulate ripening. Its role in this process is one of its best-known functions.
217. Which change is commonly promoted by ethylene during fruit ripening?
ⓐ. Permanent maintenance of green colour in the fruit
ⓑ. Prevention of sugar formation from stored food
ⓒ. Delay of softening and aroma development
ⓓ. Softening of fruit along with colour and flavour changes
Correct Answer: Softening of fruit along with colour and flavour changes
Explanation: During ripening, fruits undergo several coordinated changes, and ethylene helps regulate many of them. The fruit commonly becomes softer as cell wall materials are modified. At the same time, colour often changes because pigments such as chlorophyll decline and other pigments become more visible or are newly formed. Sugars and characteristic flavours also develop. These combined changes make the fruit ripe and edible. Ethylene is therefore closely associated with softening, colour change, and flavour development.
218. Senescence in plants refers to:
ⓐ. The early stage of rapid cell division in meristems
ⓑ. The aging process of cells, tissues, or organs
ⓒ. The sudden return of mature cells to a meristematic state
ⓓ. The irreversible increase in dry mass during growth
Correct Answer: The aging process of cells, tissues, or organs
Explanation: Senescence is the natural aging phase of plant cells, tissues, or organs. During this period, physiological efficiency declines and structural changes gradually lead toward death of the organ or tissue. It is a normal part of development rather than an accidental event in many cases. Ethylene is one of the hormones that can promote or accelerate this aging process. Senescence is therefore different from growth, differentiation, or dedifferentiation. It represents the later declining phase in the life of a plant part.
219. Ethylene promotes senescence most directly by:
ⓐ. Keeping leaves permanently juvenile and green
ⓑ. Accelerating aging and functional decline of plant organs
ⓒ. Converting mature tissues back into meristematic tissues
ⓓ. Preventing any change in old flowers and fruits
Correct Answer: Accelerating aging and functional decline of plant organs
Explanation: Ethylene is widely known for promoting senescence in various plant organs such as leaves, flowers, and fruits. Under its influence, aging processes become more rapid and tissues begin to lose functional efficiency. In leaves, for example, chlorophyll may break down and the organ may move toward yellowing and eventual shedding. This function contrasts with cytokinins, which usually delay senescence. Therefore, ethylene is regarded as a promoter of aging rather than a substance that preserves youthful activity. Its role is important in the normal life cycle of plant organs.
220. Abscission in plants is the process of:
ⓐ. Formation of new lateral roots from mature tissues
ⓑ. Increase in stem girth by cambial activity
ⓒ. Shedding of leaves, flowers, or fruits from the plant
ⓓ. Germination of seeds under suitable conditions
Correct Answer: Shedding of leaves, flowers, or fruits from the plant
Explanation: Abscission is the separation and shedding of a plant part such as a leaf, flower, or fruit from the main plant body. This occurs at a specialized region called the abscission zone. It is a normal developmental process and may also be influenced by environmental and hormonal signals. Ethylene is one of the main hormones that promote this process. Abscission is therefore not the formation of new organs or an increase in size. It is the controlled detachment of an existing plant part.
