31 Years NEET Previous Year Questions: Plant Growth & Development - 1 - NEET MCQ

The correct answer to the question is Option B, Gibberellin. Gibberellins (GAs) are a group of plant hormones that play a crucial role in regulating various aspects of plant growth and development. One of the notable effects of gibberellins is their ability to promote stem elongation by stimulating cell division and elongation.
In the case of sugarcane, a crop that is valued for its stem (from which sucrose is extracted), applying gibberellin can increase the length of the stem, thereby potentially increasing the yield of the crop. This increase in stem length due to gibberellin application is primarily because gibberellins overcome the inhibitory effects of other hormones that suppress growth; they stimulate the cells in the stems to grow larger and divide more frequently.
While Auxin (Option A) and Cytokinins (Option C) also influence plant growth and have their specific uses, such as rooting and promoting cell division, respectively, they do not primarily target stem elongation to the extent gibberellins do. Abscisic acid (Option D) is generally involved in stress responses and the inhibition of growth, making it unsuitable for promoting stem elongation.
Therefore, to specifically enhance stem length and yield in sugarcane, gibberellins are the most effective plant growth regulators among the options given.

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The process involved in the formation of interfascicular cambium from fully developed parenchyma cells is known as "dedifferentiation." This is because dedifferentiation refers to the phenomenon where mature, specialized cells revert to a more primitive, less specialized state. In plants, certain mature cells, such as parenchyma cells, can lose their specialized characteristics and revert to meristematic activity. This newly formed meristematic tissue can then differentiate into specialized tissues once again, in this case, forming interfascicular cambium.

In contrast, differentiation (Option A) is the process where cells develop from a less specialized form to a more specialized form. Redifferentiation (Option B) refers to the process where dedifferentiated cells become specialized again, but this is a subsequent step following dedifferentiation. Maturation (Option D) refers to the final stages of cell and tissue specialization, not the regression to a less developed state.

Therefore, the correct answer to the query is: Option C - Dedifferentiation

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Auxin is a plant growth hormone primarily known for its role in stimulating plant growth by facilitating elongation of cells within shoots. Its effect varies depending on the plant species and its developmental stage. The given options can be assessed to understand why grass remains undamaged in lawns treated with auxin for weed control.

Option A: promotes apical dominance.
Apical dominance refers to the phenomenon where the main, central stem of the plant grows more dominantly than the side stems due to the concentration of auxin in the apex. This is not relevant to the selective action of auxin on weeds versus grass, because while it might affect growth patterns, it doesn’t relate directly to why weeds are eliminated and grass is not.

Option B: promotes abscission of mature leaves only.
Abscission is the process where parts of a plant, such as leaves or fruit, are shed. Auxin helps delay abscission by inhibiting the process. Thus, promoting abscission does not align with the characteristic actions of auxin. Additionally, this mechanism would not be selective for weeds over grass.

Option C: does not affect mature monocotyledonous plants.
Grasses are monocotyledonous plants, characterized by having one seed leaf, parallel leaf veins, and vascular bundles scattered in stem sections. It is known that the structure and growth patterns of monocots differ sufficiently from dicots; hence, many dicot-specific herbicides (including those based on auxin-like substances) do not affect monocots. Therefore, this option provides a plausible explanation why grass, a monocot, remains unaffected by a treatment intended for dicot weeds.

Option D: can help in cell division in grasses, to produce growth.
While auxin does promote cell division and growth in plants, this is a generalized action and does not explain why it would selectively not harm grass when used as a weed killer in lawns.

Based on the analysis above, the correct answer is Option C: does not affect mature monocotyledonous plants. This option directly addresses the differential effect of auxin on monocots versus dicots, aligning with the observed outcome that grass remains undamaged while dicot weeds are targeted.

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The hormone that promotes internode/petiole elongation in deep water rice is Ethylene.
Ethylene is a plant hormone that plays a critical role in growth and development, including responses to environmental stimuli. In the case of deep water rice, when the plants are submerged, the ethylene concentration increases and promotes rapid internodal elongation, which allows the plant to keep its leaves above the water surface.
So, the correct answer is Option C - Ethylene.

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Gibberellic acid (GA) is a plant hormone that stimulates cell elongation, germination, and influences a variety of developmental processes, including maturation and seed production. In the forestry industry, it is often used to hasten the maturity period and stimulate early seed production in juvenile conifers.

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Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity e.g. heterophylly in cotton, coriander and larkspur. In such plants, leaves of juvenile plant are different in a shape from those in mature plants.
Maize does not show plasticity.

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Ethylene is a gaseous plant hormone. It induces development of adventitious roots on various types of cutting. It promotes the development of lateral roots and growth of root hairs. Cytokinin helps to overcome the apical dominance.
Auxin is used to kill dicot weeds. Gibberellin speeds up the malting process.

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Ethylene increases the number of female flowers and fruits in certain plants such as cucumber. Gibberellins are used to increase the size of fruits in some plants.

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• The ability of plant to follow different pathways and produce different structures in response to environment is called plasticity.
• During differentiation, cells lose their ability to divide and form permanent cell.
• The process where the differentiated cells again lose the ability to divide and form permanent cells is called redifferentiation.

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• Cytokinins are derived from adenine.
• Auxins are derivatives of indole compounds.
• Abscisic acid is derived from carotenoids.
• Ethylene is derived from methionine.

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2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants.

They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds (Figure 15.11). It is widely applied in tea plantations, hedge-making. Can you explain why?

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Synthetic auxins like NAA and 2, 4-D are often employed for inducing flowering in pineapple, Ethylene also stimulates Powering in pineapple and related plants. Hence, both auxin and ethylene can be applied artificially to induce flowering in pineapple plants throughout the year to increase yield.

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Plants which are difficult to flower can be made to do so by spraying them with 2, 4,  –D (2, 4 – dichlorophenoxy acetic acid) and NAA (napthalene acetic acid) which are synthetic auxins, e.g., litchi, pineapple.

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Gibberellins stimulate the synthesis of α-amylase and proteases enzyme in germinating grains of cereals. They are involved in the conversion of starch into sugar. The proteases convert an inactive β-amylase to the active form. The active β-amylase and α-amylase together digest starch to glucose which is mobilized to meet the metabolic demands of embryo. 

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Gibberellins & ABA are antagonistic with each other. ABA counteracts many effects of gibberellins like induction of hydrolases and alpha- amylases in barley seedlings.

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Phototrophic curvature is the result of uneven distribution of auxin. The experiments conducted by Charles Darwin concluded that the tip of coleoptile of canary grass contain auxin that causes the bending of the entire coleoptile towards the light source.

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Abscisic acid (ABA), also known as abscisin II and dormin, is a plant hormone. It functions in many plant developmental processes, includsing bud dormancy. Abscisic acid is a derivative of carotenoids. It was called “abscisin II” originally because it was thought to play a major role in abscission of fruits. At about the same time another group was calling it “dormin” because they thought it had a major role in bud dormancy. The name abscisic acid (ABA) was coined by a compromise between the two groups.

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NAA is a synthetic auxins. Some of the most  widely used weed killers are synthetic auxins. Synthetic auxin analogs include 1–naphthaleneacetic acid (NAA), 2, 4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5- T).

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Senescence is not confined only to whole plant,it may be limited to a particular organ such as leaf and flower or cells such as phloem and xylem. Abscission is natural shedding of leaves,foliage branches,fruits floral parts etc. According to Leopold (1967) abscission is a senescence phenomenon. Senescence as an active developmental cellular process in the growth and functioning of a flowering plant,is indicated in leaf abscission. 

So, the correct answer is 'Leaf abscission.'

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The effect of gibberellins had been known in Japan since early 1800 where certain rice plants were found to suffer from bakane or bakanae (foolish seedling) disease. Such rice plants were thin, pale green, spindle-shaped, longer by 50% than the healthy plants, and were sterile.

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Gibberllic acid stimulates cell growth of leaves and stem causing their expansion, elongation respectively and leaf fall is controlled by ABA.

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Gibberellins induces elongation of internodes.

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Ratio of cytokinins to auxins controls differentiation. If both of these are present in equal quantities, the cells divide but fail to differentiate. If there is more cytokinin than auxin, shoot buds develop. If there is more auxin than cytokinin, roots develop.

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