Plant Growth and Development

Phase of elongation is characterised by new cell wall deposition, increased vacuolation and enlargement of cell. Presence of large conspicuous nuclei is a feature of cells in the meristematic phase.

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.

Gibberellin promotes internode elongation prior to flowering in cabbage.

2, 4-D is an artificial auxin, which is widely used as herbicide.

Arithmetic growth can be expressed as :

$\mathrm{L}_{\mathrm{t}}=\mathrm{L}_0+\mathrm{rt}$ [where, $\mathrm{L}_{\mathrm{t}}=$ length at time ' t ', $\mathrm{L}_0=$ Length at time 'zero', $\mathrm{r}=$ growth rate]

$ \begin{aligned} \mathrm{L}_{7 \text { th day }} & \Rightarrow 20+30 \times 7 \\ & \Rightarrow 20+210 \\ & \Rightarrow 230 \mathrm{~cm} \end{aligned} $

The length of the stem at the end of $7^{\text {th }}$ day is 230 cm .

ABA is a plant growth inhibitor and an inhibitor of plant metabolism.

Apical dominance promotes growth of apical bud.

Statements A, B and C are correct.

The phytohormone that delays leaf senescence by promoting nutrient mobilization is cytokinin (Option B). Cytokinins

are produced mainly in the roots and transported to the leaves

maintain chlorophyll content and protein synthesis

mobilize nutrients from older tissues to growing parts, thereby postponing senescence

Abscisic acid $\rightarrow$ Helps seeds to withstand desiccation

Ethylene $\rightarrow$ Promotes female flowers in cucumber

Gibberellin $\rightarrow$ Promotes bolting in beet, cabbage etc.

Cytokinin $\rightarrow$ Helps in nutrient mobilisation

Hence, the correct answer is option (C)

F. Skoog's famous experimentation on plant tissue culture involved the proliferation of callus from internodal segments of the tobacco stem when auxin was supplied along with certain substances. The substances that supported callus proliferation included extracts from vascular tissues, coconut milk, and yeast extract. However, abscisic acid (ABA) did not support this proliferation. Hence, the correct answer to the question is Option C, Abscisic acid.

Let me explain why:

Auxins are a class of plant hormones that play a critical role in the growth and behavioral processes in a plant's life cycle. However, for the induction of callus (an unorganized mass of plant cells), auxin alone is often not sufficient. The addition of other substances can synergistically enhance its effect. This has been shown through Skoog’s observations with the following substances:

1. Extract of Vascular Tissues: Vascular tissues contain various nutrients and growth factors that can promote cell division and growth.

2. Coconut Milk: Rich in cytokinins, which promote cell division and cooperation with auxins enhances callus formation.

3. Yeast Extract: Contains vitamins and nutrients that can aid in the growth and development of plant tissues.

On the other hand,

4. Abscisic Acid (ABA): ABA is typically known for its role in inhibiting growth and promoting dormancy, not for promoting callus formation. It acts in contrast to auxins and cytokinins, often inhibiting processes like germination and cell division. Thus, it does not support callus proliferation and is not used for this purpose in Skoog’s experiments.

Therefore, the correct answer is:

Option C: Abscisic acid

The correct option is Option A: A, C, D.

Let's analyze each statement:

A. All GAs are acidic in nature.

This statement is true. Gibberellins (GAs) are a group of plant hormones that are primarily known for their role in stem elongation, seed germination, and flowering. Their chemical structure features a characteristic carboxyl group (COOH), making them acidic.

B. Auxins are antagonists to GAs.

This statement is false. While auxins and GAs can have opposing effects in some cases, they generally act synergistically, meaning they work together to enhance each other's effects. For example, both auxins and GAs can promote stem elongation, and their combined action can result in more pronounced growth than either hormone alone.

C. Zeatin was isolated from coconut milk.

This statement is true. Zeatin is a naturally occurring cytokinin, a class of plant hormones that promote cell division. It was first isolated from coconut milk, a rich source of plant growth regulators.

D. Ethylene induces flowering in Mango.

This statement is true. Ethylene, a gaseous plant hormone, plays a crucial role in fruit ripening and can also induce flowering in certain plant species, including Mango. Ethylene treatment can be used to synchronize flowering in Mango trees, leading to more efficient fruit production.

E. Abscisic acid induces parthenocarpy.

This statement is false. Abscisic acid (ABA) is a plant hormone primarily known for its role in stress responses, such as drought tolerance and dormancy. It does not induce parthenocarpy, which is the development of fruit without fertilization. Parthenocarpy can be induced by other hormones, such as auxins and gibberellins, depending on the plant species.

Therefore, the correct statements are A, C, and D, making Option A the correct choice.

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.

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:

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.

The answer is Option A - Phototropism.

The phenomenon of phototropism is the growth of organisms in response to light. Charles Darwin and his son Francis Darwin were among the early scientists to study phototropism. They found that light is perceived by the tip of the plant shoot, and this perception leads to growth further down the shoot.

Later, it was discovered that a hormone, named auxin, is responsible for this effect. Auxin accumulates on the side of the plant facing away from the light source, and causes cells on that side to elongate, thus bending the plant towards the light. This played a major role in the discovery of auxin, which is one of the main plant hormones.

While auxins do also influence other processes like root initiation, gravitropism, and apical dominance, their role in phototropism was significant to their discovery.

So, based on your question, phototropism (Option A) is the correct answer.

Option B : (A)-(IV), (B)-(III), (C)-(II), (D)-(I)

Explanation :

(A) Auxin - (IV) Promotes flowering in pineapple: Auxins are a class of plant hormones that influence cell growth by altering cell elongation. However, the role of promoting flowering in pineapple is actually played by Ethylene. Therefore, there's an error in this match.

(B) Gibberellin - (III) Increase in the length of grape stalks: Gibberellins are plant hormones that stimulate stem elongation, seed germination, and flowering. They are known to increase the length of grape stalks.

(C) Cytokinin - (II) Overcoming apical dominance: Cytokinins are plant hormones that promote cell division and delay leaf senescence. They also play a role in overcoming apical dominance, encouraging the growth of lateral buds.

(D) Ethylene - (I) Promotes female flower formation in cucumber: Ethylene is a gaseous plant hormone which regulates a wide range of biological processes. It is known to influence sex determination in certain plants, promoting female flower formation in cucumber.

Option C : "Plant growth is generally determinate" is not correct.

Plant growth is generally indeterminate, not determinate. This means that plants typically continue to grow as long as they live, unlike animals which have a determinate growth and stop growing after reaching a certain age or size. The meristematic tissues in plants (such as the apical meristem, lateral meristem etc.) allow for continuous growth and production of new cells. This is a key characteristic of plants. So, the statement that plant growth is generally determinate is incorrect.

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 B - Ethylene.

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.

$\bullet$ Cytokinins are derived from adenine.

$\bullet$ Auxins are derivatives of indole compounds.

$\bullet$ Abscisic acid is derived from carotenoids.

$\bullet$ Ethylene is derived from methionine.

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.

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.

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.