NEET Plant Growth and Development Previous Year Questions with Solutions

Auxins were the first plant hormones discovered. While IAA and IBA are natural auxins, NAA and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic auxins widely applied in agriculture and horticulture.

• • Promotes root initiation in stem cuttings and prevents premature fruit/leaf drop at low physiological concentrations.

At higher concentrations, 2,4-D acts as an effective selective herbicide. Dicotyledonous weeds possess broad leaves and distinct auxin sensitivity that leads to unregulated cellular elongation, vascular collapse, and plant death, whereas monocotyledonous grasses and cereal crops are unaffected.

• • Commonly used by gardeners to create weed-free lawns.

Kinetin is a cytokinin used to promote cell division and delay senescence. Abscisic acid (ABA) is a growth inhibitor (stress hormone) inducing stomatal closure and dormancy. Gibberellins promote stem elongation (bolting) and fruit enlargement. None of these act as weedicides.

• • Matches official NTA final answer key Option A / (1).

Plants like cabbage and beet exhibit a rosette habit during their vegetative phase, characterized by clustered leaves and compressed, stunted internodes close to the soil surface. Before entering the reproductive flowering phase, rapid and massive elongation of the internodes occurs, an event termed bolting.

• • Naturally triggered by long days or cold exposure (vernalization).

Exogenous application of gibberellins (specifically gibberellic acid, $GA_3$) induces prompt internodal elongation and flowering in cabbage and beet without requiring cold or photoperiodic induction.

• • NCERT explicitly mentions: "Gibberellins also promote bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit."

Abscisic acid (Option 1) inhibits growth, induces dormancy, and closes stomata. Indole butyric acid (Option 3) is an auxin used for rooting cuttings. Ethephon (Option 4) is an ethylene-releasing compound used to accelerate fruit ripening. Only Gibberellin promotes bolting.

• • Matches official NTA final answer key Option B / (2).

In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. This produces a constant linear elongation over time, mathematically expressed by a straight line graph ($y = mx + c$).

• • L_t = Length at time t.
• • L_0 = Initial length at time zero.
• • r = Growth rate / elongation per unit time.
• • t = Time elapsed.

Given: $L_0 = 20\text{ cm}$, $r = 30\text{ cm/day}$, and $t = 7\text{ days}$.

• • Length added due to growth = 30 cm/day × 7 days = 210 cm.

Add the growth increment to the initial stem length:

• • Looking at options: (1) 50 cm, (2) 170 cm, (3) 230 cm, (4) 460 cm.
• • Option (3) is 230 cm.

Cytokinins (kinetin, zeatin) have specific effects on cytokinesis and plant morphogenesis:

• • Help produce new leaves, chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.
• • Help overcome apical dominance imposed by auxins.
• • Promote nutrient mobilisation, which mobilises amino acids, mineral ions, and sugars towards treated tissues, thereby delaying senescence (aging) of leaves.

Understanding the opposing actions of growth promoters vs inhibitors:

• • Ethylene: Promotes senescence and abscission of plant organs, especially leaves and flowers.
• • Abscisic Acid (ABA): General plant growth inhibitor, induces dormancy, abscission, and accelerates senescence.
• • Gibberellins: Promote stem elongation, bolting in rosette plants, and breaking of seed dormancy, but do not specifically delay leaf senescence via nutrient mobilization.

The unique role of promoting nutrient mobilization to delay leaf senescence belongs specifically to Cytokinin (also known as the Richmond-Lang effect).

Analyzing the correct plant developmental principles:

• • Statement A: Auxins (e.g. IAA, IBA, NAA) applied to unpollinated pistils stimulate pericarp enlargement without fertilization, producing seedless fruits (parthenocarpy), commercially exploited in tomatoes and watermelons. Statement A is CORRECT.
• • Statement B: Plant growth regulators encompass two broad functional categories: plant growth promoters (participating in cell division, elongation, flowering, fruiting) and plant growth inhibitors (mediating dormancy, abscission, and stress responses). Statement B is CORRECT.
• • Statement C: In plant development, differentiated parenchyma cells lose their specialization and regain mitotic activity (dedifferentiation) to form secondary meristems (interfascicular cambium, cork cambium). The progeny of these meristems subsequently lose division ability and re-specialize into permanent secondary vascular tissues (redifferentiation). Thus, dedifferentiation is a mandatory developmental prerequisite for redifferentiation. Statement C is CORRECT.

Identifying false claims:

• • Statement D claims "Abscisic acid is a plant growth promoter". NCERT explicitly classifies ABA as a general plant growth inhibitor, functioning as an antagonist to gibberellins and promoting seed dormancy and stomatal closure. Hence, Statement D is FALSE.
• • Statement E claims "Apical dominance promotes the growth of lateral buds". In apical dominance, growing apical buds synthesize auxin which suppresses and inhibits the growth of axillary/lateral buds. Removal of the shoot tip (decapitation) relieves this inhibition and allows lateral branches to grow (practiced in tea plantations and hedge-making). Hence, Statement E is FALSE.

Only statements A, B, and C are correct, which corresponds to Option (1).

Cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions. This loss of capacity to divide is called differentiation (e.g., parenchyma, tracheary elements).

The living differentiated cells, that have lost the capacity to divide, can regain the capacity of division under certain conditions. This phenomenon is termed dedifferentiation. Formation of interfascicular cambium and phellogen (cork cambium) from fully differentiated medullary ray and cortical parenchymatous cells are classical examples.

When the cells produced by dedifferentiated meristems lose the ability to divide again and mature into permanent tissues (secondary xylem, secondary phloem, cork), it is called redifferentiation.

Synthetic auxins such as 2,4-D (2,4-dichlorophenoxyacetic acid) are widely applied in agriculture and horticulture.

NCERT explicitly notes: "2,4-D is widely used to kill dicotyledonous weeds, it does not affect mature monocotyledonous plants. It is used to prepare weed-free lawns by gardeners."

Lawns consist of grasses which are monocotyledonous plants. Because mature monocots are physiologically insensitive to the concentrations used to kill dicots, 2,4-D eliminates weeds without harming the lawn.

Sugarcane stores carbohydrates as sucrose in its stems. Increasing the physical length and mass of the stem directly translates into higher commercial sugar yields.

Gibberellins stimulate cell division and dramatic internodal cell elongation. NCERT explicitly states: "Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre."

Plant cell development follows a reversible tripartite trajectory:

Mesophyll cells are fully differentiated vegetative cells specialized for photosynthesis. When excised and placed on nutrient medium supplemented with hormones (auxin and cytokinin), they exit the $G_0$ quiescent phase, resume mitosis, and generate an unorganized callus:

The same dedifferentiation process occurs in plants in vivo during the formation of interfascicular cambium and cork cambium (phellogen) from mature parenchyma and collenchyma cells.

Gibberellins ($GAs$) mediate diverse developmental transitions in angiosperms and gymnosperms:

Conifers normally have a prolonged juvenile phase lasting several years before reaching reproductive maturity. Exogenous spray of gibberellic acid overcomes juvenility, accelerating cone formation and enabling early commercial seed collection.

NCERT states verbatim: "Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production."

In deep water rice varieties (e.g., lowland flood-prone cultivars), rising floodwaters submerge the lower plant body. Gas diffusion in water is $10^4$ times slower than in air, trapping endogenous ethylene gas inside tissues:

NCERT states under Ethylene: "Ethylene promotes rapid internode and petiole elongation in deep water rice plants. It helps leaves/upper parts of the shoot to remain above water."

2,4-D is a synthetic auxin herbicide. $GA_3$ promotes bolting in rosette plants and stem elongation in sugarcane, but is not the NCERT answer for deep water rice. Kinetin is a cytokinin promoting cell division.

Plant photoperiodic floral transition separates perception site from response site:

Photoreceptors (primarily phytochromes) localized within leaf mesophyll cells measure the critical duration of darkness and photoperiod. Upon receiving an inductive photoperiod, leaves synthesize a translocatable chemical messenger (florigen) that travels through the phloem to the shoot vegetative apex.

Plants exhibit developmental flexibility termed plasticity:

NCERT categorizes heterophylly under plasticity into two distinct classes: - Developmental (Phase-dependent): Leaves of juvenile plant differ from mature plant, e.g., cotton, coriander, larkspur. - Environmental: Leaves produced in air differ from submerged leaves in water, e.g., buttercup (*Ranunculus*).

Auxins include both naturally occurring phytohormones and synthetic chemical analogues:

At elevated concentrations, 2, 4-D acts as a systemic hormonal herbicide. It induces uncontrolled, aberrant cell division and vascular crushing in broad-leaved (dicotyledonous) weeds, leading to their rapid death. Monocotyledonous plants (such as wheat, rice, corn, and turf grasses) rapidly metabolize and conjugate 2, 4-D, remaining unharmed. Hence, it is universally applied to maintain weed-free lawns and cereal fields.

Living cells, plant organs, and microbial populations growing in natural environments exhibit a characteristic sigmoid (S-shaped) growth curve comprising three successive phases: 1. Lag Phase: Initial phase where cells adapt biochemically, synthesize enzymes, and grow slowly without massive division. 2. Log (Exponential) Phase: Following the lag phase, both daughter cells retain the ability to divide. Cell division and cell elongation proceed at an accelerated exponential rate ($W_1 = W_0 e^{rt}$), where the rate of growth reaches its absolute biological maximum. 3. Stationary Phase: As space, nutrients, and resources become limiting, the rate of growth decelerates and levels off.

- Senescence: The irreversible, terminal developmental phase of aging leading to tissue degradation and programmed death, characterized by negative growth. - Dormancy: A state of suspended metabolic development and zero mitotic growth, designed to survive adverse environmental conditions (e.g. seed dormancy).

Sugarcane stores sugar as sucrose in the parenchymatous cells of its stem. Consequently, a taller stem with longer internodes directly translates to greater sucrose yield.

Gibberellins (GA3) promote rapid cell elongation and cell division, causing elongation of intact stems and internodes.

NCERT explicitly states: "Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre."

Cytokinins promote cell division and delay senescence; Ethylene promotes fruit ripening and causes horizontal growth of seedlings; Abscisic acid is a stress hormone inhibiting growth.

In many plants, viable seeds fail to germinate even when external environmental conditions (moisture, temperature, oxygen) are favorable. This state is called seed dormancy.

NCERT explicitly identifies chemical inhibitors present in the seed coat or embryo that induce and maintain dormancy: Abscisic acid (ABA), phenolic acids, and para-ascorbic acid.

Dormancy can be overcome by treating seeds with gibberellic acid and nitrates. Gibberellins induce the synthesis of hydrolytic enzymes (such as alpha-amylase in aleurone layer of cereals) that mobilize food reserves, triggering seed germination.

Therefore, Gibberellic acid acts antagonistically to ABA as a promoter of germination, NOT an inhibitory substance governing dormancy.