Plant Growth And Development – NEET UG BiologyPractice Questions & PYQs

Phytohormones are naturally occurring organic chemical messengers produced in minute amounts within specific tissues of a plant body. Once synthesized, these signaling molecules are transported to target sites where they orchestrate and modulate a wide array of vital physiological processes. Their presence governs everything from developmental transitions to responses against environmental cues, ensuring coordinated growth throughout the plant's life cycle.

Long-day plants typically require extended periods of illumination exceeding a critical length before they can transition into their flowering phase. Plant growth regulators like gibberellins can artificially substitute for this environmental requirement, effectively triggering floral development even when the plants are subjected to restricted daylight hours. This hormonal substitution highlights how internal chemical signals can bypass specific light duration constraints to induce flowering out of season.

Many biological processes and seasonal developmental milestones in living organisms are synchronized with the changing duration of day and night in their habitat. Photoperiodism describes the physiological reaction of organisms, particularly plants, to the relative lengths of light and dark periods in their environment. This mechanism allows species to time critical events like flowering, bud dormancy, and germination to favorable seasonal conditions.

Gibberellins induce bolting, which is the dramatic internode elongation observed in rosette plants like cabbage right before they flower. By stimulating both cell division and longitudinal cell expansion within the stem internodes, this hormone forces the plant to shoot upward rapidly. This specific physiological trigger is exclusive to gibberellins among standard plant growth regulators.

Flowering in many plants is normally strictly regulated by environmental triggers such as day length and seasonal temperature variations, which dictate when reproductive development can occur. When a tree flowers multiple times throughout disparate seasons with vastly different photoperiods and temperatures, it demonstrates complete independence from these environmental cues. Such behavior indicates that the plant's internal transition from vegetative to reproductive growth proceeds regardless of external light or thermal conditions.

Directional growth movements in plants in response to physical contact with a solid object are mediated by touch-based sensory mechanisms. When plant tendrils make contact with a support structure, the cells on the untouched side elongate faster than those on the contact side, causing the tendril to coil securely around the object. This specialized directional response that enables climbing plants to utilize physical support is termed thigmotropism.

Abscisic acid functions as a primary inhibitory regulator that helps plants cope with environmental stress and unfavorable seasons. A major physiological role of this compound is maintaining developmental arrest in embryos until external conditions improve. Consequently, high internal concentrations prevent premature germination while promoting other survival adaptations.

Cytokinins are plant growth hormones renowned for their ability to delay protein degradation and chlorophyll breakdown in aging tissues. By promoting nutrient mobilization and keeping stomata open, this phytohormone effectively inhibits the natural senescence process in detached leaves and other plant organs. It achieves this anti-aging effect largely by counteracting the senescence-promoting action of ethylene.

Phytohormones govern diverse physiological responses, where auxins successfully trigger seedless fruit development known as parthenocarpy, and plant growth regulators generally act as versatile agents capable of both promoting and inhibiting various developmental processes. Furthermore, specialized mature cells must first undergo dedifferentiation to regain meristematic capacity before they can redifferentiate into new specialized tissues. In contrast, abscisic acid functions as a major stress-induced growth inhibitor rather than a promoter, and strong apical dominance actively suppresses rather than encourages the growth of lateral buds.

Cell division and delayed cellular aging in plants are primarily regulated by cytokinins, which possess a unique capacity to direct the active transport of nutrients toward specific tissues. By mobilizing amino acids, sugars, and other essential metabolites to treated leaves, these hormones effectively prevent chlorophyll degradation and metabolic breakdown. This targeted redistribution of resources sustains cellular metabolism and successfully prolongs the functional lifespan of aging foliage.

Synthetic auxins function as selective weed killers because dicotyledonous and monocotyledonous plants exhibit distinct physiological sensitivities to these compounds. Mature monocotyledonous plants remain largely unaffected by standard agricultural concentrations of auxins due to physiological differences in uptake, translocation, and rapid degradation pathways. Consequently, gardeners can successfully eliminate broad-leaved weeds from lawns without causing any phytotoxic damage to the grass turf.

Plant growth regulators play distinct physiological roles, starting with abscisic acid which induces seed dormancy and helps seeds withstand desiccation. Ethylene further influences plant development by increasing the formation of female flowers in crops like cucumber, while gibberellins stimulate internode elongation to promote bolting in rosette plants such as beet and cabbage. Finally, cytokinins delay leaf senescence and direct nutrient transport by facilitating nutrient mobilisation throughout the plant tissues.

Plant tissue culture experiments originally demonstrated that auxin alone is insufficient for sustained cellular proliferation, requiring specific biological supplements to stimulate growth in tobacco stem explants. Organic growth factors such as coconut milk, yeast extract, and vascular tissue extracts provide essential purine derivatives and complex nutrients that synergize with auxins to drive cell division. Because abscisic acid acts as an inhibitor of growth and stress regulator rather than a cell-division factor, its addition fails to induce callus proliferation alongside auxins.

Gibberellins are characterized structurally as acidic plant growth regulators that influence various developmental processes throughout a plant's lifecycle. Cytokinins like zeatin were originally discovered and isolated from natural plant sources including coconut milk and corn kernels. Gaseous plant hormones such as ethylene play a crucial role in agriculture by effectively inducing uniform flowering and ripening in crops like mango. Meanwhile, statements regarding auxins antagonizing gibberellins or abscisic acid causing parthenocarpy are scientifically incorrect because abscisic acid acts primarily as a growth inhibitor while auxins are typically responsible for parthenocarpic fruit development.

Plant cells that have temporarily or permanently lost the ability to divide can under certain physiological conditions regain meristematic activity. This biological phenomenon where fully differentiated parenchyma cells revert to an actively dividing state to form secondary meristems is known as dedifferentiation. The resulting interfascicular cambium exemplifies this remarkable cellular plasticity, enabling plants to undergo secondary growth and increase their stem girth.

Gibberellins are phytohormones renowned for stimulating cell elongation and division in the internodal regions of plant stems. Since sugarcane crops store their carbohydrate reserves as sugar within these elongated stem structures, treating the fields with gibberellic acid significantly boosts overall vegetative growth. This targeted application stretches the internodes without altering the thickness, ultimately resulting in a substantial increase in sugar yield per acre.

Submerged aquatic plants such as deep water rice rely on gaseous signaling molecules to cope with environmental stresses like seasonal flooding. The rapid accumulation of ethylene inside the submerged tissues triggers accelerated internode and petiole elongation, lifting the upper parts of the plant above the water surface. This adaptive growth mechanism ensures that leaves remain in contact with the air for proper gas exchange and survival during prolonged inundation.

Plant growth regulators play a crucial role in altering phase changes and accelerating developmental transitions in trees. Among these hormones, gibberellins are particularly effective at overcoming the juvenile phase in conifers, which naturally have a prolonged immature period before reproduction. Applying gibberellic acid artificially prompts these young gymnosperms to transition rapidly into the adult reproductive stage, resulting in precocious cone formation and early seed production.

Living plant cells that have already matured and specialized often lose their ability to divide, remaining in a differentiated state. However, under specific in vitro conditions, these mature cells can reverse their fate and regain mitotic division capability to form an unorganized mass of cells called a callus. This reversion from a specialized, non-dividing state back to a meristematic, dividing condition is known as dedifferentiation.

Plant hormones are chemically diverse organic molecules synthesized from various metabolic precursors. Cytokinins represent a specific class of these growth regulators that are structurally characterized as adenine derivatives, typically featuring an isoprenoid or aromatic side chain attached to the adenine ring. This unique nitrogenous base structure allows them to actively promote cell division and coordinate various morphogenetic processes alongside auxins.