Plant Growth And Development – NEET UG BiologyPractice Questions & PYQs

Ethylene acts as a volatile plant hormone that significantly influences root architecture and epidermal cell differentiation. By stimulating the formation of adventitious roots, promoting lateral root development, and accelerating root hair elongation, it substantially broadens the underground surface area available for water and nutrient uptake. This targeted physiological response distinguishes it from other hormones like auxins or gibberellins, which govern distinct developmental milestones.

Plant plasticity, or heterophylly, describes the developmental phenomenon where a species alters its leaf morphology in response to environmental conditions or life phases. Species such as cotton, coriander, and buttercup prominently display this adaptive trait by producing juvenile leaves that look drastically different from their mature foliage. Maize lacks this specific developmental flexibility, maintaining a consistent leaf shape throughout its life cycle regardless of external influences.

Environmental factors heavily influence plant morphogenesis, allowing certain species to alter their structural pathways depending on surrounding conditions. This remarkable adaptability enables a single genotype to produce entirely different physical forms or leaf shapes when exposed to contrasting habitats like aquatic versus terrestrial environments. Such versatile developmental flexibility in response to environmental cues is defined as plant plasticity.

During standard plant tissue culture protocols, mature permanent tissues like parenchyma are isolated and exposed to specific nutrient media containing exogenous growth regulators. These mature, specialized cells, which normally do not divide under natural physiological states, re-enter the cell cycle and resume mitotic divisions. This biological process of a differentiated cell losing its specialized restriction and regaining meristematic activity is termed dedifferentiation.

Photoperiodism relies on specialized photoreceptor pigments located within the foliar organs of plants to measure day length. The leaf acts as the primary sensory organ that intercepts light signals and synthesizes flowering hormones, which are subsequently transported to the shoot apex to trigger floral transition. While other parts of the plant respond to environmental cues, only the leaf perceives the photoperiodic stimulus.

Certain synthetic auxins function selectively as weedicides in agricultural practices. Among them, 2,4-dichlorophenoxyacetic acid is specifically applied to eliminate unwanted broad-leaved dicotyledonous weeds without harming monocot cereal crops. Other compounds like IAA and IBA occur naturally, whereas NAA serves different horticultural purposes such as preventing premature fruit drop.

Plant growth regulators frequently influence internodal elongation and overall vegetative biomass in commercially important crops. Gibberellins stimulate cell division and elongation in the stem tissue, which substantially increases the overall length of the plant. Applying this hormone to fields of sugarcane significantly boosts the total harvestable yield by producing taller and bulkier stalks.

Seed dormancy is maintained by various natural growth inhibitors that suppress germination until environmental conditions become favorable. Abscisic acid, phenolic acids, and para-ascorbic acid all act as inhibitory chemicals that accumulate and enforce this resting state. In contrast, gibberellic acid functions as a primary plant hormone that actively breaks dormancy by triggering the synthesis of hydrolytic enzymes like alpha-amylase.

Apical dominance is a physiological phenomenon where the terminal bud produces auxins that inhibit the growth of nearby lateral buds. Pruning or removing the shoot tips eliminates this high concentration of apical auxins, thereby releasing the lateral buds from suppression. This encourages dense branching and bushiness, which ultimately maximizes the harvestable surface area of tea leaves.

Photoperiodism relies on specialized photoreceptor pigments located within the green lateral appendages responsible for capturing solar radiation. When these specialized organs detect the critical day-length duration, they synthesize a mobile floral stimulus hormone known as florigen. This chemical signal is subsequently translocated through the phloem to the apical meristem, triggering the structural transition from vegetative growth to floral bud formation.

Plant growth regulators play a crucial role in commercial horticulture by overriding natural seasonal constraints on reproductive development. Specifically, synthetic auxins are widely utilized to promote uniform flowering in crops like pineapples, while ethylene gas or its releasers are concurrently applied to synchronize fruit set and maturation. Combining these two specific hormonal treatments ensures a reliable, year-round harvest schedule and maximizes agricultural productivity.

Stalk elongation in certain grass crops is directly regulated by specific endogenous hormones that promote cell stretching and division. Gibberellins are uniquely capable of stretching the internodes of plants without altering their developmental morphology. Cultivators leverage this property by spraying sugarcane with gibberellins to extend stem length and significantly elevate overall crop production.

Abscission, the natural detachment of leaves and immature fruits from a plant, is heavily regulated by hormonal balances across the separation zone. Applying specific growth-promoting compounds helps maintain cellular integrity and suppresses the enzymatic breakdown of cell walls at the petiolar base. Consequently, this targeted chemical intervention inhibits premature organ shedding and improves overall crop retention.

Phytochromes are specialized sensory photoreceptors that enable plants to monitor and respond to changes in light quality and duration in their environment. Structurally, these photoreceptor molecules consist of a light-absorbing pigment moiety covalently bound to a protein structure. This dual architecture allows the pigment to undergo reversible conformational changes upon absorbing red and far-red light, thereby regulating numerous physiological processes like germination and flowering.

In plant tissue culture, organogenesis depends heavily on the relative proportions of specific growth regulators supplied in the nutrient medium. A balanced interplay between cell division factors and elongation-promoting agents dictates whether undifferentiated callus tissue will develop root structures or shoot buds. Manipulating this precise hormonal ratio allows biotechnologists to successfully regenerate complete, functional plantlets from explants.

Plant growth regulators are quantified using specific biological response tests known as bioassays. The Avena curvature test measures the unequal growth response induced by the hormone indole-3-acetic acid. When an agar block containing this specific auxin is placed unilaterally on a decapitated coleoptile stump, it promotes differential cell elongation. This asymmetrical growth causes the coleoptile to curve, providing a reliable quantitative measure of the hormone.

Plant development under normal conditions follows a characteristic S-shaped trajectory featuring distinct phases of rate increase and plateau. This pattern begins with a slow lag phase, transitions into a rapid exponential phase of vigorous cell expansion, and eventually levels off into a stationary phase as growth reaches maturity. Plotting this cumulative growth against time yields a classic curve that reflects the changing rate of cellular division and elongation throughout the life cycle.

Biological assays rely on specific living tissue responses to determine the presence and concentration of plant hormones. Auxin activity is classically measured using the curvature response of oat coleoptiles. In this setup, hormone-laden agar blocks applied to one side of a decapitated tip stimulate localized cell elongation. The resulting bending angle directly correlates with the concentration of the applied auxin.

Phototropism in plants is regulated by the lateral redistribution of growth hormones in response to directional light cues. When unilateral light hits a stem, auxin molecules migrate away from the illuminated side toward the shaded region. This higher concentration of the hormone on the darker side accelerates cell division and elongation there. Consequently, the shaded flank grows faster than the lit flank, forcing the entire shoot to curve toward the light source.

Abscisic acid acts as a primary plant inhibitor that accumulates rapidly when a plant encounters environmental adversities such as drought or soil salinity. By triggering the swift closure of stomatal pores, this hormone drastically minimizes water loss through transpiration during dry conditions. Consequently, its role in conferring resilience against harsh environmental factors has earned it the designation of the stress hormone.