Plant Growth and Development

🎯 Key Points

• 5 classical hormones: Auxin (elongation, phototropism, apical dominance), Gibberellin (stem elongation, germination), Cytokinin (cell division, delays senescence), ABA (stress/dormancy/stomatal closure — "stress hormone"), Ethylene (ripening, abscission — the only gaseous hormone)
• Growth has 3 phases: meristematic (active division) → elongation → maturation (differentiation) — strictly in that order at any growing point
• Photoperiodism: flowering response to relative length of the DARK period (not light period as commonly assumed); plants classed as short-day, long-day, or day-neutral
• Vernalization: cold treatment required before a plant can flower (e.g. wheat, barley) — distinct from photoperiodism, which is about day/night length, not temperature
• Newer hormones: Jasmonates (defense against herbivory/pathogens), Strigolactones (inhibit shoot branching, promote mycorrhizal symbiosis), Florigen (the FT protein, made in leaves, moves via phloem to trigger flowering at the shoot apex)

Phases and Patterns of Growth

• Meristematic phase: cells at root/shoot tips actively divide, rich in protoplasm, thin walls
• Elongation phase: cells enlarge significantly (vacuolation, new cell wall material), just behind the meristematic zone
• Maturation phase: cells differentiate into specific tissue types, attaining their final functional form
• Arithmetic growth: only one daughter cell continues to divide, the other differentiates — linear growth rate (e.g. a root elongating at constant length per unit time)
• Geometric growth: both daughter cells retain the ability to divide — growth starts slow (lag phase), accelerates rapidly (exponential/log phase), then slows as nutrients become limiting (forming the classic sigmoid/S-shaped curve seen in most natural growth)

Plant Hormones

• Auxin (IAA): promotes cell elongation; causes phototropism (bending toward light, due to asymmetric auxin distribution) and apical dominance (auxin from the shoot tip suppresses lateral bud growth); transported in a strictly POLAR (basipetal, away from the apex) manner via PIN efflux carrier proteins; used commercially to induce rooting in stem cuttings
• Gibberellins: promote stem elongation (dramatically so in genetically dwarf plants), seed germination (by inducing α-amylase to mobilise stored starch), and bolting/flowering in some species; used to produce seedless grapes with larger fruit size
• Cytokinin: promotes cell division, delays leaf senescence, and promotes lateral bud growth — functionally antagonistic to auxin's apical dominance effect; the auxin/cytokinin RATIO (not either hormone alone) determines whether tissue-cultured cells form roots (high auxin) or shoots (high cytokinin)
• Abscisic Acid (ABA): the principal "stress hormone" — induces stomatal closure during water stress (via Ca²⁺ signalling in guard cells), and promotes seed and bud dormancy, generally antagonising the growth-promoting hormones
• Ethylene: the only gaseous plant hormone; promotes fruit ripening, leaf/fruit abscission, and the "triple response" (reduced stem elongation, increased stem thickening, horizontal growth) seen in etiolated seedlings exposed to it

Newer/Lesser-Known Hormones

• Jasmonates: activate plant defence responses against herbivore attack and pathogens, regulate pollen development, and generally inhibit growth under stress
• Strigolactones: inhibit shoot branching (synergistic with auxin, antagonistic with cytokinin) and promote symbiosis with mycorrhizal fungi by stimulating hyphal branching — also act as a germination stimulant exploited by parasitic plants
• Florigen: now identified as the FT (Flowering Locus T) protein; synthesised in leaf phloem companion cells under an inductive photoperiod and transported through the phloem to the shoot apical meristem, where it triggers the switch to flowering

Photoperiodism and Vernalization

• Photoperiodism: the flowering response to the relative length of day and night; despite the name, the critical variable is actually the length of the uninterrupted DARK period, not the light period; plants are classed as short-day (flower when night is longer than a critical length), long-day (flower when night is shorter), or day-neutral
• Phytochrome: the photoreceptor underlying photoperiodism; exists in two interconvertible forms — Pr (absorbs red light ~660nm, biologically inactive) and Pfr (absorbs far-red light ~730nm, biologically active); the Pr/Pfr ratio controls germination, flowering timing, and shade-avoidance responses
• Vernalization: the requirement for a period of cold exposure before a plant becomes able to flower (e.g. winter wheat, barley); distinct from photoperiodism since it depends on temperature history, not day length
• Seed dormancy: a temporary suspension of growth that allows seeds to survive unfavourable conditions and germinate only when conditions improve; commonly maintained by ABA and broken by gibberellins, light, or temperature cues depending on species

Conditions and Measurement of Growth

• Growth is defined as an irreversible, permanent increase in the size of an organ, its parts, or even an individual cell, and is generally accompanied by an increase in dry weight; it is intrinsically linked to energy-requiring metabolic processes
• Plants show indeterminate growth throughout their life because of meristems located at certain positions (apical, lateral/cambial and intercalary) whose cells retain the capacity to divide — this ability is called the "open form" of growth
• Conditions for growth: water (for cell enlargement and turgidity), oxygen (for energy-releasing metabolism), nutrients, and an optimum temperature and light environment
• Measurement of growth: growth can be measured by many parameters — increase in fresh weight, dry weight, length, area, volume or cell number; for instance, a maize root apical meristem may add more than 17,500 new cells per hour, while a watermelon cell may enlarge in size some 3,50,000 times
• Growth rate: the increased growth per unit time; it may be expressed as absolute growth rate (total growth measured per unit time) or relative growth rate (growth per unit time expressed on a common basis, such as per unit initial size)

Differentiation, Dedifferentiation and Redifferentiation

• Differentiation: cells derived from the root apical and shoot apical meristems and from the cambium mature to perform specific functions, undergoing major structural changes in their walls and protoplasm; e.g. to form a tracheary element, the cell loses its protoplasm and develops a strong, lignified wall to conduct water over long distances
• Dedifferentiation: living differentiated cells that had lost the capacity to divide can regain it under certain conditions; e.g. the formation of the interfascicular cambium and the cork cambium from fully differentiated parenchyma cells
• Redifferentiation: the cells produced by dedifferentiated tissue once more lose their ability to divide and mature to perform specific functions; e.g. the secondary xylem and secondary phloem formed by the vascular cambium
• Growth, differentiation and development are therefore closely interlinked events in the life of a plant

Development and Plasticity

• Development is the sum of all the changes an organism passes through during its life cycle — from seed germination through growth, differentiation, maturation, flowering, senescence and death; it is controlled by both intrinsic factors (intracellular genetic and intercellular hormonal factors) and extrinsic factors (light, temperature, water, oxygen and nutrition)
• Plasticity: plants can follow different pathways of development to form different kinds of structures in response to their environment or phase of life; e.g. heterophylly in cotton, coriander and larkspur, where the juvenile leaves differ in shape from the mature leaves
• In the buttercup (Ranunculus), the leaves formed in air are broad and quite different from the finely dissected leaves produced under water — another striking example of plasticity driven by the surrounding environment

Discovery of the Plant Growth Regulators

• Auxin: first isolated by F.W. Went from the tips of coleoptiles of oat seedlings; its discovery traces back to the observations of Charles Darwin and Francis Darwin that the coleoptile of canary grass bends towards a source of unilateral light
• Gibberellin: the "bakanae" (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi, led E. Kurosawa to the discovery of gibberellic acid
• Cytokinin: the search for a factor that stimulates cell division (cytokinesis) led to the discovery of kinetin (a modified form of adenine) from autoclaved herring sperm DNA; the natural cytokinin zeatin was later isolated from maize kernels and coconut milk
• Ethylene and abscisic acid (ABA): ethylene was discovered from studies on the ripening of fruits, while ABA was discovered independently as inhibitor-B, as the "abscisin" that promoted abscission and as the "dormin" that promoted dormancy — all later found to be the same compound
• The plant growth regulators are broadly grouped as growth promoters (auxins, gibberellins and cytokinins) and growth inhibitors (abscisic acid, and the gaseous ethylene, which has mixed roles)

🚀 NEET Advanced Edge

Hormone interactions (synergism/antagonism) are what's actually tested at the advanced level: Auxin and cytokinin together control root-vs-shoot regeneration in tissue culture (high auxin/cytokinin ratio → roots; low ratio → shoots); strigolactones act synergistically with auxin but antagonistically with cytokinin on shoot branching. Single-hormone "promotes X" facts are the easy layer; combinatorial logic between hormones is the harder layer.

Why phytochrome's Pfr form (not Pr) is the "active" signal: Far-red-rich light (e.g. under a forest canopy, or at dusk) converts Pfr back to Pr, switching OFF phytochrome-mediated responses like germination and flowering induction — this is the basis of the shade-avoidance response, where a plant detects the far-red-enriched light reflected/transmitted by neighbouring plants and adjusts its growth (e.g. elongating faster) to compete for direct sunlight.

Worked reasoning: A short-day plant requires a continuous dark period of at least 12 hours to flower. If a breeder interrupts the middle of a 14-hour night with a brief flash of red light, will the plant flower? Answer: No — interrupting the dark period with light (especially red light, which converts Pr to Pfr) resets the "dark timer," so the plant effectively never experiences the required uninterrupted dark period, and flowering is suppressed. This is exactly how commercial growers manipulate flowering time in practice.