Plant Growth — Concepts, Formulas & Examples

Plant growth is irreversible increase in size, and it is controlled by five major groups of hormones plus environmental signals like light and temperature. CBSE Class 11 and NEET both test this heavily — expect two questions a year on hormones alone.

Core Concepts

Phases of growth

Cell division (meristematic), cell elongation, cell differentiation. Growth is measured as increase in length, volume, mass or cell number. Growth curve is sigmoid — slow, rapid, then slow again.

Three phases of the sigmoid growth curve:

Lag phase: Initial slow growth — cells prepare for division, synthesise enzymes and materials
Log (exponential) phase: Rapid growth — cells divide actively, resources are abundant
Stationary phase: Growth slows, then stops — resources become limiting, mature cells differentiate

Growth rate can be measured two ways:

Arithmetic growth: One daughter cell continues dividing, the other differentiates. Linear growth: $L_t = L_0 + rt$
Geometric growth: Both daughter cells divide. Exponential growth: $W_t = W_0 e^{rt}$ . Root and shoot tips initially show geometric growth.

Plant hormones summary

Hormone	Chemical nature	Main site of production	Key function	Commercial use
Auxin (IAA)	Indole derivative	Shoot apex, young leaves	Apical dominance, cell elongation	Rooting of cuttings, weedicide (2,4-D)
Gibberellin (GA $_3$ )	Terpenoid	Root tips, young leaves	Stem elongation, seed germination	Brewing (malting), seedless grapes
Cytokinin	Adenine derivative	Root tips	Cell division, delay senescence	Tissue culture
Abscisic acid (ABA)	Terpenoid	Leaves, roots	Stomatal closure, dormancy	Stress tolerance research
Ethylene	Gas (C $_2$ H $_4$ )	Ripening fruits, wounded tissue	Fruit ripening, senescence	Controlled ripening of fruits

Auxin

First hormone discovered. IAA (indole-3-acetic acid) is the main natural auxin. Functions — apical dominance, root initiation, parthenocarpy, cell elongation. Used to prevent premature fruit drop and for rooting cuttings.

Apical dominance: The auxin produced by the apical bud suppresses the growth of lateral buds. Remove the apex → laterals grow (bushier plant). Apply auxin to the cut surface → laterals stay dormant.

Phototropism: Auxin migrates to the shaded side of a stem. Higher auxin concentration on the dark side causes more cell elongation there, bending the stem towards light. In roots, high auxin inhibits growth — so roots bend away from the auxin-concentrated side.

Synthetic auxins: 2,4-D (2,4-dichlorophenoxyacetic acid) is used as a selective weedicide — it kills dicot weeds but not monocot crops (like wheat and rice) because monocots metabolise it differently.

Gibberellins

GA $_3$ is the most studied. Functions — stem elongation (bolting in rosette plants), seed germination, breaking dormancy, parthenocarpy. Used in brewing (malting) and to increase fruit size.

Bolting: Rosette plants (like cabbage) have very short internodes. GA $_3$ causes rapid elongation of internodes, causing the plant to “bolt” — produce a tall flowering stem. This is a classic NEET question.

Seed germination: GA $_3$ stimulates the aleurone layer of cereal seeds to produce $\alpha$ -amylase, which breaks down starch in the endosperm to sugars. The embryo uses these sugars for growth. This is exploited in the malting process for beer production.

Cytokinins

Promote cell division. Delay leaf senescence. Help break apical dominance. Kinetin and zeatin are examples.

Auxin-cytokinin ratio determines what grows in tissue culture:

High auxin, low cytokinin → roots form
Low auxin, high cytokinin → shoots form
Equal ratio → undifferentiated callus

This ratio is the basis of all plant tissue culture and micropropagation — a concept NEET tests directly.

Abscisic acid

The stress hormone. Closes stomata under water stress, promotes seed dormancy, causes leaf and fruit abscission. Works opposite to gibberellin.

Stomatal closure mechanism: ABA triggers efflux of K $^+$ ions from guard cells. Loss of K $^+$ reduces osmotic pressure → water leaves guard cells → they become flaccid → stomata close. This conserves water during drought.

ABA is called the stress hormone because its levels rise sharply under water stress, salinity, and cold. It is also responsible for seed dormancy — it prevents premature germination.

Ethylene

A gaseous hormone. Promotes fruit ripening, leaf senescence, flower fading. One bad apple ruins the barrel because ethylene from one ripe apple triggers ripening in others.

Triple response in seedlings: Ethylene causes (1) inhibition of stem elongation, (2) thickening of stem, and (3) horizontal growth of stem. This was the classical test for ethylene presence.

Commercial applications: Ethephon (2-chloroethylphosphonic acid) is sprayed on crops — it slowly releases ethylene. Used for synchronised fruit ripening (bananas, tomatoes), thinning of cotton bolls, and promoting female flowers in cucumbers.

Photoperiodism

Flowering response to day length. Short-day plants (chrysanthemum) flower when nights are long. Long-day plants (wheat) flower when days are long. Day-neutral plants (tomato) are insensitive.

Critical detail: What plants actually measure is the night length, not the day length. Short-day plants are really long-night plants. If you interrupt a long night with a brief flash of red light, the short-day plant will not flower — proving it needs uninterrupted darkness.

Phytochrome is the photoreceptor involved. It exists in two forms:

Pr (absorbs red light, 660 nm) → converts to Pfr
Pfr (absorbs far-red light, 730 nm) → converts to Pr
Pfr promotes flowering in long-day plants and inhibits it in short-day plants

Vernalisation

Some plants need a period of cold treatment to flower. Winter wheat must experience a cold spell before it can produce flowers in spring. This prevents premature flowering in autumn.

Worked Examples

Remove the apex of a pea plant and lateral buds start growing. Apply auxin to the cut surface and they stay dormant. Proves auxin from the apex suppresses laterals.

Ripe fruit releases ethylene, which triggers ripening in nearby unripe fruit. Commercial shipping uses artificial ethylene for controlled ripening.

A plant biologist wants to produce shoots from a callus. She should increase cytokinin relative to auxin. If she wants roots, she increases auxin. The ratio determines the developmental fate.

Short-day plants need long, uninterrupted nights. A flash of red light during the dark period converts Pr to Pfr. Pfr inhibits flowering in short-day plants. Even a brief interruption resets the clock, and the plant “thinks” the night was short.

Common Mistakes

Confusing auxin and cytokinin. Auxin is apical dominance and elongation; cytokinin is cell division and breaking dominance.

Saying abscisic acid promotes growth. It inhibits it.

Calling ethylene a liquid. It is a gas.

Saying short-day plants measure day length. They actually measure night length. A short-day plant with a long night will flower; break the night and it will not.

Confusing bolting with etiolation. Bolting is GA-induced rapid stem elongation in rosette plants leading to flowering. Etiolation is abnormal stem elongation in the dark due to absence of light (not hormone-driven).

Exam Weightage and Revision

NEET 2023 asked about the auxin-cytokinin ratio in tissue culture. NEET 2022 tested ethylene’s role in fruit ripening. CBSE boards ask about photoperiodism and phytochrome regularly. Plant hormones give 2-3 guaranteed NEET questions.

When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.

Make a five-row table — hormone, one main effect, one commercial use. Covers most PYQs.

Practice Questions

Q1. What is the role of GA $_3$ in seed germination?

GA $_3$ stimulates the aleurone layer of cereal seeds to produce the enzyme $\alpha$ -amylase. This enzyme breaks down starch in the endosperm into sugars (maltose), which the embryo uses as an energy source for germination and early growth.

Q2. How does 2,4-D work as a selective weedicide?

2,4-D is a synthetic auxin that causes uncontrolled growth in dicot plants (broadleaf weeds), leading to their death. Monocots (cereal crops like wheat, rice) metabolise 2,4-D quickly and are not affected. This selectivity makes it useful for weed control in cereal fields.

Q3. What is the triple response of ethylene?

When pea seedlings are exposed to ethylene: (1) stem elongation is inhibited, (2) stems become thicker (radial expansion), (3) stems grow horizontally (lose gravitropism). This was the classical bioassay used to detect ethylene before chemical methods were available.

Q4. Name one short-day and one long-day plant.

Short-day plant: Chrysanthemum (flowers when nights are long, typically in autumn/winter). Long-day plant: Wheat (flowers when days are long, typically in spring/summer). Remember: the classification is based on the minimum night length needed, not the day length.

Q5. Why is ABA called the stress hormone?

ABA levels increase dramatically in response to environmental stresses like drought, salinity, and cold. Its primary stress response is closing stomata (by causing K $^+$ efflux from guard cells) to reduce water loss. It also promotes seed dormancy to prevent germination under unfavourable conditions.

FAQs

Do plants feel pain? Plants do not have a nervous system, so they do not experience pain in the way animals do. However, they do sense and respond to damage — wounding triggers ethylene and jasmonic acid production, which activates defence responses. This is a biochemical response, not a conscious experience.

Can we grow plants without hormones? Not effectively. Plant hormones are produced naturally and are essential for normal growth and development. Without auxin, stems would not elongate. Without gibberellin, seeds would not germinate properly. Without cytokinin, cells would not divide. Hormones coordinate all aspects of plant life.

Why do fruits ripen in a paper bag? Placing fruit in a closed paper bag traps the ethylene gas it produces. The accumulated ethylene accelerates ripening. Adding a ripe banana (high ethylene producer) to the bag speeds things up further.

Plant growth is hormone orchestra. Each hormone has one clear lead role, but they work in concert — and the balance is what matters.