Plant Growth and Development — Growth Curves, the Five Hormones, Photoperiodism
The heart of this chapter is the five plant growth regulators, and they are best learnt as a set of antagonistic pairs rather than as five separate lists. Auxin promotes apical dominance; cytokinin overcomes it. Gibberellin breaks dormancy; abscisic acid imposes it. Cytokinin delays leaf senescence; ethylene accelerates it. Once you see the oppositions, a question naming an effect usually points at one hormone and rules out its partner in the same breath. Around that core sit two smaller topics that are pure recall but reliably examined: the shapes of growth curves, and photoperiodism, where the surprise is that a plant is measuring the length of the night rather than the day.
The Five Plant Growth Regulators
| Hormone | Examples | Effects |
|---|---|---|
| Auxin | Natural: IAA, IBA. Synthetic: NAA, 2,4-D | Apical dominance; root initiation in cuttings; prevents fruit and leaf drop; induces parthenocarpy in tomato; flowering in pineapple; xylem differentiation; 2,4-D as a herbicide against dicot weeds |
| Gibberellin (GA₃) | Discovered from the fungus Gibberella fujikuroi | Elongates the axis — grape stalks; enlarges fruit and delays senescence in apples; bolting in beet and cabbage; malting in brewing; speeds maturity in juvenile conifers; breaks seed and bud dormancy |
| Cytokinin | Kinetin from herring sperm DNA; zeatin, natural, from corn kernels and coconut milk | Promotes cell division; lateral shoot growth and adventitious shoots; overcomes apical dominance; delays leaf senescence (the Richmond–Lang effect) |
| Abscisic acid (ABA) | The stress hormone | Closes stomata; inhibits growth and metabolism; induces and maintains seed dormancy; increases tolerance of drought; antagonistic to gibberellin |
| Ethylene | The only gaseous PGR; ethephon is the commercial source | Fruit ripening; senescence and abscission of leaves and flowers; breaks dormancy in peanut seeds; promotes root hair formation; epinasty in tomato; rapid internode elongation in deep-water rice |
🧠 Learn the three antagonisms and half the hormone questions answer themselves
Three pairs do most of the work. Auxin against cytokinin on branching: auxin from the shoot tip suppresses the lateral buds — apical dominance — which is why pruning the tip of a hedge makes it bushy, and cytokinin does the reverse by promoting lateral shoots. Gibberellin against abscisic acid on dormancy: GA breaks seed and bud dormancy and ABA imposes it, so the balance between the two decides whether a seed germinates now or waits for a better season. Cytokinin against ethylene on ageing: cytokinin delays leaf senescence while ethylene hastens senescence and abscission, which is why a cut flower treated with cytokinin lasts longer and one stored near ripening fruit does not. Two practical notes follow from the ethylene pair. Ripening fruit releases ethylene, so one ripe apple ripens the whole basket — a genuine autocatalytic effect used commercially to ripen bananas and tomatoes in transit. And the same effect is why ethylene-sensitive produce is stored away from ripening fruit rather than with it.Photoperiodism, Vernalisation and Dormancy
| Type | Flowers when | Note |
|---|---|---|
| Long day plant | The photoperiod is longer than a critical duration | Equivalently, when the dark period is shorter than critical |
| Short day plant | The photoperiod is shorter than a critical duration | Needs a long uninterrupted night — this is the examinable subtlety |
| Day neutral plant | Regardless of day length | Flowering is controlled by other cues such as age or size |
⚠️ The night-break experiment: what a short-day plant is actually measuring
Give a short-day plant its short day and long night and it flowers. Now interrupt that long night with a brief flash of light in the middle — and it fails to flower, even though the total hours of daylight are almost unchanged. That single experiment shows the plant is not measuring the day at all but the length of the continuous dark period, which is why "short-day plant" is a slightly misleading name and why a question phrased in terms of the dark period is the more precise one. Two further points are examinable. The leaves, not the shoot apex, are the organ that perceives the photoperiod — a plant with its leaves removed does not respond — and a hormonal signal, historically called florigen, is inferred to travel from leaf to the site of flowering. And the response depends on the plant's own critical duration, so the same twelve-hour day can be a long day for one species and a short day for another; the terms are relative to the species, not absolute clock times.ℹ️ Vernalisation and seed dormancy — two ways a plant refuses to hurry
Vernalisation is the dependence of flowering on a period of low temperature, and its purpose is to prevent a plant from flowering before winter is over. The standard examples are the biennials — sugar beet, cabbage, carrot — which grow vegetatively in one season, need cold, and flower in the next; and the winter varieties of wheat, barley and rye, which if sown in spring without cold treatment grow but do not flower. Seed dormancy works towards the same end by a different mechanism: germination is blocked by abscisic acid, an impermeable or hard seed coat, or the presence of inhibitors, and it is released by chilling, light, or gibberellin. Read both as timing devices rather than as defects. A seed that germinated the moment it fell would sprout into the wrong season and die, so dormancy is an adaptation, not a failure to germinate — a framing that answers the "why is dormancy advantageous" question directly.Key takeaways
- Arithmetic growth gives a straight line (one daughter divides); geometric growth gives a sigmoid curve (both divide).
- Differentiation → dedifferentiation → redifferentiation; development = growth + differentiation.
- Heterophylly in cotton and coriander is age-dependent; in buttercup it is environment-dependent.
- Auxin causes apical dominance and 2,4-D is a herbicide; cytokinin overcomes apical dominance and delays leaf senescence.
- Gibberellin breaks dormancy, causes bolting and malting; ABA is the stress hormone that closes stomata and imposes dormancy.
- Ethylene is the only gaseous PGR: fruit ripening, abscission, epinasty, deep-water rice elongation. Ethephon is its source.
- A short-day plant measures the length of the continuous DARK period; a night break prevents flowering. Leaves perceive the photoperiod.
- Vernalisation is the cold requirement for flowering — biennials such as sugar beet, cabbage and carrot, and winter cereals.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
A short-day plant fails to flower when its long night is interrupted by a brief flash of light. This shows that the plant measures:
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Answer: A — The length of the continuous dark period
The flash barely changes the total daylight but does break the continuous darkness, and that alone abolishes flowering — so the dark period is what is being measured. This is why "short-day plant" is a slightly misleading name, and why questions framed in terms of the dark period are the more precise ones.Removing the shoot tip of a hedge makes it bushy because it removes the source of:
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Answer: A — Auxin, which was suppressing the lateral buds
Auxin from the apex causes apical dominance, holding the lateral buds dormant; removing the tip removes that suppression and the laterals grow. Cytokinin does the opposite — it promotes lateral shoots — so naming it here inverts the antagonistic pair the question is built on.Which plant growth regulator is a gas, and is used commercially as ethephon?
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Answer: A — Ethylene
Ethylene is the only gaseous PGR, responsible for fruit ripening, abscission, epinasty in tomato and internode elongation in deep-water rice, and ethephon is the liquid source used in orchards. Zeatin is a natural cytokinin, so it belongs to the opposing hormone in the senescence pair.Abscisic acid and gibberellin are described as antagonistic because ABA:
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Answer: A — Imposes and maintains seed dormancy, which gibberellin breaks
The balance between ABA imposing dormancy and GA breaking it decides whether a seed germinates now or waits. The second option reverses their roles on elongation — GA is the elongating hormone, not ABA — and ABA closes stomata rather than opening them.Vernalisation refers to the promotion of flowering by:
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Answer: A — A period of exposure to low temperature
Vernalisation is specifically the cold requirement, seen in biennials such as sugar beet, cabbage and carrot and in winter varieties of wheat, barley and rye. Responses to day length are photoperiodism — a different phenomenon that the last option describes, and the two are routinely offered together.Geometric growth produces which shape of curve when plotted against time?
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Answer: A — A sigmoid, S-shaped curve
Both daughter cells continuing to divide gives an initial lag, then a log phase of rapid increase, then a stationary phase as resources become limiting — the classic sigmoid. The straight line belongs to arithmetic growth, where one daughter divides while the other differentiates.The organ that perceives the photoperiod in a flowering plant is the:
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Answer: A — Leaf
Leaves perceive the light and dark periods, and a hormonal signal historically called florigen is inferred to move from the leaf to the site of flowering; a plant stripped of leaves does not respond to photoperiod at all. The shoot apex is where flowers form, but forming is not perceiving — which is the distinction this question rests on.Interfascicular cambium arising from mature medullary ray cells is an example of:
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Answer: A — Dedifferentiation
Cells that had already matured and lost the power to divide regain it — that is dedifferentiation by definition. Differentiation is the original loss of dividing capacity, and redifferentiation is what the resulting secondary tissues then undergo when they mature again, so the three form a sequence rather than alternatives.