Dormancy and germination – Growth and Development in Plants and Animals (Senior 4 Biology) | Ganzaa

Growth and Development in Plants and Animals

Lessons

Senior 4 · Biology

Growth and Development in Plants and Animals

5 lessons

Lesson 1 of 5

Dormancy and germination

25 min

Seeds that wait

A farmer keeps bean seed in a dry sack from one season to the next. The seeds are alive, but they do not sprout in the sack. Sow them in moist soil and within a few days roots appear. Irish potatoes behave differently: tubers dug up in a field in Musanze will not sprout at once even in a damp store; their buds stay asleep for weeks before green shoots appear. The beans were only waiting for water; the potato buds were dormant. In S3 you learned the conditions seeds need to germinate. This lesson explains why some seeds, fruits and buds refuse to grow even when conditions are good, and what happens inside a seed when it does germinate.

Definition

Dormancy: a state in which a living seed, fruit or bud does not grow even when water, oxygen and temperature are suitable, because of a condition inside it. Quiescence: not growing only because outside conditions are unsuitable (for example a dry seed).

Germination: the resumption of growth by the embryo of a seed, ending when the radicle has broken through the seed coat (the seedling then grows on).

Imbibition: the uptake of water by a dry seed, which swells and softens its coat.

Scarification: scratching, cracking or softening a hard seed coat so that water and oxygen can enter. Vernalization (or stratification): a period of cold that some seeds and plants need before they will germinate or flower.

Epigeal germination: the cotyledons are carried above the ground. Hypogeal germination: the cotyledons stay below the ground.

Seed and fruit dormancy

Dormancy lets a seed survive dry seasons, cold or fire, and spreads germination over time, so that not all seedlings are killed by one bad spell of weather. It is broken when a particular signal shows that conditions will favour the seedling.

Cause of dormancy	How it works	How it is broken
Hard, impermeable seed coat (many legumes, such as acacias)	Water and oxygen cannot reach the embryo	Scarification: abrasion by soil, fire, passage through an animal's gut; in practice, nicking the coat or soaking in hot water
Growth inhibitors, mainly abscisic acid (ABA), in the seed coat or the fruit around it	ABA blocks the growth of the embryo; in fruit dormancy, inhibitors in the fruit flesh or wall keep the seeds inside from germinating	Rain washes inhibitors out (leaching); ABA falls and gibberellin rises
Immature embryo	The embryo is not fully developed when the seed is shed	After-ripening: time for the embryo to mature
Need for light (many small seeds)	Phytochrome must be switched to its active form (lesson 4)	Exposure to red light at the soil surface
Need for cold (temperate species)	Ensures seeds wait until after winter	Vernalization: weeks of cold, moist conditions

Bud dormancy

Buds are small, undeveloped shoots protected by bud scales. In temperate trees, shortening days in autumn trigger dormancy: ABA builds up, scales close over the bud and the leaves fall. Cold and lengthening days later break it. Potato tubers are swollen stems whose "eyes" are buds; these stay dormant for some weeks after harvest, which is why seed potatoes are stored until they sprout before planting. Gibberellin breaks bud dormancy; ABA keeps it.

Worked example 1: breaking a hard coat

Three groups of 50 acacia seeds were kept moist and warm for 14 days (illustrative results): untreated, 8 germinated; coat nicked with a file, 41 germinated; soaked in water just off the boil for one minute and then cooled, 37 germinated. (a) Calculate the percentage germination for each. (b) What do the results show?

(a) Untreated: $\frac{8}{50} \times 100 = 16\,\%$ . Nicked: $\frac{41}{50} \times 100 = 82\,\%$ . Hot water: $\frac{37}{50} \times 100 = 74\,\%$ .
(b) Damaging the coat greatly increases germination, so the main cause of dormancy is an impermeable seed coat that keeps out water and oxygen. Some untreated seeds germinated, probably those with naturally damaged or thinner coats. All groups had the same water, oxygen and temperature, so these were not the cause.

What happens during germination

Imbibition. The dry seed takes up water through the micropyle and coat, swells and may double its mass; the coat softens and splits.
Activation. Water activates enzymes and the embryo releases gibberellin (GA).
Mobilisation of food. In a cereal grain, GA travels to the aleurone layer, whose cells make amylase; this hydrolyses starch in the endosperm into maltose and glucose (Figure 1). In beans and groundnuts, proteases and lipases in the cotyledons break down stored protein and oil.
Respiration and growth. The soluble products are carried to the embryo, where respiration releases energy and cell division and elongation begin.
Emergence. The radicle grows out first, anchoring the seedling and absorbing water; then the plumule grows up towards the light.

Figure 1. Mobilising the food store of a cereal grain (simplified longitudinal section). This is the same process that makes malted sorghum and barley sweet.

Feature	Epigeal germination	Hypogeal germination
Part that elongates first	Hypocotyl (stem below the cotyledons)	Epicotyl (stem above the cotyledons)
Position of the cotyledons	Pulled above the soil; they may turn green and photosynthesise briefly	Stay in the soil
Examples	Common bean, groundnut, sunflower	Maize, sorghum, pea

Measuring the changes

The fresh mass of germinating seeds rises at once, as they take up water. The dry mass (mass after drying in an oven at about 100 °C to remove all water) tells a different story (Figure 2): it falls at first, because the stored food is respired and lost as carbon dioxide, and rises only once the young leaves make more food by photosynthesis than the seedling uses.

Figure 2. Dry mass of germinating bean seedlings (illustrative data).

Worked example 2: reading the graph

Use Figure 2. (a) Calculate the percentage decrease in dry mass from day 0 to day 7. (b) Twenty dry seeds had a mass of 10.0 g; after soaking for one day their fresh mass was 19.0 g. Calculate the percentage increase. (c) Why does fresh mass not show the use of food?

(a) $\frac{10.0 - 7.6}{10.0} \times 100 = 24\,\%$ .
(b) $\frac{19.0 - 10.0}{10.0} \times 100 = 90\,\%$ .
(c) Fresh mass includes water, which the seeds take up in large and varying amounts, hiding the small loss of organic matter. Dry mass measures only the organic matter, so it shows the food being respired.

Neil Bromhall: "French Bean germinating underground", a time-lapse filmed against glass so you can see below the soil. Watch the radicle emerge first, then the hooked hypocotyl pull the cotyledons up: epigeal germination.

Warning

Common mistakes

Calling a dry seed in a sack "dormant". It is quiescent: give it water and it grows. A dormant seed will not grow even with water, oxygen and warmth.
Saying the plumule emerges first. The radicle comes first, to anchor the seed and absorb water.
Saying the dry mass of a seedling rises from the start. It falls until photosynthesis outpaces respiration.
Mixing up hypocotyl and epicotyl: hypo- means below the cotyledons, epi- above.

Tip

Memory hook. "GA says Go, ABA says Abort": gibberellin breaks dormancy and switches on germination; abscisic acid keeps seeds and buds dormant. For the cereal grain, remember the chain water → embryo → GA → aleurone → amylase → starch → sugar → embryo.

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Practice: dormancy and germination

Distinguish between dormancy and quiescence.
Give three causes of seed dormancy and, for each, one way it is broken in nature.
State two advantages of seed dormancy to a plant.
Describe the roles of the embryo, gibberellin and the aleurone layer in the germination of a maize grain.
Fifty seeds had a dry mass of 25.0 g. After 5 days the dry mass of 50 seedlings was 21.5 g. Calculate the percentage decrease and explain it.
Compare epigeal and hypogeal germination, with an example of each.
A pupil soaks maize grains in a solution that blocks gibberellin. Predict and explain the effect on germination.

Show answersHide answers

A dormant seed (or bud) will not grow even when external conditions are suitable, because of an internal block; a quiescent seed is ready to grow and is held back only by unsuitable conditions, such as lack of water.
Any three: hard coat (abraded by soil, fire or an animal's gut); inhibitors such as ABA (leached out by rain); immature embryo (after-ripening with time); need for light (exposure at the soil surface); need for cold (a cold season).
Any two: survives unfavourable conditions such as drought; germination is spread over time, so not all seedlings die in one bad event; seeds can be dispersed far before germinating; germination happens when the seedling is likely to survive.
After imbibition, the embryo releases gibberellin, which diffuses to the aleurone layer; aleurone cells make amylase, which hydrolyses starch in the endosperm to maltose and glucose; these sugars pass through the scutellum to the embryo, which respires them for energy and uses them to grow.
$\frac{25.0 - 21.5}{25.0} \times 100 = 14\,\%$ . The seedlings respire their stored food, releasing carbon dioxide, and are not yet photosynthesising enough to replace it.
Epigeal: the hypocotyl elongates and lifts the cotyledons above ground (bean, groundnut). Hypogeal: the epicotyl elongates and the cotyledons stay underground (maize, pea).
Germination would fail or be very slow: without gibberellin the aleurone layer would not make amylase, so the starch store could not be broken down to sugars and the embryo would lack energy and materials to grow.

Quick check

Put the events in a germinating maize grain in order.

Tap the cards in the right order.

Key points

Dormancy is an internal block on growth (hard coat, inhibitors such as ABA, immature embryo, need for light or cold); it helps seeds and buds survive bad conditions and spreads germination over time.
Germination: imbibition → enzymes and gibberellin activated → food mobilised (GA → aleurone → amylase in cereals) → respiration and growth → radicle, then plumule, emerge.
Epigeal: hypocotyl lifts cotyledons above ground; hypogeal: epicotyl grows, cotyledons stay below.
Dry mass falls during early germination and rises once photosynthesis outpaces respiration.

Sources and credits

6.3.3: Pollination and Fertilization (sections "Seed Germination") and 6.1.7: Plant Sensory Systems and Responses, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for dormancy, germination and the roles of gibberellin and abscisic acid. Adapted by Ganzaa.
Figures 1 and 2, the tables, worked examples (the germination and mass data are illustrative), practice and quick check: written by Ganzaa.
Photos: JonRichfield, CC BY-SA 3.0; Bapt-francois, CC BY-SA 4.0; both via Wikimedia Commons.
Video: "French Bean germinating underground. Epigeal germination time lapse", Neil Bromhall, embedded from YouTube.

1Seeds that wait

2Seed and fruit dormancy

3Bud dormancy

4What happens during germination

5Measuring the changes

Seeds that wait

Seed and fruit dormancy

Bud dormancy

What happens during germination

Measuring the changes