Seed dormancy and germination
How a seed comes to life, what it needs to germinate, the kinds of dormancy and how to break them, how germination is tested, and how seed is kept alive.
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A dry seed is a plant on pause: an embryo with a food store, wrapped in a protective coat, with its metabolism almost stopped. Given water, oxygen and the right temperature, most seeds of crops come back to life within days. Many wild plants and weeds, and some crops, do not: they are dormant, and will germinate only after something particular has happened.
Germination and dormancy decide how well a crop establishes, how long seed can be stored, and why weeds keep returning. They are standard topics in agronomy, seed technology and plant physiology examinations.
Germination step by step
Germination begins with the uptake of water by the dry seed and ends when the embryonic root, the radicle, emerges through the seed coat. Physiologists describe three phases:
- Imbibition: rapid water uptake by the dry seed, driven by the very low water potential of dry tissue; the seed swells, membranes reorganise, and respiration restarts within minutes.
- Lag (activation) phase: little further water uptake, while the seed repairs DNA and membranes, activates enzymes and makes new ones. In cereal grains, gibberellin from the embryo signals the aleurone layer to make α-amylase, which breaks down starch in the endosperm.
- Growth: the radicle elongates and breaks through the coat; water uptake rises again as the seedling grows.
For an agronomist, the crop is "established" only when the seedling emerges and becomes self-supporting, which needs the seed's reserves to last until the shoot reaches the light.
Epigeal and hypogeal germination
- Epigeal: the hypocotyl elongates and carries the cotyledons above the soil, where they turn green: common bean, cotton, groundnut, castor, cucurbits, onion. Epigeal seedlings struggle to push through crusted soil and are usually sown shallower.
- Hypogeal: the cotyledons, or the grain, stay below ground and the shoot rises through elongation of the epicotyl (pea, chickpea, broad bean) or the coleoptile and mesocotyl (maize, wheat, rice).
What germination needs
- Water: enough to bring the seed above a critical moisture content; seeds in dry soil, or in saline soil where water is held osmotically, imbibe slowly or not at all.
- Oxygen: for respiration. Waterlogged or compacted soils starve seeds of oxygen; rice is unusual in being able to germinate under water, sending up a coleoptile to reach air.
- A suitable temperature: each species has a minimum, an optimum and a maximum. Cool-season crops such as wheat, peas and lettuce germinate at a few degrees above freezing; warm-season crops such as maize, sorghum, cotton and cucurbits need warmer soils, often above about 10 to 15 °C.
- Light or darkness, for some species. Small-seeded species, including many weeds and some lettuce varieties, need light, a response mediated by phytochrome; a few need darkness.
Dormancy
A dormant seed does not germinate even when given water, oxygen and a suitable temperature. Dormancy spreads germination over time and protects a species from germinating at the wrong moment, for example during a brief warm spell in autumn. In crops it is usually bred out; but some post-harvest dormancy is useful in cereals, because it prevents pre-harvest sprouting of grain in the ear in wet weather.
The classification most used today (Baskin and Baskin) recognises:
| Type | Cause | How it is broken | Examples |
|---|---|---|---|
| Physiological | the embryo's own physiology, often a high ABA to gibberellin balance | after-ripening in dry storage, cold or warm stratification, gibberellin, light, nitrate | many cereals and grasses after harvest, lettuce, many weeds |
| Physical | a coat impermeable to water ("hard seed") | scarification: mechanical abrasion, acid, hot water; in nature, fire, heat, gut passage | many legumes such as lucerne, clovers, Acacia, Leucaena; okra, cotton (some) |
| Morphological | the embryo is underdeveloped at shedding and must grow first | time in warm, moist conditions | carrot family, some palms |
| Morphophysiological | both an immature embryo and physiological dormancy | combinations of warm and cold stratification | some temperate trees and shrubs |
| Combinational | physical plus physiological | scarification followed by stratification | some legumes and others |
Some common treatments:
- Scarification of hard seeds by rubbing with sand or sandpaper, clipping, brief soaking in concentrated acid (in laboratories and nurseries) or hot water.
- Stratification: holding moist seed cold (often 1 to 5 °C) for weeks to months, mimicking winter, for many temperate trees, shrubs and fruit rootstocks.
- Leaching inhibitors by soaking in running water.
- After-ripening: simply storing dry seed for weeks or months.
- Chemicals: gibberellic acid and potassium nitrate solutions are used in laboratories and nurseries.
Testing germination and vigour
Seed laboratories test germination under standard conditions set out in the ISTA rules (International Seed Testing Association): usually four replicates of 100 seeds on moist paper, in sand or in soil, at a set temperature, with a first and a final count at set days. Only normal seedlings, with all essential structures, count as germinated. Abnormal seedlings, dead seeds, hard seeds and fresh ungerminated (dormant) seeds are recorded separately.
- The tetrazolium test gives a quick estimate of viability: living tissue turns red when soaked in a tetrazolium salt solution, because respiring cells reduce it to a red dye.
- Vigour tests, such as accelerated ageing (storing seeds warm and humid for a few days before a germination test), cold tests and electrical conductivity of the soak water, predict performance in difficult field conditions better than germination alone.
Germination percentage and mean germination time
In a test of 100 seeds, new normal seedlings were counted daily: 10 on day 2, 30 on day 3, 40 on day 4 and 10 on day 5; no more after that.
- Germination = (10 + 30 + 40 + 10) ⁄ 100 = 90 per cent.
- Mean germination time = Σ(n × t) ⁄ Σn = (10×2 + 30×3 + 40×4 + 10×5) ⁄ 90 = (20 + 90 + 160 + 50) ⁄ 90 = 320 ⁄ 90 ≈ 3.6 days.
Two lots with the same final germination can differ in speed; the slower one usually emerges less evenly in the field and is more exposed to soil pests and crusting.
Keeping seed alive: longevity
Seeds of most crops are orthodox: they tolerate drying to low moisture and live longer the drier and cooler they are kept. Seeds of many tropical trees and some crops, such as cocoa, mango, avocado, rubber and many palms, are recalcitrant: they die if dried and cannot be stored for long by ordinary means. Some are intermediate.
For orthodox seed, Harrington's rules of thumb are widely quoted: within normal storage ranges, each 1 percentage point fall in seed moisture, or each 5 °C fall in temperature, roughly doubles storage life; and the sum of storage temperature in degrees Fahrenheit and relative humidity in per cent should be below 100 for safe storage. They are approximations, but they explain why gene banks store seed dried to about 3 to 7 per cent moisture at about −18 °C, where many species keep for decades.
Key points
- Germination: imbibition, activation (enzymes, gibberellin, α-amylase in cereals), and radicle emergence.
- Epigeal germination lifts cotyledons above ground (bean, cotton); hypogeal keeps them below (pea, maize).
- Requirements: water, oxygen, suitable temperature and, for some seeds, light or darkness.
- Dormancy types: physiological, physical (hard seed), morphological, morphophysiological and combinational; broken by after-ripening, stratification, scarification, light, gibberellin or nitrate.
- ISTA germination tests count only normal seedlings; tetrazolium tests viability; vigour tests predict field emergence.
- Orthodox seeds store well dry and cool (Harrington's rules); recalcitrant seeds cannot be dried.
What the exams ask
- School-leaving and entrance papers: conditions for germination, epigeal and hypogeal germination with examples, and methods of breaking dormancy.
- Degree and research-fellowship entrances: physiology of germination, causes and classes of dormancy, hormonal control (ABA and gibberellin), ISTA testing, viability and vigour tests, and orthodox versus recalcitrant seeds.
- Seed technology examinations: germination standards for certification, storage conditions and longevity, and pre-harvest sprouting.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do some weed seeds keep coming up for years?
Many weed seeds have dormancy that is broken in stages, so only part of the seed bank germinates each year, and some remain viable in soil for decades. Preventing weeds from seeding matters as much as killing seedlings.
Why soak seeds before sowing?
Soaking (and "priming", partial hydration followed by drying) starts imbibition and the early metabolic steps, so seeds germinate faster and more evenly after sowing. It must be done carefully, because over-soaked seeds can be damaged or start germinating before sowing.
Can old seed still be used?
If it has been stored dry and cool and still passes a germination test, often yes, though vigour falls before germination does. Test before sowing and adjust the seed rate.
Sources
- Bewley, J. D., Bradford, K. J., Hilhorst, H. W. M. and Nonogaki, H. Seeds: Physiology of Development, Germination and Dormancy, 3rd edition. Springer, 2013.
- Baskin, C. C. and Baskin, J. M. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination, 2nd edition. Academic Press, 2014.
- ISTA. International Rules for Seed Testing. International Seed Testing Association, updated annually.
- Harrington, J. F. "Seed storage and longevity." In Kozlowski, T. T. (ed.), Seed Biology, Vol. 3. Academic Press, 1972.
- FAO. Genebank Standards for Plant Genetic Resources for Food and Agriculture. FAO, 2014.
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