A dormant seed is alive but does not germinate, even when water, air and warmth are given. Dormancy helps a plant survive bad seasons and spread its germination over time. The causes are a hard seed coat, an embryo that is not ready, chemical inhibitors such as abscisic acid, and a need for cold or light. Scarification, stratification, gibberellic acid, nitrate and light end it.
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A seed that waits
Keep a few bean seeds in a dry cupboard for a month. Then put them in wet cotton wool. In two or three days they sprout. The seed was alive all the time, but it was waiting for water.
Some seeds wait even when you give them water, air and a warm place. You can leave them on wet soil for weeks and nothing happens. This is seed dormancy. A dormant seed is a living seed that does not germinate, although the conditions outside are good for it.
Dormancy is not death. The embryo inside is alive and breathes very slowly. It is only held back until something tells it that the time is right.
Why dormancy helps the plant
Think of a plant that sheds its seeds in autumn. If all of them germinated at once, the seedlings would meet the winter, and they would die. Dormancy prevents this. It helps a plant in three ways.
It carries the seed through a bad season. The seed rests through cold, drought or a dry summer, and germinates when the good season comes.
It spreads the germination over time. Seeds of one plant do not all sprout together, so one bad year cannot kill the whole next generation.
It stops a seed from sprouting on the parent plant. A seed inside a wet fruit would otherwise germinate there. Abscisic acid, which you meet in the course on plant growth regulators, helps to prevent this.
Why a seed stays dormant
There is more than one reason. A seed may have one of these, or two together.
Five rows, each with a box on the left and an arrow to a box on the right. Why the seed waits, then how it is woken. A hard, waterproof seed coat is woken by scarification: sandpaper, a knife or shaking. An embryo that is not yet fully grown is woken by time, as the seed ripens after it is shed. A chemical inhibitor in the seed, abscisic acid, is woken by gibberellic acid, nitrate or washing it out. A seed that needs a cold spell is woken by stratification: moist seeds kept in the cold. A seed that needs light is woken by exposure to light.
A hard, impermeable seed coat. The coat does not let water or oxygen in. Seeds of many legumes, such as senna, are like this.
A mechanical block. The coat is so strong that a young embryo cannot push through it.
An embryo that is not fully grown. The seed is shed before the embryo is ready. It needs time to ripen, called after-ripening.
A chemical inhibitor. The seed or its coat holds a substance that stops growth. The best known is abscisic acid (ABA). Some phenolic compounds also act like this.
A need for a particular temperature. Many seeds of cold countries will not germinate until they have been through a cold spell.
A need for light. Seeds of some plants, lettuce for one, germinate only when light falls on them.
How dormancy is broken
Each cause has its own cure. A gardener and a plant breeder use these.
Scarification. The hard coat is damaged so that water and air can enter. It is rubbed with sandpaper, nicked with a knife, or shaken hard with sand. A strong acid is used for some seeds. In nature, a seed that passes through the gut of an animal, or is bruised by stones in a stream, is scarified.
Impaction. Some hard seeds are shaken or struck so hard that a small plug in the coat is knocked out. Water then enters through the gap.
Stratification. Moist seeds are kept for some weeks at a low temperature, a few degrees Celsius above freezing. This ends the dormancy of seeds of apple and of many other plants of cold countries.
Gibberellic acid and nitrate. A treatment with gibberellic acid (GA) or with a nitrate salt wakes many seeds. GA works against the effect of ABA.
Light. A short exposure to light wakes light-sensitive seeds.
Washing. An inhibitor in the coat can be washed out with water. In a desert, a heavy rain does this. A light shower does not, and so the seeds stay asleep until there is enough water for the seedling too.
The balance of ABA and GA
Two growth regulators pull in opposite directions. ABA keeps the seed dormant. GA wakes it. What counts is the balance between them.
Two small bar charts side by side, each with one red bar for abscisic acid (ABA) and one green bar for gibberellin (GA). On the left, a dormant seed has a tall ABA bar and a short GA bar, and the seed stays asleep. On the right, a seed ready to germinate has a short ABA bar and a tall GA bar, and the seed germinates.
In a dormant seed ABA is high and GA is low. As the seed is woken, the ABA falls and the GA rises. The seed then takes up water, the embryo grows, and the root comes out first.
Buds sleep too
Seeds are not the only dormant parts. A tree of a cold country forms winter buds before the cold season, and the buds do not open until spring. A potato tuber also rests for some weeks after harvest. ABA helps to keep a bud dormant. Ethylene can end the dormancy of buds and of seeds. It makes the tuber of a potato sprout, and it starts the germination of peanut seeds.
Seeds that wait for years
The length of time that a seed stays alive and able to germinate is its viability. For most seeds it is a few months or years. Some seeds are far better.
Seeds of the Arctic lupin, found frozen in the tundra, germinated and flowered after about 10,000 years.
A seed of the date palm, found near the Dead Sea, was about 2000 years old when it germinated. It is the oldest seed known to have grown.
Dormancy and a very dry, cold resting state make this possible.
What a seed needs when it does germinate
Once a seed is free of its block, it needs three things from outside.
Water. The seed soaks it up and swells. This soaking is called imbibition. The coat splits.
Oxygen. The embryo respires, and it needs oxygen to release the energy for growth.
A suitable temperature. Each kind of seed has a range in which it germinates.
You should find that the curve rises quickly near the weeks that the seed needs. Too little cold leaves most seeds asleep. More than enough wakes nearly all of them.
Try it in Python
The program matches each seed to its cause of dormancy and the cure.
Practise with new numbers
Common mistakes
Saying that a dormant seed is dead. It is alive. It is only held back.
Thinking that water, air and warmth are always enough. A dormant seed does not germinate even with all three.
Mixing up scarification and stratification. Scarification damages the seed coat. Stratification is a cold, moist treatment.
Giving ABA the wrong job. ABA keeps a seed dormant. GA ends dormancy.
Forgetting that a lack of water also stops germination. That is not dormancy. A dormant seed fails to germinate in good conditions.
Remember these
A dormant seed is alive and does not germinate in good conditions.
Dormancy gets a seed through a bad season, spreads germination and stops sprouting on the parent plant.
Causes: hard seed coat, immature embryo, inhibitors such as ABA, need for cold, need for light.