Seeds: Dormancy and Germination
When you look at a seed it does not seem to be alive. It does not move or grow, in fact, it looks dead. Even with testing for metabolic activity the metabolism of a seed is so slow that it would be hard to tell if there really is anything alive inside, but there most certainly is!
Seeds come in all shapes and sizes from about 400,000 seed bearing plants. The smallest seeds are from the Vanilla orchid. One seed weighs about 8 nanograms (0.0000008 gr.) or 35,000,000 seeds to the ounce. These seeds are so small that they look like tiny dust particles. At the other end is the largest seed. It is from a palm tree, the Lodoicea maldivica (sea coconut). It can take two or more years to germinate. The largest seed ever recorded weighed in at 17.6 kg.
A seed is produced when the right pollen comes into contact with the right flower. When the pollen makes contact with the pistil of the flower the pollen then produces a pollen tube that transports the nucleus of the pollen to the ovule. When the nucleus of the pollen enters the pollen tube it divides and forms two sperm cells. One sperm cell fuses with the egg in the ovule to create the zygote which will become the embryo. The other sperm is transformed into the endosperm. The endosperm is the stored nutrients in a seed.
A typical seed structure has three basic parts. The outer layer is the seed coat. Inside is an endosperm and a dormant embryo. The embryo also has three parts, the cotyledon or seed leaf, the epicotyl that will become the shoot and the radicle which is the root.
Common garden vegetable seeds are generally ready to sprout when they are harvested, they are not truly dormant. All they need is some moisture and temperatures warm enough to allow its life as a plant to begin. However, most seeds do have a deeper form of dormancy.
Getting a seed to germinate can be simple or complicated, depending upon the means the seed uses to stay dormant. The most common ways a seed remains dormant are: a thick seed coat that can not be penetrated by water, insufficient development of the seed embryo or inhibitors such as abscisic acid and phenolic compounds.
A thick seed coat is the most common means to keep a seed dormant. The seed coat must be broken in some way to allow moisture in, to activate the metabolism of the seed. The process is called scarification. In the wild scarification happens when an animal starts to eat the seed and gives up before it gets all the way through the seed coat. Or the seed goes through the digestive tract of a bird or animal and the seed coat is weakened by the digestive juices. Strawberries and raspberries are good examples of this. Environmental conditions that can cause scarification are exposure to alternating freezing and thawing, fire, abrasion from soil and sand or rubbing on rocks in rivers and streams. In the garden we must do the scarification by mechanical means. There are many techniques for scarifying seeds with a thick coat. Individual seeds can be nicked with knives and files or rubbed on sand paper. For large numbers of seeds a rock polisher with coarse sand works well. Another technique is to soak seeds in sulfuric acid or boiling water. I use a metal file and rub individual seeds on it.
Insufficient development of the embryo and a lack of endosperm are common to orchids and other plants that don’t invest much energy in their seeds. These seeds must find a certain fungus to feed the embryo inside the seed until the embryo has developed to a size big enough to germinate.
Many species use a hormone, abscisic acid as an inhibitor to protect their seed from germinating when they fall to the ground at the end of the growing season. Abscisic acid inhibits the gibberellins that are responsible for germination and shoot development. Over time at temperatures below 4°C the abscisic acid is degraded. When the abscisic acid has been neutralized, the gibberellins are now ready to trigger germination and seedling development. When germinating these seeds a cold treatment (stratification) will be needed for 40-180 days. Stratification can be as easy as placing a pack of seed in the fridge for a few weeks, but not always. A common stratification technique is to fill a small tub with moist sand and place your seeds on the surface, cover the seeds with 1mm of sand then put a cover over the tub and place it in the fridge. In a few weeks or months the individual seeds are removed and planted as they begin to germinate.
Phenolic compounds’ function in plants is not fully understood but they do inhibit germination and regulate plant growth. Often desert plants use phenolic compounds as inhibitors. Phenolic compounds are water soluble so when conditions are wet enough phenolic compounds leach out and the seed will sprout.
In up coming issues I will be taking a close look at germinating some difficult seeds. If there is a seed you are having a problem germinating let me know and I will try to help.
Be patient and enjoy.
do do do Fred
Indoor Gardener January 2007
When you look at a seed it does not seem to be alive. It does not move or grow, in fact, it looks dead. Even with testing for metabolic activity the metabolism of a seed is so slow that it would be hard to tell if there really is anything alive inside, but there most certainly is!
Seeds come in all shapes and sizes from about 400,000 seed bearing plants. The smallest seeds are from the Vanilla orchid. One seed weighs about 8 nanograms (0.0000008 gr.) or 35,000,000 seeds to the ounce. These seeds are so small that they look like tiny dust particles. At the other end is the largest seed. It is from a palm tree, the Lodoicea maldivica (sea coconut). It can take two or more years to germinate. The largest seed ever recorded weighed in at 17.6 kg.
A seed is produced when the right pollen comes into contact with the right flower. When the pollen makes contact with the pistil of the flower the pollen then produces a pollen tube that transports the nucleus of the pollen to the ovule. When the nucleus of the pollen enters the pollen tube it divides and forms two sperm cells. One sperm cell fuses with the egg in the ovule to create the zygote which will become the embryo. The other sperm is transformed into the endosperm. The endosperm is the stored nutrients in a seed.
A typical seed structure has three basic parts. The outer layer is the seed coat. Inside is an endosperm and a dormant embryo. The embryo also has three parts, the cotyledon or seed leaf, the epicotyl that will become the shoot and the radicle which is the root.
Common garden vegetable seeds are generally ready to sprout when they are harvested, they are not truly dormant. All they need is some moisture and temperatures warm enough to allow its life as a plant to begin. However, most seeds do have a deeper form of dormancy.
Getting a seed to germinate can be simple or complicated, depending upon the means the seed uses to stay dormant. The most common ways a seed remains dormant are: a thick seed coat that can not be penetrated by water, insufficient development of the seed embryo or inhibitors such as abscisic acid and phenolic compounds.
A thick seed coat is the most common means to keep a seed dormant. The seed coat must be broken in some way to allow moisture in, to activate the metabolism of the seed. The process is called scarification. In the wild scarification happens when an animal starts to eat the seed and gives up before it gets all the way through the seed coat. Or the seed goes through the digestive tract of a bird or animal and the seed coat is weakened by the digestive juices. Strawberries and raspberries are good examples of this. Environmental conditions that can cause scarification are exposure to alternating freezing and thawing, fire, abrasion from soil and sand or rubbing on rocks in rivers and streams. In the garden we must do the scarification by mechanical means. There are many techniques for scarifying seeds with a thick coat. Individual seeds can be nicked with knives and files or rubbed on sand paper. For large numbers of seeds a rock polisher with coarse sand works well. Another technique is to soak seeds in sulfuric acid or boiling water. I use a metal file and rub individual seeds on it.
Insufficient development of the embryo and a lack of endosperm are common to orchids and other plants that don’t invest much energy in their seeds. These seeds must find a certain fungus to feed the embryo inside the seed until the embryo has developed to a size big enough to germinate.
Many species use a hormone, abscisic acid as an inhibitor to protect their seed from germinating when they fall to the ground at the end of the growing season. Abscisic acid inhibits the gibberellins that are responsible for germination and shoot development. Over time at temperatures below 4°C the abscisic acid is degraded. When the abscisic acid has been neutralized, the gibberellins are now ready to trigger germination and seedling development. When germinating these seeds a cold treatment (stratification) will be needed for 40-180 days. Stratification can be as easy as placing a pack of seed in the fridge for a few weeks, but not always. A common stratification technique is to fill a small tub with moist sand and place your seeds on the surface, cover the seeds with 1mm of sand then put a cover over the tub and place it in the fridge. In a few weeks or months the individual seeds are removed and planted as they begin to germinate.
Phenolic compounds’ function in plants is not fully understood but they do inhibit germination and regulate plant growth. Often desert plants use phenolic compounds as inhibitors. Phenolic compounds are water soluble so when conditions are wet enough phenolic compounds leach out and the seed will sprout.
In up coming issues I will be taking a close look at germinating some difficult seeds. If there is a seed you are having a problem germinating let me know and I will try to help.
Be patient and enjoy.
do do do Fred
Indoor Gardener January 2007