I have always been fascinated with seed germination and seed dormancy. The idea that a complete plant can be grown from a small hard nugget of cells has to be one of natures best creations. As you become familiar with seeds you soon realize that the germination process is not as straight forward as you might think. For many plants you just can’t take seed, plant them, and have germination in a few days.
Every seed seems to require its own process for germinating. Some need to be stratified, some scarified, and some need cold-warm cycles. There are many options for getting seed to germinate and I’ll discuss these in a future post.
The term ‘dormancy’ is used to describe a seeds reluctance to germinate; or more correctly seed dormancy to distinguish it from plant dormancy. The seed lies dormant until environmental conditions are favorable for it to germinate – or at least that is the impression you are left with. But is the seed really dormant? What does dormancy really mean? Good questions that I’ll try to answer in this post.
Definition of Seed Dormancy
What is the definition for seed dormancy?
A good place to start is with Wikipedia, which says “Dormancy is a mechanism to prevent germination during unsuitable ecological conditions”. That sounds fairly strait forward, but what is a ‘mechanism’? Wikipedia defines mechanism (for biology) as “explaining how a feature is created”. That’s not much help. The term mechanism tends to be used when you don’t have a better term to describe something.
Here are some other definitions for seed dormancy that I found on the net:
A state in which seeds are prevented from germinating
The incapacity of a viable seed to germinate under favorable conditions
The absence of germination of an intact, viable seed under germination
favoring conditions within a specific time lapse
An innate seed property that defines the
environmental conditions in which the seed is able to germinate
Derek Bewley from the University of Guelph put this way: “Despite the fact that many researchers study dormancy, there is no unambiguous definition of the phenomenon, perhaps because it is manifest and broken in different ways in different species”.
From a gardeners perspective – what is seed dormancy? I like to think of it as a time period that starts when the seed is mature and ends with germination. During this time a combination of seed biology (i.e. seed properties) and environmental conditions, prevent it from germinating.
This is different than all of the definitions above. It is not a seed mechanism, nor is it a seed property, or even environmental condition. These terms refer to the reason why a seed does not germinate. For me the term seed dormancy is an event, a time period if you like, in the life of a plant.
Seed Dormancy – When Does It Start?
Once seed is mostly developed, it goes through a process of separating from the mother plant. It is now completely on its own. It is probably still inside a fruit or a seed pod, but it no longer has a biological connection to the mother plant. The umbilical cord has been severed.
A phase of seed maturation now starts that does not involve the mother plant. Various chemical changes are still going on inside the seed. You can’t see these but, they are there.
The seed is also going through a drying period and it can loose a significant amount of it’s moisture. It is getting smaller and harder as it gets ready to face the real world.
At some point water loss is almost finished, but the internal chemical changes continue. It really is not clear when dormancy starts.
What this means to a gardener is that ‘seed ripeness’ does not occur at a specific time. It is an ongoing process that continues even after the seed is harvested. That round dark seed that you hold in your hand is still changing – you just can’t see the changes.
Are Seeds Dormant?
If we call them dormant seeds, surely they are dormant i.e. inactive? It turns out dormant seeds are anything but dormant.
For this discussion I am going to ignore seeds that man dries for long term storage under special conditions. Even these are not fully dormant, but they are quite close to being dormant. In this discussion I am talking about seeds in nature, or seeds a gardener might collect and plant within a few months or a year. Even purchased seed is usually less than a year old and falls into this category.
Such seed is not dormant. Even after sitting around for months, there are chemical changes taking place inside the seed. The following is a list of some of these activities.
After Ripening is a term that refers to seed that looks ripe, but is not ripe internally. They are still developing even though they have separated from the mother plant and appear fully ripe.
During seed development, chemicals are produced in the seed that prevent germination. These inhibitors need to decay or be converted by enzymes so that germination can take place.
Some seed requires the presence of light, or the absence of light to germinate. Light sensitive proteins are active and when light triggers them, other chemical reactions take place that facilitate germination.
Respiration is the conversion of stored food to energy by using oxygen absorbed from outside the seed. This continues at low levels even in dry seed.
Seed dormancy appears to be very dependent of the ABA/GA ratio, two hormones called abscisic acid and gibberellic acid. These ratios change over time until they reach a point where germination can happen.
mRNA Produces Enzymes
mRNA is a form of DNA that is used to synthesize proteins and enzymes. These are used to carry out specific chemical reactions in the seed.
Water, nutrients, hormones and other chemicals can be absorbed by the seed. For example both nitrate salts and gibberellic acid have been used to speed up the onset of germination (i.e. reduce the length of dormancy).
Chemical Changes in Seed Coat
Some seed coats have a waxy outer covering that slowly decomposes. Seeds can also have physical and chemical changes take place inside the seed coat.
Overcoming Seed Dormancy
There are a number of reasons why a seed is prevented from germinating and various methods gardeners can use to overcome dormancy, which are described fully in Seed Dormancy – What is it and How do Your Break it?.
Looking At Dormancy In A New Way.
I’d like to look at dormancy from a gardeners perspective. A gardener looks at seeds from the outside. We don’t cut seed open and dissect it to identify small internal changes that may be taking place. So the following may not be ‘100% botanically correct’, but I think it makes it easier for gardeners to understand Dormancy.
I see seed development and dormancy as part of the plant development process. If we think of the life of an animal, we have several distinct phases; fertilization, embryo development, birth, childhood and adulthood. This is not unlike the phases of plant development which consist of fertilization, embryo development, seed maturation, germination, seedling stage, adult plant.
The plants equivalent stage to an animal birth is the seed maturation point; the point in time when the seed separates from the mother plant. It is now a separate entity with a life of its own. It now enters a phase we call seed dormancy but in fact it is clearly a phase of plant development. The root and shoot are undergoing chemical and physiological changes – we just can’t see them because they occur inside the seed coat.
Germination is simply the point at which we humans are able to see something happen outside the seed – it is not the birth of the plant.
The above diagram shows the development of a plant. The left side of the diagram shows the phases of plant development. The right side shows specific points in this process. The diagram focuses on the early stages of plant development with a focus on seed development and dormancy. For the purpose of this discussion the processes past this point are summarized simply as plant growth and plant maturation – these could easily be expanded into more phases.
Seed dormancy is the period time between seed maturation and germination. Many chemical and physiological processes take place during this time period. There is no period of inactivity where nothing is happening.
It is also important to understand that the complete process from fertilization to plant maturation is a continually changing process. At no point is the plant dormant.