The goal of this article is to present the different types of dormancy found in tree seeds. By tree seeds we also include shrub seeds.
Seed dormancy is a natural mechanism
found in seeds to prevent early germination in non-ideal conditions for the emergence of the seedlings. It is a form of barrier till the ideal conditions are present to trigger the germination. Studies have shown that dormancy in a tree seeds show almost no sign of life. Biochemical test have shown that life symptoms are undetectable in the seed embryo.
Dormancy will differ for different types of plants. Vegetable have little to none dormancy in their seeds, while trees are amongst the most common type of seeds with dormancy. But again, dormancy varies between different species of tree seeds. Amongst tree seeds species, maple are some of the hardest while lotus and tropical plants are the easiest with very little dormancy.
Germination dormancy comes in many flavours and combinations. Some tree seeds have multiple dormancy stages
that must be broken at same time or one after the other. Seed germination as often been simplified to three steps: Scarification, stratification and germination. But, it is more complex then this. Seed germination dormancy is mainly address by the stratification step of germination.
We will try to expose some dormancy definitions of the known type of seed dormancy. Seven different types of dormancy can be found in tree seeds. Here they are:
- Physical & Physiological
We will define characteristics of each dormancy type with a short descriptions.
Physiological dormancy comes in three types with each of them having their own characteristics.
Nondeep Physiological dormancy
The first being the nondeep dormancy. This dormancy is an easy dormancy. It requires a very little period of prechilling to break the dormancy barrier. Germination will occur at temperature above 15 celscius degrees. Light can be a factor in germination. Some known chemical compounds, like ga3, can help and improve the germination of tree seeds with a nondeep physiological dormancy.
Intermediate Physiological dormancy
The second being the intermediate dormancy. The period of dormancy and prechilling required for this type of seeds is a bit longer then nondeep dormancy seeds. Anywhere from 1 to 6 months could be required the break dormancy. An excised embryo will still grow and germinate eventually. Chemicals will also help germination for this type of tree seeds.
Deep Physiological dormancy
The third and last of physiological dormancy seeds is the deep dormancy tree seeds. Contrary to the intermediate dormancy, an excised embryo will grow. But, one exception exist, the prunus seeds are still growing after being excised even if they are considered a deep dormancy tree seeds. Deep dormancy type of seeds are requiring the longest period of dormancy period which is usually anything from 3 months to even 2 years. Some studies have shown that these type of seeds could have 2 levels of dormancy barrier.
This type of dormancy is usually absent from shrubs and is mostly found in a few temperate and almost all tropical species of trees. This is a nonexistent barrier compared to other type of dormancy. The embryo is just undeveloped and under ideal conditions it will complete is development and germinates.
This third dormancy type is a combination of the first two ones. The name is self-explanatory. As morphological, the embryo is undeveloped. It needs to develop itself to germinate, but before or at the same time, a dormancy barrier must be broken. The type of climate needed to broke the germination barrier is either moist, cold or warm. Again, nondeep, intermediate and deep type of physiological dormancy can apply with this type of dormancy.
The physical dormancy type of seeds are seeds that contain an embryo that is usually large and already contains it's own food and energy to germinate. While it is consider a form of dormancy, the embryo is actually non dormant and no barrier whatsoever are present. Thus, germination can occur at any time given it is provided with moisture and warm temperature. It is most common in the flowering plant family, being the angiosperm species family.
Physical & Physiological
Once again, this one is a combination of two types of dormancy. It is the same as the Physical types of seeds but with an added germination dormancy barrier. The three types of the physiological dormancy do still apply with this type of dormancy. Dormancy must be first broken in order for the embryo to germinate. Prechilling is the best method to break the dormancy barrier. Hot water, acid or mechanical scarification are essential before prechilling for germination. Some excised embryo will still grow.
Chemical dormancy of tree seeds are characterised by the chemicals found in the seed coat, embryo and endosperm. A chemical reaction must be achieve, mostly between the embryo and the endosperm. At that time, germination is triggered once the endosperm releases food and energy to the embryo. Seed coat removal is essential to achieve germination. This type of dormancy is amongst the hardest to break as it is often combined with a physiological dormancy barrier. Germination could occur form anywhere to a few months to a few years. Abscisic acid will help germination if applied to seed coat and embryo.
The last of the dormancy type is the mechanical dormancy. These are usually seeds with a deep physiological dormancy. They will require a very long prechilling period to break the dormancy. These seeds exposed to a warmer temperature before prechilling can show improvement in germination.
Germination of tree seeds is not as easy as we can think. Natural forces can prevent a seed from germinating and ideal conditons are always the biggest factors to help your germination success.