2/6/09

Seed Storage

I will begin talking about breeding plants for the next several weeks. An important thing in plant breeding is seed storage. If you can create a good generation of seeds, you can grow some and create clone generations of their offspring for years. If you can store the unused seed, you can then keep pulling from your seed population for years to clone, and/or a chance to find better traits. Seeds like all things have a limited shelf life. Eventually, no matter what you do, over time seeds will not be able to grow into a healthy plant. I should also mention that storing seeds will also allow you to backcross traits over multiple generations to create the best strain possible with the seeds you have.
Scientists break down seeds into two groups, those that are long lived seeds (store well) and those that are short lived. Many tree species are short lived. These seeds tend to be covered in fleshy fruit, and germinate in the fruit. Conversely, long lived seeds tend to be from plants that are annuals and live in open fields. These seeds tend to have a hard outer covering.
No matter what type of seed your plant produces, seed deterioration is inevitable. You can not stop it, but you can prolong it. There are a few simple rules that will help you improve seed storage. Temperature and relative humidity are the two most important things to keep in mind when storing seeds. But other factors like pathogenic organisms, the seeds maturity when harvested and genetics are important too.

Your first concern seeds can be attacked by various organisms. Fungus is usually the biggest problem. You should be able to keep insects out of your seeds if they are kept in containers. Yet another benefit for indoor growers, you should not have to worry about insects getting at your seeds as they mature on the plant. If you do collect seeds from outside or areas exposed to the outside world you may want to check for small holes in the seeds before storage. Concerning fungus, there are two basic types of fungus that can harm your seeds. The first type is those that affect seeds as they mature on the plant. To fight these fungi, keep the moisture below 65% while the seeds are developing. The other kind of fungus is storage fungus. There are at least two genus of fungi that feed on seeds. They can survive humidity as low as 30%, and temperatures above 0C. (A good reason to keep the humidity and temperature low.) A fungus grows from microscopic spores in the air so it is pretty hard to control. Do not microwave seeds to sterilize them, you will kill the seeds.

The second concern is that all seeds will deteriorate internally over time. So, why do seed deteriorate any way? Seeds are made up of organic molecules, proteins, carbohydrates etc. like all living things, so, free radicals damage cells over time, DNA deteriorates over time, proteins deteriorate, plant structures inside the seed deteriorate , lipids deteriorate etc… Organic molecules need energy to keep their structure. Seeds have a limited energy supply for the cells inside, and when it is gone, deterioration follows.

How do you know if your seeds are deteriorating? Good growers will notice that when your seeds are new (young), the germination period* is shorter. In other words, older seeds tend to take longer to germinate and eventually old seeds will never germinate. Once you notice the germination time lengthening, the writing is on the wall, you have a limited time to grow the remaining seeds. If you are not observant, you may not notice the length of germination, but most people notice when less seeds as a percentage germinate. Young seeds (less than one year) may have a 100% germination rate (every seed germinates). You may notice low seed germination even with young seeds if you have a bad cross*. Even the most healthy seeds age, eventually you will notice less and less of your seeds will germinate over time. One day, you will try to germinate seeds and not one will germinate.
The next observation you can make concerning seed health is that older seeds germinate but the seedlings tend to grow less vigorously. Young seeds often produce fast growing seedlings. You may also notice that more and more seedlings not only grow slower, they grow “funny” with deformed leaves or cotyledons*. Look at (http://www.htgsupply.com/images/articles/HTGSupply-Starting-Article-April-2008.pdf) for more seed dormancy information. If you have been saving seeds for years, you may need to think about creating or buying some new, younger seeds. Breeding is the only way to get “new” plants.
More Symptoms of deterioration You may also notice decreased resistance to environmental stress, and decreased yields when growing plants from older seeds. Decreased yields in some plants begin to occur even before you notice a decrease in the germination rate. If you are growing plants from seeds a few years old, you may have the need to breed. Keep in mind the loss of yield may also occur in cloned plants over time.

Ways to improves seed storage. First, keep your seeds cold. {I have some useful information under my HERB STORAGE WRAP-UP post that may be helpful in storing your seeds in the freezer}.  Cold temperature will slow the seed ageing but just as important as cold is a constant temperature. Do not put the seed on the freezer door, which can have temperature fluctuations every time you open the freezer/refrigerator. Many scientists I worked with stored seeds in special -20C freezers. Seeds at the international seed bank in Norway on Spitsbergen Island are stored at -18C which is 0 F, a bit colder than a typical freezer. The idea is that the colder the temperature the longer the seeds could survive. If you don’t have access to an ultra cold freezer as a home hobbyist, your freezer or even refrigerator is fine. No matter where you store your seeds they should be placed in labeled containers, with the date on them so that you can keep track of time. This is also a good place to list other important tid bits like the specific traits the parent(s) of the seeds had to reference for breeding, like “Tall” “Tastes fruity” or “Quick to finish flowering” etc.
Second, keep your seed dry – the lower the humidity the less chance that microbes will attack and destroy the seeds, and the less likely that the seeds will break down physiologically inside. It might be a good idea to use a chemical desiccant if you have access to one.
Be mindful that seeds within and among different species will have different longevity. At Michigan State University, there was a professor Beal who in the 1890’s stored seeds of many different species of plants in containers. These containers were buried in secret locations all over the campus. One of my dissertation committee members had responsibility for the map of where the containers were stored around the campus. One of the last containers was dug up while I was at MSU. All seed were over a century old, and only one species of plant, Verbascum thapsus (common mullein) still had some seed germination. On the other hand, I have read reports of seeds found in peatlands germinating after centuries, but I think that is the exception rather than the rule.
No matter what you are growing, your seeds and clones will only last so long, plant breeding is the only way to get new plants. As I see it, you have two choices; you can do it, or pay someone else to. If you are interested in doing it yourself, please read the next several posts about plant breeding.

Good Growing!

2 comments:

Anonymous said...

Nice info!

Happy Hussein said...

Some seeds require scarification to germinate. I have found that many seeds that are difficult to germinate will germinate after freezing, the thermal stress will greatly enhance germination rates.