I recently visited a facility where they were doing a lot of research into plant biology. As such, it was important for them to have their seeds stored at exactly the correct temperature and humidity to prevent germination or degradation of the seeds.
Seed storage in general
Around 10000 years ago when the first human beings stopped hunting and gathering wild plants, and started to cultivate on farms, preserving and storing seeds became important.
There are various reasons to store seeds, for example, simply preserving grain for consumption later in the year or for sowing during the following season. A little more complex is the collection and preservation of seeds for a longer period of time. This may be done to protect species from extinction or to ensure genetic variety for future generations. Long term storage is also necessary as a back up in case of catastrophic events, such as natural disasters, and disease outbreaks. This type of long term storage is usually done in well protected storage building called seed banks.
Inside each seed is a living plant embryo which, even in a state of dormancy, breathes through the exchange of gases across its membrane, and is constantly undergoing metabolic processes, also known as aging. The natural lifespan of a seed is influenced by several factors including: permeability of the seed coat, dormancy, and seed physiology. But one of the most important factors is the external environment the seed is exposed to. Temperature and humidity play a key role in the storage capabilities of seeds.
Facts & figures:
The oldest seed that has grown into a viable plant was a Judean date palm seed about 2,000 years old.
The Millennium Seed Bank Project in the UK is the biggest seed bank in the world. Currently they store 31880 species and 1`907`136`030 seeds.
China, with 197 million metric tons, is the world`s biggest producer of rice.
Why the need to measure humidity?
Controlling the environment in seed storage is essential for maintaining the germination capacity of seeds, or simply the quality of the seed as a food.
Every 1% decrease in the moister content will double the storage life. The same applies for every 5°C decrease of the storage temperature.
A rule of thumb: the sum of the temperature in degrees F and the % relative humidity should be less then 100 for good seed storage conditions.
Proper storage conditions maintain relative humidity levels
between 20% and 40%, giving corresponding seed moisture contents between 5% – 8%, depending on the type of seed. This range is safe for most seeds. When seed moisture content drops too low (<5%), storage life and seed vigor may decline. When seed moisture content goes above 8%, aging or seed deterioration can increase. Deterioration effects the integrity of the cell membrane, along with several biochemical processes, which overall results in loss of vigor and viability. Seed moisture contents above 12% will promote growth of fungi and insects. Most seeds cannot germinate until seed moisture contents go above 25%.
Seed preparation for long term storage (Seed bank)
To prepare for long term storage, seeds are first put in to a drying room where temperature and humidity is carefully kept at 15°C and 15% relative humidity. Under these conditions the seeds gradually dry out. They are then cleaned, counted and put into airtight containers, before being placed in a seed bank at -20°C. The seeds are then tested for viability on a regular basis.
Philip Robinson Rotronic UK