6.2 – Dormancy, Orthodox and Recalcitrant Seeds

Storing Seeds

Our agricultural systems have evolved over the years to rely on storing seeds to preserve planting stocks from one season to the next.  One of the most important reasons for storing seeds today is to accumulate desired genetic stocks for years following those of low production, called carryover seed.

• Short term storage of seeds for 10 years or less remains a significant practice.
• Long term storage (more than 20 years) is also desirable for genetic preservation programs.

The ability of humans to save and store seeds for prolonged time periods is due much in part to the inherent trait of seed dormancy.

Ultimately short- and long-term storage of seed has one goal:  to delay seed deterioration.

Dormancy

The ability of humans to save and store seeds for prolonged time periods is due much in part to the inherent trait of seed dormancy.

Seed dormancy is defined as a physical or physiological condition of a viable seed that prevents germination even in the presence or otherwise favorable germination conditions.  In other words, it is a complex resting stage influenced by genetics, environment and preharvest factors.

Dormancy traits set seed apart from nearly all other forms of life, a characteristic that evolved over time as a protective or survival measure of a species so that not all seeds germinate at the same time and therefore not all seeds are subject to a potential catastrophic event that may lead to population extinction.  However, the ability to manipulate seed dormancy is thought to have led to the development of agriculture.  The image below from Science Facts illustrates both external and internal conditions must be present for a seed to break dormancy and germinate.

Seeds can become dormant during formation and maturation in two ways. 

The first is called primary or endogenous dormancy.

•  Is triggered by genetic factors or other conditions that induce seed dormancy during the development and maturation of the seed itself.
• Primary or endogenous dormancy is the most prevalent dormancy found in seeds due to inherent properties of the seed.
  • For example, the seed may possess an excess of inhibitors that must be removed or reduced prior to germination by freezing or leaching with water.

The second way is called secondary dormancy and occurs after the seed has reached maturity.

• In secondary dormancy a non-dormant seed exposed to an environmental condition such as heat or drought, may become dormant at that time, to survive the unfavorable condition.
• This is also referred to as exogenous dormancy, or a condition in which the essential germination components (water, light, and temperature) are not available to a viable seed and it fails to germinate, but remains viable, waiting for the right conditions to appear.
  

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The length of time seed will be able to be stored will be dependent on seed species, quality of seed and how it is handled.  Seed species have been separated further into 3 categories based on how long they can be stored based on the biology of their biology.  These categories are Orthodox seed, Recalcitrant seed, and Intermediate seed.  Click on each below to learn more.

View the linked video (2 mins) for a discussion of how seed dormancy in black grass (weeds) impacts control strategies in crops.

Storage Condition Considerations

It is the composition of the seed that will determine the appropriate atmospheric conditions needed for optimum long-term storage.  Recalcitrant seeds have a much higher moisture content than do orthodox seeds at physiological maturity.  They do not tolerate cold temperatures or loss of moisture content.  As a result, recalcitrant seeds are not able to be stored for long periods of time, they do not enter dormancy and do not tolerate low temperatures, because ice crystals will form when the water held within the seed freezes and breaks cell membranes.

Causes of Seed Deterioration

Seed deterioration is an inevitable and irreversible event. Seeds can deteriorate or lose their viability over time due to various factors which can affect their ability to germinate.  Seed deterioration is a significant consideration in agricultural production.

Estimates have been made that about 25% of the annual value of seeds in inventory are lost because seeds deteriorate, leading to poor seed quality. 

Some of the primary causes of seed deterioration include the following, click on each to learn more:

Seed Moisture Content, Humidity, and Temperature

Temperature and relative humidity are the two most important factors influencing the longevity of seeds and are interrelated.   As temperature decreases in a storage facility, moisture in the air is released, because warm air holds more moisture than cold air.  Seeds are hygroscopic, meaning they will absorb the moisture in the air.

In 1972 Harrington[1] proposed a ‘Rule of Thumb’ pertaining to storage stating that the longevity of a seed is decreased by ½ for each 5 degree C increase in seed storage temperature and for each 1% increase in seed moisture content.  This rule of thumb is only true for storage conditions greater than 0 C and below 50 C.

The purpose of seed storage at minimum is to preserve seeds from one season to the next.  Seeds of most orthodox species (like wheat and other crop seeds) can be stored for several years by carefully controlling the temperature and relative humidity.

For short term storage (less than 3 years) another general ‘rule of thumb’ is that the sum of the relative humidity and temperature should not exceed 100.  For example, most seeds can be stored unsealed for short term at 50 F and 50% relative humidity.

Preserving seeds for the longest duration possible without sacrificing viability is ultimately the goal.

Depending on the length of storage needs, the value of the seed and the genetic makeup of the seed, longer term storage can be achieved by reducing temperatures to subzero (-10 to -20 C) and by placing seed in a moisture -proof container.  Ultimately seeds will need to be dried and subject to cooler temperatures for optimum long-tern storage.

Drying is an essential step in seed storage and helps maintain high quality seed.  In the drying process excess moisture is removed so that the seed can be stored at a constant temperature and moisture regardless of the length of storage to maintain seed viability.