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What is a difficult seed?

This project aims to improve the identification, handling, storage and use of ‘difficult’ seeds. We took a broad definition of the word ‘difficult’ so that we could address a range of problems with the conservation and use of seeds.

We started by asking participants at the Stakeholder and Training workshops to tell us about any species that showed poor germination during initial testing and/or during re-tests after storage. This information can help determine whether low germination is related to inherent issues such as seed storage behaviour, dormancy or seed longevity, or to handling and storage difficulties.

Inherent difficulties

Carica papaya germinating

Recalcitrant or intermediate storage behaviour

Seed banks dry seeds to low moisture levels (3-7% mc, 15-20% eRH) and store them at a low temperature (usually -20°C). Orthodox (or desiccation tolerant) seeds remain viable for many years under these conditions.

  • Intermediate seeds are generally less tolerant of desiccation than orthodox seeds. They do not withstand prolonged storage at -20°C.
  • Recalcitrant (or desiccation sensitive/desiccation intolerant) seeds do not survive any significant water removal and therefore are not suitable for sub-zero storage.

Dormant seeds

Anacardium occidentale, an orthodox species with physiological dormancy

When mature, healthy seeds fail to germinate under favourable conditions, they are described as dormant. Seed dormancy has evolved in some species to ensure survival from seed dispersal until field conditions are suitable for seedling establishment. If seeds do not germinate in routine germination monitoring, seed banks may underestimate their true viability. The seed collection may just be dormant, and simply require a dormancy breaking treatment to encourage germination.

Short-lived orthodox seeds

Under conventional seed bank storage conditions, seeds of many orthodox species will remain viable for many decades, if not hundreds or even thousands, of years. Some species however are inherently short-lived and germination re-tests will soon start to show a decline in viability.

Handling and storage difficulties

Immature orthodox seeds

Mature seeds of Cucumis melo, being extracted from their fleshy fruits, so that they can be dried

Seed maturity affects seed viability in two different ways. Most orthodox seeds acquire the ability to survive desiccation soon after they reach the point of mass maturity or maximum dry weight. Seeds collected too soon may not be fully desiccation tolerant and may die under typical dry room conditions. This would result in initial germination results being lower than expected.

Maximum storage potential is reached at a later stage of seed development, typically around the point of natural dispersal or harvest ripeness. Seeds that are not fully ripe will have reduced longevity in storage, resulting in lower re-test results than expected.

Insufficient drying and/or storage under poor conditions

If orthodox seeds have not been sufficiently dried prior to storage, they will not live as long as properly dried collections. If seeds are not stored in hermetically-sealed containers, there is a risk that they will gain moisture and lose viability through ageing. Properly-dried seeds can survive well in hermetic containers at ambient temperatures but will live much longer under cooler temperatures.

Damaged seeds

Seeds may be infested by insects either in the field or during drying and storage. Physical damage may also occur during processing, while seeds are being cleaned of plant material and debris. Any damage to the embryo, whether the root tip or the cotyledons, will affect the potential of a seed to germinate.