The following resources were developed for the Training Workshops of Kew's 'Difficult' Seeds Project. There are slides and notes for lectures, along with handouts for running practical sessions and relevant technical information sheets. Together, they cover the essential theory and practice of seed conservation.
- Other useful links: Find more useful sources of information to address problems with ‘difficult’ seeds.
- Information for farmers and community seed projects: Discover resources to support on-farm seed activities.
Seed-air moisture relations
Knowledge of the movement of moisture from seeds to the surrounding air is essential for understanding the process of drying. Seed moisture content affects the longevity of seeds in storage. Seed storage behaviour categories are related to responses to water removal.
The practical session on measuring seed moisture status uses oily and non-oily seeds that have been equilibrated to different moisture levels. Moisture content and relative humidity are measured for each seed sample and plotted against each other.
- Measuring seed moisture status - practical session (pdf)
- Measuring seed moisture status using a hygrometer - technical information sheet (pdf)
- Equilibrating seeds to specific moisture levels - technical information sheet (pdf)
Seed storage behaviour
Seed storage behaviour is one reason why seeds might be difficult to conserve and use. This lecture defines orthodox, intermediate and recalcitrant seeds, and looks at how genebanks can identify seeds with intermediate and recalcitrant behaviour.
This practical session shows how to predict storage behaviour of tree seeds from seed structure. The seed coat ratio (seed coat mass : whole seed mass) is determined and this calculation is used to predict the likelihood of the seeds being desiccation-sensitive (recalcitrant).
The following practical demonstrates a simple method to determine seed storage behaviour, using just 100 seeds. This is a much-simplified, efficient version of the large-screen approach, which requires thousands of seeds and considerable time and consumables.
- 100 seed test for desiccation tolerance - practical session (pdf)
- Identifying desiccation-sensitive seeds - technical information sheet (pdf)
‘Seed longevity’ refers to how long seeds can be stored, and remain viable, under a given set of conditions. Seeds with a high initial viability that have been well-dried and stored at sub-zero temperatures, will have maximum longevity, avoiding expensive regeneration of the collection. When monitoring viability of collections in storage, seed banks need to be aware that seeds of some species are inherently short-lived. This lecture describes the factors affecting longevity and what can be done to maximise the longevity of seed collections.
Understanding the effect of seed moisture status on seed viability and longevity is essential for maintaining seed quality. This practical session uses accelerated ageing methods to investigate seed longevity, enabling seed survival curves to be plotted. Germination percentages can be transformed to probits, to produce straight line graphs, allowing an estimation of sigma (σ - time to lose one probit) and comparison of longevity between seed lots.
- Effect of seed moisture level on seed viability - practical session (pdf)
- Protocol for comparative seed longevity testing - technical information sheet (pdf)
The following practical session uses the viability equations through Kew's Seed Information Database. Seed longevity of a particular seed lot can be predicted for different storage conditions.
- Using the viability equations - practical session (pdf)
- Using the viability equations - answers (pdf)
It is important to understand how to deal with collections immediately after harvest, once seeds have reached the point of maximum potential longevity, to ensure that seed quality is maintained. Fully ripe, orthodox seeds should be dried to a level that minimises ageing, and maintains potential longevity. Immature orthodox seeds must be allowed to ripen naturally before drying, to avoid poor viability and reduced storage life.
This practical session demonstrates use of a modified psychrometric chart as a field tool, to indicate whether seed collections are at risk of unacceptable losses in viability. Knowledge of ambient conditions at harvest time can help us make decisions on how to handle seeds.
- Post harvest handling - practical session (pdf)
- Psychrometric chart (pdf)
- Post-harvest handling of seed collections - technical information sheet (pdf)
The following practical session uses the viability tools in Kew's Seed Information Database to model the effect of post-harvest handling on initial viability. The time until viability falls to the regeneration standard can also be calculated.
- Modelling the effect of post-harvest handling on seed storage life - practical session (pdf)
- Modelling the effect of post-harvest handling on seed storage life - answers (pdf)
This lecture covers different drying methods and looks at the key factors affecting the extent and rate of drying. If orthodox seeds are properly dried before storage, they will remain alive for the longest possible time. Drying enables seeds to tolerate sub-zero temperatures, which also prolongs longevity.
- Seed drying - slides and notes (pdf)
- Seed bank design: seed drying room - technical information sheet (pdf)
The seed drying practical session can be carried out with local farmers, especially those experiencing difficulties in storing seeds in warm, humid regions. Charcoal and grains such as rice or maize are readily available and inexpensive, and once dried, can be used as desiccants, reducing harvested seeds to moisture levels lower than those attainable under ambient conditions. A simple salt test can be carried out to check the moisture status of seeds before storage.
- Seed drying using low-cost desiccants - practical session (pdf)
- Small-scale seed drying methods - technical information sheet (pdf)
- Salt test: a simple indicator of seed moisture status - practical session (pdf)
- Low-cost monitors of seed moisture status - technical information sheet (pdf)
This lecture looks at seed storage containers and cool storage options. Once seeds have been dried to safe moisture levels, they must be stored in leak-proof containers to maintain a low seed moisture content and maximise longevity. Reducing storage temperature has a beneficial effect on seed longevity and ideally, storage temperature should be about -20°C.
- Seed storage - slides and notes (pdf)
- Selecting containers for long-term seed storage - technical information sheet (pdf)
- Seed bank design: cold rooms for seed storage - technical information sheet (pdf)
Germination and dormancy
Monitoring the viability of collections, through germination tests, is an important routine task for all seed banks. The specific conditions required for germination vary considerably amongst species, and in many cases the situation is complicated by the presence of seed dormancy. This lecture explains how environmental factors affect seed germination and discusses methods to overcome seed dormancy, using examples of ‘difficult’ seeds.
The practical session on germination and dormancy examines the two most important constraints to the successful germination testing of conservation collections: physiological and physical dormancy. The differences between scarification treatments to overcome physical dormancy, and surgical treatments to remove constraints to embryo growth in physiological dormancy, are demonstrated. ‘Cut tests’ are performed to evaluate ungerminated seeds at the end of each germination test.
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