Once harvested, the ‘post-harvest handling’ of a seed collection can seriously affect the quality and longevity of the seeds.

Drying seeds is critical

Figure 1. Collecting good quality seeds takes time and effort. Protecting this investment requires careful post-harvest seed handling

Figure 1. Collecting good quality seeds takes time and effort. Protecting this investment requires careful post-harvest seed handling

Seed collections made in the field need to be transported to the TSCC as quickly as possible. At the point of natural dispersal most seeds contain too much moisture for seed bank storage. Drying seeds down and maintaining low seed moisture content is critical in order to maximise seed viability and longevity 1.

Seed moisture content (as well as temperature) determines the rate at which seeds lose viability and die. The vast majority of seeds maintain greater viability when dry. A useful rule of thumb is that seed longevity approximately doubles for every 10% reduction in relative humidity (RH)2. Therefore knowing the moisture status of seeds is crucially important in seed conservation.

Storing seeds at 15°C and 15% RH

Seed collections in the TSCC drying room

Figure 2. Seed collections in the TSCC drying room. Collections are held in cloth/paper bags or paper envelopes and placed in slatted plastic crates. This allows good air flow around the collections and ensures rapid drying.

Once collections arrive at the TSCC, they are usually placed immediately in the drying room (fig. 2). The drying room is maintained at 15°C and 15% RH, an internationally-accepted regime for drying seed collections1.

Dry seeded material collected in the field is placed directly into the drying room. However wet fruit and berries are tackled differently. Seeds, stones, pyrenes, etc. need to be extracted from wet fruit as soon as possible (fig. 3). Wet fruits are pulped in a sieve and washed with warm water to remove the fleshy tissue. Once the fruit pulp is removed, the extracted seeds can be placed in the drying room.

Figure 3 - Fleshy fruited collections


Figure 3. Fleshy fruited collections like these need to be processed immediately to extract the seeds easily.

Seeds are ‘hydroscopic’

All seeds are hygroscopic, including those with physical dormancy once the seed coat has been chipped. This means that they automatically absorb or desorb moisture from the surrounding air by diffusion. If the seed contains more moisture than the surrounding air, the seed will lose water and become drier, and if the seed conatins less moisture then the air it will absorb water. Absorption or desorption of water occurs until the seed is in equilibrium with the surrounding air.

Poorly dried seeds can age rapidly

Slow gradual gain of moisture from the air by dry seeds can accelerate seed ageing and result in a rapid loss in seed viability. It is important then that dried seeds be kept dry.

Dry seeds can be frozen

Placing seeds in a 15% RH environment causes them to gradually dry down to approximately 15% RH. Not only does this maximise seed longevity while in storage, but it is also sufficiently dry enough for seeds to be stored at -20°C without harmful ice crystals forming and destroying delicate seed tissue.

Cleaning up collections

Figure 4. Cleaning seed

Figure 4. Seed Bank coordinator James Wood supervising the cleaning of Spyridium parviflorum (Rhamnaceae) with volunteer Lorraine Cotter.

Cleaning of dry seeds takes place as soon as possible and has two main aims:

  1. To reduce collection bulk as much as possible (e.g. surplus plant material and large bulky seed covering structures are removed).
  2. To remove empty or infested seeds (this significantly improves the quality of the collection and also improves the longevity of the collection in storage).
Shaking of capsules is often all that is necessary to obtain a good clean seed collection. In some cases, the infrutescence (the fruiting stage of an inflorescence) may need to be crushed in a sieve to free seeds. Removal of debris or empty and infested seed is normally tackled using a seed aspirator. This piece of equipment winnows seed material, separating heavy and light material.

The cleaning process can significantly increase the overall viability and longevity of a seed collection, if carried out correctly. During the cleaning process, seed samples are regularly examined for quality and to determine when an optimal end point in the cleaning process has been reached.

Once dry, cleaning a seed collection can take anywhere between 10 minutes to 60+ hours to complete, but the majority of collections can be processed within 1-2 hours. Meanwhile seeds are continually stored at 15°C and 15% RH in preparation for banking at -20°C.

At the TSCC, dedicated volunteers help prepare seed collections for banking (fig. 4).


References:

  1. Probert RJ. 2003. Seed viability under ambient conditions and the importance of drying. In: Seed Conservation. Turning Science into Practice. RD Smith, JB Dickie, SH Linington, HW Pritchard and RJ Probert, eds. Royal Botanic Gardens Kew, 337-365.
  2. Roberts EH and Ellis RH. 1989. Water and seed survival. Annals of Botany 63: 39-52.