Chapter 9

Respiration in stored seeds must be kept low to prevent germination and build up of heat which would kill the seeds. This is usually done by keeping moisture levels and storage temperatures low. Most seeds (orthodox seeds) can be dried down to around 5% moisture without injury. Above 9% moisture, insects may be active and levels above 14% allow fungal growth. Above 18% heating occurs due to respiring seeds and organisms and above 40% moisture germination may take place. Reducing temperature increases storage time and can offset the adverse effects of high moisture. A set of rules are employed to calculate storage times based on the trade off between temperature and moisture content called ‘Harrington’s Rules of Thumb’:

  1. Each 1% reduction in seed moisture doubles the life of seeds for moistures between 5 and 14%.
  2. Each 5oC reduction in storage temperature doubles the life of seeds for temperatures between 0 and 44.5oC.

Modified atmospheres may also be used – for example, in vacuum-packed seed packets or containers flushed with nitrogen. Recalcitrant seeds contain 50–70% moisture at maturity and cannot be dried below 30% without injury. They tend to be seeds from tropical trees, aquatic plants, citrus and seeds with large fleshy cotyledons – for example, Castanea sativa, Aesculus hippocastanum. Valuable seed collections such as those at the Millenium Seed Bank, Royal Botanic Gardens are stored at -20oC or, for recalcitrant seeds, embryos are removed and frozen in liquid nitrogen at -196oC.