Explanation of terms used on the species profile pages
Kew's 'Difficult' Seeds project has put together a series of species profile pages for species identified by workshop participants as possessing seeds that are difficult to handle, store or germinate. The terms and the methods referred to in the species profile pages are explained below.
- Recalcitrant seeds do not survive drying to any large degree, so are not suitable for long term storage.
- Intermediate seeds are more tolerant of desiccation than recalcitrants, but generally lose viability more rapidly at low temperature.
- Orthodox seeds can be dried to low moisture contents without suffering damage. Over a wide range of storage environments, their longevity increases with reductions in both moisture content and temperature, in a quantifiable and predictable way. Orthodox seeds should be dried to 15-20% relative humidity and stored under cool conditions, ideally at -20ºC. If cool storage is unavailable, well-dried seeds in sealed containers will survive many years at ambient temperature.
Poor handling of orthodox seeds will reduce their storage life. Seeds from fleshy fruits require special attention. If the fruits are fully ripe, remove the seeds as possible and dry slowly, under ambient conditions, before placing them in a dry room. If the fruits are not fully ripe, allow them to mature before extracting the seeds.
Improper handling of oily seeds may also reduce storage life. If using moisture content to monitor drying, remember that oily seeds will have a lower equilibrium moisture content than non-oily seeds when dried at the recommended 15% relative humidity.
The relative longevity of orthodox seeds in storage has been estimated from the viability equations on the Seed Information Database and from the studies of Walters et al. (2005) and Ellis and Hong (2007).
- Ellis, R.H., Hong, T.D. (2007). Quantitative response of the longevity of seed of twelve crops to temperature and moisture in hermetic storage. Seed Science and Technology, Vol. 35 (2): pp432-444.
- Walters, C., Wheeler, L.M., Grotenhuis, J.M. (2005). Longevity of seeds stored in a genebank: species characteristics. Seed Science Research, Vol. 15 (1): pp1-20.
The information on germination requirements is mostly sourced from the Seed Information Database, which contains germination data from over 40,000 tests carried out at Kew's Millennium Seed Bank. We have also included germination conditions described in the Manual of Seed Handling for Genebanks and the International Rules for Seed Testing.
Recommended germination conditions assume that testing is carried out post-storage, after seeds have been dried and stored under cool conditions. Remember to allow containers that have been taken from cool storage to reach ambient temperatures before they are opened to remove seeds for germination tests.
- Kew's Millennium Seed Bank mainly uses 1% agar as a germination medium. Make up with distilled/deionised water (final pH 6.0 to 7.5), and pour into plastic Petri dishes to a depth of approximately 1cm.
- Germination paper, made from purified vegetable cellulose, is also a suitable medium. Seeds may be placed on ‘top of paper’ in Petri dishes or other containers, or ‘between paper’ (rolled paper towels). Wet with distilled/deionised water (pouring off any surplus).
- Clean, oven dried (100ºC) sand of uniform size (0.05-0.8 mm), at a depth of approximately 1 cm, can also be used. For small seeds (<5 mm diameter), add 5 ml water per 50 g sand; for larger seeds, add 10 ml water per 50 g sand.
Seeds should be placed apart from each other on the surface of each of these media. Washing seeds in 10% sodium hypochlorite solution for 5 mins before sowing may help prevent fungal contamination during germination. Gibberellic acid (GA3) or potassium nitrate (KNO3) may be added to germination media to promote germination in some species. Place germination containers in a closed plastic bag to prevent evaporation.
Germination tests can be carried out in a germination incubator or under ambient conditions. Illuminate by an artificial light source (preferably cool white flourescent tubes of 30-35 W) or indirect day-light. If an alternating temperature regime is recommended, use a photoperiod of 8 hours light and 16 hours dark, or 12 hours light and 12 hours dark, with the highest temperature during the illuminated period.
Carry out a cut-test on ungerminated seeds remaining at the end of a germination test to assess whether they are fresh and filled, mouldy and filled, empty or insect infested. Don’t include any empty or insect infested seeds, or those with no embryo present, in the final germination percantage calculation. This especially important with seeds of Poaceae, Asteraceae and Cyperaceae families, which tend to produce a high proportion of empty seeds.
Seed dormancy is the failure of healthy seeds to germinate under favourable conditions, and has evolved to ensure survival from the time of seed dispersal to germination. It is important to overcome seed dormancy in routine tests so that the true viability of a collection is not underestimated. There are five main dormancy types, as defined by Baskin and Baskin (2004).
- Physical dormancy is due to the seed or fruit coat being resistant to water uptake, thus preventing germination. It can also be due to the presence of inhibitors within the seed covering structure or the presence of a hard, woody fruit wall.
- Physiological dormancy is caused by a mechanism within the seed embryo, or embryo covering structure, which prevents germination.
- Morphological dormancy is due to a seed embryo which is underdeveloped at dispersal. Germination is prevented until the embryo reaches a species-specific critical length.
- Morpho-physiological dormancy occurs in seeds with underdeveloped embryos that also have physiological dormancy. The embryo must grow to a species-specific critical length and physiological dormancy must be broken before germination can occur.
- Combinational dormancy occurs in seeds that have a combination of an impermeable seed or fruit coat and a physiologically dormant embryo. For germination to occur, both types of dormancy have to be broken.
We would like to extend particular thanks to Carol Baskin at the University of Kentucky, who has provided dormancy information for many of the ‘difficult’ species.
- Association of Official Seed Analysts (2010). AOSA Rules for Testing Seeds. AOSA, USA.
- Baskin, C.C. and Baskin J.M. (1998). Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, USA.
- Baskin, C.C. and Baskin J.M. (2004). A classification system for seed dormancy. Seed Science Research, Vol. 14: pp1-16.
- International Seed Testing Association (2010). International Rules for Seed Testing: edition 2010. ISTA, Bassersdorf, Switzerland.
- Rao, N.K., Hanson, J., Dulloo, M.E., Ghosh, K., Nowell, D. and Larinde, M. (2006). Manual of seed handling in genebanks. Handbooks for Genebanks No. 8. Bioversity International, Rome, Italy.
Information on seed morphology has also largely been extracted from the Seed Information Database. The fruit types described are based on work by Spjut (1994) and the embryo classification derived from Martin (1946). Have a look at our Glossary of Seed and Fruit Morphological Terms (Pdf) for more details of the descriptions. Morphology images have been taken by our Seed Morphologist, Wolfgang Stuppy, and colleagues.
- Spjut, R.W. (1994). A systematic treatment of fruit types. Memoirs of the New York Botanical Garden, Vol. 70: pp1-182.
- Martin, A.C. (1946). The comparative internal morphology of seeds. American Midland Naturalist, Vol. 36: pp513-660.
Various website databases have been used to determine correct taxonomy for the ‘difficult’ species, including the International Plant Names Index, Tropicos and the African Flowering Plant Database, as well as Kew's own databases. Common names have been found in the Dictionary of Plant Names, the Multilingual Multiscript Plant Name Database (MMPND) and from workshop participants.
- AFPD (African Flowering Plants Database) - Conservatoire et Jardin Botaniques de la Ville de Genève, Switzerland, and South African National Biodiversity Institute, Pretoria, RSA.
- Dictionary of Common Names - The Plant Press, UK.
- IPNI (International Plant Names Index).
- MMPND (Multilingual Multiscript Plant Name Database) - Institute for Land & Food Resources, The University of Melbourne, Australia.
- Tropicos - Missouri Botanical Garden, USA.
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