Improving the identification, handling and storage of “difficult” seeds ...

Orthodox but dormant
Although less of a problem in the major cultivated crop species, dormancy is an issue in some crop
wild relatives and underutilised/lesser known species. Specific requirements for germination may
include e.g. dry after-ripening, alternating temperatures, stratification, smoke treatment,
mechanical scarification etc.
Orthodox but short-lived
Information about relative seed longevity can help gene banks better plan viability monitoring and
regeneration activities. A recent study (Walters, 2005) determined the time taken for germination to
decline to 50% for 276 species, stored from 16 to 81 years, defined as the relative longevity value.
▪ Short - Arachis hypogaea, Guizotia abyssinica, Oryza sativa, Carum carvi
▪ Medium short – Zinnia violacea, Lupinus angustifolius, Lepidium sativum
▪ Medium – Linum usitatissimum, Triticum aestivum, Sesamum indicum, Lolium perenne
▪ Medium long – Citrullus lanatus, Eragrostis tef (abyssinica), Sorghum bicolor, Melilotus alba
▪ Long – Abelmoschus esculentus, Eleusine coracana, Zea mays, Vigna radiata
Immature orthodox seeds dried too rapidly
Collections showing a low level of viability during initial testing may have been harvested too early.
Desiccation tolerance is acquired during the later stages of seed development, around, or slightly
after, mass maturity or maximum dry weight. Immature orthodox seeds behave as recalcitrant seeds
and are killed by rapid drying. It is possible that some so-called recalcitrant and intermediate
species are in fact orthodox species that were collected too soon.
Collections with good initial viability but poor viability during re-testing may also indicate a degree
of immaturity. Maximum storage potential (potential longevity) is achieved during the latter stages
of seed development in most species, typically around natural dispersal in wild species. Although
initial viability may be similar, seeds harvested too early will lose viability faster than those
harvested at the optimum stage.
Orthodox seeds insufficiently dried prior to storage and/or stored under poor conditions
Not all gene banks meet international standards (3-5% moisture content, -18ºC storage conditions)
for long-term storage. In many cases, long-term storage is not required – the species is adequately
conserved elsewhere – and short-medium term storage of active collections is the purpose. Here too,
drying and storage practices may not be optimal. Power failures can lead to fluctuating storage
temperatures, but this is not as damaging as a high seed moisture content, caused by inadequate
drying or poorly sealed containers.
Resources spent on improving these procedures will save time and resources on regeneration (the
mean regeneration need, as reported in 1996, was 50% of collections) and/or re-collecting.
Consequently, this will free up resources for much-needed characterization and evaluation.
At the farm level, ambient drying may be sufficient to dry seeds for one year’s storage or even
longer. In humid regions, farmers have more difficulties in drying and storing seeds for any length
of time.
Harrington’s rules - seed life span doubles:
▪ for every 1% reduction in moisture content (or approximately, every 10% reduction in RH)
▪ for every 5°C drop in temperature
On-farm seed losses due to insect damage
▪ 75-100% (soft floured maize seeds, Peru)
▪ >75% (beans, Amazonia, Peru)