Seed containers may indeed involve more than one material, most often two or three different ones. They may also include sealing, lids with screw, twist-off or pressure mechanisms, gaskets, joints, levers and might have fractures, pinholes, etc. (Walters, 2007). In other words, they have not only micropores in their materials but might also show macropores associated to discontinuities which are often difficult to avoid. Dilatation coefficients differ for different pieces and this is only one of the causes for such discontinuities. Therefore, any approach to their behaviour based upon the properties of their nude constituent materials is clearly insufficient.
This was the reason supporting a direct experimental approach (Gómez-Campo, 2002) which was strongly opposed but cannot be overlooked here because it contains the detailed information which is summarised in Gómez-Campo (2006). That research showed that 90% of the tested containers (36 in 40) allowed water vapour to penetrate inside. Worst of all, the four efficient ones were very rarely used in seed banks. Some tested foil bags did not perform well. Thus, the author wishes to re-affirm his opinion that 'the main reason for the failure to maintain high germination rates is the widespread use of inadequate containers' (Gómez-Campo, 2006). This is believed to be the most prudent position before starting to focus on other possible reasons for poor seed preservation.
Before continuing, some important recent developments should be mentioned. Pérez-García et al. (2007) show how seeds having been ultra-dried with silica gel and kept inside sealed glass tubes have maintained their viability practically intact (average = 98.4%) for almost 40 years in the seed bank of the Universidad Politécnica de Madrid (UPM). These figures are by far unmatched by any other seed bank of similar age. The highest viability we have been able to trace for an equivalent seed material and comparable storage period is 54.8% (Walters et al. 2005). The reasons behind this sharp difference are of paramount importance for the future of seed banking in the World. Let us make some more comments on the containers before suggesting a second possibility.
Dry seeds are highly hygroscopic and produce strong inward gradients when stored in a container. In the long term, enough time is available -many decades or centuries (yes, centuries!)- for any leak or permeability to become fatal -regardless of how minimal it could be. In an uncontrolled cold room, relative humidity is normally high and -irrespective of the outdoor climate- this will contribute to rise the gradient further. Thus, we should insist on the containers. In Walters (2007), table 3 shows how most materials allow some water vapour through at different rates. Only metal and glass are exceptions. Metallic foil bags -the only technical innovation in this field for many years- are sealed with plastics which are never fully reliable because water molecule is small enough to pass through the array of polymeric chains. The longevity that might be gained with the use of low temperature is largely lost through high moisture. Being opaque, foil bags do not additionally permit the benefits associated to the use of silica gel (easy direct control from the outside, absorption of toxic gases generated during seed aging, etc.). Sealed glass remains the only fully reliable and practical option. If it is breakable, efficient ways to protect it exist. Flame sealed glass vials may be highly useful for many species with small seeds and/or for parallel "black-box" collections, special material, etc. The finding that "Kilner" or "Scotch" jars (Gómez-Campo, 2002) are reasonably hermetic and transparent, meant nothing less that the possibility of extending the silica gel method (Gómez-Campo, 1972) to any other type of orthodox seeds.
If unsuitable containers were not the main cause explaining poor preservation, an alternative might consist of the generalised use of simple drying (4-7% moisture content, m.c.) against the possibility of ultra-drying (1-3% m.c.; ultra-dry conditions being here defined as those obtained by equilibrium with dehydrated silica gel). Both causes are closely linked because ultra-dry conditions cannot be maintained within defective containers. At this point it is highly relevant to refer to another result in the above mentioned paper by Pérez-García et al. (2007). A set of ultra-dry seeds which were stored in a closet at room temperature also for almost 40 years showed a viability close to 95%. This figure is not too distant from the viability of our refrigerated ultra-dry seeds (98.4%) and is far higher than anyone recorded from simply dried seeds in other genebanks, the smallest of which has probably spent hundreds of megawats·hour in refrigeration during that period. All this converge into the idea that although both factors matter, very low seed moisture content is more efficient (and less expensive) than very low temperature. However, the first factor has largely been neglected while the second has overly been emphasised. Favouring low temperature against low moisture might also stay behind the worldwide failure to maintain high germination rates.
After Pérez-García et al. (2007, 2008) there is very little room to doubt that the silica gel method works, even if several technical or logistic improvements could still be introduced or adapted to different situations. Pharmaceutical industry has probably much to say on larger sealable glass containers. Other sufficiently hermetic jars or flasks for use with external visual control might be found. "Black-box" collections might be used to send smaller but fully preserved seed samples into the future. The concept of base collections might perhaps be revised in accordance with the "black-box" concept. Practical tricks to facilitate the management of silica gel (such as methods for direct ultra-drying with molecular sieves or by immersion in anhydrous gases) could be worked out. Immediately, the cycles for germination tests and rejuvenation can be drastically re-adapted in connection to the new enhanced longevity.
It was nobody's fault if seeds were poorly preserved in the past because everybody did their best with the technology available at the time. No guilt exists, but it would be suicidal not to recognize the present situation because further losses of enormous amounts of genetic material will occur. If longevity is defined as the time needed to score a decay of 50% in viability, a longevity of 50 years (Walters, 2007) means that rejuvenation should have been started many years before (rejuvenation is recommended when viability declines to 85%). It is a telling sign that all banks with 40 years or more - and many younger ones - are now actively rejuvenating their material.
However, something was achieved by low temperature alone because present rejuvenation cycles can be estimated in 20-25 years (or some more for certain members of the Fabaceae, Malvaceae, etc.) while ancient plant breeders needed to sow their collections every 6-8 years. Using ultra-dry methods with only moderately low temperatures rejuvenation cycles might well be extended to 1-2 centuries or more. Savings in energy, labour and genetic material may be enormous. In short, it is firmly believed that seed preservation can be much less expensive and far more efficient than it is nowadays.
The UPM should be thanked again for providing a rapid publication in its series of Monographs. This article was written after an invitation by the Editor of SSR to participate in a "vivid and vigorous debate" but when my turn arrived the debate was suddenly stopped.