The ultimate goal of ex situ conservation by seed banking is to hold seed in storage, indefinitely. Unfortunately, even once seeds can be successfully dried and frozen, they may not remain viable in storage for a particularly long time.
Understanding how long seeds will remain alive in storage is therefore essential to ensure the integrity of the collection. As seeds approach the time when viability has decreased by a specific amount (often 50%), it’s time to collect fresh seeds to replenish the older seeds in storage.
But how do we know when seeds have reach 50% viability? Do we just keep testing the collection? What if the seeds take a really long time to germinate? The answer to these questions is quite simple. We estimate how long seeds will survive in storage by completing an accelerated aging or 'comparative longevity' experiment that allows us to rapidly estimate the rate at which seeds in the collection die.
A comparative longevity experiment usually requires the use of 500 seeds and therefore it is only suitable for large collections. If enough seed is available, then seeds, in groups of 50, are placed in an oven at 45°C, at about 100% relative humidity. These conditions are intentionally used to age the seeds thereby reducing their quality and viability. Over a period of about three months, groups of 50 seeds are removed and germinated. Each group should have slightly less germination then the last and by tracking this proportion of germination against time, we can follow the loss of viability. The results are then compared to species for which we have real aging data and this is used to identify how the number of days in the aging conditions corresponds to the number of months (or years) the seeds would be expected to survive under cold storage. Seeds that loose all viability within a few days of the aging experiment might be expected to only live a few months in cold storage while seeds that survive several months in the aging conditions would be expected to remain viable for many years.
There is very little information available on the longevity of rainforest species, but it does appear that many rainforest seeds only remain viable for short periods of time. The naturally high moisture content of rainforest seeds means that they are often ready to germinate once shed and this is believed to be the reason why they may survive as little as one week in the soil seed bank1
. Tropical species generally loose viability rapidly to extremely fast in cold storage even when using modified storage methods, while species from temperate environments may remain viable for a year2
Research on the longevity of a selection of native Australian plants has been gathered together by Merritt et al., (2014), and can be viewed here.
The methodology applied at the Australian PlantBank to test seed longevity has been developed into a technical handout by the Royal Botanic Gardens Kew and can be accessed here.