Seed drying for long-term storage
Keeping seeds for generations to come
There are two crucial components to successful seed storage: drying and freezing. By the age of 10 most of us know that seeds need water to sprout so it is no feat of genius to conclude that, if you want to store seeds and delay their germination, you need to keep them dry. Freezing them increases their longevity even further by slowing down metabolic reactions in the cells of the dormant seed meaning that the seed can remain viable for decades and, in some cases, hundreds of years.
In nature, seeds will enter a dormant state to prevent germination when the ecological conditions are unsuitable and the probability of the seedling surviving is low. Delaying the germination allows time for the seed to disperse and is one way of stopping all the seeds from germinating at the same time, therefore avoiding intense competition for resources such as light and water. It also ensures that the whole lot doesn’t get wiped out by bad weather or passing herbivores.
Seed storage and agriculture
Humans use this adaptation of seed dormancy to their advantage. Since the dawn of agriculture farmers have been safeguarding seeds to ensure the harvest for the following year. Conservation of plant genetic resources, which includes the wild relatives of crop plants, also allows breeders to create new crop varieties that are tastier or more resistant to disease. The Adapting Agriculture to Climate Change project, which is jointly run by the Millennium Seed Bank and the Global Crop Diversity Trust, is collecting the wild relatives of 29 of the most important food crops and making them available to breeders to create new crop varieties that will be better adapted to climate change. Crops such as potato, rice, lentil and banana are major food sources for millions of people all over the world and by increasing their genetic diversity and resilience, through the use of crop wild relatives, more people will have access to the food they need.
Once seeds have been collected keeping them dry is the number one priority for every seed collector. Some seeds are collected when they are still immature and they need time to ripen in ambient conditions. However, those seeds that are already mature should make their way to the nearest seed bank dry-room as soon as possible. The reality is that, out on a collecting trip, you may be days away from a seed bank and without taking some of the following advice you run the risk of ending up with a bag of soggy, mouldy seeds! To prevent this from happening (as not only is it unpleasant but the seeds may not survive and your collecting efforts may go to waste) there are a few tricks of the trade that every good seed collector keeps with them.
1. Bag it up
Seed collectors think carefully about the kind of bag they choose to keep their collections in. In many cases cloth bags are ideal because they are breathable and the drying process can start immediately, giving your seeds the best chances of longevity in storage. Paper bags tend to be more suitable for grass collection as the awns on the grass inflorescence snag onto cotton really easily and you would need the patience of a saint to pick them out. For more juicy seeds and fruits, plastic bags may be the best option to avoid the juices oozing onto everything else. Open the bags daily to let some air in to avoid mould and fermentation or lay them out to dry in a cool, airy place out of direct sunlight. If they are overripe or damaged, it may be better to remove the flesh from the fruit altogether by using a sieve and cool running water.
2. Keep an eye on the weather
Come rain or shine, seed collectors are gathering seeds whatever the weather. Besides checking to see whether you need sunscreen or your waterproofs, looking up the climate of the region, and even the weather forecast for the day is an essential part of the planning for a successful seed collecting trip. The humidity of the air will affect the moisture content of the seeds so if it has recently rained the seeds may be wetter than expected. Some seeds are particularly susceptible to deterioration from moist air conditions, such as those collected during the rainy season or those that are enclosed in an indehiscent fruit (a fruit which doesn’t split open). These 'wet seeds' need to be dried as soon as possible and this can be done by:
- spreading them out in a thin layer on some newspaper in the shade
- raising them off the ground to allow air to circulate beneath
- repacking them before nightfall to minimise moisture absorption as the ambient humidity rises
Every seed counts so we cannot afford to be cavalier with our post-harvest handling!
3. The big blue drum
One way that the MSB helps partner countries to make high quality seed collections when there is no dry-room nearby is to send them a large, 60 litre, blue drum containing sachets filled with dried-out silica gel. Dried silica is a desiccant and, when placed in the sealed container with the collections, can greatly speed up the drying process. The silica will be effective until it reaches the same relative humidity as the atmosphere. After that the only way to restore its desiccating powers is to dry it out in the sun or an oven, if there’s one handy. Not only is it a self-contained, portable dry-room but the blue drums shipped to partners of the Adapting Agriculture to Climate Change project also contain a ready-to-go tool kit for the perfect seed collecting expedition. Dissection kits, hand lenses, herbarium presses, collecting guides, GPS, and a first aid kit are among some of the goodies you will find inside.
Back at the Seed Bank
When you walk into the dry room at the Millennium Seed Bank (first passing through two airlock doors) you immediately sense the drop in temperature and humidity. Crates stacked almost to the ceiling are teeming with packets full of seeds collected from places as far away and exotic as Chile and the Dominican Republic as well as more local sites such as the Ardingly reservoir, where my colleagues and I spent a sunny autumn afternoon collecting the seeds of a small aquatic plant Littorella uniflora.
Seeds can spend up to 6 months in the dry room, only taken out for the brief time it takes for them to be cleaned and counted. Only when the seeds have reached 15% of their original moisture content are they ready to go into cold storage in the -20°C seed vault.
A final word on rebel recalcitrant seeds
Some seeds, such as mango and Coco de Mer, cannot survive the drying and freezing process and are therefore difficult to store. These seeds are known as recalcitrant or unorthodox seeds and at the moment their only hope is cryopreservation in liquid nitrogen.
There’s much more to say about seed drying (whole books have been written on the subject!), but I hope this has given you a flavour of what's involved.
- Sarah -