Image © 2006 Frans Holthuysen All Rights Reserved
Some seed such as Morning Glory can be viable for up to 50 years. The most extreme example of old seed germinating until recently was Chinese Lotus seed. The brown marble like seeds are hermetically sealed inside a thick shell. Work by Jane Shen-Miller from 1984 to date has found that they contain an incredibly tough protein able to survive temperatures up to 110o C. They also contain an enzyme, methyl transferase, which repairs damage to the seed's proteins & have high levels of ascorbic acid & glutathione, which help keep proteins fit & healthy & protect cells from damage by free radicals. Astonishingly, when recovered from a dried up lake bed, the seed germinated easily, the oldest being 1288 years old! The seedlings were deformed however, the DNA being damaged by chronic radiation damage accumulating over the centuries. Despite this damage however the seedlings still germinated, so it is being studied further to see if it can yield any secrets re seed longevity.
More recently in 2005 a date palm seed, recovered from an archaeological dig at the cliffside fortress of Masada in Israel, was planted and successfully germinated. Understandably at 2000 years old it is called Methuselah.
In contrast some Passiflora seed are thought not to keep for long at all, a year is seen as old, with reduced chances of germination. So do not store seed unless you have to. Jay Anderson (Queensland Department of Primary Industries) reports however, that refrigerated P. edulis seed up to 20 years old has been known to germinate. P. edulis may be a special case as its seed are quite big & it has been selected repeatedly for thousands of years. It is important to realise that Passiflora seed are alive & respire with water freely moving in & out of the seed through the seed coat. To keep seed dormant but viable, low temperature, moisture & a minimal opportunity to exchange gases will maximise longevity.
Correct storage will increase the time that all Passiflora seed remains viable. Before we even consider storage however we need to consider correct harvesting of the fruit, as otherwise the seed may not be viable from the start. See Passion fruit for more information on this. If possible avoid storage at all & try to germinate the fresh wet seed & send it wet if sharing. There are some exceptions to this rule.
Do NOT dry seed unless you have no choice. This will drastically reduce your chances of successful germination of some Passiflora seed. This is the principle reason that some seed is hard to germinate with commercial seed suppliers & indeed most of us storing it incorrectly. There is increasing evidence that some fresh ready to germinate seed will have dormancy induced by drying - this will then have to be broken. Some seed, especially of the tropical Passiflora, simply by being over dried may become unviable altogether ~ 5-8% moisture content being critical.
That said, some Passiflora that live in a relatively dry hot climate e.g. some foetidas, P. holosericea, P. herbertiana & P. cinnabarina are thought to be more resistant to drying & indeed may need it to become viable. In Spring/Summer 2002 I gave the same treatments to a number of dry tropical Passiflora seed (including P. loefgrenii) & dry P. herbertiana seed. i.e. passion fruit juice 3 day presoak & then into soil with heat, but no 7 day washing through after. The varied tropical seed took a long time to come up with germination rates of 0%-10%. The P. herbertiana (40-50 tiny seed) came up quite quickly with nearly 100% germination.
Dry seed storage
As a rule all dry seed will keep better refrigerated. (NOT in the freezer part) Dried arils look messy but don't waste time removing them. There is some evidence that fresh wet arils may inhibit germination in some species. This would make sense as otherwise seed could germinate while still in the fruit. Store in an airtight self seal plastic Ziplock bag squeezed flat & then put in a brown envelope to keep in the dark. That said, I am not sure re the Ziplock bag as I have found seed keeps well just in brown envelopes in the moist fridge environment. Iether way label the envelope fully e.g. P. antioquiensis (Wild collected Colombia) Oct 2002 & approximate number of seed. By convention seed are generally packed in multiples of 10 per envelope or perhaps 5 if rare. Further with the exception of the dry climate species above, adding a little moist (not wet) sand to the Ziplock bag may further improve storage times & even speed germination when out of storage. This may be important if you have received old seed or seed by air which could be near critical re dehydration.
Wet seed storage
I would be inclined to dry the dry climate species above before storage & store them dry. Other wet seed can have the bulk of the arils removed (sieve & water) & then be stored with moist sand as above. Do not let the seed dry out after cleaning though. Only remove the seed from the fridge when ready to germinate them. Note that it is a good idea to use thick plastic bags as thinner ones will be a little permeable.