Presentation Van der Burg 1 - International Seed Testing Association

Development of ISTA Pure Seed
Definitions (PSD’s)
Joost van der Burg
Plant Research International

Contents
Short history
Considerations while developing PSDs
Special cases
Empty seeds
Halfseed rule

History 1
The first requests for purity tests (+ 1870) were made to
combat adulterations:
addition of stones and other nonseed material
colouration of seeds and debris
The first descriptions of pure seed were made to exclude:
inert matter
empty seeds
seeds that will not germinate

History 2
‘Crop seed definitions’ (MacKay, 1968)
PSD’s introduced into the Rules in 1976
Agricultural species: First PSD Handbook of 1978
Many flower seeds included in the 2 nd edition 1987
Forestry seeds in the FTS Handbook of 1991
New PSD Handbook due in 2009 ..

Purpose of the purity test
To describe what is to be found in the sample
While doing that, we make quality judgements:
we distinguish between pure seed and inert matter
or: good and not good

Considerations
The PSDdilemmas:
what is possible in processing
vs
what is achievable in purity analysis
what is relevant for practice / the user

The analyst’s dilemma
Technical
possibilities
Economic
considerations
Technical
demands
Consumer
preferences
Seed Seed producer
Consumer
Price Price
Seed Seed analyst analyst
Technical
possibilities
Duration
of of the the test test

Starting points
‘User’ meaning the farmer as enduser of the seed
The farming system: intensive/extensive
(agric/vegetable/flowers/trees)
The way of planting: broadcast/diff types of sowing machines
The better equipped and managed companies

How to start developing a PSD?
What is the norm / current practice in trade
PSD’s for related species
Botanical terminology
Investigate what is possible with sorting
What is technically possible in the lab? (blowers, sieves)
What is possible ‘on the table’ (difficulty, time)
Relevance for the germination test (sowing possibilities, PLS)

Strong Method (‘Continental method’)
Quick Method (‘Irish Method’)

Strong Method
Pure seed is:
only seeds without any damage to vital parts

Strong Method
Pure seed is:
only seeds without any damage to vital parts
Subjective
Resulting in variation between analysts and labs
Actually carrying out a germination test on the
purity table

Quick Method
Pure seed is:
all pieces of seeds more than half the original size

Quick Method
Pure seed is:
all pieces of seeds more than half the original size
More objective
Less variation between analysts and labs
Germinative capacity is determined in the
germination test

1950:
After decades of discussion it was agreed that the
germination test will decide on germination:
The presence of an undamaged embryo was no longer
required
So, the Quick Method won !
Exception: empty seeds

Empty seeds
‘… unless it is obvious that no seed is present’ (achenes,
nuts, nutlets, mericarps, pods, etc.)
Grass florets
1/3 Rule (Festuca and Lolium)
(Deleted for Elytrigia repens !)

Half seed rule
The quick method: “> ½ of original size”
what is > ½?
what is original size?
is seed coat relevant, or endosperm?
do we need the embryo?

Half seed rule
No radicle-plumule axis
No testa
Less than onehalf
the original
size with testa
Seed completely
without testa

Half seed rule
>½ ≤½
√
PSD 10
PSD 11
√
PSD 11
(Fabaceae)
√

So the apparently simplest rule proves not to be so simple to
interpret after all ..

There is more to be said:
Appendages
Wings
Envelopes
MSU

Conclusion
We shall avoid irrelevant details
We must keep them easy to interpret
We shall remember that if a unit is almost like a seed, it will
be sown as such
We must also keep in mind that we have a germination test
after this

Thank you