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

� Half%seed rule

History 1

� The first requests for purity tests (+ 1870) were made to combat adulterations:

� addition of stones and other non%seed 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 2nd 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 PSD%dilemmas:

� what is possible in processing

vs

� what is achievable in purity analysis

� what is relevant for practice / the user

The analyst’s dilemma

Seed producerSeed producer

Seed analystSeed analyst

Consumer

Technicalpossibilities

Technicalpossibilities

Economicconsiderations

Economicconsiderations

Technicaldemands

Technicaldemands

Consumer preferences

Consumer preferences

Technicalpossibilities

Technicalpossibilities

Duration of the test

Duration of the test

PricePrice

Starting points

� ‘User’ meaning the farmer as end%user 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?

Seed completelywithout testa

Half seed rule

No testa

No radicle-plumule axis

Less than one-half the original size with testa

Half seed rule

PSD 10

PSD 11

PSD 11 (Fabaceae)

√

√

√

>½ ≤½

� 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