Historical Paper - Volume 2 2008 - International Seed Testing ...

H.A. JENSEN
Before an effective size grading was introduced in cleaning of Beta, it was due to
different sizes of the clusters, difficult to obtain a representative sample for the germination
analyses. Aiming to overcome this problem, the Rules of 1938 prescribed that from a
sample of 50g all extraneous matter should be removed and the remaining part of the
sample divided into 5 fractions by a set of sieves with 5, 4, 3 and 2.5mm slits. The
clusters retained in each sieve should be counted, and the number of clusters to be taken
from each sieve should be calculated, aiming to obtaining the 100 clusters necessary for
each replicate in the germination test.
The Beet Seed Committee tried various methods for drawing pure seed for germination,
but was not able at this stage to recommend a method, which could replace the timeconsuming
sieving and counting method (Stahl 1950a).
Improved cleaning techniques reduced the problem, and since 1966 pure seed for
germination have been obtained from the pure seed fraction. The use of the 2mm sieves
in purity analysis of Beta was removed from the ISTA Rules in 1995 to harmonise with
the AOSA Rules
The use of sieves as an aid in purity and number count analyses has always been a
standard procedure at most seed testing laboratories. In Norway, for example, a sieve is
shown on a photo of the purity laboratory taken around 1930 (Ellingsberg et al., 1984).
7.2 Size grading
After introduction of monogerm Beta seeds, which were planted by precise drilling
machines, a uniform size of the seeds and the seed pellets became important. Therefore,
seed grading became part of the ISTA Rules of 1976 (Appendix A).
In the Rules it is stated that “The ‘Bonn’ Screening apparatus…with the requisite
round-hole screens and automatic switch-gear for regular interruption of the reciprocations
may be used.”
This is one of the few cases where a specific brand is mentioned in the ISTA Rules.
The reason is, most likely, that is was a problem with different brands of screening
apparatus to obtain uniform results between laboratories.
A uniform and precise size of the holes in the sieves, used for the size grading, is also
very important for reproducing the results. A method to test the reliability of the sieves,
used for size grading, was developed by Kruse and Steiner (1994).
7.3 Image analyses (seed scanner)
Westerlind (1988) described the development of a seed scanner, a computer–based device
for determination of other seeds by number in cereal seeds. The seed scanner sorted
the seed sample into two fractions. One fraction contained typical seeds of the species
examined and the other deviating seeds and various impurities. Only the latter fraction,
usually representing only 0-15 pct. of the sample, needed to be examined by the analyst.
The seed scanner combined with manual examination could be calibrated to work with
at least the same accuracy as samples tested by manual analyses. A considerable amount
of time could be saved when the seed scanner was used on cereals. The system has been
used since 1988 at five Swedish seed testing laboratories and later in Norway, Denmark
and in a few other countries.
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