A manual of rice seed health testing - IRRI books - International Rice ...

5. Obtaining the submitted sample
(Rule 2.6.6)
The submitted sample shall be obtained
by reducing the composite to
an appropriate size by one of the
methods referred to in Rule 2.7.2.,
using larger equipment if necessary.
If it is difficult to mix and reduce
the sample properly under warehouse
conditions, the entire composite
sample shall be forwarded to the
seed testing station for reduction.
If the composite sample is of appropriate
size, it may be regarded as
the submitted sample without reduction.
Additional samples requested not
later than at the time of sampling, by
the owner of the lot, if granted, shall
be prepared in the same way as the
submitted sample and marked 'Duplicate'.
6. Weight of submitted sample (Rule
2.6.3.)
For rice, thc submitted sample
should be 400 g. If less, it should be
indicated that 'The sample submitted
weighs only ....g and is not in accordance
with the International
Rules for Seed Testing.'
Procedures in the laboratory
1. Obtaining the working sample
(Rule 2.7.2.)
The submitted sample received by
the seed testing station generally
needs to be reduced to a working
sample equal to or greater than the
size prescribed for each test.
The submitted sample shall first
be thoroughly mixed. The working
sample shall then be obtained either
by repeated halving or by abstracting
and subsequently combining
small random portions.
For routine seed health testing,
400 seeds are taken as working sample
(Rule 7.4.3.a.3.).
2. Instruments and methods (Rule
2.7.2.a)
a) The mechanical divider
method is suitable for all kinds of
seeds except extremely chaffy types.
The apparatus, a conical (Fig. 4.3),
soil (Fig. 4.4), or centrifugal (Fig. 4.5)
divider, splits samples into two approximately
equal parts. The submitted
sample can be mixed by passing
it through the divider, recombining
the two parts and passing the whole
sample through a second time, and,
if necessary, a third time. The sample
is reduced by passing the seed
through repeatedly and removing
one half on each occasion. This process
of successive halving is continued
until a sample of approximately,
but not less than, the required size is
obtained.
b) The modified halving method
(Fig. 4.6). Thc apparatus comprises a
tray into which fits a grid of equalsized
cubical cells, open at the top
and every alternate one having no
bottom. After preliminary mixing,
the seed is poured evenly over the
grid. When the grid is lifted, approximately
half the sample remains
on the tray. The submitted sample is
successively halved until a working
sample of approximately, but not
less than, the required size is obtained.
c) The random cup method, the
spoon method, and the hand halving
method are not applicable to rice
(ISTA 1985).
Samples and sampling 21