Agriculture Reference
In-Depth Information
Variables in Vigor Testing
The development of acceptable vigor tests is a challenging goal. Unlike germination testing, there is no one
standard test against which the merits of a vigor test can be judged. Part of this dificulty arises from the per-
ception that no universal seed vigor test exists; that is, no single test can measure all physical, physiological
and biochemical aspects of seed vigor. Presently, most vigor tests available are recommended for a speciic
crop or for testing a speciic aspect of seed vigor. It seems that one central theme should be to reduce the
range of vigor tests and focus standardization efforts on those that have the broadest application. Next, the
optimum times and procedures for each crop need to be determined and speciied.
Clearly, there remains much work to accomplish in seed vigor testing. There are also a number of
variables that affect seed vigor tests but which have yet to be adequately considered in the development of
vigor tests and the interpretation of their results. Included among these are the following:
Seed size.
Seed lots vary in seed size and this can have an effect on vigor test results. In some instances,
accommodations have been made for the effect of seed size in modifying vigor test results. For example,
bigger seeds would be expected to leak more electrolytes than smaller seeds in the electrical conductivity
test (EC) even though they may be more mature and vigorous. As a result, conductivity results are presented
on a seed weight rather than a per seed basis. Similarly, smaller seeds, because of their greater surface area
to volume ratio, absorb water at a greater rate during an accelerated aging, or EC test than larger seeds. To
account for this, a certain weight of seed is speciied for placement in the inner chamber rather than a speci-
ied number of seeds. It should also be emphasized that many vigor tests monitor some aspect of speed or
rate of germination. Smaller seeds tend to complete imbibition and initiate radicle protrusion before larger
seeds. These examples demonstrate that seed size modiies test results that are separate from an assessment
of seed vigor. A consideration of any new vigor test should ensure that seed size does not bias test results.
Seed treatment
. One of the reasons for seed treatment is to reduce the invasion of pathogens follow-
ing planting. But, how should seeds be tested for vigor: in the treated or untreated condition? Some argue
that seeds which beneit most from seed treatment are those that are low in seed vigor, thus the implication
that seed treatments “artiicially” enhance seed vigor of seed lots inherently low in seed vigor. According
to these advocates, a true test of seed vigor can occur only with untreated seed. Others contend that seeds
should be tested for vigor in the same way that they would be planted in the ield. After all, one important
component of a seed vigor test is its indication of a seed lot's ield performance potential.
Surprisingly few vigor tests provide guidelines on seed treatment. The conductivity test recommends
that seed treatments be removed prior to test (Tao, 1978 and 1980). However, McDonald and Wilson (1979)
and Loefler et al. (1988) found that fungicide seed treatment had little or no effect on electrical conductiv-
ity results of soybean, or corn (Marchi and Cicero, 2003). Further research is needed to verify the effect
of different seed treatment on various crops. Other tests (for example, cold test and accelerated aging test)
suggest that the seed be tested in the way that it is received in the laboratory. One thing is certain, seed treat-
ments can affect vigor test results, particularly when comparing the results of a treated seed lot with those
of an untreated lot. As an example, the cold test was originally developed to test the eficacy of seed treat-
ments. This issue becomes important as more and more seeds are either treated by pelleting, ilm coatings,
or physiological enhancements such as priming and biocontrol.
Seed dormancy
. Many important crop seeds do not have dormancy because this trait can be detri-
mental to stand establishment and has been eliminated through breeding. However, a study of germination
testing procedures in the AOSA Rules for Testing Seeds reveals that dormancy is still an important problem
in seed testing of numerous crops. But, how should dormant seeds be evaluated by a seed analyst conduct-
ing a seed vigor test? Since most vigor tests rely on some measure of seedling growth, does this mean
that dormant seeds are low in vigor? What would be their level of vigor once dormancy was broken? For
example, the conductivity test determines leakage from imbibing legume seeds. In some years, hardseeded-
ness (impermeability to water) in legumes is expressed and these seeds would not leak electrolytes, thus
lowering their overall conductivity value. Another approach is to ensure that dormant seeds are reported
