Agriculture Reference
In-Depth Information
etc.) and experience in differentiating normal and abnormal seeds. Once the principles and procedures for
conducting the test on one species are known, it is relatively easy to test other species, even unfamiliar ones,
although considerable experience may be necessary to develop accuracy and precision of distinguishing
between viable, abnormal, and non-viable seeds.
The TZ test is based on the presence of dehydrogenase activity in viable seed tissues during the pro-
cess of respiration. These enzymes catalyze reactions that release hydrogen ions that act to reduce (the pro-
cess of accepting the H-ions) the clear, colorless 2,3-5 triphenyl tetrazolium chloride solution and change
it to a red dye known as formazan (Fig. 7.1). Thus, when in contact with imbibed seed tissue, the viability
pattern of the seed can be evaluated by experienced tetrazolium analysts who use both the intensity of the
staining as well as the pattern of the staining to make viability determinations.
Figure 7.1. The chemical reaction involved in changing the colorless tetrazolium chloride to red formazan.
APPLICATIonS
The principal advantages of the tetrazolium test over standard germination is the speed with which seed
viability results can be obtained and the ability to determine the viability of even most dormant seeds. While
tetrazolium results are not always accepted for oficial purposes, the test has numerous applications in the
seed industry because of the short turn-around time for results. These are covered below:
1. Determining seed viability as an alternative to the standard germination test.
Some states such
as OR, UT, ID, SD, and NE have accepted the TZ test as an alternative viability indicator to the standard
germination test. The potential of using the TZ test as a stand-alone viability test is great, especially with
the fast pace of today's global seed industry. Tolerance tables for the TZ test are already included in both the
AOSA and ISTA Testing Rules. Although some research is needed to further validate and standardize the
TZ test procedures for various crops, it is possible to start with the crops that possess little or no dormancy
such as corn and soybean where the correlation between germination and TZ test results is high.
2. Determining seed viability before harvesting and conditioning.
Seed is sometimes subject to
injury from environmental factors such as freezing temperatures and cold, wet weather. When this occurs,
viability may be checked with tetrazolium to determine if the crop is still worth handling as seed or would
be better diverted to nonseed use or discarded.
3. Determining the degree of mechanical injury caused by harvesting and handling procedures
.
Seed coat cracks, seed breakage, and internal embryo cracks and bruises can be readily observed while
there is still time to make machinery adjustments to reduce the amount of damage in the rest of the crop
(Fig. 7.2).
4. Determining various types of seed damage and causes of germination failure.
In addition to
mechanical damage, other seed defects may be readily observed. These include immature embryos, insect
damage, fungal invasion and decay (Fig. 7.2).
