Research Background

The TZ test was developed in Germany in the early 1940's by Georg Lakon and introduced in the United States after World War II. The use of the test has increased and expanded since then because it can be completed quickly, usually within a few hours. This is a faster method of determining seed viability than a standard germination test and TZ results are commonly used in place of germination test results. The procedure is also used to determine the viability of ungerminated seed at the end of a germination test.

The TZ test measures the activity of the dehydrogenase enzymes used in the respiration process. Respiration is the cellular process of breaking down sugars to produce energy, carbon dioxide, and water, using oxygen. The enzymes react with substrates releasing hydrogen ions to the soluble tetrazolium chloride salt solution. The salt solution is reduced by the hydrogen ions. The colorless TZ salt solution is changed into an insoluble reddish compound called formazan. If the embryo and possibly endosperm or secondary nutrient reserve or storage tissues are actively respiring, formazan will be present and the tissues will stain red. Seed viability is determined by evaluating the amount of area stained, the intensity of staining, the pattern of staining and by evaluating other critical characteristics including turgidity, presence and damage of essential structures, abnormalities, and pathogen presence.

The estimated viability of a seed lot is based on the number of seeds per testing sample that display the required staining, structural and other characteristics necessary to classify a seed as viable. A viable seed is considered to be capable of producing a normal seedling under favorable conditions. Classifications are normal or abnormal, healthy or damaged.

The results of a TZ test are based on live seed as opposed to the germination test which is a measure of viability based on the actual number of germinants under a defined set of test conditions. A dormant seed can not germinate even if there are otherwise favorable germination conditions. Dormancy can either be structural or chemical. A seed coat that blocks the uptake of water is considered a structural barrier. Chemical or physiological dormancy means that there is a chemical pathway that is blocked by an inhibitor or that may need to be established to allow the development or transport of essential chemical compounds or nutrients that are necessary to initiate germination. Breaking or overcoming physiological dormancy can take many months of stratification or conditioning. Completing this process however does not guarantee that all of the dormant seed will germinate. Dormancy and environmental test conditions that can influence the results of a germination test do not usually affect a TZ test.