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¶ Introduction
The standard germination test is the accepted industry standard for seed quality. Vigor tests are useful in sorting seed lots into categories of desirability for planting or storage.
¶ Stages of Seed Life
A seed begins to deteriorate soon after it reaches maturity. It passes through several stages before death. The length of time from maturity to death depends on many things, including field weathering, storage conditions, mechanical damage, etc.
Figure 1 is a schematic diagram of the stages of seed life. During the early stages of life, the seed is able to emerge and continue growing under favorable conditions. It can produce normal shoots, roots, and leaves. These are the live seeds of germination counts. Seedlings produced in this stage are capable of normal emergence under the ideal conditions found in the germination chamber.
As the seed deteriorates, it can still emerge under ideal conditions, but it cannot continue growth. These are dead seeds of germination counts. Seedlings in this stage have abnormal or damaged plant parts that will not sustain normal plant growth even in the germination chamber.
The seed is incapable of emergence in the final stages of deterioration. It is still alive, because metabolic processes can be measured. However, they are at such a low level the seed cannot even initiate growth. Finally, the seed is completely dead.
¶ Figure 1. Stages of Seed Life
¶ Germination and Seed Quality
Germination can be a fairly good indicator of seed quality. Official germination counts are made from four replications of 100 seeds each. There are variations among the replicates and allowable tolerances have been established for various germination levels. The amount of allowable variation ranges from plus or minus 5% (when germination is 99%) to plus or minus 20% (when germination is 50%). If one of the replicates exceeds the allowable tolerance, it is discarded. The allowable tolerance is a tipoff to seed quality.
For example, if a seed lot averaged 50 % germination, the replicates could test from 30 % to 70 % and still be within allowable tolerance. This means that half of the seeds would test between 30 % and 50 %. This seed lot would not be acceptable for field use because of the high proportion of dead or weak seeds. Even if the planting rate was doubled to compensate for low germination, there could be large skips and gaps in the stand.
Classifying seeds into "live" and "dead" categories is not correct, because there are stages and degrees of seed life. Once the seed can no longer germinate, it can no longer produce a normal seedling. Germination is actually the final step in seed deterioration.
¶ Defining Vigor
The Association of official Seed Analysts defines vigor as, "those seed properties which determine the potential for rapid uniform emergence, and development of normal seedlings under a wide range of field conditions".
Vigor refers to the strength or speed of germination. There is a wide range in the ability of seedlings to emerge and grow during germination tests. Vigor is one way of further separating the live seeds in a seed sample. High vigor seeds would be expected to emerge and perform better under field conditions than low vigor seeds.
¶ Interpreting Vigor Tests
Seed vigor tests are used to estimate the potential field performance of seed under adverse conditions. High vigor seeds are expected to perform better than low vigor seeds in the field.
Vigor tests are best used to separate seeds into categories. Under adverse conditions high vigor seed may perform poorly. Under ideal conditions, low vigor seed may perform well. When speaking of field performance, we are referring to field emergence rather than yield.
Emergence is a rough indicator of potential yield, but it cannot predict final yield accurately. Under the right conditions, a low emerging seed lot may yield as much as a high emerging seed lot.
Vigor tests can help estimate potential field emergence of a seed lot. They cannot predict actual emergence because field conditions are impossible to predict. Vigor tests are best used to compare one lot of seed to another to help pick and choose among seed lots.
A vigor test on a single seed lot should be compared to the germination percentage. Germination and vigor test results that are close together indicate a fairly consistent seed lot that should perform consistently in the field. If results are wide apart, then there will probably be wide variations in performance under different field conditions.
Standard germination can help growers determine the quantity of seed needed to get a specified population of live plants. These decisions are easier to make with vigor test results. Seeding rates can be adjusted to reflect both seed vigor and field conditions at planting.
One problem in interpreting vigor test results is lack of standardization between seed laboratories. Laboratories may use different procedures for similar vigor tests. The Association of Official Seed Analysts has defined specific methods for various vigor tests. Vigor tests are not currently recognized as valid by state law. They must be used as a guideline, and not as an absolute value for legal trade.
¶ Testing Seed Vigor
Vigor can be tested by measuring either the speed or strength of germination or analyzing certain metabolic processes. There are three types of vigor tests: stress tests, seedling growth and evaluation tests, and· biochemical tests.
Standard germination is tested under ideal conditions. Moisture, temperature, and media are adjusted to provide every chance for a live seed to emerge and grow normally.
If seeds are stressed, weak seeds that are more deteriorated (low vigor) will not produce normal seedlings. Seeds with a high strength of germination will grow normally.
Stress tests are the most common form of vigor testing. The two most common tests are accelerated aging and the cold test.
¶ Accelerated Aging
The accelerated aging test was developed by Mississippi State University to predict the storability of soybean and cotton seed lots. Accelerated aging has been developed for use with other types of seed.
The test exposes the seeds to high temperature stress before running standard germination. Accelerated aging simulates the degree deterioration expected to occur in one year in storage.
Accelerated aging affects low and medium vigor seeds more than high vigor seeds. Seed with a high accelerated aging percentage is expected to hold germination longer than seed with a low test percentage. Accelerated aging has been compared with other types of vigor tests and has been found to do well in estimating potential field emergence.
¶ Cold Test
The cold test attempts to directly simulate field conditions. Seeds are planted in sand, soil, or a mixture. They are exposed to a period of cold temperature before the standard germination test. This exposes seeds to a triple stress of cold temperature, wet soil, and attack by soil microorganisms.
Cold testing is one of the most common vigor tests. Cold tests have been researched extensively, but are hard to standardize between laboratories. One variable is that microorganism populations vary from area to area.
It is also difficult to pack a soil uniformly in a tray when setting up a cold test. Differences in soil packing could affect seedling emergence. Differences in soil water holding capacity make it difficult to control moisture levels.
Even so, research on the cold test has shown a good correlation to field emergence and final stand of corn and soybeans. Cold tests and accelerated aging tests seem to compare well in estimating performance.
¶ Tetrazolium Tests
Colorless tetrazolium (TZ) salts react with dehydrogenase enzymes found in living plant cells to form a water-insoluble red compound called “formazan”
When seeds are treated with TZ salts the live cells are stained red and the dead cells remain colorless. TZ tests are used as quick tests for seed vigor because they take only a few hours and do not require elaborate laboratory facilities.
Specialized training is needed to interpret TZ staining patterns. Reproducible results are hard to obtain because interpretation relies on the judgement of the seed analyst. TZ is probably best used as a secondary test to help find the cause for a vigor weakness in a particular seed lot.
¶ Other Tests
Germination, accelerated aging, cold tests, and TZ tests are generally accepted in the seed trade. There are other tests that have one specific application, but are not used in general seed testing.
Conductivity testing: Frost or mechanical damage can damage seed coats or cause disorganization of cell membranes. Seeds can then leak amino acids and carbohydrates into the soil water around the seed. This nutrient rich "soup" provides a good food source for fungi and other microorganisms.
The conductivity test measures the electrical conductivity of water after seeds have been soaked in it. The more "leaky seeds” that are present in the sample, the more nutrients are released into the water, and the higher the conductivity.
Other tests include: speed of germination, measurement of seedling shoot length, and measurement of adenosine triphosphate (ATP) or glutamic acid decarboxylase activity (GADA).