Seed Lab Testing

Overview

An overview of seed testing methods in the lab.

 

Seed Testing: Germination and Vigor

Seed germination and emergence is the first step for successful production of most plants. The ability of seeds to germinate (often called viability) and to emerge in soil is the key for plant establishment. Seed testing is commonly used to assess viability (the ability to germinate under ideal conditions) and vigor (the ability to emerge in soil under stress conditions), and to determine the purity of seed lots. These then help plant producers with many management decisions, including determining seeding rates and to avoid planting weeds.

The Nebraska State Seed Law requires that all commercially sold seed lots to have the percent germination (viability), purity and weed seeds to be reported on the seed tag. Vigor tests are used by companies for research in hybrid/variety development, and to understand the ability of seed lots to emerge from soils under stress conditions or to maintain viability during storage. The percent germination (viability) is tested using the warm germination or tetrazolium tests, while the cold and accelerated aging tests assess seed vigor.

Objectives


Using these lessons:

In each of the following sections, The steps are explained in words and pictures, many sections end with a link to a movie about the process.


 

Warm Germ - Paper Towel

The warm germination test is the standard test used to assess viability of seed lots. The seed is germinated in nearly ideal conditions, and the result is the germination percentage reported on the seed tag of commercially sold seed, as required by the Nebraska State Seed Law. Ideal temperature, relative humidity and light vary depending upon the plant species being tested. The materials needed to conduct the warm germination test of large-seeded crop species, such as corn and soybean, are:  wetted thick paper towel, wetted thin paper towel, a seed counter tray, and a container to hold the seed.

Seed from the seed lot to be tested is placed in the container.

The seed is poured into the counter tray.  Seed is placed into each hole, and the excess seed is poured off.

 

The seed counter tray is placed on the wetted thick paper towel.

 

A sliding drawer is pulled, dropping 50 seeds on the paper towel.

 

Note that when the seed counter tray is removed, the seeds are evenly spaced.

 

The thin wetted paper towel is placed over the seeds and the thick paper towel, and the edges of the paper towels are pressed together to seal them.

 

The bottoms of the towels are folded to keep the seed from rolling out.

 

The towel is rolled up. This facilitates handling and helps keep the towel from drying out.

 

The seed lot number is written on the towel for identification. Special pencils that are capable of writing on wet paper are used.

 

Intermediate sized seed species, such as wheat, can be counted and placed on the paper towels using a vacuum plate. This reduces the time required. Seed are placed on the vacuum plate (tubes not showing), and each vacuum point holds a seed.

 

Extra seed is poured off, and seed remain where the vacuum pressure is present.

 

The vacuum plate is laid on the thick wetted paper towel, the vacuum is released placing the seeds uniformly on the paper towel.

 

The paper towel is rolled and marked as done for the large-seed crop species.

 

         

The germination chamber controls temperature, relative humidity (humidifier at bottom) and light.

 

The portable humidifier used for humidity control is similar to one used in bedrooms during the winter.

 

Temperature and relative humidity are recorded inside the germination chamber .

 

Temperature settings vary depending upon ideal temperatures for germination of seed from different plant species.

 

The rolled paper towels are placed plastic sacks to prevent drying, placed in a germination chamber with the proper temperature, humidity and light.

 

After the prescribed time (often one week), the towels are removed from the chamber.

 

 

    

The paper towels are unrolled, completely uncovering the germinating seedlings.

 

 

     

Ungerminated seeds and germinated seedlings are separated and counted, and the germination percentage is calculated.

 



 

Warm Germ - Box

Small-seeded crop germination is often conducted in plastic boxes with blotter paper. The materials needed are plastic boxes with lids, blotter paper folded inside the plastic boxes, paper labels, and the seed to be tested.


Due to the small seed size, it is often necessary to use tweezers

The first step is to wet the blotter paper.

Seeds are counted and uniformly distributed on the paper.

  

     

A label to identify the seed lot is prepared, and placed between the plastic box and the lid.  The lid is carefully placed over the box to prevent drying out and to hold the label in place. 

     

Germination boxes are placed in a germination chamber at prescribed temperature, relative humidity and light, which varies depending upon the plant species.

         

 

   

After the prescribed time in the germination chambers, theboxes are removed, germinated seeds are counted, and the germination percentage is calculated


               

Ryegrass Warm Germination

Germination tests of perennial ryegrass are unique, in that annual and off-type ryegrass contamination is common and cannot be determined by looking at the seed. Annual ryegrass seedlings fluoresce under a black light, thus allowing one to separate this contamination and calculate the true germination percentage of perennial ryegrass.

This test requires wax paper, a special germination towel, the seed to be tested, and a blacklight.



The perennial ryegrass seed is uniformly laid out in a single line across the germination paper, according to a premarked Ryegrass template.

        

The germination paper is covered with wax paper and rolled up.

        

The roll is labeled for identification, and two rolled up tests are held together with a rubber band.

        

The rolls are covered with a plastic bag and placed in the germination temperature at the correct temperature, relative humidity, and light.

After the prescribed length of time, the tests are removed from the chamber. You can see the seedlings have emerged through the towels.

The towels are unrolled.

And placed under the blacklight. Germinated seedlings that do not fluoresce are counted, and the germination percentage is calculated.

Tetrazolium

The Tetrazolium test measures potential germination. This is most commonly used on small-seeded crop species, but we demonstrate it on large seeded crops to facilitate viewing the test. The chemical compound 2,3,5 tetrazolium chloride is a white powder, mixed with water to a range of 0.1 to 1% solution and stored in a dark bottle (to avoid light) in a refrigerator.

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Seed are pre-conditioned to take up (imbibe) water slowly by placing in wetted blotter paper overnight. Seed that takes up water too rapidly can rupture cells, giving a false reading.

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After pre-conditioning, the seed often has to be cut to allow the tetrazolium solution to rapidly move into the seed. Corn is split in half longitudinally.

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Soybean and bean cotyledons are split apart.

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And wheat germ is split longitudinally. 

 

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Then placed into the tetrazolium solution to soak for several hours.

 

 

         

 

         

The living tissue in the seed (the germ or embryo) turns red within a few hours. The reaction that causes the change in color is related to the respiration rate. Red or pink tissue means that the tissue is healthy and is respiring normally, black that the tissue is respiring rapidly due to either injury or being a meristematic area, and white is dead tissue with no respiration.

This is a corn kernel changing color.

 

This is a wheat seed changing color.

Based upon the color and location of dead or injured tissue, a potential germination percentage can be determined.

In this case, all eight wheat seeds are healthy and would be expected to germinate.         

In this case, all five corn kernels are healthy and would be expected to germinate.

This soybean sample has some dead tissue in the cotyledon, thus its potential germination is lower than 100%. The biggest advantage of this test is that it is rapid, and can be used on plant species for which standard warm germination guidelines have not been established. The largest disadvantage is that this test requires the use of a highly trained seed technologist.

 

 

Cold Test

Sometimes cold germination tests are conducted to simulate the cold, wet field conditions that early planted summer annual crops experience. The cold test is the most common vigor test performed.

A vigor test is defined as a test that measures the ability of a seed lot to emergence under stressful conditions.

This test is conducted very similar to the warm germination test, with two exceptions. First, a soil paste is made a
nd smeared on the wetted thick paper towel.

         

Then the seed are counted and placed on the towel as in the warm germination test. The towel is covered with a wetted thin paper towel, rolled and marked as was done for the warm germination test.

The second difference is that the rolled paper towel is placed in a chamber with cold temperature, often 50F, for two or three days, then is transferred to a chamber with ideal temperature, humidity, and light. When the test is completed, seedlings that would have emerged are counted and the cold germination percentage is calculated.

Accelerated Aging

One vigor test is the accelerated aging test. The purpose of this test is to predict the storability of a seed lot. Seed companies use this test to decide which seed lots to save for sale from one year to the next. Obviously, seed lots that are vigorous and will not lose germination percentage are the ones that seed companies want to save.

In this test, seed is artificially aged by placing it in a warm, humid chamber (approximately 100F and near 100% relative humidity). The seed is artificially aged since the rate of respiration is greater in these conditions.

                       

The seed samples are placed in small plastic containers and placed on trays for two or three days.

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The seed samples are removed, and dried. Then a warm germination test is conducted, and the percent of artificially aged germination is calculated.


 

Purity


The last test commonly conducted test on seed lots is for purity. This separates the seed from all inert material, and seeds of other crops or weed species. The large seed sample is passed through a divider to separate it into sub-samples.

 

Each sub-sample is collected in a metal cup

          

 

 

Each sub-sample is then weighed

           

 

Reading the purity test is often a two step process. The first uses a relatively large amount of seed that passes through an automatic feeder and a shaking tray and viewed through a magnified video projector.

The seed technologist views the video screen to identify seeds of other species and weeds and removes them with tweezers.

 

 

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In the second step of the purity test, a small sub-sample is placed on a glass sheet with a background light.

 

   

 

This sample is viewed under a magnifying lens, and the sample is separated using tweezers.

 

 

 Other crop seeds, weed seeds, and inert material are removed and separated.

 

The seeds, other crop seeds, weed seeds, and inert matter are weighed and the percentage purity is calculated.