Determination of trueness to variety is complicated by the influence of environmental factors. Our methods consequently must be based on diagnostic characteristics, which are not affected by environmental forces; or they must control the environment; or they must compensate for the influences of the environment.

Seed characteristics that are stable despite changes in environment are used in detecting offtypes in some kinds, such as beans.

Growth chambers, in which light, temperature, and humidity can be controlled, are used for testing the trueness to variety of some forage crops. In field trials, where control of environment is not possible, seeds of known variety are planted for comparison. If the seeds under test are of the same variety as those of the authentic stocks, they will be influenced by the existing environment in the same way as the authentic stocks and reliable determinations can be made in spite of environmental influences.

Because of the influence of environment, it is apparent that any method for determining trueness to variety must be carefully tested to determine its validity before it is put into use. Some methods that have been proposed cannot be accepted as being reliable because we do not have enough evidence that the diagnostic characteristics are not affected by environment as well as by heredity.

The tests of trueness to variety that have proved to be reliable fall into three groups.

The first includes tests that can be made on seeds or seedlings in connection with ordinary laboratory purity analyses or germination tests.

The second includes tests conducted by growing seedlings under controlled conditions in a greenhouse or growth chamber.

The third is the field trial.

Relatively few kinds of seed can be tested for trueness to variety in the ordinary seed laboratory, but some determinations are possible. Off-types in beans, soybeans, and peas can often be detected by the color or shape of the seed. Common Kentucky bluegrass seeds can be distinguished from seeds of the Merton variety by microscopic examination of seed characteristics. Some offtypes in oats, wheat, and barley can be distinguished by the color and shape of the seed.

Some white and yellow oats varieties can be separated through the use of ultraviolet light, which causes radiation of light, called fluorescence. White oats have a light-blue fluorescence. Yellow oats have a dark-bronze fluorescence. The difference can be observed even when the plumes have become so discolored through weathering that it is difficult to determine their color under ordinary light.

Ultraviolet light can be used also to detect certain offtypes in soybeans and peas and to determine the percentage of seeds of annual and perennial rye-grass in mixtures. A few varieties of peas and soybeans produce seedlings whose roots exude a fluorescent material. The fluorescence produced by the roots of these seedlings can be detected under ultraviolet light.

Seedlings of red and white varieties of beets and of red and green varieties of cabbage can be distinguished readily in germination tests. The seedlings of the red varieties of beets and cabbage contain a red pigment that is lacking in the white varieties of beets and green varieties of cabbage.

Attempts have been made to develop chemical tests to determine trueness to variety, but most of them have been unfruitful. Relatively few chemical tests can be applied practically to individual seeds. Also, the chemical composition of seeds usually is influenced by the environment as well as by heredity. Chemical tests therefore tend to be unreliable.

The development of better light conditions for growth chambers has opened a new field for testing. In addition, control over other environmental factors permits conditions that produce the greatest differences in the seedlings or plants of different varieties.

The growing of seedlings under controlled environments in a greenhouse or a growth chamber offers special promise for testing forage crop seeds for trueness. The varieties generally cannot be distinguished on the basis of seed characteristics. Usually, the plants also lack diagnostic characteristics, but the seedlings and plants may be responsive to variations in temperature, daylength, and quality of light.

Seedlings of northern varieties of alfalfa, for instance, have short stems when grown at low temperatures with short photoperiods. Seedlings of southern varieties of alfalfa produce longer stems under the same conditions. The difference provides a way to distinguish between the two. Under similar conditions, Empire birdsfoot trefoil has short stems, and European-type birdsfoot trefoil has long stems. The same type of technique can be applied to other kinds of plants.

Measuring the resistance of seedlings to disease is another means of applying the controlled environment technique. This type of test has been satisfactory for detecting varietal admixture in wheat and oats.

In this test, seedlings are inoculated with a specific race of rust. After inoculation, the seedlings are observed to determine whether they show the resistant or susceptible type of reaction to the race involved. A susceptible-type reaction in a seedling which supposedly is a resistant variety is evidence that there is varietal admixture.

Techniques employing the same principle as the rust-resistance test of wheat and oats can be used to determine wilt resistance in alfalfa seedlings, yellows resistance in cabbage seedlings, mildew resistance in soybean seedlings, and resistance to numerous other diseases in various kinds of seeds.

The commonest test of trueness to variety is the field trial. It can be applied to almost any kind of seed without the development of special techniques. Also, since plants are usually grown to maturity in the field trials, a full range of varietal characteristics is available for observation.

The cost of conducting field trials is one of the disadvantages. During seasons when the weather is unfavorable, there may not be a normal expression of varietal characteristics. In fact, the trials may fail completely.

Probably the most serious shortcoming of the field trial is that it cannot be conducted in advance of the normal planting season. This prevents tests before seeds are put on the market unless it can be done in other places such as the Southern Hemisphere.

Despite their shortcomings, trueness-to-variety trials are an important part of any complete program to control the quality of seeds. They must be carefully conducted in accordance with a few basic principles, however.

WE HAVE indicated that authentic seed stocks for comparison with those under test are an essential part of trueness-to-variety trials. These authentic stocks should be included at close intervals among the stocks being tested. It is also essential to have replications in field trials. Each sample tested should be planted in at least two separate plots.

Another requirement is to have enough plants to detect admixtures with a degree of precision consistent with the purpose of the trial. Large populations are required to detect off-types with a low rate of occurrence. More serious admixtures, on the other hand, can be detected with relatively few plants.

WALTER A. DAVIDSON is Director of the Grain Division of the Agricultural Marketing Service. He was a Smith-Hughes vocational agriculture teacher before his employment with the North Dakota State Seed Commissioner and later with the Division of Seed Investigations in the Department of Agriculture. He was formerly Chief of the Seed Branch, Agricultural Marketing Service. He is a past president of the International Seed Testing Association.

B. E. CLARK since 1952 has been head of the Department of Seed Investigations, New York State Agricultural Experiment Station, Geneva, N.Y.