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 Essay - jan/feb 2007
Seed testing and much, much more!
James C. Delouche
Professor Emeritus Mississippi State University
I began to appreciate how demanding and exacting seed testing is soon after I was first introduced to the technology in the early 1950s. I was simply unable to make satisfactory analyses of the seed of Kentucky Bluegrass, Poa pratensis, one of the most important lawn and turf grasses for the temperate zone. I had problems with essentially all of the components of the purity analysis - inert matter, other crops , and weeds. In those days there were no important named cultivars of Poa pratensis but seeds of several other species of Poa were frequent contaminants, especially Poa compressa (Canada bluegrass) and Poa annua (annual bluegrass). I did learn to distinguish between Poa pratensis and Poa annua most of the time, but distinguishing between the seeds of Poa pratensis and Poa compressa proved to be beyond my visual acuity. I would just examine and examine the seed in question by turning it over and over with my fine pointed forceps until - inevitably - it squirted from the forceps onto the floor or some other place where it was lost. Although this "lost seed" method of dealing with hard-to-identify seeds in a purity analysis was very tempting to a struggling student, to my credit I recognized that it was an unethical and unacceptable practice.
While distinguishing among the seeds of the various Poa species was difficult, it was not as exasperating and intractable a problem for me as the determination of inert material in Poa pratensis purity analyses. Commercial seed lots of Poa pratensis in the early 1950s contained between about 2 and 20 % sterile or empty seeds (florets) which looked very much like the good seeds (pure seeds) but were classified as inert matter. Many of the empty seeds could be distinguished by color, size and shape variations but not all. There were usually a substantial number of questionable seeds that had to be examined more closely by squeezing them with sharp pointed forceps to determine whether they were full and thus pure seed or empty and thus inert matter. Experienced analysts made these determinations quickly, seemingly effortlessly and accurately, but novices like me with clumsy, insensitive fingers invariably squeezed too hard causing injury to any full (pure) seeds. This, of course, had an adverse effect on germination with the result that my germination tests of Poa pratensis were often as unsatisfactory and inaccurate as my purity analyses. From these exasperating experiences I learned that a seed analyst had to have great patience and a substantial endowment of manual dexterity, fingers-to-eye coordination, vision and intelligence. And, I developed an admiration and respect for seed analysts that has increased over all these many years as seed testing gradually changed to seed analysis and much, much more.
Challenges and Improvements
The problem of hard-to-distinguish empty (sterile) seeds in many of the lawn, turf and pasture grasses as Poa spp., Paspalum notatum, and Bouteloua spp was mostly overcome with the development of precision seed blowers and uniform blowing procedures. In other pasture species of Poaceae, e.g., Festuca rubra,
Dactylis glomerata, Agropyron spp., multiple units (florets) commonly occur in the harvested seeds. Special procedures for dealing with these multiple units (florets) were devised to eliminate the need to manually separate the florets for determination of pure seed and inert mater. The coating of seeds with various materials for various purposes changes the size, shape and weight of seeds and presents a huge challenge to the seed analyst. Detailed procedures for analyzing coated seeds were developed and incorporated in the rules and procedures for seed testing. They are tedious but workable.
Perhaps, the most difficult challenge to seed analysts during the past century resulted from the great proliferation of crop varieties. Instead of a few cultivars of Poa pratensis, Avena sativa, Glycine max, Triticum aestivum and other crops, there became many and even more and methods for identifying them in seed analysis had to be developed. Since the cultivars were and are developed by a variety of breeding technologies, it is not surprising that the methods and procedures to identify them range from relatively simple to the latest biotechnologies. The seeds of some cultivars can be distinguished on the basis of morphological and other visual differences, others by relatively simple chemical tests, while electrophoretic protein analysis and even more sophisticated biotechnological techniques have to be used for some of the modern hi-tech cultivators. Some cultivar purity determinations involve chromosome counts, seedling and juvenile plant examinations and analyses.
The challenges in seed analysis have not all been in the area of purity analysis and cultivar determination. Germination test procedures had to be developed for many of the species that retain some degree of dormancy for relatively long periods. Generally satisfactory procedures for germination testing of most of the important crop species were developed rather early but there is still much work to be done on germination test methodologies for native herbaceous species, trees and shrubs. Two of the more recent challenges in the germination testing area, however, led to the development of powerful new tests.
Tetrazolium (TZ) and Vigor Tests
A purity analysis can be completed and reported within a day - a few minutes to a few hours. A germination test takes 7 to 28 days and even longer. Since a period this long can be a serious impediment to orderly business operations in the seed industry, more rapid methods for determining the germination capacity of seed lots have long been sought. The TZ test for rapid determination of germination was discovered in the 1940s and developed during the next 40 - 50 years. It turned out to be not only an excellent rapid method for determining the germination capacity of seed but also a very powerful technique for establishing their physiological quality. Skilled practitioners like Drs. Charles Baskin (cotton seed) and Jose B. Franca Neto (soybeans) use the TZ test to diagnose physiological problems of seeds just as a medical doctor uses blood tests and imaging technology to diagnose the ills of his patients.
A second challenge in the germination testing area was the inadequacy of the standard germination test as a measure of the plant producing capacity of seeds under field conditions. This challenge has been met mostly in the last 50 years of so by the development of a variety of vigor tests of which the TZ test is one of the most important.
Seed Testing Today
Seed testing as noted earlier gradually changed into the broader field of seed analysis. At the time I was struggling with the purity analysis of Poa pratensis seed testing laboratories made purity tests, noxious weed examinations, and germination tests. Today modern hi-tech seed laboratories make seed tests and analyses such as purity, germination, vigor, TZ, seed health, genetic purity, hybridity, genetic trait testing and validation, zygosity, adventitious presence, DNA for various applications and so on. Many of these tests, of course, are associated with the transgenic (GMO) cultivars of maize, cotton, soybean and other crops that are now available and widely planted in some countries. Seed heath, vigor and genetic purity tests, however, are more associated with traditional aspects of seed quality than with the specific genetic traits of the GMO cultivars. The development of new tests is accelerating as a wider variety of genetic traits are incorporated in crop cultivars such as drought, cold and salinity tolerance, nutrient efficiency, and the food quality of the grains and oil seeds is altered in specialty cultivars by conventional and recombinant genetic methods, e.g., low linolenic oil soybeans. What kinds of tests and levels of precision will be needed when the GMOs with traits for production of pharmaceuticals and other specialty chemicals come on stream?
Considering all the new tests and services offered by seed testing laboratories, I vote for a new service offered by the Oregon State University Laboratory (OSU-SL) as the most interesting and innovative. The OSU-SL now offers a soil seed bank test. The test which detects identifies and determines the viability of seeds present in the soil provides valuable information for seed producers on potential weed problems, especially noxious weeds which have seeds that are hard to clean and greatly affect the marketability of the seeds produced. Wouldn't this information be valuable to a rice seed producer who wants to produce seeds that are free of the very noxious red rice?
Seed testing is indeed much, much more than it was just 50 years ago, and seed laboratories have become the essential service units of the modern seed industry.
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