Seed Testing: Principles and Practices

Chapter 4. Testing for Physical Purity

43 Testing for Physical Purity Along with germination, purity tests are one of the oldest and most common tests performed by seed analysts. Their purpose is to determine the physical composition of a seed lot by performing a detailed and precise separation on a small representative working sample. The procedure consists of separating the sample into four components (pure seed, other crop seed, weed seed, and inert matter) so the percent composition by weight of each may be determined. These components are shown in Figure 4.1. Pure seed is the portion of the working sample represented by the crop species for which the lot is being tested. In actual practice, it includes the percentage of each crop species present in levels of 5% or more, or levels less than 5% if shown on the label. Other crop seed refers to crop seeds present in concentrations of less than 5% of total sample weight. Weed seed denotes the percentage of seeds present of plants considered as weeds. Sometimes this designation may be somewhat arbitrary since a plant may be considered a crop in one state or country but a weed elsewhere. For any particular region (state, country), the analyst uses well-accepted guidelines for classifying seeds as crops or weeds. The Association of Official Seed Analysts (AOSA) includes a classification of weed and crop seeds in its seed testing rules. Inert matter denotes the portion of the sample that is not seed. It consists of materials such as chaff, stems, shells, stones and soil particles, but may also include pieces of broken, damaged, or immature crop or weed seeds that do not qualify as pure seed units. It is worthy of note that the International Seed Testing Association (ISTA) rules specify a three-way separation of components into pure seed, other seeds, and inert matter. Figure 4.1. Sample components are separated into pure seed, other crop seed, weed seed, and inert matter in the purity analysis according to the AOSA Rules for Testing Seeds. 4 44 Chapter 4 A noxious weed seed examination is usually performed in conjunction with the AOSA purity test. Perhaps the best definition of a noxious weed is a plant that is declared by law or regulation as noxious. The federal noxious weed seed list in the United States specifies weed species that are prohibited when seed is sold in interstate commerce. Moreover, each state law or regulation has established its own list of noxious weeds that are noxious within the state. State noxious weed lists include primary or secondary weeds. Primary or prohibited noxious weed seeds are those that are prohibited in seed lots sold or offered for sale. Secondary or restricted noxious weed seeds are those that are allowed in restricted levels in seed sold or offered for sale. EQuIPMEnT And THE WorkInG EnVIronMEnT Laboratory Space and Light The purity testing section of a seed laboratory should be designed with special attention to lighting. The use of natural daylight should be provided whenever possible from large windows facing to the north and extending downward to desk level. This arrangement allows minimum dependence on use of supplemental lighting and provides the best conditions for making precise determinations where small differences in color, texture, and shape are critical. It is also believed that natural light causes less eyestrain and fatigue than that from artificial sources. When supplemental light is required, it should be provided from individual, adjustable, multiple-tube fluorescent lights of the cool-white type which most nearly duplicates natural daylight. desks and Workboards Purity separations have traditionally been performed on diaphanoscopes, wooden workboards equipped with a drawer or pan in front and arm rests on either side. Such workboards should rest on desks or tables of convenient height to provide maximum comfort to the analyst throughout several hours of exacting and intense, painstaking work. Some laboratories have tables and/or chairs that allow analysts to adjust them to any desired height. Figure 4.2. Two kinds of conventional purity stations found in seed laboratories. Forceps, hand lens, desk lamps, or lighting magnifiers are basic requirements for the traditional purity work station. 45 Testing for Physical Purity Purity workboards vary greatly in design and material. Figure 4.2 (left) shows a popular design with wooden top and side arm rests. Others may be simple table tops arranged at suitable heights with adequate working space (Fig. 4.2, right). Usually, light-colored, preferably...