Seed Testing: Principles and Practices

Chapter 5. Germination and Viability Testing

59 Germination and Viability Testing Seeds are tested for germination to determine how they will perform when planted in the field, the garden, or in a seedling nursery. This information is also needed for labeling and marketing purposes or to determine if a seed lot has been properly labeled when sold or offered for sale. Although seeds have been tested for germination for hundreds of years by critical gardeners and farmers , it has only been in the past 150 years that laboratory germination has been developed to provide farmers and other seed users assurance of the quality of seed produced on their own farms as well as that purchased from others. Conducting a Standard Germination Test Reproducible germination results require test conditions that are uniformly applied. Thus, both the AOSA and ISTA rules prescribe standardization conditions of optimum substrata, temperature, light, time of evaluation , methods of overcoming dormancy, and specific criteria for determining normal and abnormal seedlings . Development of these uniform rules and procedures represents the single most important contribution to the standardization of seed testing. Thus, the rules are an invaluable asset to the seed analyst in the performance and interpretation of germination tests. The AOSA and ISTA rules define procedures for germinating seeds of different species, including substrata, temperature, timing of evaluation, duration of test, and additional directions such as the use of light and other means of breaking dormancy. Since both AOSA and ISTA rules have similar germination requirements for most species, examples cited in this chapter are mostly from the AOSA rules. SEEd SELECTIon, PrEPArATIon, And PLAnTInG Seeds for the germination test are taken from the pure seed portion of a seed lot. The counting and selection of the seeds must be made randomly without regard to size or appearance. Selection may be made by hand, a counting board, or with a vacuum seed counter. When only germination results are needed, the pure seed portion must be at least 98% pure; otherwise, pure seed must be separated by standard purity test procedures to provide seeds for germination (see Chapter 4). At least 400 seeds must be tested except that 200 seeds may be used for kinds comprising 15% or less in seed mixtures. Germination tests are conducted in replicates of 100 seeds or less to avoid crowding on the substratum. Grass spikelets or multiple units regarded as “seed units” or defined as pure seed are considered and germinated as a single seed. Only one sprout or seedling that emerges from a spikelet (e.g., Poa) or multiple unit is counted. This also applies to complex fruiting structures such as those found in garden beets or New Zealand spinach. 5 60 Chapter 5 Seed Counting and Spacing Counting boards are generally used for counting and spacing large seeds such as corn, beans, and peas. They consist of two perforated wooden or plastic boards approximately the size of the planting substratum with 50 or 100 holes slightly larger than the seeds to be counted (Fig. 5.1). The two perforated boards are off-set to permit seeds in the holes of the upper leaf to rest on the solid portion of the bottom board. Seeds are placed on the upper surface of the counting board which is shaken to allow a single seed to fall into each hole. Excess seeds are removed by tilting the board and allowing them to slide or roll off into a suitable container. The counting board is placed directly over the planting substratum and the top board is retracted until the holes in each board are aligned, allowing the seeds to fall through onto the planting surface. Vacuum counters are generally preferred for small, smooth free-flowing seeds although they can be used for large seeds if enough vacuum is available. A suction capacity of 5 to 8 ft3 /min is adequate for clovers , while larger seeds such as corn or beans require 15 to 18 ft3 /min. Counting heads have both front and back faces with a space for air passage between (Fig. 5.1). They are about the size and shape of the planting surface and made of metal (brass or aluminum) or plastic, which is preferred for large counting heads because of its lighter weight. The face is ridged around three sides of its periphery to retain the seeds and has 50 to 100 evenly spaced holes. Seeds are placed on the counting head and the vacuum turned on allowing the seeds to spread over the head surface. Although the...