After seeds are cleaned, it is a good idea to determine their quality by testing seed viability, seed germination, or both. Seed viability tests estimate the potential for seeds to germinate and grow, whereas seed germination tests measure the actual germination percentage and rate. For seeds you are purchasing or selling commercially, you may also
want to know the percentage of pure live seeds (PLS.)
Seed Viability Tests
Cutting tests, described previously, are the simplest seed viability tests and are usually performed during seed collection and often just before treating seeds for sowing. Cutting tests should also be performed on seedlots that have been stored for a long time to visually assess their condition. Cutting tests can reveal whether or not the seed is healthy, but really cannot determine anything about the potential for germination. A better test is the tetrazolium (TZ) test, a biochemical method in which seed viability is determined by a color change that occurs when particular seed enzymes react with a solution of triphenyltetrazolium chloride. Living tissue changes to red, while nonliving tissue remains uncolored (figure 8.19). The reaction takes place both with dormant and nondormant seeds, and results can be obtained within a couple of hours. Although the TZ test is easy to do, interpretation of results requires experience. For this reason, some larger nurseries or nurseries that also sell seeds send seed samples to seed analysts that have the necessary laboratory equipment and experience for testing. A third test is an excised embryo test. Embryos are carefully removed from seeds and allowed to grow independent of the seed tissue. Seeds often must be soaked for several days to remove hard seed coats, and excision of the embryo is an exacting procedure that normally requires the aid of a microscope. As when doing TZ testing, most nurseries send their seed samples to seed analysts for excised embryo testing.
Figure 8.19—Tetrazolium (TZ) tests stain living tissue red and can be used to estimate seed viability of a seedlot. Shown in this figure (left to right): dead embryo, damaged embryo, and healthy seed. From Stein and others (1986).
A seed germination test determines both germination rate and total germination percentage, and is used to determine sowing rates so seeds are used efficiently (figure 8.20). The germination rate indicates how promptly seeds germinate, whereas germination percentage indicates what proportion of the seeds eventually germinate. Germination tests are used to determine how many seeds to sow per container and how long you can expect seeds will continue to germinate after sowing. If the species being tested has some type of seed dormancy, an appropriate treatment to remove dormancy will be needed before the germination test. Many nurseries will test dormant seedlots before and after the dormancy treatment to check its effectiveness. Actual germination in the nursery may vary greatly because of the inherent variability of germination in most plant species and differences in the environmental conditions during testing and growing at the nursery.
Use the following steps to conduct a germination test:
• Select an area in the greenhouse or office that canbe kept clean.
• Line the bottom of plastic trays, Petri dishes, or similar containers with paper towels. For large seeded species, line the bottom with moist sterile sand (bake sand in the oven at 212 °F [100 °C] for at least 1 hour to sterilize it) or unmilled Sphagnum moss.
• Moisten the paper towels or other substrate withdistilled water.
• Remove equally sized seed samples from each container of the same seedlot, or, if only one container exists from the seedlot, remove the seeds from different portions of the container. Mix these samples together to form a representative sample
• From the sample, make 4 replicates of 100 seeds and spread each replicate onto the moist substrate in a container. The containers may be covered to reduce evaporation from the substrate.
• Use distilled water to remoisten the substrate as necessary, but never allow standing water to remain in the container.
• Place the containers under optimum germination conditions—ideally those in which light, temperature, and humidity can be controlled. Conditions similar to the nursery will yield more meaningful results.
• Count the number of germinants on a daily or weekly basis for up to 4 weeks on herbaceous species and up to 3 months on woody species. Uniformity of germination
timing may be advantageous, but be sure you do not exclude healthy seeds that simply germinatemore slowly from the genetic pool for that species.
Paper towels (A), sterile sand (B), and Sphagnum (C) are suitable substrates for germination testing. Photos from Vozzo (2002).
Percentage of Pure Live Seed
The percentage of pure live seed (PLS) is a seed quality index that can be calculated during seed testing (figure 8.22). When seeds are bought or sold, it is important to know the seeds have high PLS values and very low percentages of weed seeds and other inert materials. It is often a good idea to ask about where the seed was collected and to determine what weeds may be present in the seedlot.
Figure 8.21—Test seeds by collecting primary samples from an entire seedlot to make up a composite sample. The composite sample is further divided into samples tested at the nursery or submitted to a seed laboratory for testing. Illustration from Dumroese and others (2008).
Figure 8.22—Pure live seeds (PLS) is the percentage of the bulk seed weight that is composed of viable seeds. In this example, results of a purity test show 95 percent of the bulk weight is composed of seeds. The subsequent germination test indicates that 83 percent of the seeds germinated. Multiplying percentage purity by percentage germination yields 79 percent PLS. Illustration from Steinfeld and others (2007).
(Taken by permission from “Tropical Nursery Manual” by Tara Luna, and Kim M. Wilkinson)