Derek Tilley, Manager, USDA-NRCS Aberdeen Plant Materials Center: Derek.tilley@id.usda.go
For certified seed production, seed samples must be sent to an accredited seed laboratory for purity, germination, and/or viability testing. Getting these results back may take weeks to months depending on the lab, time of year, and species being tested. There is also a chance that the lab results will indicate viability less than the standard required for seed certification, in which case the seed must be re-cleaned and samples resubmitted for further testing. This can be very time consuming and expensive. In order to expedite this process, the Aberdeen Plant Materials Center (PMC) uses a novel technique that allows them to estimate seed quality during the cleaning process and thus reduce the chance that seed certification standards will not be met.
Figure 1. Hot plate mounted on a propane heater used for conducting "pop" tests at the Aberdeen Plant Materials Center.
Healthy seed contains on average 12 to 15 percent moisture content at the end of the cleaning process. When seeds are placed over sufficient heat this water is converted to a gas, which exerts tremendous pressure against the seed coat and causes an explosion. In the case of popcorn, as the gasses escape, the inner starch filled portions of the endosperm fill with air as the gas escapes and expands into a tasty treat. A more practical (but less delicious) use of this phenomenon is to estimate seed quality before sending seed off to a lab for testing.
The ability of seed to pop is dependent upon seed moisture content and the integrity of the seed coat. Even under good storage conditions, seeds lose this ability as the seed slowly loses moisture. In the case of popcorn, for example, optimum popping moisture content is approximately 14 percent. As the moisture content declines to below approximately 10 percent, popping performance goes down, and the number of unpopped kernels increases.
The PMC uses a “pop test” to get an approximation of seed viability during the seed cleaning process by sprinkling a small number of seeds on a hot plate (Fig. 1) and counting the number that pop. If the percentage of popped seed is high enough, usually above 90%, we know the cleaning system is doing a good job of removing empty or shriveled seed. If the popped percentage is low, we can adjust the air-flow and remove more light seed.
We use a hot plate made from a 1.6 mm (0.0625 in) sheet of steel welded with mounting brackets to sit over a heating element which is fueled with a 13.6 kg (30 lb) propane tank (figure 1). For best results we set the heater on high. We measured temperature using a thermal imager and found temperatures on the hot plate surface ranged from 126 to 327 °C (260 to 620 °F) at the high setting with most of the plate lying between 200 and 260 °C (400 and 500 °F). Isolated hot spots of over 315 °C (600 °F) occurred at some of the weld points and along one edge of the hot plate.
Over numerous comparisons we found the pop test to be relatively accurate compared to lab test results for a wide range of species and plant types. Grasses, forbs, and rushes tested were often within +/- 10% of lab results, often underestimating actual viability.
Although the pop test is fairly accurate for many grasses and forbs, there are some species and instances for which the pop test is unreliable. Old seed and physically damaged seed can pop and give false positive results. Our tests indicated that with proper moisture content, seed will have some physical reaction to heat, whether viable or not, but damage to the seed coat reduces popping. Popping, therefore, may indicate good seed fill, not necessarily the presence of a healthy embryo. In fresh seed, the absence of a pop or movement often indicates an empty hull or floret. In older seed, the decrease of response to heat may indicate loss of moisture, degeneration of endosperm, or a breakdown in the seed coat.
Seed of some species pop and visibly produce a popcorn-like puff such as in hardstem bulrush (Fig. 2). Others pop but the starchy puff is difficult to see. Many grasses fall into the latter category because the caryopsis is enclosed in the lemma and palea. Other species, such as Lewis flax, pop very quickly and multiple times. Some species are too small to accurately observe popping; the smaller the seed, the more quickly the popping occurs upon touching the hot plate making it difficult to obtain an accurate count. Juncus, Calamagrostis and Achillea, for example, are perhaps better evaluated using a small amount of seed and estimating the percent that popped instead of conducting an actual count.
Figure 2. Popped seed of hardstem bulrush revealing popcorn-like puff of starchy endosperm.
Several species we have looked at either do not pop or popped erratically. Fourwing saltbush could not be induced to pop or otherwise react to the hot plate, most likely because the outer walls of the seed absorb the heat and burn rather than allowing the seed to pop. Indian ricegrass also pops inconsistently, and the round shape of the seeds makes it difficult to keep them from rolling off the hot plate. Two penstemon species we have examined will visibly pop, but the results are significantly lower than viability obtained in lab tests.
The pop test can be used to estimate seed viability quickly and inexpensively. When used correctly, it can save seed producers time and reduce frustrations encountered when lots are determined to be sub-standard. It can be seen as an easy way to determine if a lot requires more cleaning before sending it off to the lab for final analysis, but will never replace actual germination or even viability testing.