Dr. Miller McDonald is professor of seed physiology in the
Department of Horticulture and Crop Science.
Rapid, uniform seed germination is an important but often neglected factor in profitable bedding plant production. Plug producers recognize that reductions in germination and the time necessary for germination and seedling establishment lower costs. To help plug producers achieve this goal, many flower seed companies focus on seed enhancements such as priming, pelleting and pregermination. Even though seed quality is very important, terms and concepts critical to seed quality are often confused. Here we help you define and clarify seed viability, germination and vigor for plug production.
VIABILITY
Seed viability and germination are often considered synonymous terms, but this isn't correct. Seed viability means the degree to which a seed is alive. Some seeds do not germinate because they are dormant despite the fact they are alive. Dormant seed aren't dead and have the capability to germinate at some point in time if dormancy is broken. So, viability is the sum of % germination and % dormant seeds in a seed lot. Viability provides growers and seed companies important seed quality information, identifying the potential germination of a seed lot.
GERMINATION
The purpose of a germination test is often confused with that of a seed vigor test. Seed germination is defined by the Association of Official Seed Analysts as "the emergence and development from the seed embryo of those essential structures which ... are indicative of the ability to produce a normal plant under favorable conditions." The purpose of a germination test is to determine how seeds perform under favorable conditions- conditions seldom encountered in the more stressful conditions of a greenhouse or field environment. These favorable conditions are specified by AOSA in the Rules for Testing Seeds. Because seeds are always tested for germination under favorable conditions, it isn't surprising that growers seldom achieve germination percentages reported on the seed label in the greenhouse.
VIGOR
Seed vigor is "those seed properties which determine the potential for rapid uniform emergence and development of normal seedlings under a wide range of field conditions." Because a seed vigor test is a more sensitive measure of seed quality, its objective is to more accurately portray how a seed lot will perform under the unpredictable environment of commercial greenhouse production. In our example, a vigor test should provide an 80% emergence result.
Unlike germination testing, vigor testing isn't a legal requirement, so seed companies can design any vigor testing protocol that best mimics the actual planting environment that seed encounters during plug production. For example, this may be nine- and 21-day counts of usable seedlings in a plug tray or computer imaging using the increase in size of seedling cotyledons during emergence in a standard plug tray.
Another approach is an accelerated aging test, which exposes seed to high temperatures and high relative humidity stress for short time periods. Poorer quality seeds deteriorate faster under these conditions. Vigor test results are important to growers who depend on rapid, uniform emergence and have a goal of 100% usable seedlings. Seed vigor also provide important benefits to seed companies, helping them identify seed lots that are the most vigorous for further seed enhancement technologies. Improved inventory management and the identification of the highest quality seed lots for marketing are also advantages.
