Advances in test equipment unveiled
More than 3,000 international scientists, professionals, educators and students revealed their work with new technologies and discussed emerging trends in agriculture, science education and related issues at a national conference in Pittsburgh last November (2009). The American Society of Agronomy, the Crop Science Society of America and the Soil Science Society of America sponsored the conference, with a theme of “Footprints in the Landscape: Sustainability through Plant and Soil Sciences.”
A plethora of informative sessions covered subjects ranging from nutrition to wines, food security, invasive species, organic agriculture and plant breeding. Dr. Gregory Welbaum, seed biologist and assistant department head in the department of horticulture at Virginia Tech, presented his research and development on LED-lighted thermo-gradient tables, used for seed germination testing.
Germination is the growth and development of seed into a seedling when water is added. Germination testing ensures that farmers are planting the highest-quality seed possible to maximize crop yield and quality.
To assess seed quality or viability and to predict performance of the seed and seedling in the field, seed germination testing is conducted in a laboratory. Via this testing, seed technicians can determine the following: if seeds can germinate and how well seeds germinate.
Factors that can affect the performance of seed in germination tests:
- Maturity at harvest
- Mechanical damage
- Deterioration during storage caused by moisture or high temperature
Treatments are available for seeds with poor germination (see previous columns in Growing for more info about seed treatments, www.growingmagazine.com). However, in most cases, a seed treatment will improve germination of seed only if the poor quality is due to seed-borne disease.
In order for a commercial seed company to sell its product, it must meet standards for purity and germinability according to the Federal Seed Act. To verify that seeds meet federal standards, they are often sent to a seed laboratory and tested according to the seed testing rules established by the Association of Official Seed Analysts (AOSA). Several different kinds of tests are available. Technicians choose the test depending on the customer’s request and the type of seed to be tested, the conditions of the test, and the potential uses of the seed.
The most common tests are the standard germination tests, which are conducted in the optimum temperature range for each type of crop tested to determine germination percentage. Several vigor tests are available to determine how seeds may germinate under stressful conditions in the field. Stress tests to assess seed vigor include the cold germination test, accelerated aging test and tetrazolium test. Each test is designed to evaluate various qualities of the seed. The Federal Seed Act only dictates standards for germination percentage and does not mandate vigor-testing standards. However, within the seed industry, vigor testing remains a popular way of assessing seed quality.
The standard AOSA germination tests are usually conducted by placing 400 seeds from each seed lot in moist conditions on blotters, rolled towels or sand. Seeds are maintained at constant temperature for a specified period according to the AOSA Rules for Seed Testing. At the end of this period, seedlings are characterized as normal, abnormal, or diseased, and dead or hard seeds. The germination percentage is calculated by dividing the number of normal seedlings from the total number of seeds evaluated.
The cold germination test is designed to measure the ability of seeds to germinate under high soil moisture content and low soil temperature. This vigor test simulates early season adverse field conditions and usually represents the lowest germination a grower could expect from a seed lot planted under such conditions. Technicians plant seeds in a sand-soil mix at high moisture content and maintain them at a temperature of 50 degrees for seven days. Subsequently, they raise the temperature on the test lot to 77 degrees for four days. The percentage of healthy seedlings that emerge at least one inch above the soil is reported.
The tetrazolium test is a biochemical test that differentiates live tissues from dead tissues of seed embryos based on dehydrogenase enzyme activity (respiration enzymes). Upon seed hydration, the activity of dehydrogenase enzymes increases, resulting in the release of hydrogen ions, which reduce the colorless tetrazolium salt solution (2,3,5-triphenyl tetrazolium chloride) into a chemical compound called formazan. Formazan stains living cells with a red color, while dead cells remain colorless. The viability of seeds is interpreted according to the staining pattern of the embryo and the intensity of the coloration.
The accelerated aging test exposes seeds for short periods to high temperature and high humidity, the two environmental variables that cause rapid seed deterioration. High-vigor seed lots will withstand these extreme stress conditions and deteriorate at a slower rate than low-vigor seed lots. Factors with the power to modify the results must be controlled during the test. The accelerated aging test subjects unimbibed seeds to conditions of high temperature (105.8 degrees) and relative humidity (around 100 percent) for short periods (three to four days). The seeds are then removed from the stress conditions and placed under optimum germination conditions.
Tests that require temperature control or evaluation of germination at multiple temperatures can be conducted on a thermo-gradient table. Thermo-gradient tables circulate warm and cold antifreeze through pipes welded to opposite ends of an aluminum plate. Enclosed in an insulated case, the plate can maintain a temperature gradient.
Professor Welbaum works with gradient tables in his lab at Virginia Tech. After posting information about gradient tables in a discussion on the Seed Biology Web site at Cornell some years ago, Welbaum received numerous inquiries about how to build tables. He built his first table in the early 1990s, but chose to improve upon the existing design by using water resistant materials, programmable baths, and LED grow lights. Welbaum created this version in 2008 and trialed it for a year before presenting the table and data from the trials at November’s conference. Response among conference attendees was strong.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.