Corn seed quickly imbibe water when exposed to its maximum within 3 hours, regardless of the temperature. Imbibition allows the swelling of membrane-bound cellular contents such as mitochondria, plastids and ribosomes. Metabolic activity in these organelles is dependent upon temperature and status of those membranes after imbibition. Some individual seed within a seed lot are more vulnerable to membrane damage, especially from the sudden swelling due to imbibition. Membranes do have the ability to self-repair as new metabolic products are activated. This repair is more rapid at higher temperatures (70°+F) but can be inhibited or very slow when temperature is near 50°F.
The standard warm test in the USA was originally designed to measure the percentage of seed that are viable and thus is conducted at above 70°F for 7-10 days. The problem arises when some individual seed being tested manage to show partial germination, with only the shoot or root showing. Others may have shoots and roots eventually growing but much later than the others in the sample. There are ‘official’ definitions of approved germinating seed but labs still need to have some way of defining these slow or injured plants. Often, they become defined as ‘abnormals’.
Even with attempts to regulate and define germinations in the warm test, and with defined methods such as substrates used and amount of water added, there remain differences among labs. Much of that is related to definition of these delayed germinating seedlings.
Cold tests are designed to predict the reality of emergence in the lower temperatures that are common to corn planting dates in the temperate zones. Usually the seed is placed immediately after watering in a 50°F environment for 7 days. The intent is to allow expression of damage that is likely to occur in the field at the earlier planting dates. Under these conditions most viable seed with little cold imbibition damage will germinate well when placed after the cold 7 days into a warm 70°F+ environment. Those seed with damage, however, will either not germinate or will be significantly delayed. Labs differ in substrates used for the cold test. Each has some difference in results and consequently companies may establish different standards for release of seed lots to their clients.
The ultimate objective of seed germination testing is to predict if seed germination quality will detract from maximum genetic expression for performance of the hybrid in the field. Very few seed lots will show 100% germination in cold tests. There are other factors influencing final plant stand and uniformity of growth in the field, but germination tests are intended to limit this factor.
The challenge is to identify the seed lots that will have sufficient germination in the field 4-6 months after lab testing. Real life biology is not always easy.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.