Growing interest by corn growers in the USA to having seed tested for germination at temperatures typical of actual planting dates has resulted in several questions. Many of these issues have been raised by seed producers for many years. Why does some seed within a seed lot germinate and others not? What causes some seed to not germinate under cold conditions although the warm test result shows they are alive? What should be the minimum acceptable cold test result?
Hybrid corn is mostly bred to produce a large amount of grain per area of soil. Most of that volume and weight is carbohydrate deposited in the endosperm. Genetics affecting the production and storage of carbohydrate is mostly biologically separate from the utilization of that stored carbohydrate into metabolism needed for growth of the plant. A hybrid in which maximum grain production per plant occurs because the combination of genetics from a specific male and female parent are a match. Those two inbreds, however, frequently differ in vulnerabilities to environments that affect germination. The inbred that becomes the female plant in hybrid seed production becomes the genetic source of some cellular components such as the mitochondria and chloroplasts. Mitochondria are especially significant in deriving the energy from carbohydrates to drive the metabolism needed for production of new cells during the germination process. This is probably the main biological reason that seed companies have identified certain parents as more reliable for germination.
Membrane systems, including those within mitochondria, are essential to cellular function. Cellular membranes are also vulnerable to damage, especially during the rehydration after seed has be dried. The damage is independent of temperature. Movement of water through the pericarp into the embryo cells at the same rate in warm wet soil as in cold soil. Sudden swelling of the membrane-bound organelles causes some breakage. Remaining metabolism in undamaged portion repairs the damaged membranes. This rate of repair is temperature dependent. Repair is slower at lower temperatures. If soil temperature is below 55°F, it is believed that very little imbibition damage to cellular membranes especially those of the mitochondria will occur. In the field with low temperatures, the damaged seed and slow growth becomes vulnerable to invasion by microbes that can further inhibit seedling emergence.
Cold germination testing is intended to identify the percent of seed within a sample that have sufficient membrane damage to inhibit or delay germination and consequently not become a productive plant. Developing reliable, repeatable test methods is a challenge resulting in multiple lab differences in results. Some variability is due to subtle aspects of temperature of water, nature of germination media and definition of damaged seedlings usually called ‘abnormals’. Added to the biological-environmental variables are the difficulties in adequately sampling. The initial source causing the imbibitional chilling damage vulnerability could have been disease or moisture stress in the production field, handling in the harvest, drying and bagging process. Genetics, especially of the female parent also interact with all of these factors. In many ways it is amazing that we produce mostly high germinating seed.
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.