It is established that uneven emergence in the field has a negative effect on corn yields. Evaluation of the cause of this problem is not always clear. Was it planter problems- too deep, too shallow, poor spacing? Or perhaps uneven soil preparation? Did planting get followed by prolonged cold wet weather? Soil too wet- or too dry? Or was it seed quality. It is not always easy to sort out the cause or causes.
Much has been written about corn emergence problems. Among them are https://www.agry.purdue.edu/ext/corn/news/timeless/emergence.html and corn.agronomy.wisc.edu/Pubs/UWEX/NCR344.pdf
If the unevenness is occurring in consecutive plants, perhaps it is some field condition that caused the problem. Seed quality problems usually only applies to a small percentage of the seed with a lot, the bad ones scattered among many good ones. Furthermore, even the weaker seed range from being dead to potentially recovering enough to emerge a few days behind the good ones. If it is a seed quality problem, it seems reasonable to expect that these delayed plants will be mostly randomly distributed in the field. Expression of a seed quality problem is likely to be greater when there is a field stress, as well, such as cold, wet weather soon after planting or heavy soils. Dead or partially germinated seed showing only the root are most likely indications of seed quality problems.
Seedlings in the field are surrounded by microbes attracted to exudates from the new roots and the carbohydrates stored in the seed. Seed treatments do ward off some of the fungi but also the living, healthy cells actively produce defense compounds to limit potential invasion by most pathogens. However, most dead or even weakened seedlings will have some fungi such as Fusarium species, complicating the analysis of cause of the poor stand. Did the fungus cause the seedling to be weak or did the weak seedling allow the fungus to invade? More aggressive pathogen such as Pythium species, favored by cold wet soils, are more likely to attack healthy seedlings if not inhibited by a seed treatment, but Fusarium species are generally more likely secondary to poor seedling development.
It is remarkable that seed producers can provide high quality seed from genetics basically developed for carbohydrate storage to be planted in environments loaded with organisms that feed on such carbohydrates. And that growers can apply techniques to provide favorable environments for each of these seed to produce hundreds more units of the carbohydrates.
Very soon after the corn seed is planted, imbibition begins. The H2O activates the membrane-bound mitochondria to respire, providing energy for protein production. The enzymatic proteins include those that digest the starch stored in the endosperm into more sugar molecules to be transported through the scutellum to other cells in the embryo, resulting in more energy available to produce structures for cell elongation. Heat energy provides a regulatory function affecting the speed of this germination process. Imbibition occurs at any temperature but metabolic activity in corn is generally thought to be very low if seed environment is below 50°F. Speed of germination increases as the temperature increases.
Membrane integrity within the seed also affects the net speed of this process. Those individual seed with more damage are slower to sufficiently activate the system and thus slower to activate the metabolism needed for cell elongation in root (radicle) and the shoot sections of the embryo. Cool environments, delaying membrane repair, may result in death of the imbibed seed before the shoot can emerge from the soil. Some of these seed, even after warmed manage only to extend the root through the outer wall for the kernel, the shoot never emerging. Other weakened seed may finally get enough momentum to push through the soil surface but days after the healthier seed have emerged, resulting in a season-long competitive disadvantage. Heat energy during germination affects the severity of the effect of membrane damaged seed.
Microbes in the soil are generally warded off by products of seed metabolism in healthy seed. Those individual seeds that are slow to generate sufficient energy for growth are also more easily attacked by microbes, further slowing the germination process. Seed treatments are useful in giving the damaged seed more time to successfully germinate. Healthy seeds can successfully produce normal seedlings despite surrounding common soil microbes but those weaker individuals need the extra protection.
Germination testing is used to assure that seed quality will not detract from final performance in the field. Warm germination tests are done by rolled towel, Kimpak and sand test. Each lab has some variation of these tests and results are used as a basis for the germination percentage posted on the tag for a bag or other container of commercial seed. Cold tests are also used for all commercial seed lots and is generally considered equally or more important predictor of field emergence, especially because most corn is planted as soon as soil temperatures approach 50°F. Various labs also have other tests in attempt to refine the evaluation of seed quality within a seed lot.
Attempts to standardize the testing systems among labs has been done by various groups including the International Seed Testing Association (ISTA) and Society of Commercial Seed Technologists. Despite efforts to standardize among labs it is common for results of testing referee samples when drawn from the same commercial bag to have a wide range. One example, a referee corn test sent to 30+ public and private labs resulted in warm test range of 86 to 98 and cold test range of 78-93%. This range is not because labs are making errors but it is more a problem that not all seed in a sample are of equal quality. Some are vigorous, some are a range of less vigorous and some are dead. It would be easier, and more consistent between labs if seed within a sample were either dead or alive.
The problem comes with those less vigorous seed. How do you predict which ones will emerge in the field? Will they emerge so far behind the neighbor plants that they won’t compete? Each lab attempts to classify those late germinating seed. Those that only show a root and not a shoot are classified as non-germinating. Those with torn coleoptile or first leaf are usually classified as ‘abnormal’ and most, I think, equilibrate them to non-germinating. The problem becomes more difficult with the seed that germinate slower than the others in the group. These individual seed are at some stage of deterioration but do eventually establish a shoot and root in the lab germination test. I think labs vary in how these seeds are classified are probably are expressed differently with different test methods. As the quality of seed deteriorates and the percentage of germinating seeds decreases, so does the percentage of seed with late germination increases. As a result, the differences among labs also increases.
Ideally a lab first works for consistency within the lab method and then attempts to relate the results to field performance. The latter requires multiple reps and locations and perhaps multiple hybrids as well.
We want things to be simple but it is amazing how often biology is not.
Just as experience grows experience, science leads to more science. Each new bit of knowledge of how plant cells work potentially leads to new concepts that potentially will increase our use of a corn crop. My graduate school days in the 60’s encouraged pursuit of new information about plants and fungi with the purpose of expanding knowledge, regardless of the eventual application. So-called basic research continues to be the foundation for eventual applications. Understanding DNA structure has led to many applications but it was originally driven by curiosity to understand how it works. Each bit of new understanding leads to desire to dig deeper, not always with expectation of eventual application but simply the realization that there is another level of mechanisms to be understood.
What causes the mitochondria in some corn genotypes to maintain the power to drive germination better than other genotypes? Can one change the mitochondria DNA in a manner to make it more reliable? Why are chloroplasts in some corn genotypes more productive than in other genotypes? Is it related only to its environment within the plant or are there chloroplast genetics involved. There is more to learn about the mechanisms that turn on or off resistance to corn pathogens, of mineral uptake by roots, of movement of carbohydrates within the plant before more efficient selection of better genetics can be developed.
We must not forget our dependence upon a balance between basic and applied research as science grows science. Corn is a major converter of light energy into carbohydrates and despite the thousands of years of human experience and science, basic science has more to learn. Applied science will follow.
Among the fun enigmas of advances in corn knowledge is how to apply new information into practical application. How to take a new understanding of plant function into increased efficiency of producing a crop. Multiple field environments within a few inches of each requiring a resilience affected by many of the 40000 genes is required for reliable field performance.
We are learning more about how to turn on and off genes by new technology. Hopefully the right gene changes still will mean increased productivity within the economics of today’s crop value. Often the single gene changes for herbicide or insect resistance have eventually lost effectiveness as the weeds and insects with single genes to avoid the corn resistance increased. These technological advances served for a time but, at least in some cases, are now not as useful.
This is not really a new phenomenon that is limited to application of a new technology. The Ht1 gene was discovered in the 1960, resulting in most commercial corn research programs integrating the gene into many hybrids beginning in the late 60’s. It was very effective in controlling the northern leaf blight pathogen, Exserohilum (Helminthosporium) turcicum until 1979 when it was apparent in a seed field with the Ht1. After being informed of this occurrence, several more observations were made across the Midwest corn belt in that season. Although occurring at a low frequency within the fungal species, individuals had a gene producing a product that blocked the Ht1 resistance factor and thus allowed normal disease development. These members of the fungal population gained momentum and now are the dominant ‘race’ in the USA, making the Ht1 gene less useful.
We are seeing the same story with weeds, insects and other pathogens. We are dependent upon variability in corn genetics as the crop is grown in a variable environment that includes genetic variability in corn’s pests. It is a continual challenge to decide which new technologies will make meaningful application to corn productivity.
That corn seed, soon to be or recently planted in the northern hemisphere field, has a history. Its ancient ancestor was chosen by someone 8-10000 years ago who found a plant, or perhaps a clump of plants in which the seeds were not totally enclosed inside the usual hard fruit covering of most Teosinte plants. This person, or people, decided to save some of this seed, perhaps because the expanded endosperm would offer a source of more food. From that choice, this annual plant became increasingly affected by people choosing mutants that met their uses. Separation of the male and female flowering structures, a decent mutation rate, annual reproduction, ease of transport by migrating humans all contributed to the variability in this new species selected by people.
People remain an important part of each generation of corn. The combination of academic theory of inbreeding and hybridization had a dramatic effect on corn production- and decisions that led to choosing that seeds genetics. Experience and academics led to better understanding of which combinations of inbreds lead to better hybrids. This combination of knowledge also contributes to the breeding techniques, the evaluations for the best hybrids and the production of high quality seed. Its history includes a major effort to reach the dual goal producing a seed meeting the need to germinate, grow into a plant in multiple environments and result in a product profitable to the grower.
As we sophisticate techniques in developing and growing this species, we shouldn't overlook that the real gains were made by many women and men over its long history that moved this weed to a productive crop. That seed we plant today remains dependent upon people, just as it did thousands of years ago.
Corn, like other grasses, is a monocot- the seed has a single cotyledon as part of the embryo. Unlike the dicot species, such as soybeans, in which the two cotyledons emerge and photosynthesize, the corn cotyledon remains underground. The shape of this thin structure led to the name scutellum, which is latin for ‘small shield’. It attaches to the rest of the embryo by a small channel with vascular tissue and is positioned between the shoot-root portions and the endosperm. The scutellum has its own enzymes that are activated with imbibition to digest the starch and oils stored in the scutellum as well as assist in the movement of sugars from the endosperm.
The scutellum is a storage location although much smaller than the endosperm. Efforts to select for high oil corn, carried on for many years at the University of Illinois, resulted in larger scutellum for more storage of oils, and smaller endosperm. Other parts of the embryo also were larger in high oil corn seeds as compared to the original ‘normal’ varieties before selections. Having a carbohydrate storage capacity and having a vascular connection with the shoot and root parts of the embryo makes it a target for infection by pathogens. This connection has been utilized by scientists to transmit trait DNA via infection by the symbiotic bacterium Agrobacterium tumefaciens carrying the DNA into the scutellum when excised from the embryo. Activity in the scutellum is turned on with imbibition as germination begins.
As the corn kernel is developing after pollination, embryo shoot and root cells are formed in a way that could quickly germinate with the moisture present in the tissue. A temporary dormancy prevents this from happening. Removal of the milky endosperm from the embryo as early as 10 days after pollination will allow the growth of root and shoots. Abscisic acid (ABA) in the endosperm appears to be the hormone involved in avoiding germination in seed before the full development. At least 10 single gene mutants are known to overcome dormancy in developing maize seeds, resulting in germination while on the ear (vipipary). Drying of the kernels initially by displacement with starch formation and then by air inactivates the dormancy.
Imbibition is the movement of water into the seed. It is a physical phenomenon, independent of the germination quality of the seed and of temperature. Movement of water into the seed is relatively fast, most occurring within a few hours. There is some evidence that slowing the imbibition by some seed treatments reduces the harm to membranes by giving them more time to repair. Rehydration of cellular tissue and chemicals cause swelling. Membranes, shrunken by drying, strengthen and activate but some solutes escape before damage from the drying is repaired. As the mitochondria are activated, ribosomes begin producing proteins needed for more cell growth, starches from endosperm are digested to form glucose molecules to be transported to the mitochondria in embryo cells. Oxygen uptake into the seed increases rapidly during imbibition- an indication of the active respiration occurring in cells. Heat, moisture and oxygen availability influence the speed in which imbibition initiates the germination of the corn seed.
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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.