Corn seed quality
Corn grain production is dependent on maximum percent of plants for its field environment and plant to plant growth uniformity. Seed quality is a major factor in determining these characteristics.
Crop agriculture is dominated by multiple environmental and biological factors. Everyone participating with corn seed attempts to define and control these interactions. Breeding procedures can mostly assure that a hybrid is genetically uniform, production methods are intended to maintain this purity and testing methods can evaluate for level of genetic purity.
Seed viability is affected by environments during seed production in the field and after harvest. Each individual seed within a seed lot has a distinct experience with this process, ultimately affecting the ability to germinate with viable shoot and root tissue. Individual seed may have some damage to membranes within cells that require metabolic repair before being able to elongate the root (radicle) part and shoot part of the seed embryo after imbibition. This may be affected by genetics, often of the female seed plant and perhaps of the mitochondria in the female parent.
Germination tests to identify seed viability, usually defined as a seeds ability to produce a shoot and root when placed in a controlled environment can be done with reasonable repeatability. Results are determined after a specific time with specific definition of a root and shoot. Defining and characterizing differences among the seed’s vigor, or the time it takes for that individual seed to produce a root and shoot is more difficult. The seed analyst may see differences in vigor among germinating seed but communicating these differences becomes a major problem.
How to characterize a seed lot that has a high percentage of seed that meet the definition of viability but does not germinate uniformly in test conditions? Generally, those seed lots with delayed germination in warm conditions have lower germination percentages when tested under cold conditions (50°F) but there are exceptions to that as well.
The ultimate goal is to reduce the possibility that seed viability and vigor affect hybrid performance in the grower’s fields where environments present their own variables. It is understood that late emerging seedlings, regardless of cause, have difficulty in competing with adjacent corn plants. They often remain less vigorous because competitors reduce light on leaves and outcompete the late-emerging plant’s roots for minerals and water. Often late emerging plants produce ear shoots later than most adjacent plants resulting in poorly pollinated ears. Genetics of hybrids probably differ in ability for late emerging plants to remain nearly fully pollinated and thus the detriments of lack of uniformity is not exactly the same for all hybrids.
Everyone in corn agriculture wants maximum performance from the seed. We attempt to remove known variables by measuring viability and vigor and by preparing planting conditions. There remain uncontrollable environments and difficulties in defining and communicating seed vigor. Late emerging corn plants detract from maximum yield potential of a hybrid, but how late is the emergence and how much is the yield reduction? Like most of life’s experiences, we wish for clear definition but often the variables make that difficult. Our germination tests at PSR report uniformity of emergence with a 1-5 rating as the seed germinates in the uniform artificial soil, heat and moisture environment of our greenhouse (www.psrcorn.com/seed-testing.html).
Water runs downhill
I recall a phrase used by the plant physiology professor in a lecture that he used to make a point. While traveling through mountains, he was arguing with his wife as to whether they were going up hill or down, confused by surrounding terrain. He claimed he got out of the car and poured water on the road to see which way the water flowed. Then, he made the point to the students “water runs downhill!”.
Moisture from soil moves into the drier tissue of the planted dry corn seed. This imbibition causes the membranes of cells and their contents to expand, sometimes damaging the membranes. Cellular membranes have the ability to self-repair, but this process occurs more quickly with heat energy. Fast movement into the seed under cold soil conditions can cause significant damage to cell function ultimately resulting in slow or no germination.
Movement of water into the seed is slowed by the outer layer of cells in the pericarp. Breakage of this layer, results in more rapid uptake of water by the seed, potentially inhibiting seed germination. Seed treatments include a polymer coating to slow the uptake speed of water into the seed, allowing for membrane repair even when planted in cooler conditions.
Small breaks to the outer layer of cells in corn seed is practically unavoidable during the movement of harvested seed, despite extreme care of the seed producer. Ultimate uniform and nearly complete germination of the seed is enhanced by polymers applied to the seed outer layer. The warm germination test distinguishes seed sufficiently injured that cannot recover but the cold test identifies seed that cannot recover under the usual cooler soil conditions common for temperate zone corn planting. Pericarp cells, derived from the female plant, have some influence affecting the speed of water imbibition. Coating of the pericarp with a polymer can slow the imbibition process and resulting in less membrane damage.
Imbibition does not require living cells but simply the physical act of water moving down hill.
Activating corn seed
Storage of seed requires very slow metabolic activity, enough to keep membranes, but not enough to cause premature death of the embryo. Low seed moisture is mostly responsible for maintaining life in this semi-dormant condition. After planting, however, we want the seed to imbibe enough water to stimulate more activity. This process of imbibition was addressed in Corn Journal blog of 4/4/2017.
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.
Humans, without knowing cellular physiology, selected for these traits for Zea mays. And we get the benefit.
Corn seed storage
Dry corn seed are alive and breathing. Respiration in the seed, like in the rest of us, occurs in the mitochondria of the cell. The process of breaking down sugar provides the energy for creation of enzymes needed to maintain membrane structures in the seed that will be needed when germination begins.
Membrane deterioration increases as the temperature and moisture increase. These interactions of storage temperature and moistures have been shown to have a drastic affect on eventual germination percent of stored seed. Even size of root and shoot length of the seedlings is reduced if the seed was stored under conditions of higher humidity and temperatures.
Membranes not only surround individual cells but also the main sites of activities in cell organelles. Enzyme activity along these are the sites of protein production in ribosomes, transfer of proteins and other products are often done via cellular membranes. Much of this activity is guided by DNA within the cell nucleus, its integrity and activity affected by the nuclear membrane. Because natural breakdown of membranes increases with temperature and moisture, the need for higher respiration rate increases when seed is stored under poor environments. Seed stored at 9% moisture content and 10°C (50°F) retained nearly 100% germination for 4 years whereas the same lot of seed stored at 15% moisture content and uncontrolled temperatures as high as 38°C (100°F) had o% germinations. Reducing the seed moisture percent to 11% even under the warm conditions increased the percent germination to 90% (Plant Physiol. (1967) 42, 1071-1076). Many seed studies and experiences since then have verified the principle that drying seed and storing under low temperatures are essential to maintaining eventual high percentage germination of corn seed.
Corn seed aleurone cells
The pericarp surrounds the whole corn kernel, affecting the insect and pathogen invasion of the kernel and water penetration in the kernel. It may be 2-20 cells thick and is, genetically, female tissue. It is also without pigments.
Surrounding the starchy endosperm portion of the corn kernel, but within the pericarp is a single (usually) layer of cells known as the aleurone layer. These cells are part of the seed, the result of fusion of one nucleus from the pollen grain with two nuclei from the egg cell in the ovule. Whereas the cells in the rest of the endosperm function mostly to synthesize and accumulate starch, aleurone cells maintain more metabolic activity. Although only a single layer of cells, it can include 30% of the total proteins of the endosperm.
Anthocyanin production occurs within the aleurone cells, resulting in red and purple or blue corn kernels. Genes for lack of anthocyanin in the aleurone, allows the yellow color of starch endosperm cells carotenoid production to show in the common yellow corn kernels. Corn genetics for lack of carotenoid production in starchy endosperm, along with genes for no anthocyanin in aleurone, results in white corn kernels.
Aleurone metabolic activity contributes to much of the seed activity. Phytosterols infuse into the pericarp, contributing to insect and pathogen resistance. Although 80% of the oil in corn kernels is located in the embryo, 12% of the oil located in that thin layer of aleurone cells. Fibers from the aleurone cells and pericarp are processed together as corn bran, the aleurone contributing the oil to the bran animal feed.
The scutellum and aleurone cells are stimulated to produce amylase when moisture and temperatures are appropriate for germination. This enzyme assists in breaking down the starch of the endosperm, and thus making energy available for the growth of the embryo.
This layer of cells is an important component of both the use of corn grain and the growth of the next generation.
Importance of kernel pericarp
This thin outer layer of the kernel, originating from the female plant becomes an important contributor to ability to fend off pathogens and insects searching for the carbohydrates stored in the endosperm of the enclosed seed.
Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The pericarp, being completely dependent of the female plant genetics, adds to the significance of the hybrid seed producer’s decision of which parent inbred to use as the female. Both parents contribute to the hybrid ‘vigor’ but the female parent determines the pericarp characteristics.
Complexity of corn ‘seed’
The fact is a corn kernel is a fruit, composed of a part of the female plant, the thin outer wall, and the remaining from fertilization of the female egg by the male sperm nuclei. The cells in the resulting embryo include organelles such as mitochondria that are duplications of those from the female plant as the male’s contribution is only half of the resulting DNA in the seed. Mitochondria genetics, as inherited from the mother plant, may have subtle affects on the hybrid plant but could be major in seed aging and germination.
The powerhouse of almost all living cells in all plants and animals is a very small, bacteria-like organelle called a mitochondrion. It is similar to bacteria in its size, shape of chromosome, organization of its DNA and function. Mitochondria presence in all from the smallest of single cell animals and plants to the largest has led to the hypothesis that it originated as symbiotic relationship with a bacterium 3 billion years ago. The clear advantage of having this organelle that could transform carbohydrates into chemical forms of energy that allowed production of proteins for growth and movement of muscles in animals is overwhelming.
Mitochondria are the size of bacteria and therefore visible only with a strong light microscope magnifying at 1000X but the details require electron microscope power at 30000-50000X. With this extreme level of magnification, mitochondria are shown to be composed of a surrounding double layer of membranes enclosing many folds of membranes. Membranes are significant to function in that these are the sites in which the enzymatic action allowing the energy holding the glucose molecule together is released and combined with nitrogen and phosphorus into another chemical compound, Adenosine triphosphate (ATP). This compound released through the membranes into the rest of the cell for normal cell metabolism. It is also the site in which CO2 is released during respiration.
The fact that mitochondria have their own DNA has had dramatic affects on corn. Individual cells may contain from a few to hundreds of mitochondria and they replicate on their own, independent of nuclear chromosomes. However, when sexual reproduction occurs in plants or animals, and the nucleus from the male donor fuses with the nucleus of the female egg cell, no mitochondria are passed along. Consequently, the mitochondria in the progeny are only those from the female parent. Although the size and genetics of the nuclear DNA is overwhelmingly greater than that of the mitochondrial DNA, and the most profound genetics remains with the nucleus, mitochondria inheritance has had some dramatic affects on plants and animals, including us humans.
Amazing how we are so dependent upon such small things like mitochondria!
THE KERNEL AS A SEED
The corn kernel is a fruit with one giant seed. We humans mostly bred and selected this grain for its use as a food source, increasing the endosperm size with carbohydrates. Selecting for desirable seed traits has been at least somewhat secondary to the grain production. On the other hand, uniform and reliable field emergence is a major contributor to corn grain production in modern corn hybrids. This is dependent on the science and experience of seed producers.
Much of the propensity for high germination is dependent upon the female seed parent. Pericarp, being totally part of the female plant, affects rate of moisture loss during drying, vulnerability to physical damage from handling of the seed and susceptibility to ear molds. Mitochondria genetics are totally inherited from the female parent. Much of the damage from rapid, cold imbibition of water at the initial stage of germination involves the mitochondrial membranes.
Maize kernels handled as grain need to be stored at 15% moisture to avoid mold. Modern hybrids are usually allowed to dry in the field well beyond the 30-32% moisture level that black layer forms and completion of movement of carbohydrates to the kernel. It is usually most economic to allow drying in the field before finishing with artificially drying.
Most maize seed begin losing germination capacity if left in the field during those final days of slow drying in the field. Studies have shown that greater germination percentages are retained if seed is harvested in the 35-40% moisture level and then is quickly dried with lots of air and less than 100°F. Retaining the higher moisture level for some time initiates metabolism in the embryo, essentially an artificial aging process. Seed dried to about 12% moisture is considered optimum of storage and retention of high germination rates.
Producing and retaining high germination is the result of research of each hybrid parent’s vulnerability as well as experience with weather and facility. Drought damage during grain fill, rain delaying harvest, drying at too high temperature and not enough air, rough handling during processing and adding too much water during seed treatment can contribute to below standard germination. One can write a manual for production of seed corn but ultimately it takes some experience to apply the science. Like most agriculture.
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.