Probably the most essential component of cells of corn, and all other living things, are submicroscopic organelles composed of RNA and proteins.  These are the site of linking together amino acids to form unique structures of proteins essential for other metabolic activity in the cells and whole organism.  DNA in the nucleus of the cell is make of the composition of the plant genetics.  This DNA of a gene is a long string of sets of three nucleotides.  When a gene is ‘turned on’ in the nucleus, the sequence of the nucleotides is translated into a string of RNA. This is now called messenger RNA (mRNA) because it migrates through the nuclear membrane into the cell cytoplasm via the string of membranes called endoplasmic reticulum to a ribosome.
 
As the mRNA moves through the ribosomes, each code attracts an amino acid, attaching them to the next amino acid, ultimately constructing a specific protein to be released into the cytoplasm.  The construct and sequence of the amino acid become important to the protein function of being an enzyme in other metabolic activity or participant in body construction.
 
All genetic inheritance is dependent on these many tiny components of the cells. Considering that corn has more than 30,000 gene, each ultimately coding for a different protein, as interpreted by ribosomes it is not surprising that these are important components of all living cells in any plant or animal.  And there remains much more to learn about cell function with the research to be done by future young scientists.

Those individual cells that make up the corn plant that we see in the fields are actually doing most of the work.  One of the important components of almost all cells is described in this corn journal blog of 2/9/2016.
 
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 alon from the male sperm.  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.

​Green is the color of leaves soon after emergence from the soil and exposure to light.  This color is the reflection off the chlorophyll molecule located in chloroplasts, among the organelles of the plant leaf cells. 
 
Plant cells have plastids, distinguishing them from animal cells. They are believed to have been derived from cyanobacteria, such as single cell blue green alga, when a symbiotic relationship with a single celled organism merged with it, perhaps a billion years ago. Like any symbiosis between organisms, each one benefits, and they often become interdependent. Plastids, like bacteria, have two surrounding membranes and DNA organized in a circular manner as opposed to the chromosomal arrangement in all other organisms with a membrane-bound nucleus.  Plastids multiply by division independent of host cell division but are carried along with new cells.  Consequently, in corn, they are present in the female egg cell. After pollination, as the fertilized egg cell divides and ultimately forms meristems, each cell includes the plastids. These are called proplastids because they are not fully developed. Those in the cells reaching the light quickly are transformed into chloroplasts.  Although plastids have their own DNA and capability to produce the many enzymes and other components of chlorophyll, as in other cases of symbiosis, they are also dependent upon the host cell to provide some proteins and plant hormones such as cytokinins needed for proper development. 
 
A major structural feature of chloroplasts is formation of multiple layers of membranes (thylakoids) with the chlorophyll molecule and thereby enhancing the capacity for photosynthesis. The plant hormones classified as cytokinins, perhaps produced more by the host cell but some from the chloroplast itself, apparently affect the size and quantity of these layers.  Host cell genetics, those inherited from both parents of a corn hybrid, thus influence the chloroplast development and function despite the fact that the proplastids are carried along in only the female parent egg cells.
 
All proplastids do not develop into chloroplasts.  Those remaining below soil surface and some others do not become green and become sites for starch storage.  Some others accumulate other pigments, contributing to other colors expressed in plants.  Some chloroplasts located near the veins in plants develop slightly different carbon-fixing methods that allows corn’s photosynthesis to be among the most efficient of plants to convert light energy into carbohydrates.
 
These small organelles in plant cells not only produce the energy to allows the plant to grow but the oxygen molecules that we need for respiration and carbohydrates for our nutrition.
 

​​It is difficult to imagine 32000 genes distributed among the 10 chromosomes in the nucleus of a single cell within the embryo of the corn seed. But the microscopic cell also contains many other substances that contribute to cell function once it is activated with germination. Proteins and lipids contribute to the function of the outer plasma membrane surrounding the cell, but membrane-like structures also are intertwined within the cells. Endoplasmic reticulum is used to transport cell products. Ribosomes are attached to the outside of ‘rough’ endoplasmic reticulum. These ribosomes are the organelles in which RNA codes, originating from the DNA, are used to link the amino acids to form proteins. Adjacent endoplasmic reticulum is used to transport the newly formed proteins to sites in the cell appropriate for that protein’s function.

Mitochondria, independent organelles within the cell, are the site of transferring glucose molecules in the chemical energy used by other cell functions. These organelles, carried along in the egg cell from the maternal parent plant, have their own DNA for genetics but are dependent on the rest of the cell and nuclear DNA to provide the glucose, proteins and lipids for structure and function. This symbiotic relationship is in all animal, plant and fungal species, originating a few billion years ago and certainly is significant in corn performance. Mutations in the mitochondria DNA are the source of cytoplasmic male sterility, at least partly because of a genetic defect in the outer membrane of the mitochondria results in defective pollen production. This sterility affect can be overcome by products coded in the nuclear DNA of corn, the male sterile restorer genes. However, the specific mutation to URF13 gene in mitochondria with T cytoplasm, not only cause sterility but also increased susceptibility to certain pathogen toxins such that produced by race T of Bipolaris maydis, resulting the disastrous epidemic of 1970 corn crop. This toxin destroyed mitochondrial function, reducing the plant’s ability to produce normal pathogen-inhibiting resistance chemicals.

DNA of chromosomes in the cell nucleus is, by far, the largest, affecting most cell functions, but the much smaller amounts of DNA in mitochondria and plastids are essential and interdependent with the nuclear genes.

It’s the activity within the corn cells that drives the growth of the corn plant.

​Rate of production of new cells produced in the corn apical meristem is largely determined by photosynthetic energy in young plants. These new cells have pliable cell walls at first, that can be stretched by the turgor pressure from water movement into these cells before the epidermal cell walls accumulate the cellulose and lignin to form strong structures. Drought stress during the first 30-40 days after germination results in smaller leaves and shorter plants for the whole season.

Plant size is not always a major contributor to final grain production. Total leaf area per land area exposed to sunlight is essential for maximum grain production per land area. Uneven plant height, allowing some plants to be shaded by adjacent plants is probably more significant to grain production. Uniform water supply within short distances in the field can influence this factor.

Stomata in leaves allows the infusion of carbon dioxide essential for photosynthesis. Corn stomata open when light causes photosynthesis in the chloroplasts of stomata. Stomata also allow the evaporation water from the leaf. This has the effect of allowing the water molecules, adhering to each other, to be drawn from the roots to the above ground corn parts through the vascular system thus providing the turgor pressure for elongation of cells. Water loss through stomata requires a constant source of water

Multiple environment factors influence water supply to corn plants but genetics also distinguish variety reactions to growth of the plant. Root size and growth pattern affect water and mineral uptake. Structure of vascular tissue from roots to leaves affect efficiency of water movement. Number and activity of stomata in leaves affect the evaporation of water from leaves. Efficiency and number of chloroplasts within the cells affect the transmission of light energy to carbohydrates, mitochondrial numbers and efficiency affect the change of this energy into ATP for use in the formation of proteins and other products needed for cell growth. Translation of chromosomal DNA to RNA that moves to ribosomes where the codes for specific amino acids are strung together for specific proteins, some of which are used as enzymes driving production of cell structure components. A large number of those 30-40000 corn genes must be participating in that early growth of a corn plant.

It is not surprising as each year's early season differs, that the ‘best’ hybrid is not the same each year nor in each field.

​Corn seedlings are vulnerable to invasion of the multiple fungi activated by spring temperatures. As the new leaves push out from apical meristem still under the soil surface, the initial 1-4 leaves begin to senesce, weakening their ability to respond to potential invaders such as the fungus Colletotrichum graminicola, cause of anthracnose.

This fungus overwinters of infected leaves and stalks from the previous. This fungus produces very small spores that are carried easily in the wind. Spores landing on the young leaves can produce hyphae that penetrate through the leaf epidermis or through stomata. Vigorous growing leaves in most corn hybrids are mostly resistant to this fungus but weakened first few seedling leaves, as the naturally senesce may lose the ability to fight off the fungus at least in the epidermal cells. The result is a small elongate lesion from which the fungus produces more spores.

These first leaves are not usually damaged enough to hurt development of new leaves and later leaves appear to be mostly resistant. As these initial leaves die, this disease will not be noted until leaf senescence starts occurring late in season after grain fill. In this sense anthracnose is not an aggressive pathogen of corn, mostly being able to attack physiologically weakened plant tissue. The seems to be little correlation between the seedling occurrence of anthracnose and its appearance of corn stalks. I have seen the rare hybrid that would get considerable number of lesions on mid-season leaves and even that hybrid was noted for having superior stalk quality.

Although this fungus will cause long black streaks on outer cells of a corn stalk, there is evidence that it is only successful in rotting the stalk when the plant died from the depletion of the stalk and root from excessive movement of carbohydrates from those tissues to the grain.
Resistance to multiple pathogens, including Colletotrichum graminicola, is a dynamic interaction between the plant physiology and the potential pathogen.

​It is amazing how humans have adapted a tropical annual plant like Teosinte to produce large starch-storing food for people and their livestock.  Along those few thousand years of selection as corn was moved to temperate zones it became adapted to shorter growing seasons and cooler environments. Among the remaining challenges is adaptation to cool spring environments and invaders.  Pathogens and other feeders of the nutrients in the corn seed and seedling have followed the adaptation to these conditions as well. Resistance to such invaders is affected by the host plants biology and environment of the young corn plant.  Sometimes it is not easy for us to sort out the significance of the outside invader of corn, the host interactions and environment.
 
The fungal species of the genus Fusarium have a complicated relationship with germinating corn seedlings. The most studied species, Fusarium verticilloides (formerly known as Fusarium moniiforme and its sexual stage as Gibberella fujikuroi commonly is found in germinating corn seeds.  It often is found in corn plants without symptoms of damage and therefore is characterized as an endophyte because it appears to live within the plant tissue but does not always cause symptoms.  It is not uncommon to see growth of this fungus from germinating seed in paper germination test.  A study published in 1997 (Plant Dis. 81:723-728) compared seedling growth from seed artificially infected with this fungal species with those that were not infected.  There was no difference in germination percentage between infected vs uninfected seed. There was a slight size difference favoring the uninfected seedlings at 7 days after planting but at 28 days those growing from the infected seedlings were slightly bigger and with more lignin in cell walls.  Is this because of a hormone (gibberellin?) produced by the fungus or because of some defense compound produced by the plant?  The fungus was easily recovered from the seedlings but less from the older leaves.  Infected plants showed no symptoms of disease. 
 
It is clear that Fusarium verticilloides can be damaging to germinating seed sometimes but I don’t think all the factors are clearly understood.  Is the difference caused by the strain of the fungus, the host plant or the environment?  I know from experience that it is so common to find Fusarium growing from a dead leaf sample that one tends to ignore it.  It seems to live in much of corn plant’s tissue.  It often leads to confusion with diagnosis of problems including stalk rot, almost as if one cannot find other fungi usually associated with rotting stalks, there is always Fusarium.  It’s complicated!
 
 
 

​Corn seedlings are frequently attacked by a fungal-like organism of the Pythium genus when the soil is extremely cool and wet. Pythium belongs in a group of organisms called Oomycetes.  These were once considered fungi, but more recent research has shown that they are more related to algae.  Other pathogenic oomycetes include those causing diseases such as downy mildew of several grasses (including crazy top of corn) and Phytopthora root rot of many crops.  Oomycetes were considered fungi because they usually produce filaments and spores plus they absorb nutrients from plants and animals.  They differ from fungi by mostly lacking individual cells within the filaments, resulting in many nuclei within the long filaments.  Most fungal cell walls are composed of chitin whereas the oomycete filament walls are cellulose, a difference that relates to the host resistance system.  Often the first signal of fungal invasion is of the chitin, triggering the production of defense systems.  Detection of an oomycete invasion requires a difference in plant chemistry.
 
Oomycetes frequently produce swimming spores that are released from overwintering spores (oospores) in the soil.  Pythium spores, with flagella, swim to host root cells on which they grow hyphae to invade the host.  Consequently, they cause the biggest problems in fields that are temporarily flooded with heavy rains and cool conditions.  Studies have shown that 55°F is optimum for most Pythium species infecting corn in Midwest USA.  The low temps probably slow down the host plant’s growth rate and inhibit potential competition from soil fungi.
 
As many as 18 species of Pythium have been shown to be pathogenic on corn.  Often, they destroy the outer layer of the new root tissue.  If this occurs before secondary roots become the main source of water uptake, the seedling will wilt. Although seed treatments can offer some protection, genetic diversity within a species often includes resistance to most treatment chemicals.  Pythium genetic diversity also includes ability to attack rotation crops such as soybeans.  Pythium species are easily isolated from soil but completely accounting for all the species or diverse genetics in regard to effective seed treatments or genetic resistance is not easy.
 
Control of the disease is best done with controlling water holding capacity of the soil.

Corn seedling vigor is affected by their environment including soil water-holding capacity, temperature and pathogens.

Seeds are planted in environments that vary every few inches for water holding capacity, organic content and microbes. Furthermore, each individual seed varies slightly in its cellular membrane status. With imbibition causing swelling of the membrane bound cell contents, some seed can have problems getting effective metabolism for early cell growth to push out the root and stem structures.

Cell metabolism includes producing the response to attacks by potential pathogens in the soil. These anti-pathogen chemicals (phytoalexins) are usually produced with a complex system of detecting the microbe and concentrating the phytoalexin into the area of the attack. Weakened seed not only are likely to release more carbohydrates and proteins into soil because of membrane injury, but also be less capable of responding to the microbes invading root and mesocotyl tissue.

Diagnosis of seedling disease becomes complicated also. Pathologists can isolate a fungus such as a Fusarium species or an oomycete like a Pythium species, but the actual cause probably involves some interaction between the microbes, metabolic quality for the ‘diseased’ seedling, and a complex environment not only providing potential pathogens but also affecting the seedlings metabolic rate. Soil organisms are affected by the environments as well. Leakage of carbohydrates directs their growth toward the seedling roots but temperatures favor some over others. Pythium’s swimming spores do well in cool wet environments but can be inhibited by certain seed treatments that have very little effect on fungi such as Fusarium species. Other seed treatments can inhibit the latter group of microbes but are less effective against Pythium. Corn seed genetics and seed quality can be greater factors than either group of chemicals. Cold wet heavy soils for a prolonged time can overcome all methods of defense.

After the stress on the seedlings is reduced, remaining plants that emerge can give normal production especially if they are uniform in growth with adjacent corn plants. The metabolism of these plants will promote the recovery and normal root growth. Those plants that survive but emerge later than adjacent plants will have difficulty competing for light and mineral uptake which will be reflected in grain productivity.

It is remarkable that we usually get a high percentage of ‘normal’ corn plants with all the potential of problems surrounding that small seedling.

​Corn seedlings have recently emerged in fields of North Central USA. It is exciting to see the new plants arrive. Most of the activity is occurring out of site, however. Beneath the soil surface, close to the seed, the primary root is growing downwards, and the apical meristem is producing new cells via cell division. Heat energy is contributing to driving the cellular activity at both ends of the young plant.

Imbibition of water into the seed leads to activation of the cytoplasm within cells. Most of those processes occur along membranous components of mitochondria, ribosomes, plastids and the endoplasmic reticulum. Hydrated proteins now acting as enzymes in breaking down starch molecules stored in the endosperm and glucose and sucrose molecules are moved thru the scutellum to embryo cells. Diffusion of these sugars through pores of these cells, with cooperation of the cellular membrane and endoplasmic reticulum, these complex molecules composed of carbon, hydrogen and oxygen atoms are transported to mitochondria where they are further metabolized to create the ATP energy needed for other cellular activity. This cellular respiration process allows further cell construction as cells divide in the root and shoot meristems. Elongation of hypocotyl cells, as well as meristem cell division pushes the tissues from the kernel.

ATP (adenosine triphosphate) results from the energy transfer from electrons holding the glucose atoms together to form ATP, releasing CO2 and H2O. This process occurs in the mitochondria. These membrane-intense organelles apparently vary in number and efficiency among corn varieties. Mitochondria, having their own DNA, and yet is dependent upon the rest of the cell for its structural components, are transmitted to the next generation only through the egg cell. This is probably why different female parents of corn hybrids vary in time for seedling emergence and vulnerability to imbibition chilling damage.

The remarkable growth from a corn seed during a few months all coming from the cellular activity as coded in the genetics of the seed and its environment as assisted by the corn grower.

​Emerging corn seedlings initially utilize the primary root for absorption of water and nutrients as this root tissue is powered initially by the endosperm and then the photosynthesis from the first seedling leaves. Initial stem nodes remain under the soil surface. Soil temperatures affect the growth rate of the seedling but by time the third leaf is visible inside the whorl of the seedling, lateral secondary roots emerge from the nodes below the soil surface. Photosynthate are moved to these root tips stimulating more cell in new root tips as hormones direct the growth down. Cells outside the dividing root tip cells develop a strong epidermis allowing the root to push through the soil.
 
Newly formed cells of the elongating roots, near the root tip includes epidermal cells with thin-walled protrusions called root hairs. These protrusions into the soil affectively expand the net root surface area of the root allowing flow of water and nutrients into the root by osmosis.
Cells in the core of the new root differentiate to form vascular tissue that connects to the stem vascular tissue through the nodes.  This vascular tissue allows transport of water and minerals upwards through the xylem and carbs downwards through the phloem. A few cells in this vascular portion of the young root maintain cell division capability, becoming stimulated by another group of hormones (cytokinins) to increase cells laterally, pushing through the epidermal cell layer becoming lateral roots with their own root meristems. As more lateral root branches form, along with their root hairs the water and nutrients shipped to the developing seedling leaves the upper leaves are formed and photosynthesis increased.
 
This early coordination of shoot and radical root emergence from the seed. Initially with energy from the seed endosperm and then from photosynthesis, allows the new plant to develop for its annual lifetime.
 

The initial roots growing from the embryo radical supply the emerging seedling with water and nutrients. Other changes then occur.
 
After the first corn leaves emerge, the hormonal message to the mesocotyl tissue is to stop pushing upwards. Apical meristem, at the tip of the mesocotyl is now below the soil surface where the first leaf is attached. Photosynthesis now drives the metabolism of the young seedling as it switches from dependence upon the seed endosperm for carbohydrates.  Cell division in the meristem produces new leaves, each attached to the young stem under the soil surface and attached in distinct clusters of newly dividing cells called nodes.  These nodes are then stimulated to produce roots at about the time the 4th leaf appears in the young seedling.  Because the roots are being produced from stem tissue, they are called adventitious roots.  As the primary root, that had grown initial seed, loses its energy source, adventitious roots become the main roots for the plant.
 
The first 4-5 nodes of the young stem remain underground, each producing the roots for the plant, even as the first leaves remain attached at the same locations.  What appears to be stem in a 4-5 leaf seedling is a compilation of leaf sheaths tightly wrapped together while the actual stem remains beneath the surface. The underground stem portion, formerly attached to the mesocotyl, with adventitious roots becomes known as the crown.  Eventually the mesocotyl deteriorates as it is deprived on nutrition and loses resistance to the many soil organisms.
 
As the stem growing point eventually emerges above the soil surface a few exposed nodes will often form the brace roots to further support the adult plant. A lot of changes in a relatively short time after seed is planted.

​As we look for those first seedlings poke up to reach the light it is easy to miss the amazing feat that has been accomplished. Breeder efforts is selecting genetics, seed producers care during production, and growers efforts with soil preparation and care in seed planting results in the biology of each seedling pushing the shoot up and root down. A cooperative effort by all results in a uniform emergence.

​The first appearance of corn seedling tissue emerging from the imbibed seed is the new root. This young seedling primary root can be considered as three regions: meristem, elongation and mature. The cell elongation area provides the main initial force for pushing through the kernel pericarp. Cell elongation and maturation involves production of many new molecules composing the cell walls as they grow. The pectins, hemicelluloses and celluloses that compose the new cell walls are composed of several sugar-related molecules joined together through specific reactions, assisted by enzymes, heat energy and chemical energy such as from ATP.

Energy and components for the biosynthesis of these cell wall components comes mostly from the endosperm. Starch in the endosperm is broken down with enzymes into sucrose molecules, moved to root cell cytoplasm where other enzymes break down the sucrose into glucose and fructose. With other enzymatic action, the fructose is made into more glucose. Modification of these sugars allows other new carbohydrate based molecules that become linked to form the more complex polysaccharides such as pectin, hemicellulose and cellulose for the new cell walls.

We see what looks like a rather simple process- seed swells, root protrude and a few days later the stem emerges from the seed. What we don’t see is a complex utilization of stored energy, production of complex proteins some of which act as enzymes assisting in linking molecules together and thus giving outer strength to cells. Also, unseen is production of anti-microbial compounds to ward off the many organisms attracted to the very molecules stored and manufactured in the seed. We don’t witness the genetics that programs for these processes. Humans successfully selected for these features from a wild plant species, adapting it to worldwide growing environments. The complexity of corn seed germination still can be challenged in a cold, wet spring such as being experienced in the central U.S.A. spring, but it is amazing that these processes work even under tough environments

​​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. (Corn Journal 4/24/2018)

​While we wait for the first signs of emergence from the soil with the coleoptile poking up to the light, the real action has been happening within the cells of the shoot and roots of the seed embryo. It is cell division and cell elongation that pushes the shoot tissue up and root down. That action is occurring as the cell organelles are activated

​It is difficult to imagine 32000 genes distributed among the 10 chromosomes in the nucleus of a single cell within the embryo of the corn seed. But the microscopic cell also contains many other substances that contribute to cell function once it is activated with germination. Proteins and lipids contribute to the function of the outer plasma membrane surrounding the cell, but membrane-like structures also are intertwined within the cells. Endoplasmic reticulum is used to transport cell products. Ribosomes are attached to the outside of ‘rough’ endoplasmic reticulum. These ribosomes are the organelles in which RNA codes, originating from the DNA, are used to link the amino acids to form proteins. Adjacent endoplasmic reticulum is used to transport the newly formed proteins to sites in the cell appropriate for that protein’s function.

Mitochondria, independent organelles within the cell, are the site of transferring glucose molecules in the chemical energy used by other cell functions. These organelles, carried along in the egg cell from the maternal parent plant, have their own DNA for genetics but are dependent on the rest of the cell and nuclear DNA to provide the glucose, proteins and lipids for structure and function. This symbiotic relationship is in all animal, plant and fungal species, originating a few billion years ago and certainly is significant in corn performance. Mutations in the mitochondria DNA are the source of cytoplasmic male sterility, at least partly because of a genetic defect in the outer membrane of the mitochondria results in defective pollen production. Integrity of the membrane of the endoplasmic reticulum, ribosomes, mitochondria, nucleus and outer cell membrane after imbibition of water is essential to that early activation of the seed embryo and the emergence that we anticipate.

​ 
Each seed in a single cross hybrid corn bag is slightly different in its biological and even genetic history.  Rarely are the two inbred parents in a seed field 100% homozygous plus some of those plants may have their own mutations.  Fortunately, drastic mutants are usually caught by the seed producer before pollination. An individual female plant in the seed field may produce 200-300 seed. And one 80000 kernel bag of seed corn must represent seed from 300-400 individual plants.
 
We want to think of all single cross seed in a bag are the same, but they are not identical genetically or in germination quality.  Even with multiple generations of selfing in development of the parent seed, some mutations occur with each generation of seed increase prior to planting in the hybrid seed field.  Most often these mutations are non-consequential to hybrid performance and especially not visible in the field where many small environmental effects are affecting appearance of the plants.  The closer we get to discerning DNA differences the more difficult it becomes to distinguish inconsequential differences from the drastic ones.
 
Seed quality differences among the seed in that one bag of hybrid seed also shows differences.  A warm test may show 95% germination but even beyond the non-germinating ones, there will be some that are slower to germinate than others even when all environment is uniform.   As the percent germinated gets lower, more late ones become evident.  A cold test, especially like ours at Professional Seed Research, Inc. in which we cover the seed with 3/4 inch of artificial soil, nearly always show lower percent germination and more late emerging plants than the warm test.  Why are all the seed not with the same quality when produced in same field?
 
Unfortunately, not all the seed on a single seed field ear have the same environment. First silk, coming from the ovules near the base of the ear, emerge 3-6 days before the final silk.  Successful pollination by the correct male parent is dependent of many variables, including factors associated with maturity for the male and female in the seed field.  In general, pollen timing is affected more by accumulation of heat units whereas silking is favored by water.  Cool wet pre-flowering weather can lead to silks being exposed before pollen.  This not only makes the seed more vulnerable to contamination by outside pollen from field corn, resulting in outcrosses, but also to infection from fungi such as Fusarium or Diplodia species traveling down the silk channel before it closes after pollination.  The opposite can happen with hot dry weather, in which the silk emergence is delayed, causing the pollen to be spent before all the silk emerges.  Consequently, there often are differences in germination (and outcrosses) among seed positions on the ear.  

We humans have adapted a tropical plant (Teosinte) and created a species adapted to temperate zone called Zea mays. Along the way, selections were made to allow germination of seed and early growth and at very non-tropical, low temperatures. It also requires careful seed production, storage and planting.

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.

​Each plant of a single-cross hybrid has the identical genetics, including those affecting the speed of seedling growth.  Uniform seed quality and environment will allow uniform environment. Hybrids differ in the speed of emergence but uniformity of emergence within a field becomes an important component of maximum grain yield.
 
Corn embryo’s being planted on May 1 are about ¼ inch (0.6 cm) in length. The future shoot portion of the embryo is half the size of the embryo. Two to three months later that shoot length has been multiplied by 800-1000.  Within the embryo are cells with organelles such as mitochondria, plastids, ribosomes and other membranous structures needed to carry out this remarkable growth rate. Within the nuclei of these cells are the 10 pairs of chromosomes with the 30-40000 genes, coded by long strings of nucleic acids.   Within a few hours of water imbibition, the few genes in the mitochondria are activated. Appropriate codes within their DNA produce RNA strings of nucleic acid, that are moved to ribosomes, producing proteins appropriate to enzymatically remove the energy binding the carbon and oxygen molecules in glucose and moving that energy into adenosine triphosphate (ATP).  This energy is utilized in manufacturing the other structures for rapid cell elongation and cell duplication, pushing the seedling shoot to the soil surface.
 
With exposure to light, some cellular plastids with guidance from their own DNA and supplies from the other cell components, produce chlorophyll. This pigment allows absorption of light frequencies providing energy to drive the capture of carbon, oxygen and hydrogen molecules in the process of photosynthesis.  Resulting glucose molecules are moved to the growing cells that utilize the new molecules in manufacture of structural complex molecules such as fatty acids and proteins used for cell metabolism and cell wall structures such as cellulose and lignin.

 
It is easy to be amazed when we seed the rapid growth of young corn plants and even more impressed to know that we are only seeing the result of remarkable interactions occurring at the cellular level.  
 

​I find it interesting to realize that much of what we see in a corn plant from germination to maturity is happening out of the sight of most of us. After germination, the root shoot heads downwards and the shoot grows upwards, regardless of orientation of the seed in the soil. Plant hormones auxins, cytokinins and gibberellins are primary in affecting growth directions of these tissues. Those interactions are affecting the cell growth in those tissues, especially whether they elongate or not. The affects do not really end with germination.

Cytokinins and auxins are operative during all of the corn plants life, including the movement of sugars to the young kernels. These two kinds of hormones have different roles in origin and effect on corn growth. Cytokinins are mostly produced in root tips in root meristems and transported through the water distribution in the xylem tissue. Auxins are mostly produced in stem meristems and distributed in the phloem system. Cytokinins are associated with increasing cell division in the stem meristems whereas auxins are involved in cell elongation. Apical dominance resulting in the corn plant usually having only one upright stem is because of the interactions of the auxins produced in the apical meristem. Removing that stem tip in early corn development and thus reducing auxin production tips the balance towards more cytokinin and stimulation of cell division in the lateral buds of the corn plant, resulting in branches.

Pollination of the multiple ovules in the corn ear results in attraction of cytokinins to each developing kernel. Moisture stress during the first 10 days after pollination is known to cause early death to some kernels, perhaps because of reduction transportation of cytokinins to the most immature embryos (my conjecture!). Cell division in the new embryo meristems establishes the movement of sugars through the phloem to the kernels. Much of the sugar is deposited into the endosperm portion where it is changed to more complex carbohydrates and thus allow the osmotic pressure for more sugar movement towards the kernels.

More is known about the effect of these plant hormones on plant growth than all of the mechanisms involved with those effects. Auxins involvement in cell growth involves softening cell walls, making elongation of cells easier. Cytokinins have been shown to prevent protein breakdown and activating protein synthesis.

Cytokinins produced in root meristems are transported to and stimulate the cell division in the kernel embryos. Meristems of those embryos produce auxins. Auxins are associated with production of ethylene which has been associated with formation of abscission tissue as leaves and fruit mature. It is assumed that the auxins are associated with formation of the black layer at the base of kernels, resulting in stoppage of movement of material to the kernels.

We know that these plant hormones are associated with the growth of corn tissues including the formation of kernels but there remains lots to learn of the actual molecular interactions that allows this to happen. Meanwhile, corn breeders, agronomists and growers attempt to coordinate it all by selecting the genetics that maximize grain production. AND, new knowledge is coming as those interested in the science continue to research into the processes that most of us witness but don’t see the minutia at the cellular level.