​Corn has been selected by humans from the wild plant Teosinte to move more carbohydrates than is need for only plant reproduction but to also store excessive amounts in form for human consumption. This over-production is a metabolic process goes on after pollination as shifts in the flow of sugars within the corn plant changes.
 
​Corn plant growth is greatly affected by a broad class of complex chemicals called hormones. Two of the kinds of hormones related to grain fill are the cytokinins and abscisic acid (ABA).  These two hormones have opposing functions in plants, including the development of corn kernels.  Cytokinins function is to increase cell division and delay senescense of tissue.  They are produced in roots and transported via the xylem to meristems such as in each kernel. They also may be produced in seed embryos also but evidence for that is elusive.  Regardless, cytokinins accumulate in developing seeds where they are responsible for stimulating cell division.  Cytokinins are also linked with the transportation or at least the attraction of sugar to the developing kernels.
 
Abscisic acid, on the other hand, is associated with cutting off of translocation to tissue basically by causing a layer of thick-walled cells impervious to movement of materials.  Abscisic acid production increases when the plant is stressed. The black layer at the base of mature corn kernels and at the base of husk leaves in a mature corn ear are stimulated by abscisic acid. 

Freshly pollinated ovules have a balance of these two hormones.  A non-stressed corn plant normally has a balance favoring the cytokinins stimulating more cell division and, consequently, flow of sugars to the individual kernels.  However, if the plant is under heat or drought stress the balance tends to favor abscisic acid.  The affect can be abortion of those kernels.  Corn kernels within the first 10 days after pollination are most vulnerable, perhaps because the accumulation of cytokinin is too great to be overcome by a short-term increase in abscisic acid.
 
Genetics and environments influence the production of these two critical hormones affecting grain yield in a corn field.

​The products of meiosis in the male and female flowers of the corn plant are ready for action after the flowers are extended with anthers dangling on the top of plant and silks extending from the ear shoot mid-way up the plant.
 
Female flowers have extended the stigmata (silks) through the husk exposing them to the air. Male flowers, beginning first from the oldest florets, have extended anthers emerging and filled with pollen grains.  With a drop of relative humidity, the oldest anthers will open at the lower tip, releasing the pollen grains. The grains are sufficiently dry to be viable and yet float with the slightest of a breeze.  Some claim corn pollen can travel ½ mile in 15 minutes with sufficient wind but considering all the variables, i.e. genetics, relative humidity and amount of wind, it becomes difficult to generalize.  Seed producers, attempting to produce pure hybrids are well aware of the influence of pollen distribution. 
 
New corn pollen has a light-yellow color but as it ages and desiccates in dry air it becomes dark yellow. Pollen will germinate when moistened by growing a germ tube. Pollen landing on the silk hairs (trichomes) produce enzymes that allow penetration of the germ tube into the silk.  Nutrition in the pollen grain is sufficient to grow about ¾ inch (2 cm). Nutrition from the silk is needed to allow continued growth down the several inches of silk channel to the ovule.  Although several pollen grain may initially penetrate the silk only one usually is allowed to reach the ovule, as the silk channel basically collapses as the germ tube progresses. Pollen grain penetration of a silk occurs within 5 minutes but germ tube growth to the ovule may require 40-60 minutes.
 
Once the germ tube reaches the micropyle of the ovule, the ovule causes the germ tube to burst, releasing the sperm. One sperm cell migrates to the egg cell with its monoploid nucleus fusing with the monoploid egg cell nucleus to form a diploid zygote.  The other sperm nucleus enters the central cell, fusing with its two monoploid nuclei forming a triploid endosperm.
 
This complex process continues millions of times within a single corn field.
 

​Cell division in corn growing points including the division of the nuclei by mitosis, in which each of the paired chromosomes are duplicated, resulting in the same genetic codes for each cell. Within the flowers in the tassel and ear meristems, however a different nuclear division occurs resulting in the genetic diversity that has allowed corn to be adapted to multiple environments.
 
One or more of the lateral meristems, which are located at each base of each leaf but attached to the stem node, is stimulated by hormones to produce female flower parts.  In corn each node of the modified lateral meristem includes two ovules, one of which degenerates. The ovule diploid cell undergoes meiosis, initially producing 4 monoploid nuclei but three degenerate, leaving a megaspore cell with one monoploid (haploid) cell. This single set of 10 chromosomes on hybrid plants represents a random mix of chromosomes from each of the hybrid plant’s parents.  Thus, just as with pollen, there is a minimum of 1028 different sets of genetics among the ovules on a single plant.
 
The nucleus of the megaspore cell undergoes three successive mitotic divisions resulting in 8 nuclei and a total of 7 cells.  Most important of these is the egg cell with a single monoploid nucleus and a large central cell with 2 monoploid nuclei.  The central cell is destined to become the endosperm after pollination. Two of the other cells (called synergid cells) adjacent to the egg cell apparently produce attractants to guide the pollen tube to the egg cell. A small opening, called a micropyle, at the tip of the embryo sac, is conveniently located where the silk is attached to the ovule. This composes the embryo sac of the female.
 
Corn apical meristem switches to producing male and female flowering parts, but quickly changes to male development only.  Each glume in the tassel is an individual floret containing three anthers.  Within these immature anthers are hundreds of microspore mother cells in which meiosis occurs.  As a result, each of these cells with 2 sets of the 10 chromosomes (diploid) before meiosis now contain 4 microspores, each with only 1 set of the 10 chromosomes (monoploid).  Whereas the diploid stage in hybrid corn, included 1 set from the parent male parent and 1 from the female, after meiosis, each microspore includes a random mix of two parents.  There are a minimum of 1024 different combinations of the two parental genetics among the microspores. The 4 microspores separate over a 4-day period and begin to become separate pollen grain with thicker walls. Nutrients are absorbed from the liquid contents of the anther during the microspore and pollen grain stages over about 10 days, at least in one study. During this period, the anther dehydrates as it is filled with pollen grain.  By the end of this period, the pollen grain has many starch granules, two haploid nuclei, a thick outer wall and a thin inner one. Total time from beginning of microspore production to mature pollen is 14-17 days. Each pollen grain remains viable for only about two days after maturity and less when under high temperatures.
 
A pore at the end of the anther opens to release the pollen.  This process involves dehydration and is affected by drops in surrounding relative humidity. There is no release during rain and pollen release is common in mornings as relative humidity drops with rising daytime temperatures.
 
Each floret of the tassel has slightly different time of development as the apical meristem expanded.  Consequently, each finishes the process of pollen development at different times, frequently over 10 days. A typical hybrid tassel has about 6000 anthers, although hybrids and environments vary. It is common for a single tassel to produce millions of pollen grains.
 
Meiosis sets the potential for new genetic mixes within the ear shoot and tassel.

​It is midseason for temperate zone corn plants. Terminal buds have pushed out the male flowers, the tassel, and the lateral branches have extended exposing the the female flowers of the corn plant.
 
​The ovary is formed from the diploid tissue of the mother plant.  Like other flowering plants the female sex organ is called the pistil, consisting of the ovary, a style and stigma. The style, like in other flowering plants allows the movement of the pollen sperm to be transmitted to the ovule.  In corn, this style is exceptionally long and is known as the silk.  Towards the outer end of the silk is a portion that has many hairs (trichomes) that aid in capturing pollen and encourage them to germinate.  This is known, botanically, as the stigma.   Each silk is part of a single flower of the female plant and thus leading to a single ovary with its enclosed ovule.  Cells making up the silk elongate basically due to osmotic pressure as water is transported to the cells as well as photosynthetic sugars for energy. Environmental conditions including soil moisture, leaf disease and light intensity interact with genetics to influence the movement of essential elements to the growing silk cells. The oldest ovaries at the base of the forming ear are the first to develop and elongate, but they also have the furthest to go before emerging from the surrounding leaves. First to emerge often is those a short distance from the base of the ear.
 
Corn silk emergence may occur over a 10-day period as those at the tip of the developing ear eventually emerge. Without pollination or stresses, an individual silk remains viable for about 10 days. A viable pollen grain germinates within minutes of adherence to the silk. Growth of the pollen germ tube into the silk initiates the halt to that silk’s elongation. As the pollen tube progresses down the silk channel towards the ovule, silk cells dehydrate and collapse, effectively inhibiting infection by fungi.  Timing of the pollination and silk emergence is essential to successful fertilization of the ovule cells.  Water pressure being more essential to silk emergence than the production of pollen, makes corn seed production very dependent on field conditions. Genetics vary for vulnerability to stress related silk extension.  Inbreds and hybrids vary in root growth patterns for absorption of water from soil as well as the tendency to move water to the developing silks.  Duration of silk emergence without pollination also influences the vulnerability to ear mold fungi. Aspergillus infection, often causing aflatoxin, is related to drought delaying silk emergence and thus poor pollination.  Diplodia ear rot is often related to long silk emergence periods without pollination when rain inhibits movement of viable pollen to the silk, adding to the vulnerability of the silk to infection by this fungus.  Insect feeding of fresh silk also is linked to fungus infection.
 
Environment and genetics greatly influence the biology of flowering in corn.
 

​Maize male and female flowers are on separate branches of the corn plant, thus the species is called monoecious, as opposed to the dioecious flowers of soybeans.  Both the ear-forming branch and the terminal tassel is composed of multiple flowers.  Each kernel that forms in the ear traces to a single flower with a single ovule within the fruit wall, the ovary.  Both male and female flowers of corn begin as dioecious but the male portion in the ear and the female flower in the tassel are aborted very early in the development of each.  A mutation or an environmental factor can overcome the abortion, resulting in tassel seed or terminal tassel on and an ear.
 
A corn tassel may include up to 1000 spikelets, each one including 2 florets.  These individual flowers are enclosed in the modified leaves called glumes.  Each of the florets have three stamens, consisting of filaments and anthers.  Each anther includes multiple cells called microspore mother cells or microsporangia.  Meiosis occurs in these diploid cells resulting in 4 haploid microspores per mother cell.  This occurs over a period of 3 days.  Microspores become free of each other as they grow for a few more days.  The individual haploid nucleus in each microspore undergoes mitosis, resulting in two haploid cells within the individual pollen grain.  The pollen grain secretes a pollen wall within another 7 days.  Starch crystals accumulate within the pollen grain during that wall formation period as the cytoplasm of the pollen grain dehydrates.   A small pore is formed in the pollen wall. 
 
Pressure from the growing pollen grains and dehydration of anther walls causes the split that allows the release of pollen grains.  One thousand spikelets each with 2 florets with three anthers each with hundreds of pollen grains easily produces a cloud pollen.  Production of the spikelets over a period of days results in daily release.  Pollen longevity may only be a few hours in high heat but the release over consecutive days in a field of corn usually assures viable pollen reaching most viable female stigma.
 
The remarkable human selection and development of maize adapted to multiple environments because of available genetic diversity is largely due to the separation of male and female flowers.

​Leaves of all higher plants have leaves attached to the stem at a place called nodes. Monocotyledons including grasses like corn, have one leaf per node.  At the base of the attachment of the leaf is a branch meristematic growing point.  Genetics and environment determine whether the meristematic cells at an individual node divide and develop into another stem, some environments such as wide plant spacing with some genetics encourage the lower node buds to develop a branch stem that we call a tiller.  Corn was selected from Teosinte plants often had tillers at lower nodes of central stem with many upper plant node buds developing into specialized branches as flowering structures.  These became small ears at numerous upper nodes. 
 
Humans selected, over several thousand years, genetics that generally inhibited the growth of most nodal meristems, selecting for plants without nodal branches except for one specialized branch for female flowers at a node convenient for humans use.  That branch consisted of leaves surrounding a series of nodes each of which had a female flower in which an embryo was surrounded by specialized tissue and from which a long, specialized tissue extended beyond the leaves. This specialized tissue could be penetrated by corn pollen from other plants or from the other specialized tissue at the top of the plant, the tassel.  Each tiny flower within the specialized nodal branch produces its own embryo surrounded by tissues that ultimately, after pollination, develops into a seed within the fruit that we call the kernel.  Genetics and environment determine the number of nodes with these specialized buds and the number of flowers within these buds.  
 
Accumulation of heat or day length interact with genetics determine the timing of these ear shoots and environment affect on photosynthesis the number of specialized flowers created within an individual nodal branch we call an ear. Humans have selected genetics concentrating the nodal buds into specialized structures stored with carbohydrates in forms that are convenient for human consumption.
 
That reminds me, I wonder if the sweet corn in our nursery is ready for eating?

Corn belt of USA 2021 weather has featured a cool April and May and hot June and early July. Most corn planting was done in April, as usual.  Those temperatures and genetics interact to affect the timing of the apical meristem switching from producing more leaves to those specialized cells of the tassel. 
 
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature.  Once again, we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years.
 
Photoperiodism in plants is evident as we see many species that bloom at the same time every year.  A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex.  There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812.  Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud.  Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date.  Our company does some breeding projects with tropical material.  Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet.  Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers.
 
One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids.  This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop.  Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater.  Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting.  Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering.

​Those tassels that we see emerging in corn fields now, in mid-July, were initiated several weeks ago as determined by genetics and heat of the early corn season.

​Whereas movement of water in corn is mostly a passive action through the dead cells of the xylem that form a narrow tube within the vascular system, distribution of carbohydrates from the sources of photosynthesis to elsewhere in the plant requires living cells of the phloem tissue.
 
The sugar sucrose is formed from the glucose product of photosynthesis and becomes the sugar for distribution within the corn plant. Movement of carbs between cells can be simple diffusion through those small ‘holes’ in cell walls, the plasmadesmata, as the molecules move from a high concentration to a low concentration.  It is a little more complicated with travel through membranes by osmosis, but the basic principle is the same.  Water is involved because It is the solvent of the sugar.  Greater concentration of water equals less concentration of sugar. Water molecules are also affected by the principles of movement from high to low concentration, setting up dynamics for what is called turgor pressure within each plant tissue.
 
As the sucrose molecules move into a ‘sink’ such as newly formed cells of a growing point more complex molecules such as starch and thus maintains the osmotic pressure for more movement of sucrose into the kernels. Other sinks, such as biologically active tissue of all living plant cells, consume the sucrose in cellular respiration and formation of essential amino acids and cell structures.  These various sinks are not all in the same direction from the carbohydrate sources where the photosynthesis occurred and consequently flow among phloem cells may not be in the same direction.
 
Whereas water movement in xylem tissue of the vascular bundles of a corn plant is mostly upwards from the roots, movement of the products of photosynthesis is affected by concentrations of various sugars in the sinks, allowing bi-directional flows.  

​All plants are dependent upon water to live.  It is essential to nearly all physiological functions in the plant.  This is due to the unique properties of water as a solvent allowing movement of metabolites within cells and among cells.  Not only is water a good solvent but also has the cohesive property of water molecules tendency to adhere to each other. Corn plants are very dependent upon these characters of corn.
 
Water has a complicated interaction with corn physiology and function. It moves through root hairs via osmosis, water moving from a high concentration through cell walls where sugars and minerals reduce the water concentration.  It is a physical phenomenon.  Osmosis further causes water molecules to move to the xylem vessels.  This pressure pushes water up the vessels. Leaf stomata open during the day because photosynthesis in the two curved cells surrounding the produce sugars resulting is swelling, again due to osmosis drawing in water.  This causes opening to the air, allows movement of CO2 into the leaf and oxygen into the air. Water evaporates in the opening below the stomata, moving into the air, again moving by relative concentration of water molecules. Dry and windy air increases the rate of transpiration.  Water molecules tendency for cohesion, causes water to be pulled upwards, as each molecule transpires through the stomata is replaced by a molecule pulled from the xylem.  It's a push from below the soil surface and a pull through the stomata that moves water through the plant. 
 
During cell elongation, before flowering in corn, water movement into new cells largely determines length of cells and ultimately affects plant height. At flowering, this cell elongation process become critical to the timing of ear silks pushing out of ear husk tissue for exposure to pollen.  Pollen production and distribution is less dependent on water concentration and therefore timing of pollen and exposure of silk may not match.  Poorly pollinated ears are the result of drought conditions.
 
Photosynthesis utilizes water as H2O is combined with CO2 to make glucose (C6H12O6).  Drought conditions during the growth period can reduce ultimate leaf area and thus photosynthesis. Severe drought can result in stomata not opening, reducing the CO2 available but this appears to be most significant before pollination. The biggest cause of grain yield loss from drought stress is not reduction of photosynthesis but it is the lack of place to put its products.


Movement of the glucose from the leaf tissue to grain is determined mostly by the hormones produced in the newly formed embryos in the pollinated ovules.  Lack of water reduces elongation of silk causing them not to be exposed to pollen and consequently fewer embryos.  Sugar molecules accumulate in leaf tissue, triggering production of anthocyanins in leaves, turning the leaves red.  The pigment change reduces photosynthesis.
 
Corn biology is dependent upon adequate water supply for nearly all functions from being a solvent for movement of sugars and minerals, providing turgor pressure for cell expansion, a coolant as it evaporates from leaf tissue and contributor to photosynthesis.

​Early June 2021 weather in western USA corn belt featured hot and dry conditions.  Affect of moisture on corn discussed in last blog can result in shorter corn plants than normal.  Affect of heat on time to flowering on temperate-zone corn was discussed in Corn Journal Blog 7/12/2016.
 
 
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature.  Once again, we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years.
 
Photoperiodism in plants is evident as we see many species that bloom at the same time every year.  A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex.  There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812.  Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud.  Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date.  Our company does some breeding projects with tropical material.  Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet.  Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers.
 
One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids.  This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater.  Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting.  Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering.
 
There will be some short corn plants, flowering extra early in parts of USA in 2021.

​Every corn growing season differs in two important factors affecting plant height and flowering of corn plants.  The growing point of the corn plant continues to produce new cells until the V5 (5 visible leaf collars). At that time, the new cells differentiate to produce the tassel cells. It is elongation of those cells that determines the plant height.  Pressure from the water transported through the xylem to the upper plant tissue and newly formed cells causes the cells to elongate. Heat energy, interacting with the genetics, determined the change in cells produced by the apical meristem from producing more stem and leaf cells to tassel cells. This genetically affected trait has allowed a tropical plant to be adapted to temperate zones around the earth.
 
Seasons and environments differ in both water and heat with the consequence of important corn productivity factors including leaf area, accumulations of carbohydrates, timing from planting to harvest, uptake of minerals, number of kernels per ear and ultimately grain yield.  Early season hot and dry weather results in shorter plants and early flowering.  Continuation of excessive heat increases evaporation of water through stomates, potentially dehydrating leaf cells, affecting photosynthesis in chloroplasts and flow of glucose through phloem in vascular system to roots and newly formed kernels.
 
Timing of excessive heat or drought is significant to the final season affect on grain production and standability of the corn crop. Corn breeders select genetics that tend to perform well in multiple environments, but all are affected when heat is unusually hot and soils are extra dry.

​Photosynthesis in all plant species involves multiple steps with many enzymes that can be divided into two main steps occurring in the membranes making up most of the content of chloroplasts. 
 
Light energy is used to split H2O molecules into its hydrogen and oxygen atom components freeing up electrons. These electrons allow provide the energy to unite the oxygen and carbon form the glucose molecule (C6H12O6), releasing the excess O2 molecules that eventually escape through the stomata. Electrons binding the components of glucose later are released in the respiration processes within the cells, providing energy for synthesis of proteins for growth in plants plus movement in animals.
 
This photosynthesis process is present in most plant species.  The release of oxygen through stomates is dependent upon open stomates and therefore is dependent of light being absorbed by the stomate guard cells and sufficient water transported from the roots to maintain those cells to swell.  Thus, at night and during droughts most plants no longer can absorb the CO2 needed for more glucose synthesis and the excess oxygen is consumed in nighttime metabolism.  This is a character of most plants with C3 photosynthesis processes.
 
Some species, including corn, have evolved a system to avoid this wasteful system.  Chloroplasts in the corn leaves make normal photosynthesis process but then break down the C3 molecules, have them transfer them to the specialized, vascular bundle cells surrounding the vascular system that are loaded with special chloroplast for C4 molecules.  These molecules are then enzymatically combined to make sugar which is moved elsewhere in the leaves and other parts of the plant.  This system occurs in species that are native to dry, hot environments such as that of corn’s central America origin.  The ultimate advantage is that corn can continue to produce carbohydrates despite environments that cause stomates to close.  Whereas most C3 plants such as soybeans, wheat and rice do not utilize light intensity greater than 3000 foot-candles, corn photosynthesis rate keeps increasing with light intensities to our maximum sun brightness of 10000 foot-candles.  Those few cells surrounding the xylem and phloem of a corn leaf vein have a special role in allowing the photosynthetic efficiency of maize.
 
This type of photosynthesis drives the rapid growth of a corn plant and ability to store excess glucose as starch in the grain.

Chloroplasts in corn provide the structures for the corn plant to be the most productive of crops in converting light energy into usable forms of energy. Selection of features by humans over the past 8000 years assisted but natural selection of origin in hot dry Central America gave great assistance by evolving a C4 photosynthesis metabolism. Most plants have a photosynthesis system with an inefficiency that limits its productivity.  This system, labeled as C3 photosynthesis, peaks in its ability to fully use total light intensity to about 3000 foot candles where-as unclouded sunlight has 10000 foot candles.  In corn, with it C4 photosynthesis, it continues to produce carbs in direct relation intensity of the light with maximum photosynthesis in bright sunlight.
 
Carbon dioxide enters plants through holes in leaves called stomata. These structures also allow oxygen to escape from leaves to the benefit of all of us.  Water vapors also go through the same stomata.  Stomata open and close.  At night they close with the benefit of avoiding unnecessary loss of water when photosynthesis cannot occur.  But when plant tissue is stressed from lack of water, these stomata also close, limiting the water loss but also interfering with uptake of carbon dioxide for photosynthesis.  C3 photosynthesis doesn’t make carbohydrates out of all the CO2 it absorbs, using some of it in other molecules.  No problem when environment provides plenty of moisture, is generally cool and have long summer days, but some plant species that evolved under hot dry conditions evolved systems to overcome that limitation.
 
Teosinte, the species of origin for corn in Central America, has a C4 photosynthesis system.  Plants with this character have additional structures in their leaves surrounding cells that perform photosynthesis. These cells function to reduce the loss of CO2 by causing these molecules to be recycled into more carbohydrates. The combination of extra enzymes and structures comes at some energy cost but the net gain is both more net carbohydrate and better utilization of CO2, even if stomata are closed. 
 
Fortunately, corn that was moved out of the original dry hot environment, kept that C4 photosynthesis system. Along with that came the C4 photosynthesis advantages and its superior production of carbohydrates.  Sorghum and sugar cane also are C4 plants but wheat, rice and soybeans are C3 and will not be able to match corn in carbohydrates per acre because of this trait. Although only about 3% of all plant species are C4, it does occur in a few plants in many plant families, suggesting that it can evolve independently. Researchers of other crops, such as rice, are trying to use genetic engineering methods to develop C4 photosynthesis, but it is not an easy task.
 

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!