​Stalk rot of corn occurs when sufficient root tissues dies of starvation. This happens when its energy source, carbohydrates stored in the stalk, are depleted because of excessive draw to the grain versus the supply from leaves and stalk. Size of daily movement to grain is determined by genetics and environments.  Soil moisture during time of silk extension is significant to more ovules being pollinated.  Supply of carbohydrates during the season is determined by multiple factors:  Hours and days of intense light, utilized by it C4 photosynthesis method to supply energy for more plant growth, storage of carbs in the stalk tissue as well as sugars to move to the grain after pollination.  Individual plant environments such as competing with adjacent plants for light and mineral uptake can influence success in normal completion of transport of sugars to all pollinated ovules on each plant.
 
Seasons with extreme late planting dates in temperate zones have extra factors.  Some fields will be fewer stresses in early season, resulting in less ovules pollinated, resulting in less grain but less stalk rot.  Late planting dates still requires about 55 days after pollination to complete movement of sugars to the grain.  It can be stopped with freezing the phloem within the stem tissue, with the result of light grain weight.  Such a freeze may not result in stalk rot as remaining stalk tissue may remain intact.
 
If the below freezing temperatures were not sufficient to kill the stem phloem but did result in leaf death, depletion of the sugars in the stalk are intensified.  This can increase death of stalk pith tissue, allowing stalk rotting fungi to digest the cells and weakening the stalk strength.  
 
We should expect variable field results in Midwest USA in 2019.

​Severity of below freezing temperatures while a corn plant is moving sugars to the grain is dependent on what tissue freezes.  Leaves are most vulnerable because of exposure to the cold air.  Ice crystals form in the leaf cells, killing the individual cells including those with chloroplasts.  Among these chloroplasts are those in guard cells of the stomata. Without photosynthesis the day after freezing, the guard cells do not open to allow transpiration.
 
Water movement from the root tissue to the upper plant is a physical phenomenon in which each molecule evaporating through open leaf guard cells, is replaced by a molecule of water because of water’s molecular structure causes bonding with each other.  Water is essentially pulled from the roots through the xylem structures of the vascular system because of this bonding.  If water is not utilized or transpired, leaves do not receive new supplies of water, further causing wilting of the leaves after frost or a more severe freeze.  
 
Sugars are transported through living phloem cells in vascular tissue.  During grain fill, sugars are drawn from the leaves and the stored reserves in the stem.  Death of leaves eliminates movement from the leaves.  If the low temperatures were not severe enough to kill the phloem and other stem cells, movement from stem to grain continues. 
 
Below freezing temperatures during grain fill will cause some loss of expected grain weight, but the severity will depend upon whether stem phloem is killed and the supply of sugar reserves in the stalk.

​As if the stresses that reduce photosynthesis isn’t enough to offset the balance of movement of sugars during grain fill, the 2019 USA Midwest had extreme water in the early season, even after corn was planted.  Sugar movement, stimulated by hormones produced by meristem tips, goes to apical meristem of shoot prior to flowering and to root tips.  After pollination the concentration of apical meristems in the ear redirects the flow towards the grain, competing with the flow towards the root tips.
 
If root growth was inhibited by extreme moisture in the soil, perhaps by low oxygen supply, does it result in fewer root tip meristems?  If so, does this reduce its capacity to attract sugars during the season and does this increase the vulnerability to the root pathogens.  This would result in increasing probability of the plant wilting during grain fill as well.  Stalk rot follows after plants wilt.
 
Deterioration of stalk quality follows plant wilting during the grain fill period. Wilting is caused by root tissue unable to absorb enough water to be transported to leaves.  Loss of water from leaves occurs through leaf stomata via evaporation.  Dry, windy environment around leaves causes more rapid transpiration.  Early season environment affects root growth and mid-season environment affects the size of grain sink.  Genetics determine how the plant reacts to these environments.  These multiple factors determine whether the grain successfully completes normal grain fill on plants with green stalks or not. 

Individual corn plants that have brown lower stalks are invaded with several fungi that can be identified in the dead stalk tissue.  Some, such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium verticilloides, become obvious in the deteriorating stalk tissue and therefore the naming of the stalk rots as Diplodia, Gibberella, Fusarium and Anthracnose. Although these are the most frequent and most easily identified fungi found in those plants with rotted stalks, the underlying cause of the plant death involved more complicated biology of carbohydrates to the root tissue as the plant moves sugars to the grain.  If the movement to grain is too great for the supply from the leaves and stored carbs in the stalk tissue, roots suffer without sufficient energy to meet the root metabolism needs.  Eventually deteriorating root tissue succumbs to destruction by soil microbes, resulting with the plant wilting.  This allow many fungal species to advance into the dying and dead stalk tissue, destroying the structural strength of the stalk.
 
Beyond naming the dominant fungus present in the dead stalk, it is important to identify the cause of insufficient supply of carbohydrates.  Potential causes are shading from other plants, leaf disease destroying leaf tissue, insufficient sunlight, leaf removal from corn borer, or hail damage.
 
Stalk rot of corn is a problem in some fields somewhere each year. Complexity of environments and genetics makes conclusions of causes very difficult. Predicting whether this year’s performance is likely to reoccur is not easy.
 
More perspectives on corn stalk rot can be found by using the search on this page in Corn Journal.  It is a subject that I (and others) have studied for a long time.

​That wilted corn plant, often surrounded by green plants with carbohydrates still being transported to the grain, did not have sufficient supply of carbohydrates available to meet the draw to the ear.  This may have been due to reduced photosynthesis because of leaf tissue destruction from leaf disease or hail, shading from adjacent plants, insufficient potassium available or dark cloudy days. 
 
Movement of carbs to the grain is directed by hormones produced in growing points, each embryo having an apical meristem producing auxins to direct the flow.  Genetics affects the amount per day and number of meristems affects the total flow per day.  If daily photosynthesis in leaves is insufficient to meet this demand, reserves stored in the corn stalk are drawn upon.  Sugars that are being stored in grain are also required to maintain life in the root cells.  Depletion of stalk sugar reserves available to roots, eventually weakening those cells ability to resist invasion by soil microbes. 
 
Eventually, root rot reduces water uptake and transport that water availability to transpiring leaves causes desiccation of all plant tissue.  All leaves on this plant show the wilting symptom by a gray appearance and pointing downwards.  Wilting also causes the pith tissue, previously attached to the inner layer of the rind tissue to shrink away from that attachment.  Stalk cells die.  Chloroplasts in the outer rind cells die, resulting in the lower stalk tissue turn from a green color to yellow-green and eventually brown, while adjacent plants continue to have a green color.
 
Abscission layers form at base of all leaf tissues immediately after tissue wilting.  That includes the formation of black layer at base of each kernel.  Consequently, these kernels will not have as complete grain fill as the adjacent green plants that continue to receive the flow for the 55 days after pollination.  If the major cause of the early wilt was from producing more kernels than the adjacent green plants, the total grain weight on the wilted plant may not be much different although the weight per kernel will be less.
 
Cause and effect of wilting corn plants is dynamic with multiple interactions between the corn biology and environments. Assessment of these potential causes when they occur could be useful in preventing significant yield and harvest problems in the future. 

​As the corn plants approach completion of grain fill about 40-50 days after pollination, some individual pants will change in color.  These changes can indicate the effect of the season on the plants.
 
Plants in which all leaves are gray, then brown, pointing towards the ground, have wilted because the roots could not provide sufficient water to meet transpiration needs.  These have root rot caused by lack of sufficient sugars moved from the leaves, allowing soil microbes to invade and destroy the root tissue.  Movement of limited supply of sugar moved to the grain was preferred over movement to roots.  That individual plant did not produce sufficient photosynthesis to maintain both the root life and grain fill.  Such plants will develop hollow stalks and be invaded with fungi.   These plants are likely to lodge by harvest time.
 
Some individual plants may turn red gradually during this time period.  These plants usually have only a few kernels.  Sugars accumulate in the leaves because not enough hormone driven transport is causing the sugars to be moved, or perhaps because of interference of movement because of insect damage to the vascular system. Chemical processes in the cells with accumulating sugars transpose the sugar molecules to anthocyanin, providing the red color in the leaves. This is usually an indication of poor pollination, perhaps because this individual plant emerged late as a seedling and therefore missed much of the pollen from other plants.
 
Nitrogen deficiency is indicated when several plants have yellow lower leaves while upper leaves remain green.  This is common in areas of fields that were water-logged.
 
Desired color of maturing plants indicating a successful yield has white ear husks while other leaves remain green and turgid all the way to completion of grain fill, about 55 days after pollination.  The occasional wilted plant can be a sign that the field got maximum grain allowed by that season’s environment.

Corn in the Midwest USA is approaching the time of season in which scattered plants show whole plant death.  It does cause confusion as to if it involves an aggressive pathogen or is it more of a physiological nature.  Diagnosis is further complicated by presence of fungi such as Fusarium species or Colletotrichum graminicola, the cause of anthracnose.
 
If the symptoms include all leaves wilting and stalk color becoming yellow green then brown, the problem is caused by roots not being able to supply sufficient water to leaves to remain turgid.  Resulting wilt results in all physiological activity in plant to stop.  Abscission layers (black layer) develop at base of each kernel.  It is tempting to call the disease by the fungi found in the dead tissue but the real problem is rotting roots.
 
Roots become susceptible to the many microbes in the soil as sugars from the leaf tissue after pollination is insufficient for the root cells to produce the metabolites needed to ward off the microbe invasions.  As more root tissue is destroyed, uptake and transport of water declines.  If the transport to leaves is insufficient to meet the loss of water via transpiration, the leaves wilt.  The visibility of this wilting process occurs relatively sudden.  Close observation may show a slight discoloration one day and complete wilt the next day.
 
The individual plant that wilts was probably determined by size of root during season, supply of carbohydrates after pollination and volume of carbohydrates moved to the grain.  Each of these involve multiple factors. Plant density, plant uniformity, light intensities, number of kernels, leaf diseases are among them.  Genetics and environments are obviously interacting.  It is better to consider those variables than simply associate it with a disease. 

​As the flow of carbohydrates to the developing grain intensifies, cells in leaves begin senescence.  Along with these changes in cellular metabolism it is probable that production of the chemicals associated with limiting microorganisms with potentially destroying cells as they feed on carbohydrates and protein contents.  The nature of the battle between the living plant material and surrounding micro-organisms.
 
Vigorous cells of corn plants before pollination were only overcome by microbes with specific metabolites that overcome the resistance mechanisms. Senescing cells are overcome by others that are no longer inhibited.  Definition of pathogens becomes more ambiguous as these organisms attack the tissue in senescing tissue.  Some fungi such as Fusarium species may have been present in the plant for much of the season, but not destroying much tissue until natural senescence begins.  At what point do we call it a pathogen and not just a saprophyte?
 

Sampling of aspects of crop agriculture is difficult and care must to be used to draw conclusions from results of tests of samples.  Biology of the plant and varying environments affect the predictability of the sample’s test result.  Nearly everyone participating in agriculture realizes this problem within a short time of exposure, although it is not always expressed.
 
Seed producers are aware that each seed within the production field did not have exactly the same environment and that each seed can be potentially with a different parent and environmental interaction during and after the growing season. Seed producers attempt to use field and facility methods to limit potential problems that could eventually affect performance of the hybrid in grower’s fields.   After using these efforts, the next challenge is to predict the success of these efforts to have good purity and germination. 
 
Sampling of seed usually begins after ears are dried and shells.  Methods are used to take general bulk sample by some randomizing technique. This bulk is the sized that includes those that represent different portions of the ear, the rounds tending to be at both ends and the flats in the center. This essentially is allowing checking seed with differing pollination dates that could affect purity. Shapes of the kernels also potentially affect germination viability.
 
Seed sizes are submitted to purity and germination test often before final bagging procedures have begun.  Number of seed included in sub-sample to be tested varies by testing method. Effectiveness of the test in predicting the eventual seed effect on field performance is dependent upon the sampling accuracy, sample size and testing accuracy in evaluation.  
 
Even if the initial sampling of the seed lot is done with care, there remains a randomness factor with test sample size. The percentage of seed germinating in a lot, or percent of outcross plants actually in the lot determined by the test is affected by the test size.  As summarized in https://www.statisticshowto.datasciencecentral.com/probability-and-statistics/find-sample-size,
a germination or purity test of 100 seeds showing 100% has a 95% probability of actually being between 96-100% where as if the test size was 400 seeds showing 100% purity or germination, the actual has 95% probability of being 99-100%.  If test result showed 96%. on a 100 seed test, the actual has a 95% probability of actually being between 90-99% where as a 400 seed test is probably between 94 and 98%.

Sampling of seed lots and testing methods including number of seed tested affect the accuracy of predicting the actual germination and purity of a seed lot.
 

​Obtaining reliable predictions of percentage of occurrence of any biological feature within a population is extremely difficult.  Hybrid seed corn in which two parent inbreds, rarely perfectly homozygous for all genetics, needs to be evaluated for potential problems with purity problems due to contamination within the parent seed or outside pollen fertilizing the ovules. 
 
Seed producers use all reasonable approaches to limit these possibilities but environments within the seed field can affect the purity as well.  Extreme dry areas can delay silk emergence but rarely delay pollen production by the male inbred.  Consequently, female silks remain viable for potential fertilization by pollen from hybrid fields.  Such outside pollen can be genetically segregating, resulting in genetics varying from the correct hybrid, but with each of the resulting plants different from the correct hybrid and different from each other.  Corn pollen can remain viable while carried by wind for at least a mile.  Lack of timely distribution of correct male inbred pollen, increases the potential contamination by foreign corn pollen.
 
Stressed plants in a hybrid production seed field also may cause delayed tassel production leading to the possibility of missing a few plants from having tassels removed from the female inbred parent. This can lead to self-pollination of the female parent resulting in inbreds within the hybrid seed corn.
 
Hybrid seed corn producers are well aware of these potential problems and use multiple methods to avoid purity problems.  Despite their field management and care, there are circumstances that are difficult to overcome.  Consequently, checking the purity and germination of the resulting seed needs to be done after the seed is harvested.
 
Each kernel of seed corn can be distinct in origin.  Those at the base of the female parent ear were probably fertilized a few days earlier than those at the tip.  It is possible that the source of pollen could be different simply because of timing conditions at that location of the field.  Seed producers are aware of these possibilities and significant problems to hybrid corn performance are rarely released to sales.  Testing for purity of the hybrid seed sizes allows the eventual discard of any highly contaminated seed sizes from those being sold.  
 
Seed companies give considerable effort to produce and sell pure hybrid seed. These are tasks easily overlooked as one views uniform hybrid corn fields from the roadway.

​A few corn plants on the edges of fields are showing the deformed tassels as predicted with the very wet spring.  Corn Journal summarized the main factors of standing water relationship with this disease in the issue dated 6/18/19 and 5/30/17 blogs.  It is always somewhat surprising because the disease symptoms are most evident after tassels develop but the infection occurred a few months previous to symptom development.  Here is a copy of the 6/18/19 blog concerning this disease.

​Excessive rain in much of the USA Corn Belt in 2019 with ponds of water in areas of fields can encourage infection by a fungus that has swimming spores.  Corn Journal blog of 05/30/17 may be appropriate for this year as well.
 
One of the effects can be infection by an organism called Scleropthora macrospora.  This is a fungus-like organism belonging to a group of organisms called Oomycetes. Also, in this group are pathogens causing Downy Mildew and Pythium diseases of corn and other plants.  Common among these are the ability to form thick walled spores to withstand stress environments that can release swimming spores when in water-saturated soil.  S. macrospora infects more than 140 grass species in addition to corn.  
 
The source of infection of corn is often grasses near a low spot or edge of a field.  Oospores in the flooded living and dead leaves release swimming spores (zoospores) when close to the corn submerged leaf tissue these zoospores release a germ tube that infects the plant. The filaments (hyphae) grow towards the meristems throughout the life of the plant.  This can initially be seen as fine stripes in the leaves, but the most obvious symptom is proliferation of leafy aberrations of the tassel- the crazy top symptom.  Scleropthora macrospora also can grow to the ear bud meristem, causing similar multiple ears from a single node- but no grain.
 
Related oomycetes occurring in warmer, subtropical and tropical environments can cause similar symptoms.  These downy mildew diseases can also cause the proliferation of the tassels and ears. Susceptible genotypes can have severe grain loss from these diseases.  Scleropthora macrospora infection is usually limited to a very small area near grass in a low part of the field.
​
Infection occurs when the plants have less than 6 leaves.  Symptoms that show late in the season, but the problem began with excessive rain that occurred only a few weeks after planting.  That early moisture that may contribute to large yields can allow this pathogen to form these unusual corn structures in a few spots of the field.  In addition, it is just part of the interesting biology of corn.

​Individual plants in corn fields are starting to show red leaves as the season approaches the final days of grain fill.  Close observation of these plants reveals that they have very few kernels, generally because of poor fertilization.  These plants probably were silking only after most pollen in the field was gone, perhaps because of late emerging seedlings or other causes of late silk emergence such as moisture stress.
 
Red pigments are caused by production of a flavonoid known as anthocyanin.  Many plant species produce anthocyanins, especially in reaction to stresses such as low temperatures, diseases or insect damage. Anthocyanin compounds accumulate in cells as water soluble compounds in cell vacuoles. It is derived from glucose in a synthesis and is linked to accumulation of glucose within the cells. It has the effect of absorbing light with effect of reducing photosynthesis.  
 
It is not completely clear the advantage to the corn plant to reduce glucose production by absorbing less light for photosynthesis.  Perhaps it reduces the callose development in phloem tissue that could reduce the flow of glucose to the few developing kernels.  It is clear that it is related to accumulation of glucose in the leaf tissue because of reduction of transport to grain.

​Corn has benefited from human’s selection process annually for the past 10000 years.  This has occurred over multiple environments with preference towards stability of desired characteristics of the grain. This usually led towards increase in grain storage of starch.  With the realization of value of hybrids between parents from unique heritage, the combination of those genetics added to the greater grain yield.  Combinations of 30000 genes from both parents creates a stability in multiple environments.
 
Genetics affecting root size and direction, essential to water and mineral uptake for the plant is also influenced by genetics affecting efficiency of transport of carbohydrates from the leaves to supply energy for the growth.  Volume of carbohydrates produced in leaves is influenced by multiple genes affecting leaf size and intracellular dynamics.  Even resistance to most leaf diseases involves 3-4 genes directly limiting the pathogen.
 
Genetics affect the timing of the production of pollen and emergence of female stigmata (silk).  Genes contribute to movement of water to the ovules for extension of the silk from the leaves surrounding the ear shoot.  Number of ovules, potential size of endosperms, quantity and strength of hormones causing the flow of carbohydrates to the pollinated ovules is affected by genes.  
 
All of these genes are selected for stability under multiple environments, some with annual extremes of mineral, water and sunlight supplies.  Multiple genes contribute to stable performance of successful corn hybrids.  

​Annual plants such as corn undergo physiological changes after flowering, especially in corn that is genetically selected to maximize capture of products of photosynthates in the grain. Flow of carbohydrates within the plant are directed by hormones produced in meristems.  Before flowering that flow went to growing leaves and roots near meristems.  Excess carbs were stored in parenchyma cells in stalk tissues.  After flowering, hormones direct the flow towards the developing kernels.
 
Genetics and environments influence the intensity of the flow. Hybrids that tend to have more total starch in the ear either because of more kernels or larger kernels are favored by humans but risk early death of roots and leaf tissue that still require the energy provided by carbohydrates for cellular metabolism.  Environments that reduce optimum photosynthesis during the grain fill period accelerate the depletion of carbohydrate reserves stored in the stalk tissue.  In some hybrids, perhaps all, the depletion becomes most evident in the stalk tissue near the flag leaf, eventually resulting in an abscission layer to form at the base of the flag leaf, cutting off water to that leaf and eventual wilting of the leaf.  Fungi such as Colletotrichum graminicolaare able to invade the outer rind of that small stalk tissue with typical anthracnose symptoms.  This loss of productive photosynthetic tissue in the small leaf is insignificant and could be indicating good grain fill. Loss of significant root tissue is more important.
 
The challenge of the corn breeder is to select hybrids that have the balance of maximum grain production capturing all carbohydrates available without causing too much damage to needed life functions in the plant.  The challenge of the grower is to provide environments that maximize this possibility.

​It not easy to correlate all environmental and genetic factors to predict time between pollination and formation of the abscission layer (black layer) at base of kernels when the season is as abnormal as 2019 in the USA corn belt.  It is established that heat determines the differentiation of growing point into flowering structures instead of leaves in temperate corn hybrids.  We mostly classify relative corn maturities among hybrids either by days to black layer or by total heat units, somewhat assuming that the difference between total heat units to black layer and heat units to flowering is the heat units needed to fill the grain.
 
But are heat units as important for that fill period as it was for determining differentiation of apical meristem?  Photosynthesis rate is mostly constant and optimum when temperatures are between 72°F-82°F, slower at 55° and above 85° F, according to one study (Journal of Experimental Botany, Volume 28, Issue 3, June 1977, Pages 519-524).
 
General metabolism in the corn plant is affected by temperature but it generalized that the optimum is between 55° and 85° as calculated by the GDU’s.   However, does that apply to the metabolism directly involved in translocation of carbohydrates from leaves and stem to the grain?  Or is it simply about 55 days from pollination to end of grain fill even if cooler than usual for corn even when pollination was later in season in 2019>
 
Temperatures affecting grain fill during this late flowering seasons probably will give us new information concerning factors affecting black layer formation in corn.
 
 

​Prior to pollination corn plants are dependent upon roots to absorb minerals and water to be transferred to the developing above-ground growth.  Size and efficiency of roots of each plant are major in supporting the growth.  Genetics and environments are major factors in successful leaf and stalk development.  Photosynthates produced in the leaves are transported to the roots through living phloem cells to supply energy for growth of the roots and active uptake of minerals.  Living cells of root cells, including the root hairs that are active, uptake of mineral ions requires the energy to move them from a lower concentration in the soil to a higher concentration in the root cells. 
 
Water uptake from the soil is moved to the xylem cells, that are essentially non-living tubes, in which the water tension character allows water to be pulled into the root and up to the stem and leaves.  As a molecule of water either transpires through leaf stomata or used in new upper cell development, a molecule of water is pulled into the root.  Water absorption and transport to above plant parts is a physical process mostly carried out in dead cell tissue but is dependent upon continuation of the tubes.
 
Mineral transport into roots, and transport of energy for this process, is directed by plant hormones produced in meristems.  After pollination, the hormones produced in the seed embryos, direct sugar transport to the developing kernels.  As this redirection increases with growth of kernels, transport of energy to the root cells decreases.  Water transport system continues but mineral absorption from the soil is reduced. Root expansion stops.  One of the functions of the living cells is to ward off potential fungal invaders by actively producing anti-fungal chemicals.  As the energy for these cells is reduced, the resistance to microbes also is reduced, increasing vulnerability of xylem being plugged by fungal mycelium.
 
Now it becomes a race to the finish.  Did the plant produce and transport sufficient carbohydrate to root cells before pollination to sustain enough living functions in roots to avoid destruction by fungi?

​Every year, fields of corn are different from other years.  This year, in the USA, it is more different than usual. Most of it is related to extreme delays in planting time and some by soil moisture extremes.  After the first 10 days after pollination, kernels begin to absorb carbohydrates at about 2% of their final total per day.  This rate continues for the next 40 days, drawing on the new photosynthetic production plus reserves stored in the pith cells of the stalk.  Genetics and environments before and after pollination contribute to the amount transferred each day to the kernels.  The number of kernels on the plant affect the total being transferred. 
 
Uneven environments associated with late planting and extreme wet soils results in wide differences among individual plants, resulting in uneven numbers of pollinated ovules, and plant to plant differences in numbers of kernels per plant within a field.  Individual plants that emerged later than adjacent plants may have same number of kernels as adjacent plants but will not have same photosynthesis rate because of shading from adjacent plants.  Competition for uptake of minerals will probably also be inferior.  Hybrids that have high number of pollinated ovules, despite this early stress, may have a larger draw of carbohydrates to the ear but the delayed emergence plants of these hybrids may deplete supply stored in stalks, developing early death of roots, leading to stalk rot.
 
2019 corn production in the USA will be different.  Biology is the same, the environment is different.

​Energy for corn growth comes from carbohydrates soon after seedling emergence. Multiple physiological, structural and environmental factors affect the production of those carbohydrates during the development of each mature plant.  Size and directions of the leaves affects penetration of light into the canopy. Minerals affect the conversion of light energy into chemical energy. After pollination these factors influence ability of the plant to produce sufficient carbohydrates to each developing kernel with minimal transfer of carbohydrates stored in the stem tissue.  This Corn Journal blog of August 16, 2016 addresses the relationship between light intensity and photosynthesis in corn.
 
Our eyes don’t allow us to be aware of differences in light intensity, but the photosynthesis rate in corn leaves is very sensitive to this factor.  Having a C4 photosynthesis system, the rate of photosynthesis in an exposed chloroplast increases directly with light intensity up to full sunlight.  C3 plants, such as soybeans, can only use 3/10ths of full sunlight hitting the leaves.  Several years ago, I carried a light meter to measure foot candles in many corn environments to help me understand light intensity differences.  Measuring in foot candles is not quite as good as PAR (Photosynthetically Active Radiation) measurements as done with more recent instruments but it did demonstrate to me the differences in light intensity within corn environments.
 
Full, unobstructed sunlight has intensity of 10000 fc (foot- candles) when measured in Ohio, Illinois and in Colorado.  However, the measurement in the shadow of a single corn leaf is only 1000 fc.  That portion of the leaf in the shade of the single leaf is only producing 1/10th the carbohydrate as the same area of the leaf fully exposed to direct sunlight.  The lowest leaf in a canopy receives only about 50fc, hardly enough to sustain its own tissue and is one of the reasons that these leaves disintegrate.  As shown in the graph above, cloudy days also reduce the photosynthetic rate in corn plants.

It is a precarious balance between sugar demands established in the ear after pollination and factors affecting photosynthesis in fields.  Growers are in control of some factors such as hybrid selection, fertilizers and plant density but others such as rain affecting kernel numbers, or leaf diseases potentially removing effective leaf area and cloudy weather can drastically affect whether the corn plant stays alive until the completion of grainfill.
 

The biology of the corn plant after the initial ten days of movement of glucose to the developing kernels is consistent with nearly all hybrids.  This Corn Journal blog of August 2016 applies even in the USA Midwest wild summer of 2019.

​As the corn embryo develops, and the cytokinins accumulate in the pollinated ovule after the first 10 days, there is a constant translocation of sugars to each kernel for each day.  The total daily movement continues for about the next 40 days, almost regardless of daily variable rates of photosynthesis due to cloudy weather or leaf damage from disease.  Sugars are drawn from all leaves and even those stored in the stalk pith tissue.  The total draw to the ear is determined by genetics of the variety, environmental factors including minerals and the number of kernels.  The number of kernels is also determined by genetics and environment factors such as minerals and especially water available during ovule formation and pollination. 
 
The daily transfer of sugars during days 50 to 60 of grain fill is greatly reduced until the abscisic acid affect causes thick cell walls to form at the base of the kernel, cutting off the sugars transfer into the kernel and the movement of water from the kernel.  This is known as the black layer.
 
Sugars translocated to the ear are sugars not available to other living tissue in the plant.  Roots are especially dependent on the same sugars to support metabolism functions, including warding off the potential microbe invaders.  Starving roots, as they rot, eventually reduce water uptake and, if insufficient water to meet the transpiration rates from leaves, a permanent wilt will occur.  With the wilt, movement of sugars to the kernels is stopped, abscisic acid takes over, causing the black layer to form a base of kernel.  The consequence is light grain weight on the affected ear.

​Two major hormones in corn are cytokinins and auxins.  Cytokinins affect cell division and auxins affect cell elongation.  Cytokinins are produced primarily in root tip meristems and transported via xylem to other meristems such as those developing in each pollinated ovule.  As these embryo meristem cells divide, the attraction of cytokinins increases.  Concentration of cytokinins in these meristems also affects translocation of glucose molecules to each developing embryo, as this carbohydrate moves through the phloem from leaves and stem pith tissue to the new cells.  Excessive stress affecting water for xylem transport, or, reducing sugar production can reduce the constant flow of cytokinins to developing kernels.  If this occurs during the first 10 days after pollination, another hormone, abscisic acid (ABA) accumulates at the base of the ovule.  This hormone causes development of thick-walled cells, blocking transport of cytokinins and sugar into the kernel.  As a result, the kernel does not develop further. 
 
Genetics and environment have a great effect on the balance of hormones during corn grain development.