​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.