If a corn plant draws more carbohydrates to the ear during those critical 60 days after pollination than it can supply with current photosynthesis and storage in the stalk, it depletes the supply needed to keep root cell’s metabolism. The deterioration of root cell metabolism allows invasion of organisms in the soil and inability of the roots to transport water to the plant parts above the soil. Transpiration from the leaves continues until all available water is gone. Then the plant wilts. Symptoms of the wilt become slightly visible for a few days before all leaves turn gray and droop. We call this premature death. These wilted plants occur as individuals, often surrounded by green plants that did not overdraw on its carbohydrate supply to fill its kernels. Often these individual wilted plants have more kernels than those adjacent green plants or have excess photosynthetic stresses.
The wilted plants initially have green outer rind to the lower stalk, but the pith tissue inside the stalk has puled away from the outer rind, as part of the wilting process. This weakens the stalk strength by one third. The outer rind slowly turns from a green color to yellow and shows symptoms of invasion of fungi as they digest the remaining cellulose and proteins in the stalk cells, further weakening the stalk strength.
The critical stresses leading to stalk rot occur during the kernel fill period, those 60 days after pollination. If a plant makes it through that time without wilting, it probably will not get premature root ands stalk rot by normal harvest time. Inspection of corn plants about 60 days after pollination allows the grower to access the to predict probability of stalk rot and lodging in the field that season. Individual gray plants with yellow and brown color to outer rind of the lowest 2-3 above-ground nodes are most likely to lodge with slight wind pressure. These plants easily fall with slight pressure. One can also easily pull the plants up from the soil as the dead roots offer little resistance.
Why did that plant commit more to kernel fill that its photosynthesis could supply? Was it shaded by adjacent plants because of inadequate spacing of seed, did it have excess damage to leaves from pathogens or insects. Did the environment of the roots cause less root mass or destruction from pathogens or insects? Perhaps the genetics of the hybrid encouraged a higher kernel number in that environment than could be supported under the photosynthetic stress of the season.
Having a few early wilted plants in a field can be a sign that the hybrid maximized its ability to produce grain yield for that season’s environment. One can learn a lot with inspection of the field shortly after the 60-day kernel fill period.
Sugars are transported to each kernel for days 10-50 after pollination if most corn plants if field conditions are favorable. Then the transport slows for another 10 days.
Each kernel in a corn ear is a fruit. As with most other fruits, sugars are transported to the kernel through the vascular system from the leaves and the stem. Plant hormones like auxins and gibberellins produced in the seed embryo meristems actively guide the sugars to the seed within the fruit. Corn kernels have only one seed. Although much of the sugar is moved outside of the embryo to the endosperm, sugar also provides energy for growth and development of the embryo. As the embryo matures, auxin production is reduced. Consequently, physiological demand for sucrose supply to the kernel is reduced.
Reduction of sucrose in the cells at the base of the kernel causes the balance of ethylene and auxin to change. Ethylene increase causes the layer of parenchyma cells closest to the kernel base to lose cell wall contents, while those adjacent cells away from the kernel gain wall thickness. Eventually an abscission layer forms cutting off all movement of sugar into the kernel and water movement away from the kernel thru the stem tissue (the cob).
Research by J.J. Afuakwa, Crookston and R.J Jones (Crop. Sci. 24. 285-288) showed that reduction of sucrose available to the kernel was a major factor in induction of the abscission layer (black layer). Although it is mostly related to maturation of the embryo, it could be induced by other factors reducing sucrose supply to kernels. Reduction of photosynthesis by leaf disease or frost damaged leaves could result in shortened time to black layer. Early plant death, perhaps from root rot, causing leaves to wilt and thus removing sugar supplies induce black layer within a few days.
Plants with green leaves 60 days after pollination now will undergo slow maturation with abscission layers forming at the base of each leaf. Those abscission layers cut off the transport of sugars from the leaves and the transport of water to the leaves and removal of water from the plant via transpiration. Reduced competition with kernels for sugars stored in the stalk pith tissue allows roots to slowly age.
Plants that did not manage to make it the full 60 days with green leaves likely had photosynthetic stress reducing the ability to meet the demand by kernels for sugars. As a result, root tissue on that plant lost its ability to adequately battle the multiple potential pathogens. Loss of living root tissue during the kernel fill period results in less water uptake and consequently early wilting of the entire corn plant. Kernel filling stops at that time.
Competition for carbohydrates between kernels in developing ear and plant metabolism sites especially in the roots of a corn plant is established 10 days after pollination. This is a battle being fought individually by each plant in a corn field, as each plant can have slightly different environmental factors influencing its ability to produce carbohydrates and different numbers of kernels.
Net supply of carbs is influenced by environments, like light intensity, in which the rate of photosynthesis drops directly with less light. Cloudy days result in lass carbs produced. Leaf disease, reducing photosynthesis due to less leaf area, as does shade from adjacent plants. We have selected genetics programed to move carbs to non-photosynthesizing tissue like the roots with excess stored in stalk. During the grain fill time of the corn plant, newly produced carbs are transported to the kernels as well as those stored in the stalk pith tissue. The draw to kernels is constant, influenced by genetics and minerals, regardless of daily photosynthesis factors. Competition between kernel development and root cell metabolism for carbohydrates reserves stored in the stalk tissue becomes more intense if daily photosynthesis is reduced during the 40-day period of maximum draw by developing kernels.
Energy supplied by the carbohydrates pulled to the root tissue is used in its cells’ metabolism for normal function including producing the compounds needed to ward off the multiple organisms in the soil attempting to devour the root tissue. Defense of the living, functioning root tissue is essential to the rest of the corn plant, as the minerals and water absorbed by roots and transferred upwards are essential to function.
It is a battle essentially between roots and kernels for carbs that is fought by each individual plant in a corn plant especially between day 10 and day 50 after pollination. If carb supply is not sufficient to meet both needs, kernels win the race. The roots degenerate prematurely and are unable to supply the water to leaves to match the loss of leaf water due to transpiration. As a result, the plant wilts. Kernels’ win is temporary, as the wilted plant no longer can transport carbs to kernels.
Photosynthesis is the engine driving corn from the seedling to maturity. Previous generation's photosynthesis provided the energy for the seedling to emerge from the soil. Current generation photosynthesis provides more than sufficient energy for the metabolism to build tissue to construct a plant with expansive roots, large leaves, and 6-9 feet of stalk within a few months of seedling emergence. The corn plant not only provides the energy for the building materials for its new structures and daily metabolism but also has excess carbohydrates that are stored in the stalk pith cells.
Then, at midseason, it produces flowers. After pollination for the female flowers, hormones in the plant shifts the direction of flow of the carbohydrates from leaves and stalk pith cells to the the new embryos and its storage compartment, the endosperm. Environment and genetics determine the rate of flow of carbohydrates to the new fruit of the corn plant. The number of fruits (kernels) also becomes a big influence on the total draw from the carbohydrate supply.
Flow for first 10 days after pollination is relatively slow but then it speeds to a faster, relentless pace for the next 40 days. That daily pace of movement of carbs to the ear on the plant continues regardless of daily variation in photosynthetic rates due to environmental variables. If cloudy weather slows photosynthesis, the reserves in the stalk provide the difference. If leave disease reduces leaf area available to light energy conversion to carbohydrates, storage from early days is called upon for movement to the new kernels.
This high rate of flow of carbs to the new kernels slows after day 50 for about 10 days until it is cut off with the formation of an abscission layer at the base of each kernel. If the other plant parts such as the root tissue survived the competition for stored carbohydrates in the stalk, slow senescence occurs in the plant cells. This is the outcome desired by all corn growers but environmental stresses can interfere with the completion of maximum deposit of carbs in the kernel on living plants.
About 5-6 weeks after pollination is a good time to evaluate corn hybrids for resistance to leaf disease. This Corn Journal blog from 2017 discusses some of the dynamics in resistance to leaf diseases.
Leaf epidermal cell’s walls and the waxy leaf surface provide the first line of defense against microbes. Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf. This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader. Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth. Much initial reaction is limited to host cells adjacent to the infection site.
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight)
Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen. These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another. On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
The system described above is referred to as general or horizontal resistance. It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen. Horizontal resistance is expressed in corn plants by fewer leaf disease lesions. Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure. Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.
Every growing season, every field, every hybrid has some difference in dynamics affecting corn leaf diseases. The primary process, however, remains the same. Fertilization of the of the embryos in the ear induces physiological changes in the leaves as movement of carbohydrates emphasizes the developing kernels.
Annual plants such as maize are genetically programmed to salvage nutrients from leaves after pollination, resulting in senescence of the leaves. One study (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0115617) maintained that it begins in the ear leaf of corn as early as 14 days after pollination. These authors found at least 4500 genes involved in the biosynthesis of the senescence process as degradation of leaf cell components and new proteins were made, resulting in the regulated movement of nutrients and sugars from the leaf to the developing ear.
This phenomenon is not unlike the reabsorption of leaf nutrients that occurs in many plants species and is being expressed in the Northern Hemisphere currently in deciduous trees, as the senescing leaves lose nutrients and chlorophyll, eventually abscising from the branch. The similar process in corn begins with the degradation and removal of leaf cellular contents, transportation to the fruit (grain) and eventual development of thickened cells at the base of the leaf, cutting off all movement of water and nutrients in and out of the leaf.
Varieties of corn surely differ in the timing and effectiveness of this senescence process. These differences may be reflected in leaf cellular activity such as leaf disease resistance or drought stress. The modified leaves composing the ear husk undergo the same senescing process, eventually effecting the opening of the husk needed for the evaporative drying of pre-harvest grain. Eventually, the senescing leaves are cut off from the rest of the plant by the development of thick walls in the parenchyma cells at the base of the leaf, similar to the abscission layer known as the black layer at the base of a mature kernel. The cutting off of the water into the leaf after the abscission layer is evidenced by the sagging of the leaf.
Leaf disease in a particular field may be different than last season but will be similar in some other field but the dynamics will remain.
The move from a wild weed Teosinte to a productive grain crop that we know as corn was the result of humans observing and selecting plants in the field that best fit the desires of the grower. The breadth of genetics available was largely due to the separation of male and female flowers on the corn plant, encouraging cross pollination, and humans’ movement of the species to many environments modern corn has nearly 40000 genes located on its 10 pairs of chromosomes.
Corn has attracted the interest of plant scientists with many specialties such as plant pathology, entomology and cellular biology. Some biologists were interested in trying to isolate corn cells from the plant into cultures of the cells that could be replicated, experimented with and then finally manipulated to produce normal plants. Techniques were discovered with some plant species in the 1950’s but some species including corn were difficult to go from cultured cells to mature plant. In the early 1970’s, the combination the right mix of plant hormones and corn varieties, allowed the stimulation of cultured corn cells to produce mature plants. This opened the way for manipulating corn cells in culture to produce unique plant characteristics.
Entomologists, and growers, were finding the more intense cropping of corn was allowing insects to damage the crop. Among those was the corn borer, a moth that produced larvae caterpillars capable of entering the corn stalk and ultimately causing the stalk to break, making the machine harvest difficult. A similar insect causes damage to other plants such as tomatoes. People had discovered that spraying a culture of a bacterium called Bacillus thuringiensis on tomato plants killed the caterpillars on tomatoes. This bacterium from the soil had genes that produced a protein that disrupted the gut of the insect larvae.
Geneticists were able to isolate the gene in this bacterium, giving the label of BT, and set off on the task of getting the gene into corn cells. Eventually a modified gun was used to fire a pure culture of the BT gene into a corn cell culture, inserted the gene into the corn chromosome. After screening in labs for the insertion in right position in a chromosome, the cell produced the protein toxic to the corn borer insect. Stimulation of that cell to grow into mature plants allowed it to be crossed to more agronomically productive corn varieties.
Combinations of scientists with varied specialties, commercial and public, and corn growers has allowed the current use of genes transferred from other species into corn. Probably genes were exchanged between species in the evolution of all living forms over time, but humans have found ways to accelerate and manipulate this for their use.
I have been thinking (I Know, that is dangerous!) about the long history of corn and all the generations of people that have participated in corn as we know it today. A lot of what we see today, adaptation of this species to all continents on our planet as well as individual field environments is due to its unique biology and interactions with growers researcher.
About 10000 years ago, someone or maybe several people came upon a mutant in a variety of Teosinte in which the seed (kernel) was larger than most Teosinte seed. Probably already using this species for food, grinding the seed into a flour. This mutant must have been attractive and worth propagating. Further enhancement was encouraged by the fact that this and surrounding plants had sufficient distance from male and female parts that cross fertilization was common, allowing further mixing of genes. Selection by those people in Southern Mexico continued to select plants with a combination of genes that had bigger and more kernels, as well as other plant features to support the kernels. Furthermore, these kernels could easily be transported by people passing through and carried further south and north, the genetic breadth and mutations allowing eventual selection of varieties adapted to the northern order of USA, through the tropics of South America to its most temperate environments.
When first European discovered the Americas, they found that native tribes had this plant species as a food crop. As each tribe had multiple years of maintaining and selections among genetic variants, there was already a large genetic pool that would eventually allow the adaptation to other continents.
Today’s growers are doing the same thing, selecting the corn hybrid that most fits their environment and the seed producers have the incentive to respond by searching for the genetic combinations best matching these environments. The broad genetic base of about 40000 genes spread across the corn’s 10 chromosomes allows for continual selection of the best fit.
Corn characteristics of its photosynthesis utilization of highest light intensity, separation of male and female flowers, annual maturity, efficient movement of carbohydrates to kernels supported with a broad genetic base has allowed this crop to become an international source of nutrition for humans.
Summer of 2021 features more rain variability in USA corn growing areas than usual. Leaf diseases such as northern leaf blight are greatly affected by the weather and consequently, so is the corn crop.
The disease is caused by the fungus Exserohilum turcicum (Setosphaeria turcica). The fungus lives in infected, dead or live leaves. It asexually produces spores (conidia) when the diseased tissue is moist and temperatures are proper for corn growth. The conidia have 4-6 cells arranged in a row and are light enough to be distributed by air currents within a corn field. After landing on a corn leaf, with a little moisture, in 3-6 hours the cells on both end of the conidia, begin dividing, emerging as germination tubes. These new hyphae quickly form a base on the surface called an appresorium, from which the fungus grows into the leaf epidermis. Within 12-18 hours after the conidia have landed on the leaf, it has successfully penetrated the leaf. There appears to be no difference in time to leaf penetration between the susceptible and resistant corn hybrids.
Chloroplasts near the infection point soon lose pigments as nearly 100 cells die, perhaps because of enzymatic activity of the fungus. This can be observed when small (0.5-1cm) circular, yellow spots show in the leaves in 24-48 hours after infection. From this initial location, the hyphae grow between cells towards the vascular bundles. Penetration of the vascular bundles is followed by plugging the xylem causing further death of surrounding cells dependent upon the water supplied through these tubes.
Resistance systems appear to begin after initial infection and perhaps mostly once the fungus has reached the vascular system. There does seem to be differences in that initial yellow spot in a couple days after infection and I wonder if the brighter color is not associated with greater quantitative resistance. Could part of quantitative resistance be self-destruction of cells near the infection point, depriving the fungus of nutrition? It does seem that initial spot is less obvious and possibly smaller in the more susceptible corn genotypes.
The wilted areas surrounding the plugged xylems eventually are depleted of living host cells. The fungus responds by producing new conidia within 14 days of the initial infection, ready to spread to more live leaves. Exserohilum turcicum remains viable in dry leaves for a number of years in dry environment, ready to produce conidia within 24 hours after moistened. Tillage, crop rotation and hybrid resistance become major factors in crop damage from this disease.
Annual plants such as corn, change physiology within a few months of the growing season. Corn has been bred to rapidly move maximum carbohydrates to the developing kernels. This has a profound affect on leaf disease resistance.
A few weeks after pollination, dynamics affecting resistance to leaf pathogens changes. Cytokinins are increasingly concentrated in the developing grain embryos, causing more translocation of sugars from leaf tissue to the ear, reducing availability for cellular metabolism in the leaf tissue. Leaves lower in the canopy, in the shadow of upper leaves have reduced photosynthetic rates due to receiving less than 5% of the light intensity as those exposed to full sunlight. Not having sufficient energy to maintain its cells, senescence of these leaves begins. Among those cell functions is the production of anti-pathogen biochemical that limit leaf pathogens.
Disease pressure increases in lower leaves with the higher humidity and longer dew periods that favor leaf pathogens. Cool, cloudy and wet weather in those 50 days of grain fill after pollination further favors the fungal leaf pathogens. This increased disease pressure on as leaf tissues occurs at the time in which they are losing the ability to react to invading organisms. Lowest leaves senesce first as the lower photosynthetic rate and increased disease kills tissue. Even weak pathogens, such as Fusariumspecies, invade the vascular tissue of such leaves causing the leaf to wilt, while the upper canopy leaves remain green and fully functional.
The senescence pattern progresses up the plant as it gets closer to meeting the active translocation period of 50 days after pollination. If there was exposure to pathogens such as Exserohilum turcicum, the cause of northern corn leaf blight, earlier in the season, the disease appears to move up the plant. This can cause difficulty in comparing resistance levels among hybrids varying in maturity such as in research plots. Those with earlier pollination dates may appear to be more susceptible, especially if the environment favored the disease, simply because the leaf senescence was more advanced than the later hybrids. This can be misleading not only in determining differences in innate resistance levels but also in predicting potential grain yield or increased stalk damage from the disease.
This corn growing season’s environment differs from previous ones, affecting physiology within the corn plants. Photosynthesis rates are affected by light intensity and nutrients, leaf areas and other factors such as disease. Timing of transformation of growing points to reproduction organs is affected by heat. Water affects the timing and number of ovules formed and success of pollination.
Heat is a major energy factor influencing the development of corn plants and the ultimate grain yield. Cellular respiration rates increase as temperatures go up. Photosynthesis rates also respond to increased heat as well. It seems reasonable to assume that practically every physiological function in the corn plant is affected by heat energy.
This includes the transformation of the apical meristem from producing leaf buds to production of the tassel. This happens in corn plants at about the V6 stage. Many, many years ago, I dissected young corn plants of hybrids of nearly all maturities sold by a major seed company looking for this change in the apical meristem. The change visible under a microscope, was nearly perfectly correlated with our final classification of the relative maturities of the hybrids. This is consistent with the view that the first influence of temperature on corn maturity occurs early in the season. It is probable that temperatures further affect further development of the differentiated apical cells into mature tassels. We attempt to express the daily temperatures that could affect the timing of pollination with averaging high and low daily temperatures but accurately depicting the duration of a high or a low temperature is difficult. We know that it does affect, but like much of growing crops, we know of the principles but not all the specifics.
Grain fill period seems mostly fixed to about 55 days but there are studies that show low night temperatures can extend the period to formation of the abscission layer, thus increasing grain yield (Elmore, R. 2010. Reduced 2010 Corn Yield Forecasts Reflect Warm Temperatures between Silking and Dent. Integrated Crop Management. Iowa State University, 9 Oct. 2010). It is likely that each hybrid differs in its reaction to temperature during this period.
Given the difficulty of accurately measuring the specifics of temperature interactions of corn plant morphological development, cellular function such as photosynthesis, respiration rates and translocation rate of sugars It is best that we simply compare
Among the features of corn that makes it a great carbohydrate crop is the C4 photosynthetic system that allows utilization of higher light intensity than C3 photosynthesis plants. We have further selected genetics that combine large kernel numbers with a strong draw of carbohydrates to each kernel, especially in days 10-50 after pollination. The combination of high response to light intensity, strong draw of carbohydrates to the ear and high numbers of plants per acre can result in maximum grain yield per unit of land area.
Of course, multiple factors interact for this to happen. Minerals and water to allow maximize plant structure and function and leaf area per land area are major. A balance in distribution of available carbohydrates to allow continual living root tissue as well as the transport of carbs to the kernels is essential especially during those 50 days.
And then there is importance of light to drive the photosynthesis. Smoke haze has been a notable feature of the 2021 summer in the western USA corn belt. This haze reduces the intensity of sunlight by scattering the light waves. This could have an affect beneficial to corn in that it may overcome the shadowing from leaves, allowing more light penetration to the lower leaves in the canopy. On the other hand, lower intensity of wave lengths in critical to photosynthesis in a C4 plant like corn could reduce total carbohydrate production in the plant.
Reduction in total photosynthate in a corn plant plus genetics that favor strong movement of available carbs from leaves and elsewhere to the developing kernels increases to the probability of root deterioration. This deterioration leads to vulnerability to invasion by microorganisms and eventual inability to transport water from soil to upper plant. This will lead to wilting of the plant and stalk rot.
A lot of dynamics interact a corn crop and the 2021 environment will include smoke haze from the forest fires in Western USA.
We have selected corn genetics to favor storage of carbohydrates in the fruit (kernels) of the corn plant. Most corn varieties follow the same pattern for this process.
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.
Corn has been selected by humans from the wild plant Teosinte to move more carbohydrates than is needed for not only plant reproduction but to also store excessive amounts in form for human consumption. This over-production is a metabolic process that goes on after pollination as it 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.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.