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.
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.