This fungus more commonly known as Bipolaris zeicola is usually an insignificant corn pathogen that is a good example of the complexity of dynamics between corn and potential diseases. Both the host and fungus genetics interact within their environments. The fungus is saprophytic digesting and growing on dead plant tissue, especially in grasses. Some genetic variants of the fungus produce a toxin that kills a small area of a living leaf, forming a lesion, allowing the fungus to receive nutrition. The plant tissue responds by limiting the fungus from further growth. The fungus responds by producing spores, to spread to more potential host areas. This interaction is common throughout nature. Corn interaction is probably clearer because of the extreme genetic variability of the host across years and environments.
Race 0 of B. zeicolacauses very small flecks on most corn varieties. It apparently can be found on dead corn tissue, perhaps on dead tissue on living plants that were killed by other causes. Race 1 of this pathogen shows up periodically as a susceptible inbred is grown. It produces a toxin that kills leaf tissue in area of about 1 cm in length and 0.5 cm wide before the plant successfully stops the pathogen but spore production spreads it to more leaves. It also can invade the kernels on ears. Susceptibility must be genetically simple, as it appears occasionally in breeding programs. It can be significant to seed production. Race 2 became notable especially on a different set of inbreds, such as W64A especially in the northern USA corn belt. Those inbreds susceptible to Race 1 were not susceptible to Race 2 and vice-versa.
Race 3 became apparent after invasion of the US corn belt by Race t of a related species Helminthosporium maydis(Bipolaris maydis) in 1969-1970. The two species have been shown to cross in culture and it is hypothesized that the mix of the two species with the northern exposure to the other species allowed for new genetic combinations. Race 3 of causes longer more narrow lesions on susceptible inbreds. Race 4 of B. zeicola became apparent in 1980 in seed production fields with B73 derived inbreds. These lesions differed from Race 3 with wider lesions and significant losses in seed production fields that spraying is needed. There is evidence that this race, and probably others, successfully invades the production field by producing initial spores on nearby grasses.
Genetic variability in potential pathogens, multiple hosts and genetic variability in the corn species and among ‘new’ varieties can result in unexpected corn diseases. We usually don’t become aware of the new interactions until the incidences are large enough to get attention.
Exserohilum turcicum (Helminthosporium turcicum) has been known as a pathogen of corn probably as long as corn has been cultivated in the humid fields. This fungus overwinters in diseased leaves, produces spores that spread to new crop leaves that germinate and penetrate the leaf tissue. The mycelium grows in the leaf tissue, plugging the veins until the plant resistance system restricts the growth, causing the fungus to produce a lesion upon which the fungus produces spores and thus spreads further on the same plant and nearby plants. Time from infection to lesion formation is about 2 weeks, although the plant’s resistance response may affect this time frame.
Corn genetics is a major factor in preventing this fungus from reaching the vascular system within the leaf. Selection of genetics for reduced number of lesions is practiced by most corn breeding programs. This system allows the fungus to reproduce but at a slower and less successful rate than more susceptible genetics. Three or four genes are involved in this type of resistance. This type of resistance is called horizontal or multigenetic resistance.
Environments favorable to the fungus, such as diseased leaf tissue on surface of non-tilled field and frequent rain may increase initial spore production and thus more infection, resulting in increased lesions on even the more horizontally resistant hybrids.
A type of resistance that allowed the fungus to reach the leaf vein but prohibited normal lesion production with fungal sporulation was identified in about 1960 in a variety of popcorn. It was inherited by a single gene referred as Ht1. Plants with this gene may have poor horizontal resistance allowing successful invasion but prohibiting the spread from the infected leaf tissue by spores became a major factor in controlling the disease. Most USA corn breeding programs quickly adapted the use of this gene as crossing in the single gene was a much simpler process than selection for horizontal resistance.
A seed production field in 1979 in Indiana, planted with a Ht1 inbred was found to have multiple susceptible lesions caused by this fungus. After being identified in that field, similar reactions were found in several locations that summer in the Midwest. The fungus had a gene that overcame the single gene resistance in corn and produced normal susceptible lesions, sporulating and spreading to others. This became known as Race 1 of Exserohilum turcicum. Other single genes for resistance have been identified (Ht2, Ht3 and HtN) and likewise so has races of the fungus been found to overcome these individual genes.
This is not a new lesson. Single gene resistance, especially one that restricts pathogen reproduction puts considerable selection pressure on the pathogen to favor the variants that can overcome the resistance.
One of the more recent ‘new’ corn diseases was first noted in USA in 2016 is bacterial leaf streak caused by Xanthomonas vascicolapv vasculorum. This disease was known in South Africa since 1949 but in its first year of identifying in the USA, it was seen in several counties of Nebraska and Iowa and Illinois. There are indications that it was also seen in Argentina in that same year. If it was distributed by seed from South Africa, why did it suddenly appear in so many places in one year? This bacterium is a variant of a more common plant pathogen, Xanthomonas campestris. A recent publication by U. Nebraska plant pathology extension
(https://cropwatch.unl.edu/2019/bacterial-leaf-streak-corn-nebraska) listed 16 common grasses in the areas that also host the bacterial leaf streak pathogen. Did the outbreak develop as a mutant of another variant of this species, infecting grasses and building up sufficient intensity until brought to the attention of plant pathologists? Perhaps continuous corn cropping, limited tillage and hybrid susceptibilities also contributed to the ‘sudden’ appearance of this corn disease.
It is frequent that we are surprised by a new distribution of a corn disease and analysis of the cause of the change is often difficult. Genetic variation of pathogen, adaptation of new corn varieties, susceptibility of other hosts, distribution of insect vectors, change in environments including weather and farm culture practices are all potential contributors to distribution changes. Movements of the pathogen can occur with wind, infected seed, grain shipments, equipment movements and even people clothing can be factors. All of these potential contributors probably could account for the initial introduction of a new pathogen and generally goes without knowledge for a few seasons before intensity is sufficient to get attention. After initial identification and understanding significance, breeders can select sufficient resistance to overcome the worst effects of the ‘new’ disease.
I think every corn disease that was identified in the last 47 years of my experience was not first found in only a single location but in multiple locations the same season of initial identification.
Extreme variation in weather patterns in recent weeks in temperate zones worldwide will have an effect on corn plants development resulting in new disease pressures. Southern hemisphere crop has been delayed in harvest because of excessive rain, allowing more time for ear rotting fungi to grow. Northern hemisphere crops have been exposed to unusual temperatures and rain, causing delays in planting, new opportunities for pathogens with swimming spores, and increased distribution of bacterial and fungal pathogens. Corn Journal blog written in September 2018 seem appropriate for July 2019. Is this a new pattern?
Variables affecting corn leaf disease damage nearly always involves moisture and temperature within a corn growing region during a critical corn growth period. Moisture of debris from a previous corn crop is usually critical to spore production by potential pathogens causing many leaf blights. Very slight air movement within a field is sufficient to move spores of many pathogens the short distance from the soil surface to young plant whorls where moisture is usually available, allowing germination and penetration into the leaves. Further distribution from the initial infection can be associated with gentle winds associated with rain storms. Violent storms with hail cause physical damage to leaf tissue, allowing entrance of some pathogens such as the cause of Goss’ Bacterial wilt. Long distance distribution of pathogens is often associated with direction of storms as spores of some pathogens are easily carried in these winds. Pathogens dependent on reproduction on living corn plants are moved from those areas to more temperate zones by storms.
Air temperatures during the corn growing season affects corn leaf diseases as well. Warm and dry environment general inhibits fungal spore production. Cool evening temperatures are usually associated with dew forming on corn leaves, providing the moisture for germination of fungal spores and penetration of the pathogen into the corn leaf epidermis. Warm and humid summer evenings is ideal for some pathogens like Cercospora zeae-maydis, cause of gray leaf blight. Frequent rain favors the spread and infection of pathogens such as Exserohilum turcicum, cause of northern leaf blight.
Vectors of virus diseases are also affected by weather as aphid intensity is associated with drier weather. Corn flea beetles, vector of the bacteria causing Stewarts Bacterial Wilt, movement from environments where the bacteria are maintained on other grasses to new corn planted as the soils warm. Distribution of the insects are often affected by direction of wind.
Annual fluctuations in weather not only affect a corn variety’s physiology and resulting grain production, but also the significance of resistance to a specific disease. A variety may be regarded as adequately resistant to a specific pathogen when under usual low intensity of that pathogen but inadequate when the weather factors change. If we are entering into a period of more erratic weather patterns, we should expect some surprising vulnerability of some varieties to diseases. Corn, as a species, appears to have adequate resistance within its genetics to any pathogen, but it requires time and effort by many people to identify the cause of a new disease occurrence, to identify the source of resistance and incorporate the resistance into productive corn varieties.
Ht1 gene for resistance to Exserohilum turcicum, cause of northern corn leaf blight, was incorporated in most US hybrids during the 1970’s. This gene was responsible for reacting to initial infection by this fungus by producing a toxin to the fungus in the area of infection that prevented the fungus from producing spores to spread the disease to other plants within the field. In 1979, it was reported that a seed field of corn with that gene had been attacked by E. turcicum producing lesions with spores. Inspection of fields in many locations in the USA discovered the same phenomenon. A gene in the fungus, apparently not previously frequent among the genome, was greatly favored over those variants of the fungus with limited production due to the Ht1 gene in corn, had built up an intensity until it was noticed by the right humans to identify the new race.
This type of genetic battle between host species and potential pathogens is continual. Most potential pathogens are controlled effectively by resistance mechanisms in corn either intentionally by corn breeders screening for resistance when the crop is intentionally exposed to the pathogen. Most damage occurs when new environments favor a variant of the pathogen that allows it to overcome host resistance systems. It is probable that the pathogen variant occurred for several seasons only to gradually build up intensity to be noticed and identified.
Corn Lethal Necrosis, also known as Maize Lethal Necrosis, develops into a damaging corn disease when Maize Chlorotic Mottle Virus (MCMV) and another virus infect the same plant. Both viruses have separate insect vectors. Although MCMV had not been found in the USA until the outbreak of damage from the disease, it was later found that this virus was found in multiple locations, but absence of other viruses prevented notable disease development.
Race T of Bipolaris maydis, cause of the epidemic of southern corn leaf blight had a gene that attacked the mitochondria associated with male sterility of corn. The bacterium causing Goss wilt probably is a mutant of a common grass pathogen. Pathotype 4 of Bipolaris carbonum, cause of very minor pathogen of corn, had a gene that specialized in attacking a common inbred used in many corn hybrids.
There are multiple examples in the many places that corn is grown where the genetics of the pathogens, the corn and environments result in development of what appear to be new diseases. In all cases, efforts by corn specialists identify these factors and reduce the damage by selection of resistance and/or changing the environment that favors the pathogen.
There is no reason to believe that this will not continue. We benefit from the broad genetics in Zea mays.
The 2019 early corn growing season will probably feature unusual disease movements. Having personally seen many of the diseases shortly after being first identified, I do often reflect on how they got there. Eyespot was first identified in Japan in 1959 but more than 10 years later it was found in North Central USA, Argentina, Brazil, Europe and New Zealand. Was it a minor pathogen (Kabatiella zeae) so weak on corn that it only became noticed after more susceptible inbred like Wf9 and W64A became widely used?
Goss wilt bacterium, Clavibacter michiganensis sp. nebraskensis, was first identified in Nebraska in 1970 where it was especially virulent on some genotypes. Switching to more resistant hybrids quickly reduced the damage but the disease was later found in several states. These pathogenic bacteria were found to overwinter in diseased corn debris but how did it spread?
Corn Lethal Necrosis (also known as Maize lethal Necrosis caused by a combination of Maize Chlorotic Mottle Virus MCMV and either Maize dwarf Mosaic virus or Wheat Streak Mosaic virus was first identified in 1976 in Nebraska and Kansas. The virus had been identified in Peru in 1974. Since then MCMV has been found in Argentina, China, Thailand and central Africa. This virus is transmitted by beetles and thrips. Severe symptoms show only when MCMV plants are also infected with the other viruses. Seed transmission has been shown at an extremely low rate. Resistance has been identified.
Damages races of more common corn pathogens such as race T of Bipolaris maydis, Races 1 and 3 of Bipolaris zeicola, Race 1 of Exserohilum turcicumall were related to use of specific
susceptibility genes. Grey leaf spot became notable as susceptible genotypes became widely used combined with less crop rotation and more previous crop debris.
Bacterial Leaf Streak, caused by Xanthomonas campestris pv. zeae, was first identified in Nebraska in 2016. Previously it was only known to occur in South Africa. Is has since been found in 7 USA State and probably in Argentina. Was it always in these areas as a minor pathogen of other grasses but environment and host susceptibility allowed it to increase sufficiently to be identified? Tar spot has been recently noted in north central states after previous identification in higher altitude South American areas. Caused by synergism of two fungi species, much is still be learned about the disease.
There remains much to be learned on mechanisms of susceptibility, environmental factors and movement of pathogens involved in occurrence of ‘new’ corn diseases. It is a non-ending problem in a widely grown crop.
Midwest corn belt planting delays will affect more than only normal yield dynamics but also corn diseases. Corn escapes damage from viruses such as maize dwarf mosaic because the damage occurs when the virus reaches the growing point before the v6 stage of development. Spread of the virus via aphids usually is limited close to the overwintering source of this virus as the aphids tend to spread further north as the temperatures become warmer. Although the June temperatures in the Midwest in 2019 have been cooler than normal, it seems reasonable to assume the aphids can feed on corn at very early development stages further north than usual. Frequent rains could mitigate this aphid survival affect, however. Other virus vectors likewise may have unusual distributions onto corn young plants.
Fungal leaf diseases, such as northern leaf blight, are usually favored by wet conditions. Sporulation on corn debris from the previous year is encouraged by the frequent rain, water in leaf whorls and high humidity allows quick invasion of leaves in the young plants. A few weeks later these lesions become sources of new spores that are easily distributed by winds to new fields. The dynamics of having later planted fields nearby could produce unusual northern leaf blight pressure, especially on those fields.
Gray leaf spot fungus also produces spores on infected leaves from the previous season, but infection of new crop is more favored by humidity than rain. Will excessive rain in June result in higher humidity than normal for the summer of 2019?
Common Rust and Southern Rust spores are carried in storms originating in the Southwest and Caribbean Islands. Corn is most vulnerable before flowering, as the leaf whorl provides an inoculation chamber for the spores to germinate and infect the leaves. The mix of planting dates within an area increases the potential for damage to the later plantings.
This unusual planting season will provide new disease pressures on corn that will require special attention this summer.
Excessive rain in much of the USA Corn Belt in 2019 with ponds of water in areas of fields can encourage infection by a fungus that has swimming spores. Corn Journal blog of 05/30/17 may be appropriate for this year as well.
One of the effects can be infection by an organism called Scleropthora macrospora. This is a fungus-like organism belonging to a group of organisms called Oomycetes. Also in this group are pathogens causing Downy Mildew and Pythium diseases of corn and other plants. Common among these are the ability to form thick walled spores to withstand stress environments that can release swimming spores when in water-saturated soil. S. macrospora infects more than 140 grass species in addition to corn.
The source of infection of corn is often grasses near a low spot or edge of a field. Oospores in the flooded living and dead leaves release swimming spores (zoospores) when close to the corn submerged leaf tissue these zoospores release a germ tube that infects the plant. The filaments (hyphae) grow towards the meristems throughout the life of the plant. This can initially be seen as fine stripes in the leaves but the most obvious symptom is proliferation of leafy aberrations of the tassel- the crazy top symptom. Scleropthora macrospora also can grow to the ear bud meristem, causing similar multiple ears from a single node- but no grain.
Related oomycetes occurring in warmer, subtropical and tropical environments can cause similar symptoms. These downy mildew diseases can also cause the proliferation of the tassels and ears. Susceptible genotypes can have severe grain loss from these diseases. Scleropthora macrospora infection is usually limited to a very small area near grass in a low part of the field.
Infection occurs when the plants have less than 6 leaves. Symptoms that show late in the season, but the problem began with excessive rain that occurred only a few weeks after planting. That early moisture that may contribute to large yields can allow this pathogen to form these unusual corn structures in a few spots of the field. In addition, it is just part of the interesting biology of corn.
Much of the 2019 corn growth in the USA is erratic due to wild swings in water and temperature, affecting planting timing and plant densities. And that is only what we see! Internally many interactions are also occurring and are affected by these environments.
Much of the above-ground growth for the first 30-40 days after seedling emergence is due to cell elongation within leaves. This not only allows the expansion of leaf blades to increase the mass of leaf tissue, but also elongation of the leaf sheaths, pushing up the plant height. Cell elongation is not only occurring in the outer tissue, but internal cells also grow in size during this time.
Cell elongation is driven by energy allowing production of cell components such as cell wall cellulose and lignin but also increase in the membranes, ribosomes, mitochondria and chloroplasts needed to drive the growth. Immature cells, before tightly constricted by deposits of solid cell walls, expand with water pressure during this growth pressure. Consequently, soil water and root development become major factors affecting the size of the corn plant during this pre-flowering stage. Root development not only affects the absorption and movement of water into these growing leaf cells but also uptake minerals needed for the general metabolism. Expansion of leaf blades during this time also increases the absorption of light driving photosynthesis, providing more energy for cell function and growth.
Multiple environment factors influence water supply to corn plants, but genetics also distinguish variety reactions to growth of the plant. Root size and growth pattern affect water and mineral uptake. Structure of vascular tissue from roots to leaves affect efficiency of water movement. Number and activity of stomata in leaves affect the evaporation of water from leaves. Efficiency and number of chloroplasts within the cells affect the transmission of light energy to carbohydrates, mitochondrial numbers and efficiency affect the change of this energy into ATP for use in the formation of proteins and other products needed for cell growth. Translation of chromosomal DNA to RNA that moves to ribosomes where the codes for specific amino acids are strung together for specific proteins, some of which are used as enzymes driving production of cell structure components. A large number of those 30-40000 corn genes must be participating in that early growth of a corn plant.
It is difficult to predict the final grain production of fields under these circumstances and it is probably that all hybrids will not react the same, even if the principles of biology will apply to all.
After emergence and successful elongation of the first true leaves, photosynthesis becomes the energy source for future growth. Consistent with its tropical origin, corn photosynthesis is negatively affected by low temperatures. Leaves grown at 14°C (57°F) have 30% of the photosynthesis rate as those grown at 25°C (77°F). (Plant Physiol. (1995) 108: 761-767). Much of this reduction is recovered within a few hours if the leaves are returned to the higher mid-70 temperatures.
Light energy absorbed by chlorophyll causes an electron to be moved within the chloroplast but if it does not ultimately get utilized in synthesis of carbohydrates, it can damage a critical protein needed in photosynthesis. Corn chloroplasts react by producing a yellow pigment (zeaxanthin) protein that is active in the quick recovery after the heat returns. Chlorophyll molecules are relatively unstable especially in high light intensity and low temperatures, further contributing to reduced photosynthesis at the lower temperatures.
Another protection system in corn, and other plants, that develops in the cell outside of the chloroplast is the pigment anthocyanin. This red pigment absorbs the blue light spectrum of sunlight and thus reduces photosynthesis. Anthocyanin forms after the sugars reach a high concentration. This often happens in seedlings when sugars are unable to be translocated to the roots, again because lack of the heat energy needed to move the sugars. Hybrids vary in the tendency to produced anthocyanin, occasionally causing alarm to the grower but return to warmer temperatures results in disappearance of the red color and normal photosynthetic rates in the seedling leaves. (Corn Journal, 5/5/2016)
Leaf epidermis cells provide important functions beyond providing a tight layer of cell walls surrounding the inner mesophyll cells of the leaf. Epidermal cells also produce a polysaccharide layer outside the outer cell walls and a fatty acid layer of wax further outside. Synthesis of these cuticle and wax substances begins in plastids within the cytoplasm of the epidermal cells. These newly manufactured compounds are moved via the endoplasmic reticulum eventually being deposited on the outer surface of the epidermal cell walls. The fatty acid wax is moved further outside forming a waxy surface to the cuticle.
Multiple genes are involved in production of these complex molecules as synthesis requires linking simple products of photosynthesis (glucose) with inorganic materials to form new compounds. The basic process is common to all land plants as they adapted to life outside of the aqua environment of algae. Further selection for adaptation to varying corn environments allowed for selection of genetics affecting responses to environmental stress.
Outer wax causes water to run off the surface, taking pathogen spores with it. Chemicals applied by growers usually include a surfactant to overcome the water resistance by breaking the tendency of the water molecules to form drops, thus reducing this feature of wax. Pathogenic leaf fungi enter the leaf either by establishing a ‘drilling station’ on the surface from which hyphae extension (appresorium) is pushed through the wax and cuticle layers on the epidermal cells. Other fungi and some bacteria, unable to penetrate the wax and cuticle, avoid the problem by entering through the stomatal openings.
Wax also prevents water loss. Corn genotypes vary in this response to dry environments, some making thicker layers of wax than others when in a dry environment. Leaf surfaces of hybrids grown in the less humid environments of western corn belt have a different texture than the same hybrid grown in the more humid eastern US corn belt. Wax production differences among varieties is probably one of the components to more drought resistance.
Cell division in the meristem establishes the eventual structure of the new leaves unfolding in the young seedlings. Corn has several unique leaf structures that contribute to it’s ability to be one of the most efficient crops in capturing CO2 from the atmosphere. One contributor is part of the epidermis.
The single layer of cells on both the top and bottom of corn leaves are mostly non-pigmented cells tightly bound together, restricting water loss. Further protection comes from a wax covering the outside of these cells. The exception to this tight wall structure comes from some unique cells interspersed within the epidermis on both sides of the leaf. These cells not only have chloroplasts with chlorophyll but are shaped differently. The two guard cells of the stomata are shaped in a manner that allows only one side of each cell to swell with water, with the affect of making a small pore in the epidermis between the two cells. The swelling occurs during photosynthesis within these cells. This process essentially results in the import of potassium ion into the cells, causing an increase of solutes and thus, through osmosis, transfer of water into the cells. The result is stomata pores are open.
This is essential, of course, to allow diffusion of CO2 into leaves for photosynthesis in other leaf cells. Open stomates allows O2 to be released to the atmosphere but also water loss. Water evaporation through stomates (transpiration) is affected by the relative humidity in surrounding atmosphere as the water concentration within the leaf spaces is nearly 100%. Cohesiveness of water molecules 'pulls’ water up to the leaves so that essentially every molecule of water that goes out the stomata is replaced by one from the root tissue.
During the day, stomata are open, carbon dioxide moves into the leaf, oxygen moves out and so does water. At night, photosynthesis in the guard cells stops, water moves out of the guard cells causing the swelling to be reduced and the pore is closed. More references on the links below.
2019 corn season in USA has started with unusual stress from wet fields in much of the corn growing areas. Not only was planting delayed but effects on seed environments has resulted in uneven emergence in some fields. Although nearly every plant in the single cross hybrid is genetically identical, too much water, or lack of water, seed quality, tillage, and soil compaction and inconsistent planting depth all may contribute to uneven emergence of these seed. Multiple studies have attempted to evaluate the effect of uneven emergence on final yield. One study published in 2012 in Journal of Plant Nutrition 35:480-496, 2012 (http://www.tandfonline.com/loi/lpla20) the yields and nitrogen uptake of plants from seeds planted between earlier planted seeds, finding that these individual plants yielded significantly less grain than adjacent plants of the same hybrid.
The multiple variables interacting with studies of delayed emergence makes the exact determination of effect emergence on final yield very difficult. Shading of leaves by adjacent plants reduces photosynthesis. Delayed silk emergence may miss pollen timing. Competition for nutrients may be inhibited by earlier and greater root growth of adjacent plants. Genetics of each hybrid may be affecting the reactions of each hybrid differently. Although the exact affect in each hybrid-field environment should be expected to differ.
In the early 1970’s I was attempting to understand why stalk rot occurred in only some individual plants of a single cross hybrid and not in other adjacent plants. If the cause was a fungus that was common, why did one plant develop stalk rot but not the genetically identical other plant? My first hypothesis was that these were late emerging plants. I marked some of these plants and followed their development though the season. Instead of developing stalk rot these plants had very narrow stalks, flowered later than adjacent plants, had deformed tassels with abnormally few glumes and very small, poorly pollinated ears. Not being sure that these plants were not inbred impurities in that hybrid, I intentionally planted seed between earlier emerged seedling. This was done at the plant densities of that time with 5 commercial hybrids. The effects on plant develop was the same as observed the previous season, confirming that genetically identical plants are affected by interactions with adjacent plants.
My brief experiments were done with hybrids of the 1970s, commonly bred for much lower densities than is common in the USA today. It should be expected that each hybrids reaction to delayed emergence will be different as well as each field environment will be different. We can acknowledge that uniform emergence is optimum but prediction of the exact result on final grain yield is complicated.
At about the V3 stage of development, the primary root function begins to be replaced by the nodal, secondary roots. Energy provided by photosynthesis in young leaves, and heat, drive the production of the metabolites for cell division and cell elongation in these young root tissues. Whereas auxin hormone causes increased cell elongation in stem and leaf cells, auxin reduce this activity in the root cells. Consequently, although the nodal roots initially emerge horizontally from the stem nodes beneath the soil surface, gravity causes more auxin to accumulate on the lower root epidermal cells. This results in longer epidermal cells on the upper side than on the lower side, effectively turning the root growth downwards.
Root tip meristem cells rapidly divide, producing the root cap cells below to protect the dividing cells as it pushes through the soil and functioning root cells above the dividing cells. Outer layer root cells composing the epidermis are thin-walled and porous to water via osmosis. A short distance from the meristem of the root tip, epidermal cells form protrusions (root hairs), effectively expanding the surface area exposed to water and minerals of the soil.
Cells in the core of the new root differentiate to form vascular tissue that connects to the stem vascular tissue through the nodes. This vascular tissue allows transport of water and minerals upwards through the xylem and carbs downwards through the phloem. A few cells in this vascular portion of the young root maintain cell division capability, becoming stimulated by another group of hormones (cytokinins) to increase cells laterally, pushing through the epidermal cell layer becoming lateral roots with their own root meristems. (Corn Journal (6/6/2017)).
The biological entity that we call a corn seed is more complex than it appears. Each seed of each hybrid may appear to be identical upon first glance, but a closer study reveals external differences in terms of size and shape and, perhaps, external damage. Each seed within a container may have been produced by pollination by the same male parent onto the silk of the same female parent. They may have identical genetics. But position on the ear in the production field may reflect slight differences in environmental exposures ranging from pathogens in the seed production field to handling during seed harvest, drying, shelling and bagging. Seed treatment application, including the important drying process, may not be equal for each seed. Potentials for variation continue as the seed is distributed to growers with varying storage conditions.
Internal biology of each seed can be affected in each step. Even a dry non-germinating environment, the critical cellular membranes are vulnerable to damage that only becomes exposed when imbibition allows cell activity.
Moisture is needed for germination but too much water, especially in some soils, can suppress availability of oxygen needed for cellular respiration. Membrane function is essential to all cellular activity. RNA produced with enzymatic activity in the cell nucleus is transmitted through the nuclear membrane to the membrane intense ribosome. Among these enzymes are those that split the starch molecules in the endosperm in to glucose molecules that are moved to the mitochondria. The membranes in mitochondria become the site in which enzymes utilize oxygen, water and glucose to produce the energy source known as ATP, that provides energy for other cell functions including the elongation and duplication of cells for seed germination.
Once planted, the seed engages many field environment variables that potentially could interfere with normal germination and emergence from the soil surface. Temperature and moisture extremes, absorption of damaging chemicals, pathogens, insects and soil hardness can be factors interfering with normal emergence from the soil.
Everyone involved in corn seed attempts to limit the risks of poor field emergence. Genetics of the hybrid, especially of the seed parent, are selected for reduced vulnerability to seed damage. Seed production methods are adjusted to limit physical damage to the seed. Growers use tillage and planting methods to provide best soil environments for the seed. In most cases all these efforts come together with a good uniform emergence in the field. Uncontrollable weather can be involved when all the efforts have failed. Surely production of a biological entity like a corn crop is more complicated than production of inanimate things.
Excessive rain soon after corn seed germination, especially in low areas of fields with heavy soils, is frequently associated with stunted plants. Much of that is caused by lack of oxygen to the roots. Oxygen is needed to maintain metabolism in root cells not only for production of new root tissue but also of other functions including defending against potential pathogens.
A comparison of corn seedling root structures growing in aerated and non-aerated conditions showed that the cells between the outer epidermis layer and the inner vascular tissue tended to collapse in the seminal roots lacking oxygen. These cells tended to be empty of cytoplasm but instead became empty spaces separated by the cell walls. Lack of cytoplasm was apparently the cause of reduced active uptake of potassium and assumedly other minerals by the seminal roots.
This study (Plant Physiol. (1980) 65, 506-511)showed that corn seedlings in oxygen deficient media tended to develop nodal roots sooner than those with adequate root oxygen as an apparent reaction to stress of the seminal roots. Prolonged oxygen stress ultimately resulted in less total root volume.
Symptoms of mineral deficiency in young corn plants in excessive, prolonged water areas of fields is associated with oxygen deficiency in corn roots. This results in less mineral uptake into roots and transfer of the minerals through the vascular system to the shoots. Prolonged oxygen deficiency results in reduced total root volume, less minerals available for shoot growth and potentially less water uptake in late season dry environment.
A cool seedling environment often results in purple leaves in some field. Isolated corn plants with extensive purple colored leaves and stalks late in the season have some relationship in cause. Purple coloration in plants is the result of anthocyanin pigments. Anthocyanin pigments absorb the green spectrum of natural light, reflecting much of the blue and red portions. Chlorophyll pigments absorb the blue and red wave lengths, reflecting the green.
Anthocyanins are water soluble pigments that tend to accumulate in the vacuoles of the epidermal cells. Reflection of higher energetic light blue and red wavelengths of the light could be reducing the availability of that energy to the chloroplasts in the mesophyll, perhaps offering some protection from overload of sugars in those cells. Absorption of the higher energy wavelengths of sunlight by anthocyanins has been hypothesized as protecting cellular membranes from photo damage.
Anthocyanin synthesis is associated with the accumulation of sugars in leaf tissue. Conditions that favor photosynthesis but inhibit distribution of the sugar from the leaf tissue can trigger synthesis of anthocyanins. Seedlings inhibited by soil conditions reducing root growth and thus movement of sugars to root tissue. Reduced phosphorus available, perhaps because of reduced roots growth, inhibits transport of sugars from the photosynthetic cells. Cooler temperatures may inhibit metabolism involved in sugar transport. Insect or pathogen damage to leaves that block the phloem cells of the leaf vascular cells, can cause accumulation of sugars in leaves. Purple color is common in plants with poor pollination a few weeks after flowering as sugars have no place to be transported.
Corn hybrids vary in anthocyanin synthesis genetics, some with a higher tendency to develop purple colors when experiencing sugar transport stresses. Early season development of purple leaves in most plants of a field probably involves cool temperatures and will be only a temporary condition. Even hybrids not tending to show purple could be undergoing a similar stress but not have the genetics for the anthocyanin response.
It is good to observe occurrence of purple leaves in corn for analysis of possible causes. If temporary and generally distributed in a field, it is probably temperature related. If scattered, then it may lead to treatments reducing the occurrence this season or the next.
Low temperatures have been a major feature of this spring in northern US Corn Belt. Initial concerns are effects on germination and emergence but after emergence, heat energy affects several aspects of the corn plant. Studies have shown that hybrids vary in tolerance to cold temperatures. Much of this involves the cellular membranes, including their ability to repair after membrane damage. This affects loss of electrolytes and their function of nearly all cellular processes from photosynthesis to protein synthesis. Many of the proteins are used as enzymes active in photosynthesis and cell duplication and elongation. This generally results in reduced plant height and smaller leaves if plants grown at low temperatures than at higher normal plants. Root volume is also less when corn plants grow under lower temperature environments. This may be caused by a reduction of photosynthates reaching the root tissue.
Chlorophyll levels are lower when corn leaves are at lower temperatures, perhaps because of membrane damage and reduced syntheses of chlorophyll. Reduction of photosynthesis is hypothesized to account for reduction of stomata and thus transpiration as well as CO2 movement into the plant. Cellular respiration in which the glucose from photosynthesis is changed to a metabolically useful chemical energy (ATP) is also reduced, contributing to the reduced cellular elongation and duplication.
A study comparing commercial hybrids for tolerance to growth under low temperature environment was published in 2015 in Canadian Journal of Plant Science and can be found at
Temperature is one of the factors that favor different hybrids each year.
Initial nodal roots, growing from the first nodes of stem in the crown area tend to grow laterally before turning downwards. Each branch of each lateral nodal root has its own root tip. Nodal roots are formed near to stem vascular system of the stem, even in the crown, allowing efficient transport of minerals and water into the stem, through the xylem and then further to the expanding leaves. Photosynthesis products are moved through the phloem into the vascular system of the roots to allow expansion of the nodal roots.
Temperature influences the metabolism rate in the root tissue and thus affects the growth rate of the nodal roots. Although water movement into the root tissue and movement up the plant is mostly influenced by other physical factors such as outside relative humidity, transport of nutrients such as glucose from leaves to roots in phloem is slower if temperatures are reduced. Auxins influencing the root tip expansion are produced in leaf tips, moved through phloem and therefore are slower to reach root tips if temperatures drop.
Genetics influence the pattern of nodal root growth, some hybrids with more lateral roots than others. It is common for corn hybrids to develop roots from 6 underground and 3 above ground nodes during the life of the corn plant. Genetics and environments influence the actual number. Soil types and depth, organic distribution in soil and water distribution all become factors in determining the most efficient nodal growth pattern appropriate for a field. A change in any of these factors may favor a different set of genetics the next 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.