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.
First root to emerge from the germinating corn seed is the radical extension of the embryo. It is mostly unbranched and growing downwards. Next are the seminal roots emerging from the tissue between the radical and the mesocotyl where the scutellum is connecting the endosperm to the growing embryo. Those roots often are branched. The radical and seminal roots are essential to collecting water and minerals for a few weeks as the mesocotyl pushes upwards to the soil surface.
With the light penetrating a short distance below soil surface, the tip of mesocotyl forms the first node and the coleoptile pushes upwards. The base of coleoptile forms the first stem node. Within the coleoptile is the first leaf connecting to the stem at the second node. By the time the first leaf is fully expanded, and the 2nd and 3rd leaves exposed, all connected to the underground stem, the crown, lateral nodal roots extend. These nodal roots grow laterally first but eventually gravity effect on the cell elongation causes these roots to point downwards. Lateral roots may develop from crown nodes 1-5 and eventually above ground nodes as well.
Lateral roots become the main source of water and mineral absorption as the mesocotyl is cut off from the energy from photosynthesis in the leaves. Seminal roots served their functions to support the early emergence of the shoot. Genetics and environment now become the major influences on the direction and branching of the main root system of the corn plant.
An interesting summary of root development of young corn can be found at http://www.kingcorn.org/news/timeless/Roots.html
Much of the USA corn growing areas are experiencing a wet, cool spring. It is likely that corn seed already planted in these soils will experience temperature and excessive water stress and that some soils will become compacted, leading to not all seedlings emerging at the same time. PSR's seed tests also detect some seed samples that have a biological problem in which not all the seed germinate at exactly the same time.
It is not clear why some individual seeds within a seed lot are slower to elongate the shoot and root, allowing the seedling to emerge from the soil. One hypothesis is that the slower seedlings had deterioration of the membrane structure in embryo cells, possibly in the mitochondria. This is enhanced with imbibition swelling the membranes, resulting in tearing the tissue. Membranes in mitochondria are the sites of cellular respiration in which sugar molecules are enzymatically changed into ATP molecules, the energy source for almost all cellular activity, including cell elongation. Other cellular membranes are major locations for transport of components needed for cell elongation as well.
There are multiple cells in the corn embryo, each with many mitochondria. Complete death of the seed occurs when too many membranes are destroyed for the seed to grow. Sufficient amount of functioning cell organelles, however, can lead to repair of damaged membranes, eventually gaining enough momentum for cell elongation in the shoot and root tips. This can result in eventual emergence from soil surface. Heat energy and oxygen in soil are important to this process as well.
Seedlings emerging later than adjacent seedlings can struggle for the whole season, as they are shaded from light and at competitive disadvantage for minerals in the soil. This can also result in plants with delayed flowering, possibly because of insufficient moisture to push out the female silk, missing the normal pollination.
Detection of these individual seeds with internal membrane damage is not easy and may not be detected with standard germination testing. When planted in artificial soil mixes, most seed lots with high germination percentages emerge uniformly. Those with non-emerging seedlings will also tend to have more late emerging seedlings as well. Rarely one finds a seed sample in which the percent emergence is 95% but includes most at three leaves exposed while some only show the coleoptile. Even in these cases, there will be some with each level of development, showing only the ‘spike’, or one leaf, or two or three. How to classify germination of these seed? They are alive and they germinated but how will the delayed emergence of these plants detract from field performance of the seed lot?
Uneven emergence in the field could have several causes. Analysis of main factors is difficult. Seed biology is one of those factors.
Crop agriculture is dominated by multiple environmental and biological factors. Everyone participating with corn seed attempts to define and control these interactions. Breeding procedures can mostly assure that a hybrid is genetically uniform, production methods are intended to maintain this purity and testing methods can evaluate for level of genetic purity.
Seed viability is affected by environments during seed production in the field and after harvest. Each individual seed within a seed lot has a distinct experience with this process, ultimately affecting the ability to germinate with viable shoot and root tissue. Individual seed may have some damage to membranes within cells that require metabolic repair before being able to elongate the root (radicle) part and shoot part of the seed embryo after imbibition. This may be affected by genetics, often of the female seed plant and perhaps of the mitochondria in the female parent.
Germination tests to identify seed viability, usually defined as a seeds ability to produce a shoot and root when placed in a controlled environment can be done with reasonable repeatability. Results are determined after a specific time with specific definition of a root and shoot. Defining and characterizing differences among the seed’s vigor, or the time it takes for that individual seed to produce a root and shoot is more difficult. The seed analyst may see differences in vigor among germinating seed but communicating these differences becomes a major problem.
How to characterize a seed lot that has a high percentage of seed that meet the definition of viability but does not germinate uniformly in test conditions? Generally, those seed lots with delayed germination in warm conditions have lower germination percentages when tested under cold conditions (50°F) but there are exceptions to that as well.
The ultimate goal is to reduce the possibility that seed viability and vigor affect hybrid performance in the grower’s fields where environments present their own variables. It is understood that late emerging seedlings, regardless of cause, have difficulty in competing with adjacent corn plants. They often remain less vigorous because competitors reduce light on leaves and outcompete the late-emerging plant’s roots for minerals and water. Often late emerging plants produce ear shoots later than most adjacent plants resulting in poorly pollinated ears. Genetics of hybrids probably differ in ability for late emerging plants to remain nearly fully pollinated and thus the detriments of lack of uniformity is not exactly the same for all hybrids.
Everyone in corn agriculture wants maximum performance from the seed. We attempt to remove known variables by measuring viability and vigor and by preparing planting conditions. There remain uncontrollable environments and difficulties in defining and communicating seed vigor. Late emerging corn plants detract from maximum yield potential of a hybrid, but how late is the emergence and how much is the yield reduction? Like most of life’s experiences, we wish for clear definition but often the variables make that difficult.
Politicians can sum up ambiguity in a simple phrase. The rest of mere humans must only attempt to evaluate and communicate what we think is happening within a corn seed lot sample.
As soon as the seed is placed in the soil, the season begins. Multiple variables that will affect the final grain productivity of this corn crop begins with those interacting with the biology of the seed. Each seed of a single cross hybrid may bring the same genetics but could have a slightly different biological condition, depending on its individual history. Biology of seed germination, described below, is from Corn Journal, 4/25/2017,
Very soon after the corn seed is planted, imbibition begins. The H2O activates the membrane-bound mitochondria to respire, providing energy for protein production. The enzymatic proteins include those that digest the starch stored in the endosperm into more sugar molecules to be transported through the scutellum to other cells in the embryo, resulting in more energy available to produce structures for cell elongation. Heat energy provides a regulatory function affecting the speed of this germination process. Imbibition occurs at any temperature but metabolic activity in corn is generally thought to be very low if seed environment is below 50°F. Speed of germination increases as the temperature increases.
Membrane integrity within the seed also affects the net speed of this process. Those individual seed with more damage are slower to sufficiently activate the system and thus slower to activate the metabolism needed for cell elongation in root (radicle) and the shoot sections of the embryo. Cool environments, delaying membrane repair, may result in death of the imbibed seed before the shoot can emerge from the soil. Some of these seed, even after warmed manage only to extend the root through the outer wall for the kernel, the shoot never emerging. Other weakened seed may finally get enough momentum to push through the soil surface but days after the healthier seed have emerged, resulting in a season-long competitive disadvantage. Heat energy during germination affects the severity of the effect of membrane damaged seed.
Microbes in the soil are generally warded off by products of seed metabolism in healthy seed. Those individual seeds that are slow to generate sufficient energy for growth are also more easily attacked by microbes, further slowing the germination process. Seed treatments are useful in giving the damaged seed more time to successfully germinate. Healthy seeds can successfully produce normal seedlings despite surrounding common soil microbes but those weaker individuals need the extra protection.
Part of the genetic history of the seed parent includes selection for reduced vulnerability to damage during germination as expressed in yield trials and experience of the corn breeder. Seed production environments further affect the biological condition of each seed begins the season. Timing of emergence of the seedling in relation to its adjacent plants affects final growth and grain production as each plant competes for light and minerals during the remainder of the season. Uniform emergence within a field is an important component of grain production for the season.
Modern corn hybrid fields are most productive if the field has a consistent ‘stand’ of plants, evenly spaced and equally developed. Lots of factors are involved is a successful establishment of this uniform growth. Field conditions, germination environment conditions, seed uniform vitality and genetics are the main interacting contributors to uniformity of seedling emergence. Some of these are controllable by the corn grower and the seed producer, some are measurable prior to planting, but temperatures and rain variables are always part of the unknowns at the start of a corn season.
Determining the vitality of seed at the time of planting is not always as accurate as it might seem to everyone. Each individual seed has had its own experience from initial pollination in the seed production field, stresses during seed maturation, exposure to potential pathogens, roughness during shelling, moisture addition during seed treatment, shipment to farm and finally placement in field.
Multiple attempts to evaluate the percentage of individuals within a seed lot that are likely to not emerge in the field is made by germination tests. Attempts are made to standardize the tests among labs, but referee samples in which multiple labs germinate sister samples drawn from the same commercial seed bag show slightly differing results with warm tests and greater differences among cold tests. These lab to lab differences become greater as the average quality of a seed lot is lower, with some labs having percent germination meeting most company standards and others determined as failing.
Added to the difficulties of determining acceptable seed quality of a seed lot, each individual seed is at a different stage of losing its cellular integrity - they are all aging but potentially at varying rates. Seed producers have the very difficult task of determining what is the rate of deterioration within a seed lot. When do they stop testing and start shipping? Multiple tests can be done to determine germination quality of a seed lot but there still can remain those that fail to germinate adequately in the field for optimum hybrid performance. Sometimes this becomes a major reasons that the grower decides the hybrid yield capacity is poor, blaming the genetics of yield instead of the seed quality. It is complicated!!
Seed germination factors are only the beginning of those that eventually effect the performance of the crop. All corn seed is aging, as the physiology involved in digestion of starch in the endosperm allows glucose to be moved to the mitochondria in the embryo cells where it is processed into the ATP needed for production of proteins and specialize structures as the seed grows shoot and root tissue. Each seed within the seed lot is aging at a different rate, resulting in uneven emergence and inter plant competition during the remainder of season.
Soil consistency, water amounts and timing, temperature, crop debris and micro-organisms all interact with the young plant development. Hybrid genetic differences affect the reactions to these variables as well. Vulnerability to aging is mostly inherited through the female parent but genetics of the hybrid influences the reaction to these environmental factors.
Potential pathogens of the corn seedling are also affected by these environmental factors. Cool, wet soils favor Pythium species while slowing the corn metabolism. Anthracnose fungus (Colletotrichum graminicola) and the pathogen Cochliobolus carbonumcausing northern leaf spot are examples of 2 minor pathogens of young emerging corn leaves favored by warm, wet weather. Most virus diseases of corn only become damaging if infected early. This usually is dependent upon an insect vectoring the virus from other grasses. Environmental factors such as presence of adjacent hosts, temperatures and wind are big factors in the virus infection.
Northern temperate zone corn season is beginning now. Interactions of plant and other organism’s biology and physical environments will affect the harvest performance. We will search for a single factor to explain that performance, but it will most likely be complex.
It is human to prefer that a manufactured product meet certain expectations in structure and performance. Our expectations may be greater than described by the manufacturer or seller advocated. The product may perform exactly as we expected. Many manufactured products perform within our expectations within a defined and consistent environment within our home, for example. Sure, we did not expect the kid to throw a baseball into the TV screen, but we don’t blame the TV manufacturer for a broken screen.
Crop agriculture always includes variables, many of which interact, to affect the final productivity of the crop. Many of the variables are biological. Bacteria, fungi, nematodes and insects in soil may be beneficial or detrimental to the young corn seedling. Soil consistency, temperature swings and moisture extremes further contribute to the environmental variables affecting corn seed and seedlings.
Each corn seed has its own biological history beginning in the seed production field that ultimately affects its ability to withstand the stresses involved in imbibition by repairing broken membranes within its cells. Environmental stresses during the development of that seed ultimately influence the life and vigor of each seed. Although the genetics of each seed within a single cross hybrid may be identical, seed production factors include environmental factors outside of the control of the manufacturer that can shorten the life and vigor of some of the seed. Corn seed lots are sampled systematically, attempting to correctly characterize the germination and purity qualities of the seed lot. It is necessary to assume the sample is representative of the lot, but it is reasonable to assume that small variances will not always be detected in the samples.
We want to think that a germination percentage based upon samples accurately depict all the seed in the lot, at least within the germination test conditions at the time of the test. Unknown variables affecting the sample, affecting the seed after testing, environments after planting ultimately result in the actual emergence of each seedling in the corn field. We celebrate the appearance of a uniformly emergence of the seedlings in the field and have difficulty analyzing the cause when that does not happen. At least we have strong suspicions when we find a baseball inside the TV with a broken screen.
Dynamics of this disease serves as a reminder of the complexity of host resistance, environment and pathogen biology are common with agricultural crops. Although the disease had been identified before 1970 it was not regarded as damaging to the corn. Host and environments in USA changed in the 1970’s. Conservation tillage allowed more corn leaf and stalk debris left on the soil surface and wide use of the inbred B73 as a female parent changed in favor of the fungus (Colletotrichum graminicola). It had difficulty surviving when buried in soil but has special sporulation advantages over other organisms with survival of winter stresses when on surface of soil. This allowed for early infection of seedling leaves. Infected leaf debris continues to produce spores during the season. Although most corn genotypes are somewhat resistant to older leaf infection, presence of this fungus and its spores allowed for eventual infection of the stalk rind cells. A few hybrids are susceptible enough to be actively killed by infection in the root and stalk but this fungus is mostly an aggressive invader of senescing cells in the mature plant.
The B73 connection was linked to its contribution of high yields in hybrids. Some of that high yield component was tendency to produce large deposits of carbohydrates in the grain, sometimes at the sacrifice of adequate reserves for maintenance of living cells in the stalk and root tissues. Colletotrichum graminicolais favored in the environment of senescing cells, often speeding to the death of these tissues.
Reducing the infected corn debris by deep tillage or crop rotation can greatly reduce the disease but reducing the environmental stress is also important. Breeders affected this reduction by selecting hybrids with less grain fill per plant and, perhaps, more net photosynthesis per plant. This effectively lengthened the time of vigorous cells in corn stalks with ability to hold off this fungus until harvest. Anthracnose remains present in USA corn fields but damage is less now than 30 years ago.
Like much in agriculture, corn disease development is the result of a complexity of factors. It is human nature to want simple explanations but each of the factors, like those mentioned in this brief blog have sub-factors. Fortunately, human and government problems are not complex so politician’s simple answers surely will solve all of them (L.O.L.).
Cold weather of temperate zone winters can be harsh on fungi in the previous crop debris left on the soil surface after harvest. Low temperatures kill most spores (conidia) capable of spreading and infecting new crop corn plants. Although spring moisture can encourage production of new spores from infections in the old leaves, inconsistent temperatures and relative humidity plus sun exposure of the young seedlings can cause result in many potential fungal pathogens to fail infection of the young plants.
Colletetotrichum graminicola (cause of anthracnose) produces spores on surface of infected leaves in mucilaginous matrix that offers protection of the spores on the infected debris from temperature fluctuations and dehydration. This allows survival of spores for quick distribution to seedling leaves. Spores germinate and hyphae quickly form appressoria, allowing penetration in the first few seedling leaves. Corn varieties vary in resistance to further spread of the fungus to the growing point or roots. Killing of seedlings can occur in a few varieties but not in most.
Most studies have shown that there is not a strong correlation among susceptibility to the anthracnose seedling disease, anthracnose on mature leaves and anthracnose stalk rot. This fungus’ ability to overwinter in minimally tilled, continuous corn fields with anthracnose in the previous season are most vulnerable to this seedling disease.
An interesting study of this phenomenon can be found at:
Moving corn from its tropical origin to temperate zones required adaptations for many characters. One character needed to advantage of the full summer season, including in some areas of the USA, was to plant as early as possible to avoid pollination problems caused by extreme heat during flowering and to avoid killing frost stopping grain filling. It is common to observe that every field does not emerge and seedlings equally fast, but the many environmental factors complicate drawing conclusions as to cause. Was it seed quality or was it due to soil content difference?
One study (https://dl.sciencesocieties.org/publications/cs/articles/55/2/851) attempted to compare hybrids under controlled temperature and environments for leaf and root weights under differing temperature environments. Results supported the hypothesis that hybrids did differ in tolerance to cold temperatures after planting. Methods and results in this presentation cited are a good read.
There are genetic differences for tolerance to cooler, early seasons, but the significance must be always be put in perspective of final hybrid performance. This character is only one of many influencing the performance of best hybrid for a season. It among the many genetic-environmental reasons that rarely is the same hybrid the best in all fields or in all years. Genetics, seed production and environments interact each year as we have taken this species of tropical origin to temperate (and tropical) fields around the earth.
Interactions between seed physiological ‘vigor’, infection by fungi such as Fusarium species, environmental pressures including potential damaging organisms and seed treatments are complex.
A low percentage of seed within a seed bag are either dead of having sufficient cellular damage that all embryo cells do not function, perhaps with elongation of seminal root cells but no growth in the mesocotyl cells. Cell membranes damaged during seed maturation or with imbibition can self-repair, but this may result in delay of mesocotyl growth, delaying emergence compared to other seedlings and allowing more time for potential invasion by soil inhabiting fungi. Leakage of nutrients from the seed may also attract the fungi towards the germinating seed.
Fusarium species in the seed are not the only potential pathogens but also others are in nearly all soils. Fusarium verticilloidesis one that tends to invade corn tissue after germination, perhaps growing between cells as the seedling extends beyond the soil surface. A few, such as F. graminearumoften occupy the shoot base (crown), but it is not always clear if they significantly damage the plant. There is some evidence that presence of fungi in the emerging seedling correlates with reduced photosynthetic rate in leaves of the young plant.
Corn germinates and emerges more uniformly and quicker at 25°C (77°F) but temperate zone growers want to take advantage of the longer growing season by planting when soil temperature are only above 10-15°C. If the temperature remains low after planting, imbibitional damage to membranes is slow to repair and overall physiologic processes are slowed. Although Fusarium species are not favored by the low temperatures, the damaged tissue exudes nutrients to attract the fungi towards the tissue. Low temperatures also slow the production of resistance factors, allowing increased invasion of the tissue. This applies to the nodal roots that emerge after the seedling emerges as well. Soil components also affect the duration of exposure of mesocotyl if it has trouble pushing through the soil surface.
Seed treatments are intended to prevent or inhibit damage from seed-borne fungi and those potential pathogens infecting initial germinating seed. Polymers either added to the chemical fungicide treatments or even if used independent of the treatments can be helpful by slowing down the imbibitional process, potentially reducing the cell membrane damage. Most commercial seed treatments include a mix chemicals aimed at inhibiting fungi within the seed and a few components become somewhat systemic in the young seedlings. Application of seed treatments does require some care to make sure the seed does not absorb too much water and thus overcome the dormancy initiated by drying the seed. An interesting summary of Fusarium control by seed treatments can be found in a thesis at https://lib.dr.iastate.edu/rtd/15394
Among the human accomplishments of developing corn from a tropical grass (Teosinte) to extreme temperate zone environments has been the ability to get successful growth under less than perfect environments. This occurred with efforts of breeders selecting genetics, seed producers developing methods and growers working environments.
Pythium and Fusarium species get the most attention as being associated with seedling rot. These fungi are only marginally aggressive, however, mostly infecting plants that are weakened either from damage to the seed or from environmental stress.
Each seed within a lot has a separate experience as it develops in the production field. Position on the ear and synchrony of silking on the female parent and pollen production from the male parent affects the shape of the corn kernel. Resulting shape and size influences the vulnerability to damage during harvest, drying and handling within the production facility. Genetics of the female parent also affects the vulnerability to imbibition damage as well.
Environmental factors affect the duration from germination to eventual independence upon the transport of nutrients from the seed endosperm and water and minerals via seminal roots. Water is needed but too much water can limit oxygen supply to the submerged cells and thus limiting growth. Heat energy greatly affect the speed of metabolism and resulting cell elongation of the mesocotyl cells. Heavy soils with high clay content can present physical barriers. Feeding by soil insects and nematodes can offer avenues for soil fungi to enter the mesocotyl tissue.
Nearly everyone involved from the beginning of seed production to planting the seed can affect the establishment of a good final stand in the field. Good luck with the uncontrollable environmental factors also helps.
Several Fusarium species infect corn at all stages of the host’s life cycle. These fungi often are not aggressive pathogens in the sense of attacking vigorous cell tissue but are ubiquitous in soils and on decaying plant materials allowing opportunity to invade living, weakened plant tissue. Among those are senescing silks still attached to freshly pollinated corn ovules. This can result in obvious kernel rot on the ear but also some internal infection of developing seed. The fungus remains in the seed after drying and revives after seed is moistened. This seed may not be sufficiently damaged by the fungus to inhibit germination. Imbibitional damage to embryo cell organelles may allow more damage by the fungus, especially if cold stresses inhibit membrane repair, but germination, although delayed, may still proceed.
Cool soils during germination also gives advantage to Fusarium species in the soil to invade weakened seminal roots and mesocotyl tissue as the coleoptile is being pushed to the soil surface. If sufficient damage is made to the mesocotyl, cutting off the supply of nutrients from the seed endosperm and seminal roots, coleoptile and enclosed new leaves will die, as the very young seedling wilts shortly after emergence. If the young plant survives this early infection, the Fusarium can continue to spread within the growing plant, perhaps having no visible effect on the plant.
The genus Fusarium is the asexual stage of certain members of fungi known for their sexual stage as Ascomycetes. Asexual stages of these fungi often reproduce and spread through asexually produced spores called conidia. They allow aerial spread of the fungus but also can move within the vascular system of the plant. Sexual reproduction in most ascomycetes requires the combination of two mating types and thus is less frequently found. Consequently, often a fungus is identified by its asexual stage, such as Fusarium, and only associated with its sexual stage after study by specialists. Fusarium graminearum, commonly identified in seeds is the asexual stage of Gibberella zeae and Fusarium verticilloidessexual stage is Gibberella fujikuroi.
Gibberella zeae is associated with Gibberella stalk rot because the sexual stage (perithecia) is expressed at the outside of the dead stalk. On the other hand, fusarium stalk rot is associated only with the asexual stage of a different species, Fusarium verticilloidesbecause that is the stage most frequently found in the dead stalk.
Ubiquitousness of Fusarium fungi in corn often allows them to be associated with dying or dead tissue although they may not be the main cause. This is true of seedling rots and of stalk rots of mature plants often involves environment stresses but these fungi quickly spread to damaged tissue and thus become associated with the problem.
Corn seedlings are most vulnerable to damage before the emergence of the secondary roots. Pythium species are the most frequent pathogens at this stage, especially invading the mesocotyl tissue and thus interfering the transfer of minerals and carbs to the coleoptile before in emerges.
Pythium looks like a fungus, but it differs in structure and chemical constituents, making it have a closer relationship to brown algae. Among the distinctive features are swimming spores. Another feature common to Pythium and its relatives is an oospore produced after fertilization of the egg cell. Oospores develop thick walls allowing the organism to withstand dry and cold conditions in soil until water returns. Oospores germinate when exposed to water, producing filaments similar to fungal hyphae except Pythium filaments are mostly without septa that wall off individual cells. While exposed to water, sporangia form within the filaments to produce swimming spores called zoospores. These spores feature two flagella allowing them to swim towards potential host roots, assumedly stimulated by exudates from potential host roots.
Many Pythium species infect grass roots, including corn with a range of ability to overcome host tissue resistance. A study in southeastern Iowa in 2012 Showed Pythium torulosumas the most frequent Pythium species found among the nine found associated with seedling disease in season favoring the disease (https://crops.extension.iastate.edu/cropnews/2013/04/nine-species-pythium-associated-corn-seeding-blight-southeastern-iowa).
Mesocotyl tissue is generally most vulnerable to attack by Pythium species. Cool wet weather delays the elongation of the mesocotyl to the surface, prolonging the potential exposure and damage from this pathogen and yet encouraging Pythium to produce zoospores. After infecting the seminal roots and mesocotyl tissues, basically causing collapse of the tissues. Results could be seedling death or weak growth. It is a primary cause of weak corn stands and uneven growth in areas of fields that have prolonged early season wetness.
Although Pythium species can invade more older corn plants when flooded for some time, it is mostly a seedling pathogen. Seed treatments can protect the germinated seedling from Pythium, but there are isolates that are resistant to most seed treatments. Although these pathogens are common in soils, conditions for significant damage is not common.
Cell elongation in the area of the apical meristem results in the upward growth of the area between the seed and seminal roots and eventual emergence of the first leaf from within the coleoptile. That mesocotyl tissue functions as a passage for energy (glucose) and minerals supplied by the seed endosperm and seminal roots to the newly forming upper tissue.
Mesocotyl growth continues until the coleoptile emerges from the surface of the soil where reactions from light, especially the blue wavelengths, results in more lignin deposition in the cell walls of the mesocotyl, essentially stopping its elongation. The mesocotyl continues to function as a conduit to supply the development and emergence of the first leaves developing in the tightly wrapped coleoptile.
Metabolism in the mesocotyl cells is greatly affected by heat energy. Growth is slower in colder soils and could also be slowed with low oxygen supply when in fully water saturated soils. Heavily encrusted soil surfaces may also interfere with ability of mesocotyl ability to cause the coleoptile to emerge, resulting in twisted mesocotyl and even leaves to push through the tip of coleoptile before emergence.
The secondary roots, also called nodal roots, form at the base of the coleoptile just below the soil surface as it releases the first new leaves. This new source of carbohydrates along with the minerals and water absorbed the new roots as the utility of the kernel, seminal roots and mesocotyl is reduced. The relatively short performance of these seedling parts is essential in establishing timely starts to the crop affecting not only number of plants but also the uniformity of the crop.
Corn embryo cells are stimulated with imbibition as chemical energy in the form of ATP, allows for the production of complex molecules for elongation of existing cells and production of new ones. The radicle, located near the tip of the kernel, breaks through the pericarp to form the non-branching, seminal root. It is attached to the scutellum at a node. As this seminal root pushes away from the kernel, tropism from gravity, causes the root to go downward.
As the stem apical meristem elongates to form the first leaf tissue, the node at the base of the coleoptile, remaining in the soil, allows the growth of the lateral seminal roots. These roots branch, forming branches of a fibrous system. These lateral seminal roots also turn downwards because of tropism.
Energy for growth of seminal roots mostly comes via the scutellum from the endosperm. These carbs are moved to mitochondria for ATP production and to other cell organelles to form the new cytoplasm contents and cellulose cell walls as these roots grow. Residual carbs and ATP in the embryo are sufficient to get some germination, however, as shown when embryos excised from the endosperm germinate. The growth is limited, however, unless carbs are supplemented by artificial media.
Seminal roots absorb water and minerals from the soil further supplying the early growth of the seedling. After the apical meristem pushes the green leaf tissue through the soil surface, allowing photosynthesis to produce the carbohydrates, new nodal roots develop at base of the first several leaves. With this development, the seminal roots stop growing and start to senescence. Dependence on the stored carbohydrates in the endosperm ends as new sources are now available. Nodal roots now take over the role of water and mineral absorption for the plant.
Imbibition of water into dry maize seed occurs within a few hours after exposure, initiating cellular activity, initially in the mitochondria. Examination of mitochondria structure using electron microscopy show poor developed membrane structures in the dry seed. Mitochondria in dry seed have very low amount of oxygen uptake and low activation of enzymes. These characters change with imbibition.
Following 24 hours of imbibition, the mitochondrial membranes show more normal structures. Among the mitochondria, however, there are some that appear to not recover normal structures and function. It is easy to conjecture that these not recovering were either inadequately formed or were damaged during drying process. This may be related to either the nuclear genetics or mitochondrial genetics of the seed parent. Enzymes needed for membrane synthesis in the mitochondria is synthesized in ribosomes and thus dependent upon nuclear DNA and thus indicative of the significance of both hybrid parent genetics, but the mitochondrial DNA is only from the female parent.
These numerous organelles in each cell activated after imbibition become drivers of cell elongation to push the first root tissue to emerge from the seed. More info on this initial activity can be found at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC64868/
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.