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.
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.).
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.