Seed viability vs seed vigor
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.
Corn growing season begins
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.
Establishing a corn stand
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!!
Corn performance variables
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.
Corn seed variability
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.
Corn anthracnose factors
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.).
Anthracnose seedling disease
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.
Corn seed treatments
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.
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.