Corn seedling vigor
Speed and consistency of seedling emergence is the first observation of the spring season. We often use the term vigor to describe this phenomenon. Hybrids, lots and seed sizes will vary in ‘vigor’ but also environments. Usually it is not easy to clarify which of these factors were major. Hybrid vigor can be affected by genetics. In some cases, it is affected by which of two parent inbreds was used as seed parent because the female parent contributes the mitochondria, and their genetics, to the cells and thus the major source of converting the stored carbohydrates into the metabolic energy needed for cellular division and enlargement. Total genetics of hybrids also has an effect on growth speed.
Seed lots of identical hybrids can vary in seedling vigor. Stresses in seed field when the seed was produced can influence some seed to have poorer germination and early growth. It could be as simple as rain delaying timely harvest that allows metabolism in the embryo to continue whereas earlier harvest and drying would prevent this process. Storage at less than optimum temperatures and humidity can also allow aging to occur in some lots.
Field conditions with uneven consistency of soil components and surface debris also will affect uniformity of emergence. Too wet or too dry also varies.
Each spring brings its own field stress and challenges. There is no reason to think this year will be different.
Seedling emergence problems
Those first few weeks in the spring when the corn seed germinates and emerges can cause grower anxiety as the hypocotyl pushes upwards. Multiple species of fungi surround the tissue, attempting to enter the fresh growth. 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 verticilloides is 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. graminearum often 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.
Elongation of seed embryo radical becoming the primary root system is often the first to emerge from the seed. Geotropism causes it to turn downwards. Shoot portion of the embryo emerges with a hypocotyl with a shoot meristem at its tip. Its direction of growth upwards is also part of the geotropism phenomenon. The timing of the growth in each direction is affected by heat energy and efficiency of the metabolism in the embryo cells. That energy is stored in the endosperm and translated into metabolic energy by the cell mitochondria. Deterioration of the cells through aging, moisture imbibition and physical damage to the corn embryo can affect the efficiency of these initial growths from the seed. Warmer temperature can minimize the damage by increasing the repair of damaged membranes in the cells.
Primary root growth has limited life as the seed source of energy is depleted. Hypocotyl upwards push towards soil surface until it detects far red light close to the soil surface. The final emergence coincides with emergence of the first collared leaf. This followed by a series of leaves. These leaves provide the energy not only for development of new green tissue but also the energy for new secondary roots growing from the first node of the stem at the base of the shoot in the soil. At this time the energy from the seed endosperm is no longer utilized as the primary root deteriorates.
The process for successful shoot emergence is dependent upon seed quality factors including genetics, especially of the female parent as the main supplier of mitochondria genetics, seed damage, field conditions including temperature.
It is amazing that the complex process is usually successful.
Pythium Seedling Disease
Corn seedlings are frequently attacked by a fungal-like organism of the Pythium genus when the soil is extremely cool and wet. Pythium belongs in a group of organisms called Oomycetes. These were once considered fungi, but more recent research has shown that they are more related to algae. Other pathogenic oomycetes include those causing diseases such as downy mildew of several grasses (including crazy top of corn) and Phytopthora root rot of many crops. Oomycetes were considered fungi because they usually produce filaments and spores plus they absorb nutrients from plants and animals. They differ from fungi by mostly lacking individual cells within the filaments, resulting in many nuclei within the long filaments. Most fungal cell walls are composed of chitin whereas the oomycete filament walls are cellulose, a difference that relates to the host resistance system. Often the first signal of fungal invasion is of the chitin, triggering the production of defense systems. Detection of an oomycete invasion requires a difference in plant chemistry.
Oomycetes frequently produce swimming spores that are released from overwintering spores (oospores) in the soil. Pythium spores, with flagella, swim to host root cells on which they grow hyphae to invade the host. Consequently, they cause the biggest problems in fields that are temporarily flooded with heavy rains and cool conditions. Studies have shown that 55°F is optimum for most Pythium species infecting corn in Midwest USA. The low temps probably slow down the host plant’s growth rate and inhibit potential competition from soil fungi.
As many as 18 species of Pythium have been shown to be pathogenic on corn. Often, they destroy the outer layer of the new root tissue. If this occurs before secondary roots become the main source of water uptake, the seedling will wilt. Although seed treatments can offer some protection, genetic diversity within a species often includes resistance to most treatment chemicals. Pythium genetic diversity also includes ability to attack rotation crops such as soybeans. Pythium species are easily isolated from soil but completely accounting for all the species or diverse genetics in regard to effective seed treatments or genetic resistance is not easy.
Control of the disease is best done with controlling water holding capacity of the soil.
Corn Journal Blog 03/22/2018
The Ht1 gene for resistance to the fungus causing northern leaf blight, Exserohilum turcicum, was effective during the 1970’s. This was before new races of the fungus that overcame the single resistance gene. One could detect the presence of the gene by putting the fungus on greenhouse-grown seedlings and evaluating for the distinctive resistance lesion that developed in about 10 days. As more inbreds, hybrids and segregating breeding materials were evaluated, it became clear that each corn genotype had distinct leaf morphological characters expressed in these young seedlings. Inbreds that had been deemed as pure from years of self-pollination had exactly identical expression of these characters, but segregating materials showed lack of uniformity in seedling morphology.
Previous experience with observations of late emerging plants that eventually resulting into deformed mature plants due to competition with adjacent corn plants led to the conclusion that one could evaluate for seed purity by observing seedlings closely in controlled greenhouse conditions. This led to a series of experiments and observations and eventual understanding that close observations of seedlings for certain key morphological features would allow evaluating genetic purity of hybrid and inbred seed lots. This technology has been applied by Professional Seed Research Inc. (PSR) since 1988 to multiple temperate and tropical hybrids for evaluation of purity (Seedling Growout®). Each corn genotype is distinct with seedling leaf morphology.
Close observation of seedling morphology also is implemented in corn breeding. As a trait is being crossed into an inbred, the segregates are formed with each generation of crosses, plants expressing the trait and having the most characteristics of the original parent can be selected for the next cross. Because the method allows observation of a large number of plants in a small space, this method can cut the number of generations needed to recover the desired inbred with the trait. PSR calls this corn backcrossing service Phenotype Select™.
Strange and fascinating how curiosity of those late emerging plants in the field and observations of seedlings could lead to new technology. Variables in crop-growing has provided many opportunities for participants to develop machinery and specific methodology for improving crop production.
Late-emerging corn seedlings
I was pursuing an explanation of why did some plants wilt and develop stalk rot when adjacent plants stayed green and complete grain fill on solid stalks. Were these plants individuals that were stressed for the whole season because they emerged late? Why these individual plants? In the early 70’s we had a cold wet spring in the corn hybrid observation plot, with uneven seedling emergence and an opportunity to tag those individual plants that adjacent to earlier emerging plants. Those individual plants were observed during the season. They were obviously less ‘vigorous’ than adjacent plants, had narrow stalks, and delayed ear shoots. I asked the seedstock manager to look at these tagged plants to see if they looked like inbreds, created by selfing in the hybrid seed production field. He replied that he had seen similar plants before and said it does make evaluation of hybrid purity in winter ‘grow outs’ difficult. Final notes at the end of that season showed that these late emerging plants had significantly fewer kernels, if any at all.
These plants did not develop stalk rot. Were they impurities in the seed lot, perhaps inbreds? The following year, hybrid seed was hand planted with extra space between plants. When these seedlings emerged, the same hybrid seed was planted between the seedlings. Those late planted seedlings were observed during the season. As suspected, Late-emerging hybrid plants performed as those in the previous year, having spindly stalks and few kernels but no stalk rot.
Those observations led this young observer of corn to look at other possible explanations for why stalk rot of corn and more experiments and eventual conclusion that the individual stalk rot plants had produced more grain yield than those individual plants environment could support, essentially depriving the roots of needed photosynthesis products to keep them alive.
Late emerging plants were detracting from yield and not the ones that develop stalk rot.
Roots down, stems up
At this time of the year in northern temperate zones, as growers are waiting for soil temperatures to be high enough for planting, one can contemplate why, no matter the position of the seed, does the root go down and the shoot go up. This has been studied by many, including Charles Darwin in 1880. The total mechanism is still not completely understood.
Geotropism is the response to gravity and phototropism is response to light. Root’s downward growth is affected by the tissue in the root tip meristem area. The root cap cells (outside the meristematic cells) effect the tropism. Removal of the cap appears to disorient the growth direction. Removal of cells on one side of the cap, causes the root to grow towards the remaining cells. It is possible that inner cell structures such as the endoplasmic reticulum, and other cell organelles tends to become more concentrated on the lower side of the cells and thus produces metabolites on that side, resulting in cell growth in that direction.
Plant hormones such as auxins and gibberellic acid are involved in plant tropism, but all of the exact mechanisms are still not understood. It has been proposed that auxins produced in the root tip are distributed to cells behind the root tip, affecting the elongation of exterior cells and thus direction of growth. Differences among corn varieties in root growth direction tendencies, some with more deep, narrow growth and others with more lateral growth, is evidence that genetic factors are influencing metabolism that affects root growth direction.
Phototropism is also affected by plant hormone production and distribution. Specific wave lengths in natural sunlight affect the auxins involved in the differential cell elongations, ultimately resulting in growth direction towards the light source. Exact mechanisms involving photo receptor compounds that allow specific wave lengths of light to turn on the growth cells is not clear, despite many researcher’s attempts to study specifics.
Tropism in plants is obvious and complex. Surely some of those 30,000-40,000 genes in corn are involved, and mostly we can only screen for the effects of the phenomenon with selecting the best performing corn plants and admire the fact that tropism exists.
(Corn Journal 4/19/2018)
Effect of uneven emergence
Corn genetics and planting rates have changed a lot in the last 50 years. Breeding and selection of hybrids when planted in the USA Midwest at 22000 plants per acre required different genetics than when planted at 36000 plants per acre. Plants had more kernels supplied by more photosynthesis per plant. Higher densities with those hybrids would certainly develop the photosynthetic stress dynamics resulting in stalk rot because of insufficient carbs to supply both the grain and roots. More plants per acre, and slightly smaller ears per plant allowed a total of more grain, compensating for the reduction in photosynthesis per plant. On the other hand, these types of hybrids are more reliant on getting maximum number of productive plants. This is not only the necessity of high number of germinating seed but also that the seedling emerge uniformly. This was discussed in Corn Journal on 3/10/2016.
40 years ago, when pursuing the question of why one plant died early with stalk rot and the adjacent plant did not, I hypothesized that the dead plant was that one emerged late as a seedling. When most of the plants in the test plots showed their 5th leaf, a tag was put near those that had only 3 leaves and another marking those with only a spike. Notes were taken of these plants and their adjacent plant during the season. At pollination, it was clear that even those tagged at three leaves were not silking in time with adjacent plants and tended to have more slender stalks. Many of those tagged as spiking no longer were present, but those that survived were far off in pollination timing, had very small, narrow stalks and, eventually, small tassels. Ears were harvested at end of season and kernel numbers were counted. Those tagged with three leaves had only 20% of kernels of adjacent plants and those tagged as spike only were barren. Delayed emerging plants did not develop stalk rot but clearly the delay affected yield.
To eliminate the possibility that these delayed plants were not ‘selfed’ inbreds instead of hybrid plants, an experiment was performed the next season to confirm that emergence delay was the main factor. Seed was planted with twice the normal plant-to-plant spacing. When those seedlings spiked, the same hybrid seed was planted between the seedlings. This would be an unusual delay, but the effect was the same as the first observation. Barren plants, skinny stalks, small tassels were characteristic of the delayed emergence. Apparently, plant competition for late emerging plants has a drastic affect.
Others have done similar experiments before and after these done by a young guy beginning to learn about corn. My conclusion was that individual plants developing stalk rot were not the late emerging ones and that uniform emergence was an important factor in corn yields. Also, it was interesting that those late emergers could be confused with selfs, as confirmed by the fellow who normally evaluated hybrid purity in winter growouts. That eventually led to developing a different purity test method.
My experiments were done in the 70’s with hybrids and plant densities common at that time. It would be useful if similar experiments were done with more recent hybrids selected for consistent ear development at the higher plant densities used today. It is notable that experiments done by others have shown that among germination test methods, the cold test is the best predictor of field emergence and that it accounts for about 70% of differences among seed lots in field emergence.
Seed deterioration and emergence
Corn seed germination quality is always temporary. Female seed parent genetics, especially that of the mitochondria, area factor. Seed with dormancy because of low moisture content remains alive as long as the mitochondria have a low level of respiration to maintain membrane integrity, it becomes more difficult if kept in humid environments at temperatures greater than 50°F. We have seen many examples of seed kept on farms because 2019 weather prevented planting. Some will show in the field this spring.
Perhaps, for the first 8 centuries of corn culture, the percent of seedlings emerging from seed planted was not as significant as the amount and type of grain produced. Planting in hills or even by planters at low densities, favored genetics that produced larger ears if adjacent seed did not germinate. Modern terminology attempts to identify hybrids that adjust of lower density by producing larger ears defining them as ‘flex’ ear types. It is doubtful that there are only two types, flex and non-flex, but hybrids do have different tendencies when plants are less dense. There are hybrids that will tend to go barren if planted too dense and there are others that will only show competitive yields if planted at high densities. There are some that do not cut back on kernel numbers if crowded but will develop high percentage of stalk rot if planted too thick. Recent years of corn culture in the USA has led to hybrids that tolerate planting a 33% higher density than 40 years ago- and give more stable high yields than hybrids of that era.
These modern hybrids may have genetics for more photosynthesis per acre, at least due to increased leaf area, but also consistent silking when under the stress from competing plants. This may also favor selection of hybrids with less ‘flex’ and consequently a need for higher plant density. Although best knowledge of the ideal plant density in any season only becomes apparent after harvest, there is more pressure now than 40 years ago to have a uniform emergence percentage of 90+% every season.
Every seed within a single cross hybrid may be nearly genetically identical, but not with identical germination quality and not planted in identical microenvironments. Highest quality seed will tend to emerge uniformly, and seed producers make large efforts to produce such seed. However, even with best genetics, timely seed harvest and care in handling the seed it is rare that field emergence is perfect. It is a challenge to produce all seed lots with great quality and for testing systems to correctly predict the field emergence the following spring.
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.