Abscission layers in corn
Nearly all plant species shed plant parts with the function of allowing continuation of the species. Temperate zone deciduous tree leaves reduce the supply of nutrients to the chloroplasts as summer ends. Nitrogen and other nutrients are transported from the chloroplasts and other leaf cell contents to plant parts that will survive the winter. This physiological change reduces movement of auxin hormones to the cells at the base of the leaf where it is connected to the stem. This stimulates the production of lignin and suberin in cells immediately below a layer of cells called the abscission layer. This thickening of cell walls essentially cuts off all translocation of water and nutrients from the stem to the leaves. It also causes the leaves to break from the stem at the point of the abscission layer while preserving the moisture in the stem.
The same process occurs in all plants, including in the tropics, although not as remarkably simultaneously colorful as with trees in the temperate zone at this season in north America. Similar physiological phenomena occur as fruit are released from the base plant. This was also true for the Teosinte species selected 10,000 years ago by someone because, contrary to most Teosinte plants, this genotype formed abscission layer at the base of the fruit (kernel) but did not quickly allow the kernel away from the parent plant. Maintaining the attachment of the kernel to the rachis (cob) even after abscission layer (black layer) formed allowed more efficient harvest for those folks 10,000 years ago, as it does for the modern farmer. Selection of this character by corn breeders since that time have attempted to maintain appropriate attachment of the kernel (fruit) to the cob thru harvest.
Leaves surrounding the corn ear, the husks, undergo similar abscission layers after the black layer is formed in the kernels. This cuts off the water movement into the husk leaves, usually causing them to withdraw from being tightly wrapped around the ear. This is associated with faster evaporation of water from the kernels. Likewise, abscission occurs in cells at the base of the cob. This further removes water movement into the cob, allowing for further drying of cob and kernels. Corn breeders challenge is to select genetics that allow rapid kernel drying while maintaining attachment of the ear to the corn plant until harvest. Stress conditions may cause premature formation of abscission layers in kernels and husk leaves. This has been associated with ears dislodging from the plant. Pests such as European Corn Borer have been associated with premature dropping of ears in the field.
The balance of timely formation of abscission layers to allow drying of grain, maintaining adequate attachment of kernels to cob and ears to the plant when grown in multiple environments continues to be a complex genetic feat. Combining these characters with maximum translocation of sugar to the grain before formation of the abscission layer at the base of the kernel adds to the corn breeders’ accomplishments for the past 10,000 years.
Stalk rot resistance genetics
Corn stalk rot involves nearly all biology of the corn plant. Consequently, the genetics of resistance is complex. The common fungi associated with stalk rot such as Gibberella zeae, Fusarium verticilloides and Diplodia zeae are often identified as the main fungi associated with destruction of stalk tissue because of their obvious symptoms but no specific genetic resistance to these fungi has been identified. Anthracnose stalk rot, caused by Colletotrichum graminicola, is associated with a few specific resistance genes that can be identified but this has minimal effect on the overall occurrence of the corn stalk rot.
Resistance to premature deterioration of the corn stalk involves nearly all aspects of the plant, at least post-flowering. Identity of the fungi most obvious in the dying stalk tissue is mostly irrelevant. Genetics of resistance involves those affecting root growth and function in that season’s environment. This could involve resistance to feeding damage from soil insects, nematodes and pathogens. Genes affecting the structures for water translocation to the stem or the metabolic efficiency of the living root cells influence the plant’s ability to function when under stress.
Photosynthesis rates during the season are affected by genetics of leaf shape and size. Leaf disease resistance is affected by both the general physiology and genes specific to each potential pathogen. Translocation of sugars to the roots and to the grain after pollination involves many physiological and structural aspects of the corn plant, each influenced by different gene interactions.
Movement of sugars to the grain is influenced by the number of ovules pollinated. Genes for establishing that number are multiple. Rate of movement to each kernel is affected by genetics not only within the embryo but also those affecting the vascular structures to carry the sugars. Multiple genes must be involved in the timely extension of the silks allowing successful pollination and fertilization of the ovule.
Successful maintenance of living pith cells in corn stalks until completion of translocation of sugars to kernels involves multiple genes affecting most parts of the corn plant in its environment. It is unlikely that a single gene for stalk rot resistance combined with acceptable grain yield will ever be identified. We will continue to realize that stalk rot resistance involves multiple genes influencing most of the corn plant’s biology just as does grain yield.
It is historic!
Ten thousand years ago, individuals in central Mexico harvested this new version of grain, derived from a wild plant, Teosinte that remained attached to the central rachis of the flower instead of shattering. They realized the increased carbohydrate increased the flour available for cooking. The annual plant spread by humans throughout the Americas for the next eight thousand years as others moved the seed to their environment, annually selecting genetics that performed best in their environment and their culture. This history of annual selection for adaptation in multiple places, separation of male and female flowers and unique C4 photosynthesis has attracted people to this species each harvest season since that initial discovery in Mexico.
All corn people from the first to those in 2018 have to be impressed with what they saw with corn. Kernel color or hardness, or amount of grain variation allowed selecting and saving desired genetics. Superior harvest was supported by the variety having resistance to local disease pressure, root structure adapted to local soils, timing of pollination matching weather conditions and maturity adapted to time between freezing.
It became apparent over a hundred years ago that crossing varieties adapted to different environments could result in new corn varieties superior to either of the two parents. This ultimately led to concentrated efforts to match breeding populations that contribute genes resulting in superior hybrid performance. With nearly 40,000 genes to work with corn breeders have realized the wealth of genetic diversity that this history has wrought.
We humans have pushed this energy converting crop to increasing yields in multiple environments. Every corn grower will be evaluating the current varieties to choose hybrids for the next season. Every corn breeder share in the excitement at harvest time as they witness the progress within the genetics that they select. Corn gains each year in some aspect of production due to the tremendous genetic diversity due to people’s selection for their preference over the past 10,000 years. It is historic!
corn ear rots
There are many reports of ear rot this year in the US corn production. Ear rot is caused by several fungal species, each influenced by the environment at the time of infection. In most cases, infection occurred at pollination time. Environments that were unfavorable to successful pollination when the silks extended beyond the ear husks but were favorable to a fungus with potential for feeding on the silks is linked to infection.
Rainy days influence rapid extension of the silks, exposing them to pollen. However, release of pollen grains from anthers involves desiccation of the anther sac and dry air influences the movement of corn pollen. Consequently, rain and extremely high humidity allows the unpollinated silk to be exposed to fungal spores for a prolonged time. Pollen normally grows rapidly down the silk channel towards the ovule, and the silk collapses behind it, leaving a dry mass of lignified tissue not easily and quickly penetrated by fungi. Delay in pollination results in a more nutritious substrate for fungi such as Fusarium verticilloides, Gibberella zeae and Stenocarpella(Diplodia) maydis.
Aspergillus flavus and the smut fungus Ustilago maydis are favored by an extreme dry pollination period. This stress can cause delay of silk extension. Pollen maturity and release is not as affected by drought and consequently may be mostly spread before the silks are available for pollination. The drier environments favor the spread of these two fungal species.
The fungi grow on the surface of the silk to the ovules. If the infected silk channel does later support pollination or if adjacent ovules are pollenated, the mycelium spreads to adjacent, newly-formed kernels. Some ears may have infection limited to a few kernels, but others may have mycelial growth covering the whole ear.
The smut fungus often grows within the infected kernel, replacing it with a gall of mycelium and spores. These spores (teliospores) overwinter in the soil, germinate the following summer to produce basidiospores that infect plants of the next season. Infection of leaf tissue can result in gall formation on leaves or stems, providing inoculum for infection of ears.
Infection of a few ears in a corn field by any of these fungi is not unusual as each plant’s environment is slightly different. Genetic susceptibility evaluation is difficult because it involves tendencies for silking and pollen production during stressful conditions. This is especially confusing in small plots with multiple hybrids. A hybrid that has silk emergence timing different than most other entries may have more vulnerability to fungal infection.
Ear rots are result of host and pathogen biology interacting with environment variability. Timing is everything concerning corn ear rots.
Corn stalk strength
Harvest of corn for grain depends upon the stalk strength to maintain upright plants. Environment and genetics were major factors influencing the ability of the plants to be upright at harvest time.
Photosynthesis supplied the carbohydrates deposited as the strong lignin, cellulose and hemicellulose components of the outer rind cells. These molecules not only provide physical strength but are difficult for digestion by invading microbes. Factors influencing photosynthesis during the season affect the supply of carbs for formation of these complex carbohydrates. Plant density affects the light intensity per plant. Leaf disease can detract from carb production. Minerals influence affect photosynthesis and construction of these complex carbohydrate molecules in the outer rind of the corn stalk.
Net production and distribution of carbohydrates in the corn plant influences the stalk strength not only by effecting the deposition in the outer rind but also by maintaining the connection of the pith tissue to the rind. Monocotyledon plants like corn differ from dicotyledons in the distribution of the vascular bundles. Xylem cells, with their solid outer walls of lignin are organized in the center of the dicot plant such as the wood of a tree. Xylem cells in monocots are in separate vascular bundles scattered among weak-walled parenchyma cells in the center of the stem. Parenchyma cells in a corn stalk are living cells that become temporary sugar storage cells. Pith contributes to the stalk strength only when it is physically connected to the rind. If the plant wilts before completion of grain fill, dehydration of these cells results in withdrawal from the rind tissue, not only reducing a rod to a tube, but reducing the strength contribution of the vascular tissue.
Genetics effects photosynthesis, formation of complex molecules in the rind tissue and maintenance of pith tissue’s connection to the rind. Environments influence photosynthesis. Harvest time allows us to observe how these factors were in action for this season.
Root lodging vs stalk lodging
2018 harvest in US Corn belt is showing some lodging problems. Analysis of cause(s) should begin with distinguishing between what is called root lodging and that named as stalk lodging.
Root lodging generally occurs anytime during the growing season when the lateral roots of the plant are insufficient to avoid the plant leaning when faced with winds. This vulnerability can be associated by feeding on roots by insects, soil texture or moisture inhibiting growth of the lateral roots or simply inherited tendencies for deeper ‘taproot’ that may be a favorable character for reaching water during drought conditions but a disadvantage if water soaking in upper soil softens the soil. If root lodging occurred before flowering, geotropism will cause new growth to result in curved plants. These plants and those that root lodged later in the season usually maintain green, lower unbroken stalks. Pre-flowering lodging may cause incomplete pollination and therefore reduced grain yield and later lodging may complicate harvest.
Stalk lodging has different causes and symptoms than root lodging. Symptoms of stalk lodging begins about 15 days before kernel abscission layer (black layer) forms. Causes involve the supply and translocation of carbohydrates to the roots, stalk and grain. Nearly always, root tissue death causes the plant to wilt because insufficient water is absorbed from the soil to meet the water loss from transpiration in leaves. Wilt causes cell death in all tissue including the stalk. Dessication of the stalk pith cells causes withdrawal from the outer rind, reducing the stalk strength. Various fungi invade and further weaken the rind tissue. Late season wind now can cause the stalk to break at the lower internodes. Early wilt results in stoppage of translocation of sugars to the grain and therefore light grain density. Broken stalks maybe difficult to harvest.
Inspection of the stalks before harvest can discern the difference between root lodging and stalk lodging. Lower internodes of root lodged plants will be green whereas stalk lodged plants will have brown outer rind color in the lower internodes. Genetics involved in these two types of lodging are different. Root lodging involves genetics affecting root type of growth whereas stalk lodging genetics involve factors affecting photosynthesis and sugar translocation factors including multiple plant biology interactions. A deep-rooted hybrid may be favored in drought seasons because it reached water, favoring silk extension and therefor more kernels than the hybrid with more lateral roots towards the soil surface. The latter may suffer with fewer kernels because of silk extension missing pollen. On the other hand, if the late season is unfavorable for photosynthesis, the deep-rooted hybrid could be vulnerable to stalk lodging.
Competition between roots and grain for products of photosynthesis can result in root death followed by premature wilting and death of the corn plant. This is followed by collapse of stalk tissue, followed by invasion by stalk rotting fungi. Increasing grain yield while maintaining stalk quality obviously requires increasing photosynthesis either per plant and /or per acre. We have seen progress in this during the past 30 years by increasing the leaf area per acre, reducing the grain per plant but increasing the grain per land area. Selection for better stalk quality along with grain yield under higher plant density also could be selecting the genetics for improved photosynthetic efficiency.
Some genetic input to leaf structure that affect capturing more light could be effective. Chromosomal genetics and chloroplast genetics affect the number of chloroplasts and their productivity. Mitochondrial genetics influences the efficiency of converting the carbohydrates into useful energy need to drive cellular physiology as well as building of the plant structures. Some beneficial characters such as leaf disease resistance are visible, but much is not obvious. A recent paper (Plant Physiology:10.1104/pp.18.00176) identifies mutants affecting the opening and closing the stomata, which affects the loss of water when open but also allows the CO2 movement into the leaf for carbohydrate synthesis. There must be multiples of other mutants to critical aspects of increasing net photosynthesis per acre that are available but unknown.
We currently mostly select for these mutants by yield testing for improved grain yield and stalk quality. We don’t know the specifics of what genetics are responsible, but we have witnessed that this improvement has occurred and expect that it will continue even if we do not which mutations in corn’s 30000-40000 genes were mostly responsible.
Agronomic practices are also contributing to improved yield and stalk quality affecting total plant physiology. Weather also contributes as it affects the plants directly, nullifying some genetic advantages and favoring others. Fortunately, the breadth of genetic variability available in this species allows general improvement in grain production.
Genetic diversity works for everyone
Humans benefited from the diversity of genetics in corn. Completion of a generation within months, easy transportation of the result of reproduction, cross pollination allowed quick selection of genetics fitting the corn growing environments. Corn breeding techniques continue to be developed to further exploit this diversity.
Other organisms affecting cultivation of corn also have genetic diversity. Selection pressure to survive and reproduce encourages favorable genes in pathogens of corn. As the Ht1 gene was widely used for resistance to Helminthosporium turcicum (Exserohilum turcicum), a gene within this fungal species population increased in the USA within about 10 years. The corn gene severely suppressed the reproduction of the more prevalent previous race, allowing quick increase in the new race. Small genetic changes in several pathogens has resulted in appearance of new corn disease problems. Goss’s wilt bacterium may have become a corn pathogen after a gene change that allowed it to move from a minor grass pathogen to causing a major disease on a few corn genotypes. Race t of Helminthosporium maydis(Bipolaris maydis) was a mutant that produced a toxin causing disfunction of the mitochondria of corn that was utilized by seed corn producers to facilitate seed production. Multiple races of Helminthosporium carbonum(Bipolaris zeicola) have evolved adaptation to specific corn inbreds while being maintained on other grasses. Diversity in potential pathogens will continue to challenge corn and diversity in corn will continue to allow selection of genotypes to resist the new race.
Hybrid corn does benefit from the genetic diversity between the two parents. Selection for grain yield when each parent covers the other’s genes that have a negative influence on plant performance often includes resistance factors to a specific race of the pathogen. Often parent inbreds are more susceptible than the hybrids. Pathogens often have the advantage of rapid reproduction, but corn has the advantage of active human’s ability to select for resistance within the diverse genetics available.
Advantages of genetic diversity applies to all species- and we should not forget that it also applies to our species. Cultural diversity among humans is also beneficial.
It is corn harvest time in the Northern hemisphere. Benefits of genetic diversity in this species becomes increasingly evident at harvest time. Grain yields vary among the hybrids and environments. Differences were the culmination of the multiple genetically influenced morphological features. Roots that extended deep in the drier environments provided access to more water, increasing the turgor pressure for silk elongation during the time of exposure to pollen. On the other hand, if the post flowering environment was extremely wet this tap-rooted hybrid type could be vulnerable to root lodging. The hybrid with shallow, branching roots favored in organic rich soils favored by frequent rain during the season, may not have had enough turgor to push out these silks during much of the pollen shedding time, harming the grain yield.
Leaf size and shape differences were factors in photosynthetic rates among hybrids. These morphological features interacted with plant density, fertilizer and sunlight intensity of the summer to affect carbohydrate production by the plants. Leaf disease resistance affected net photosynthesis during the season. Genetic differences among hybrids for reaction to accumulated heat influenced the timing of pollination. Longer time to flowering allowed for more accumulation of carbs in the stalk and more reserve for grain fill if late season stresses reduced photosynthesis. Genetically influenced differences in vascular tissue structure affected the rate of movement to sugars to the grain. Varieties differed in hormone influenced rate of movement to each kernel. Duration of sugar translocation after pollination was also affected by genetics. Ability of roots to maintain life when under the competitive pressure from grain filling was another difference among hybrids.
Kernel feature differences become visible, especially to corn breeders as they harvest nurseries. Endosperm texture differences are evident. Yellow colors vary in intensity, with simply inherited traits such as blue, red and white endosperms being most extreme. Shape and length of the ear are inherited traits contributing to the grain yield and drying characters of the hybrid. Less obvious genetically influenced character such as pericarp thickness are important to grain drying after formation of the abscission layer at the base of the kernel. Genetics affected the number and length of husk leaves surrounding the ear. This affected protection of grain from pests but also field drying of the grain.
As we observe the diversity of corn genetics we should appreciate that 8000 years of human interaction with this species as resulted in a diverse crop adapted to multiple environments across the earth. Each season, in each field, may favor different specific genetics but diversity is the key to the long-term success of this efficient converter of light into useful stored energy.
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.