Corn belt of USA 2021 weather has featured a cool April and May and hot June and early July. Most corn planting was done in April, as usual. Those temperatures and genetics interact to affect the timing of the apical meristem switching from producing more leaves to those specialized cells of the tassel.
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature. Once again, we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years. Photoperiodism in plants is evident as we see many species that bloom at the same time every year. A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex. There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812. Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud. Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date. Our company does some breeding projects with tropical material. Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet. Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers. One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids. This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater. Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting. Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering. Those tassels that we see emerging in corn fields now, in mid-July, were initiated several weeks ago as determined by genetics and heat of the early corn season. Whereas movement of water in corn is mostly a passive action through the dead cells of the xylem that form a narrow tube within the vascular system, distribution of carbohydrates from the sources of photosynthesis to elsewhere in the plant requires living cells of the phloem tissue.
The sugar sucrose is formed from the glucose product of photosynthesis and becomes the sugar for distribution within the corn plant. Movement of carbs between cells can be simple diffusion through those small ‘holes’ in cell walls, the plasmadesmata, as the molecules move from a high concentration to a low concentration. It is a little more complicated with travel through membranes by osmosis, but the basic principle is the same. Water is involved because It is the solvent of the sugar. Greater concentration of water equals less concentration of sugar. Water molecules are also affected by the principles of movement from high to low concentration, setting up dynamics for what is called turgor pressure within each plant tissue. As the sucrose molecules move into a ‘sink’ such as newly formed cells of a growing point more complex molecules such as starch and thus maintains the osmotic pressure for more movement of sucrose into the kernels. Other sinks, such as biologically active tissue of all living plant cells, consume the sucrose in cellular respiration and formation of essential amino acids and cell structures. These various sinks are not all in the same direction from the carbohydrate sources where the photosynthesis occurred and consequently flow among phloem cells may not be in the same direction. Whereas water movement in xylem tissue of the vascular bundles of a corn plant is mostly upwards from the roots, movement of the products of photosynthesis is affected by concentrations of various sugars in the sinks, allowing bi-directional flows. All plants are dependent upon water to live. It is essential to nearly all physiological functions in the plant. This is due to the unique properties of water as a solvent allowing movement of metabolites within cells and among cells. Not only is water a good solvent but also has the cohesive property of water molecules tendency to adhere to each other. Corn plants are very dependent upon these characters of corn.
Water has a complicated interaction with corn physiology and function. It moves through root hairs via osmosis, water moving from a high concentration through cell walls where sugars and minerals reduce the water concentration. It is a physical phenomenon. Osmosis further causes water molecules to move to the xylem vessels. This pressure pushes water up the vessels. Leaf stomata open during the day because photosynthesis in the two curved cells surrounding the produce sugars resulting is swelling, again due to osmosis drawing in water. This causes opening to the air, allows movement of CO2 into the leaf and oxygen into the air. Water evaporates in the opening below the stomata, moving into the air, again moving by relative concentration of water molecules. Dry and windy air increases the rate of transpiration. Water molecules tendency for cohesion, causes water to be pulled upwards, as each molecule transpires through the stomata is replaced by a molecule pulled from the xylem. It's a push from below the soil surface and a pull through the stomata that moves water through the plant. During cell elongation, before flowering in corn, water movement into new cells largely determines length of cells and ultimately affects plant height. At flowering, this cell elongation process become critical to the timing of ear silks pushing out of ear husk tissue for exposure to pollen. Pollen production and distribution is less dependent on water concentration and therefore timing of pollen and exposure of silk may not match. Poorly pollinated ears are the result of drought conditions. Photosynthesis utilizes water as H2O is combined with CO2 to make glucose (C6H12O6). Drought conditions during the growth period can reduce ultimate leaf area and thus photosynthesis. Severe drought can result in stomata not opening, reducing the CO2 available but this appears to be most significant before pollination. The biggest cause of grain yield loss from drought stress is not reduction of photosynthesis but it is the lack of place to put its products. Movement of the glucose from the leaf tissue to grain is determined mostly by the hormones produced in the newly formed embryos in the pollinated ovules. Lack of water reduces elongation of silk causing them not to be exposed to pollen and consequently fewer embryos. Sugar molecules accumulate in leaf tissue, triggering production of anthocyanins in leaves, turning the leaves red. The pigment change reduces photosynthesis. Corn biology is dependent upon adequate water supply for nearly all functions from being a solvent for movement of sugars and minerals, providing turgor pressure for cell expansion, a coolant as it evaporates from leaf tissue and contributor to photosynthesis. Early June 2021 weather in western USA corn belt featured hot and dry conditions. Affect of moisture on corn discussed in last blog can result in shorter corn plants than normal. Affect of heat on time to flowering on temperate-zone corn was discussed in Corn Journal Blog 7/12/2016.
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature. Once again, we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years. Photoperiodism in plants is evident as we see many species that bloom at the same time every year. A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex. There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812. Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud. Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date. Our company does some breeding projects with tropical material. Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet. Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers. One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids. This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater. Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting. Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering. There will be some short corn plants, flowering extra early in parts of USA in 2021. Every corn growing season differs in two important factors affecting plant height and flowering of corn plants. The growing point of the corn plant continues to produce new cells until the V5 (5 visible leaf collars). At that time, the new cells differentiate to produce the tassel cells. It is elongation of those cells that determines the plant height. Pressure from the water transported through the xylem to the upper plant tissue and newly formed cells causes the cells to elongate. Heat energy, interacting with the genetics, determined the change in cells produced by the apical meristem from producing more stem and leaf cells to tassel cells. This genetically affected trait has allowed a tropical plant to be adapted to temperate zones around the earth.
Seasons and environments differ in both water and heat with the consequence of important corn productivity factors including leaf area, accumulations of carbohydrates, timing from planting to harvest, uptake of minerals, number of kernels per ear and ultimately grain yield. Early season hot and dry weather results in shorter plants and early flowering. Continuation of excessive heat increases evaporation of water through stomates, potentially dehydrating leaf cells, affecting photosynthesis in chloroplasts and flow of glucose through phloem in vascular system to roots and newly formed kernels. Timing of excessive heat or drought is significant to the final season affect on grain production and standability of the corn crop. Corn breeders select genetics that tend to perform well in multiple environments, but all are affected when heat is unusually hot and soils are extra dry. Photosynthesis in all plant species involves multiple steps with many enzymes that can be divided into two main steps occurring in the membranes making up most of the content of chloroplasts.
Light energy is used to split H2O molecules into its hydrogen and oxygen atom components freeing up electrons. These electrons allow provide the energy to unite the oxygen and carbon form the glucose molecule (C6H12O6), releasing the excess O2 molecules that eventually escape through the stomata. Electrons binding the components of glucose later are released in the respiration processes within the cells, providing energy for synthesis of proteins for growth in plants plus movement in animals. This photosynthesis process is present in most plant species. The release of oxygen through stomates is dependent upon open stomates and therefore is dependent of light being absorbed by the stomate guard cells and sufficient water transported from the roots to maintain those cells to swell. Thus, at night and during droughts most plants no longer can absorb the CO2 needed for more glucose synthesis and the excess oxygen is consumed in nighttime metabolism. This is a character of most plants with C3 photosynthesis processes. Some species, including corn, have evolved a system to avoid this wasteful system. Chloroplasts in the corn leaves make normal photosynthesis process but then break down the C3 molecules, have them transfer them to the specialized, vascular bundle cells surrounding the vascular system that are loaded with special chloroplast for C4 molecules. These molecules are then enzymatically combined to make sugar which is moved elsewhere in the leaves and other parts of the plant. This system occurs in species that are native to dry, hot environments such as that of corn’s central America origin. The ultimate advantage is that corn can continue to produce carbohydrates despite environments that cause stomates to close. Whereas most C3 plants such as soybeans, wheat and rice do not utilize light intensity greater than 3000 foot-candles, corn photosynthesis rate keeps increasing with light intensities to our maximum sun brightness of 10000 foot-candles. Those few cells surrounding the xylem and phloem of a corn leaf vein have a special role in allowing the photosynthetic efficiency of maize. This type of photosynthesis drives the rapid growth of a corn plant and ability to store excess glucose as starch in the grain. Chloroplasts in corn provide the structures for the corn plant to be the most productive of crops in converting light energy into usable forms of energy. Selection of features by humans over the past 8000 years assisted but natural selection of origin in hot dry Central America gave great assistance by evolving a C4 photosynthesis metabolism. Most plants have a photosynthesis system with an inefficiency that limits its productivity. This system, labeled as C3 photosynthesis, peaks in its ability to fully use total light intensity to about 3000 foot candles where-as unclouded sunlight has 10000 foot candles. In corn, with it C4 photosynthesis, it continues to produce carbs in direct relation intensity of the light with maximum photosynthesis in bright sunlight.
Carbon dioxide enters plants through holes in leaves called stomata. These structures also allow oxygen to escape from leaves to the benefit of all of us. Water vapors also go through the same stomata. Stomata open and close. At night they close with the benefit of avoiding unnecessary loss of water when photosynthesis cannot occur. But when plant tissue is stressed from lack of water, these stomata also close, limiting the water loss but also interfering with uptake of carbon dioxide for photosynthesis. C3 photosynthesis doesn’t make carbohydrates out of all the CO2 it absorbs, using some of it in other molecules. No problem when environment provides plenty of moisture, is generally cool and have long summer days, but some plant species that evolved under hot dry conditions evolved systems to overcome that limitation. Teosinte, the species of origin for corn in Central America, has a C4 photosynthesis system. Plants with this character have additional structures in their leaves surrounding cells that perform photosynthesis. These cells function to reduce the loss of CO2 by causing these molecules to be recycled into more carbohydrates. The combination of extra enzymes and structures comes at some energy cost but the net gain is both more net carbohydrate and better utilization of CO2, even if stomata are closed. Fortunately, corn that was moved out of the original dry hot environment, kept that C4 photosynthesis system. Along with that came the C4 photosynthesis advantages and its superior production of carbohydrates. Sorghum and sugar cane also are C4 plants but wheat, rice and soybeans are C3 and will not be able to match corn in carbohydrates per acre because of this trait. Although only about 3% of all plant species are C4, it does occur in a few plants in many plant families, suggesting that it can evolve independently. Researchers of other crops, such as rice, are trying to use genetic engineering methods to develop C4 photosynthesis, but it is not an easy task. Probably the most essential component of cells of corn, and all other living things, are submicroscopic organelles composed of RNA and proteins. These are the site of linking together amino acids to form unique structures of proteins essential for other metabolic activity in the cells and whole organism. DNA in the nucleus of the cell is make of the composition of the plant genetics. This DNA of a gene is a long string of sets of three nucleotides. When a gene is ‘turned on’ in the nucleus, the sequence of the nucleotides is translated into a string of RNA. This is now called messenger RNA (mRNA) because it migrates through the nuclear membrane into the cell cytoplasm via the string of membranes called endoplasmic reticulum to a ribosome.
As the mRNA moves through the ribosomes, each code attracts an amino acid, attaching them to the next amino acid, ultimately constructing a specific protein to be released into the cytoplasm. The construct and sequence of the amino acid become important to the protein function of being an enzyme in other metabolic activity or participant in body construction. All genetic inheritance is dependent on these many tiny components of the cells. Considering that corn has more than 30,000 gene, each ultimately coding for a different protein, as interpreted by ribosomes it is not surprising that these are important components of all living cells in any plant or animal. And there remains much more to learn about cell function with the research to be done by future young scientists. Those individual cells that make up the corn plant that we see in the fields are actually doing most of the work. One of the important components of almost all cells is described in this corn journal blog of 2/9/2016.
The powerhouse of almost all living cells in all plants and animals is a very small, bacteria-like organelle called a mitochondrion. It is similar to bacteria in its size, shape of chromosome, organization of its DNA and function. Mitochondria presence in all from the smallest of single cell animals and plants to the largest has led to the hypothesis that it originated as symbiotic relationship with a bacterium 3 billion years ago. The clear advantage of having this organelle that could transform carbohydrates into chemical forms of energy that allowed production of proteins for growth and movement of muscles in animals is overwhelming. Mitochondria are the size of bacteria and therefore visible only with a strong light microscope magnifying at 1000X but the details require electron microscope power at 30000-50000X. With this extreme level of magnification, mitochondria are shown to be composed of a surrounding double layer of membranes enclosing many folds of membranes. Membranes are significant to function in that these are the sites in which the enzymatic action allowing the energy holding the glucose molecule together is released and combined with nitrogen and phosphorus into another chemical compound, Adenosine triphosphate (ATP). This compound released through the membranes into the rest of the cell for normal cell metabolism. It is also the site in which CO2 is released during respiration. The fact that mitochondria have their own DNA has had dramatic affects on corn. Individual cells may contain from a few to hundreds of mitochondria and they replicate on their own, independent of nuclear chromosomes. However, when sexual reproduction occurs in plants or animals, and the nucleus from the male donor fuses with the nucleus of the female egg cell, no mitochondria are passed alon from the male sperm. Consequently, the mitochondria in the progeny are only those from the female parent. Although the size and genetics of the nuclear DNA is overwhelmingly greater than that of the mitochondrial DNA, and the most profound genetics remains with the nucleus, mitochondria inheritance has had some dramatic affects on plants and animals, including us humans. Green is the color of leaves soon after emergence from the soil and exposure to light. This color is the reflection off the chlorophyll molecule located in chloroplasts, among the organelles of the plant leaf cells.
Plant cells have plastids, distinguishing them from animal cells. They are believed to have been derived from cyanobacteria, such as single cell blue green alga, when a symbiotic relationship with a single celled organism merged with it, perhaps a billion years ago. Like any symbiosis between organisms, each one benefits, and they often become interdependent. Plastids, like bacteria, have two surrounding membranes and DNA organized in a circular manner as opposed to the chromosomal arrangement in all other organisms with a membrane-bound nucleus. Plastids multiply by division independent of host cell division but are carried along with new cells. Consequently, in corn, they are present in the female egg cell. After pollination, as the fertilized egg cell divides and ultimately forms meristems, each cell includes the plastids. These are called proplastids because they are not fully developed. Those in the cells reaching the light quickly are transformed into chloroplasts. Although plastids have their own DNA and capability to produce the many enzymes and other components of chlorophyll, as in other cases of symbiosis, they are also dependent upon the host cell to provide some proteins and plant hormones such as cytokinins needed for proper development. A major structural feature of chloroplasts is formation of multiple layers of membranes (thylakoids) with the chlorophyll molecule and thereby enhancing the capacity for photosynthesis. The plant hormones classified as cytokinins, perhaps produced more by the host cell but some from the chloroplast itself, apparently affect the size and quantity of these layers. Host cell genetics, those inherited from both parents of a corn hybrid, thus influence the chloroplast development and function despite the fact that the proplastids are carried along in only the female parent egg cells. All proplastids do not develop into chloroplasts. Those remaining below soil surface and some others do not become green and become sites for starch storage. Some others accumulate other pigments, contributing to other colors expressed in plants. Some chloroplasts located near the veins in plants develop slightly different carbon-fixing methods that allows corn’s photosynthesis to be among the most efficient of plants to convert light energy into carbohydrates. These small organelles in plant cells not only produce the energy to allows the plant to grow but the oxygen molecules that we need for respiration and carbohydrates for our nutrition. It is difficult to imagine 32000 genes distributed among the 10 chromosomes in the nucleus of a single cell within the embryo of the corn seed. But the microscopic cell also contains many other substances that contribute to cell function once it is activated with germination. Proteins and lipids contribute to the function of the outer plasma membrane surrounding the cell, but membrane-like structures also are intertwined within the cells. Endoplasmic reticulum is used to transport cell products. Ribosomes are attached to the outside of ‘rough’ endoplasmic reticulum. These ribosomes are the organelles in which RNA codes, originating from the DNA, are used to link the amino acids to form proteins. Adjacent endoplasmic reticulum is used to transport the newly formed proteins to sites in the cell appropriate for that protein’s function.
Mitochondria, independent organelles within the cell, are the site of transferring glucose molecules in the chemical energy used by other cell functions. These organelles, carried along in the egg cell from the maternal parent plant, have their own DNA for genetics but are dependent on the rest of the cell and nuclear DNA to provide the glucose, proteins and lipids for structure and function. This symbiotic relationship is in all animal, plant and fungal species, originating a few billion years ago and certainly is significant in corn performance. Mutations in the mitochondria DNA are the source of cytoplasmic male sterility, at least partly because of a genetic defect in the outer membrane of the mitochondria results in defective pollen production. This sterility affect can be overcome by products coded in the nuclear DNA of corn, the male sterile restorer genes. However, the specific mutation to URF13 gene in mitochondria with T cytoplasm, not only cause sterility but also increased susceptibility to certain pathogen toxins such that produced by race T of Bipolaris maydis, resulting the disastrous epidemic of 1970 corn crop. This toxin destroyed mitochondrial function, reducing the plant’s ability to produce normal pathogen-inhibiting resistance chemicals. DNA of chromosomes in the cell nucleus is, by far, the largest, affecting most cell functions, but the much smaller amounts of DNA in mitochondria and plastids are essential and interdependent with the nuclear genes. It’s the activity within the corn cells that drives the growth of the corn plant. Rate of production of new cells produced in the corn apical meristem is largely determined by photosynthetic energy in young plants. These new cells have pliable cell walls at first, that can be stretched by the turgor pressure from water movement into these cells before the epidermal cell walls accumulate the cellulose and lignin to form strong structures. Drought stress during the first 30-40 days after germination results in smaller leaves and shorter plants for the whole season.
Plant size is not always a major contributor to final grain production. Total leaf area per land area exposed to sunlight is essential for maximum grain production per land area. Uneven plant height, allowing some plants to be shaded by adjacent plants is probably more significant to grain production. Uniform water supply within short distances in the field can influence this factor. Stomata in leaves allows the infusion of carbon dioxide essential for photosynthesis. Corn stomata open when light causes photosynthesis in the chloroplasts of stomata. Stomata also allow the evaporation water from the leaf. This has the effect of allowing the water molecules, adhering to each other, to be drawn from the roots to the above ground corn parts through the vascular system thus providing the turgor pressure for elongation of cells. Water loss through stomata requires a constant source of water 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 not surprising as each year's early season differs, that the ‘best’ hybrid is not the same each year nor in each field. Corn seedlings are vulnerable to invasion of the multiple fungi activated by spring temperatures. As the new leaves push out from apical meristem still under the soil surface, the initial 1-4 leaves begin to senesce, weakening their ability to respond to potential invaders such as the fungus Colletotrichum graminicola, cause of anthracnose.
This fungus overwinters of infected leaves and stalks from the previous. This fungus produces very small spores that are carried easily in the wind. Spores landing on the young leaves can produce hyphae that penetrate through the leaf epidermis or through stomata. Vigorous growing leaves in most corn hybrids are mostly resistant to this fungus but weakened first few seedling leaves, as the naturally senesce may lose the ability to fight off the fungus at least in the epidermal cells. The result is a small elongate lesion from which the fungus produces more spores. These first leaves are not usually damaged enough to hurt development of new leaves and later leaves appear to be mostly resistant. As these initial leaves die, this disease will not be noted until leaf senescence starts occurring late in season after grain fill. In this sense anthracnose is not an aggressive pathogen of corn, mostly being able to attack physiologically weakened plant tissue. The seems to be little correlation between the seedling occurrence of anthracnose and its appearance of corn stalks. I have seen the rare hybrid that would get considerable number of lesions on mid-season leaves and even that hybrid was noted for having superior stalk quality. Although this fungus will cause long black streaks on outer cells of a corn stalk, there is evidence that it is only successful in rotting the stalk when the plant died from the depletion of the stalk and root from excessive movement of carbohydrates from those tissues to the grain. Resistance to multiple pathogens, including Colletotrichum graminicola, is a dynamic interaction between the plant physiology and the potential pathogen. It is amazing how humans have adapted a tropical annual plant like Teosinte to produce large starch-storing food for people and their livestock. Along those few thousand years of selection as corn was moved to temperate zones it became adapted to shorter growing seasons and cooler environments. Among the remaining challenges is adaptation to cool spring environments and invaders. Pathogens and other feeders of the nutrients in the corn seed and seedling have followed the adaptation to these conditions as well. Resistance to such invaders is affected by the host plants biology and environment of the young corn plant. Sometimes it is not easy for us to sort out the significance of the outside invader of corn, the host interactions and environment.
The fungal species of the genus Fusarium have a complicated relationship with germinating corn seedlings. The most studied species, Fusarium verticilloides (formerly known as Fusarium moniiforme and its sexual stage as Gibberella fujikuroi commonly is found in germinating corn seeds. It often is found in corn plants without symptoms of damage and therefore is characterized as an endophyte because it appears to live within the plant tissue but does not always cause symptoms. It is not uncommon to see growth of this fungus from germinating seed in paper germination test. A study published in 1997 (Plant Dis. 81:723-728) compared seedling growth from seed artificially infected with this fungal species with those that were not infected. There was no difference in germination percentage between infected vs uninfected seed. There was a slight size difference favoring the uninfected seedlings at 7 days after planting but at 28 days those growing from the infected seedlings were slightly bigger and with more lignin in cell walls. Is this because of a hormone (gibberellin?) produced by the fungus or because of some defense compound produced by the plant? The fungus was easily recovered from the seedlings but less from the older leaves. Infected plants showed no symptoms of disease. It is clear that Fusarium verticilloides can be damaging to germinating seed sometimes but I don’t think all the factors are clearly understood. Is the difference caused by the strain of the fungus, the host plant or the environment? I know from experience that it is so common to find Fusarium growing from a dead leaf sample that one tends to ignore it. It seems to live in much of corn plant’s tissue. It often leads to confusion with diagnosis of problems including stalk rot, almost as if one cannot find other fungi usually associated with rotting stalks, there is always Fusarium. It’s complicated! 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 seedling vigor is affected by their environment including soil water-holding capacity, temperature and pathogens.
Seeds are planted in environments that vary every few inches for water holding capacity, organic content and microbes. Furthermore, each individual seed varies slightly in its cellular membrane status. With imbibition causing swelling of the membrane bound cell contents, some seed can have problems getting effective metabolism for early cell growth to push out the root and stem structures. Cell metabolism includes producing the response to attacks by potential pathogens in the soil. These anti-pathogen chemicals (phytoalexins) are usually produced with a complex system of detecting the microbe and concentrating the phytoalexin into the area of the attack. Weakened seed not only are likely to release more carbohydrates and proteins into soil because of membrane injury, but also be less capable of responding to the microbes invading root and mesocotyl tissue. Diagnosis of seedling disease becomes complicated also. Pathologists can isolate a fungus such as a Fusarium species or an oomycete like a Pythium species, but the actual cause probably involves some interaction between the microbes, metabolic quality for the ‘diseased’ seedling, and a complex environment not only providing potential pathogens but also affecting the seedlings metabolic rate. Soil organisms are affected by the environments as well. Leakage of carbohydrates directs their growth toward the seedling roots but temperatures favor some over others. Pythium’s swimming spores do well in cool wet environments but can be inhibited by certain seed treatments that have very little effect on fungi such as Fusarium species. Other seed treatments can inhibit the latter group of microbes but are less effective against Pythium. Corn seed genetics and seed quality can be greater factors than either group of chemicals. Cold wet heavy soils for a prolonged time can overcome all methods of defense. After the stress on the seedlings is reduced, remaining plants that emerge can give normal production especially if they are uniform in growth with adjacent corn plants. The metabolism of these plants will promote the recovery and normal root growth. Those plants that survive but emerge later than adjacent plants will have difficulty competing for light and mineral uptake which will be reflected in grain productivity. It is remarkable that we usually get a high percentage of ‘normal’ corn plants with all the potential of problems surrounding that small seedling. Corn seedlings have recently emerged in fields of North Central USA. It is exciting to see the new plants arrive. Most of the activity is occurring out of site, however. Beneath the soil surface, close to the seed, the primary root is growing downwards, and the apical meristem is producing new cells via cell division. Heat energy is contributing to driving the cellular activity at both ends of the young plant.
Imbibition of water into the seed leads to activation of the cytoplasm within cells. Most of those processes occur along membranous components of mitochondria, ribosomes, plastids and the endoplasmic reticulum. Hydrated proteins now acting as enzymes in breaking down starch molecules stored in the endosperm and glucose and sucrose molecules are moved thru the scutellum to embryo cells. Diffusion of these sugars through pores of these cells, with cooperation of the cellular membrane and endoplasmic reticulum, these complex molecules composed of carbon, hydrogen and oxygen atoms are transported to mitochondria where they are further metabolized to create the ATP energy needed for other cellular activity. This cellular respiration process allows further cell construction as cells divide in the root and shoot meristems. Elongation of hypocotyl cells, as well as meristem cell division pushes the tissues from the kernel. ATP (adenosine triphosphate) results from the energy transfer from electrons holding the glucose atoms together to form ATP, releasing CO2 and H2O. This process occurs in the mitochondria. These membrane-intense organelles apparently vary in number and efficiency among corn varieties. Mitochondria, having their own DNA, and yet is dependent upon the rest of the cell for its structural components, are transmitted to the next generation only through the egg cell. This is probably why different female parents of corn hybrids vary in time for seedling emergence and vulnerability to imbibition chilling damage. The remarkable growth from a corn seed during a few months all coming from the cellular activity as coded in the genetics of the seed and its environment as assisted by the corn grower. Emerging corn seedlings initially utilize the primary root for absorption of water and nutrients as this root tissue is powered initially by the endosperm and then the photosynthesis from the first seedling leaves. Initial stem nodes remain under the soil surface. Soil temperatures affect the growth rate of the seedling but by time the third leaf is visible inside the whorl of the seedling, lateral secondary roots emerge from the nodes below the soil surface. Photosynthate are moved to these root tips stimulating more cell in new root tips as hormones direct the growth down. Cells outside the dividing root tip cells develop a strong epidermis allowing the root to push through the soil.
Newly formed cells of the elongating roots, near the root tip includes epidermal cells with thin-walled protrusions called root hairs. These protrusions into the soil affectively expand the net root surface area of the root allowing flow of water and nutrients into the root by osmosis. 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. As more lateral root branches form, along with their root hairs the water and nutrients shipped to the developing seedling leaves the upper leaves are formed and photosynthesis increased. This early coordination of shoot and radical root emergence from the seed. Initially with energy from the seed endosperm and then from photosynthesis, allows the new plant to develop for its annual lifetime. The initial roots growing from the embryo radical supply the emerging seedling with water and nutrients. Other changes then occur. After the first corn leaves emerge, the hormonal message to the mesocotyl tissue is to stop pushing upwards. Apical meristem, at the tip of the mesocotyl is now below the soil surface where the first leaf is attached. Photosynthesis now drives the metabolism of the young seedling as it switches from dependence upon the seed endosperm for carbohydrates. Cell division in the meristem produces new leaves, each attached to the young stem under the soil surface and attached in distinct clusters of newly dividing cells called nodes. These nodes are then stimulated to produce roots at about the time the 4th leaf appears in the young seedling. Because the roots are being produced from stem tissue, they are called adventitious roots. As the primary root, that had grown initial seed, loses its energy source, adventitious roots become the main roots for the plant. The first 4-5 nodes of the young stem remain underground, each producing the roots for the plant, even as the first leaves remain attached at the same locations. What appears to be stem in a 4-5 leaf seedling is a compilation of leaf sheaths tightly wrapped together while the actual stem remains beneath the surface. The underground stem portion, formerly attached to the mesocotyl, with adventitious roots becomes known as the crown. Eventually the mesocotyl deteriorates as it is deprived on nutrition and loses resistance to the many soil organisms. As the stem growing point eventually emerges above the soil surface a few exposed nodes will often form the brace roots to further support the adult plant. A lot of changes in a relatively short time after seed is planted. As we look for those first seedlings poke up to reach the light it is easy to miss the amazing feat that has been accomplished. Breeder efforts is selecting genetics, seed producers care during production, and growers efforts with soil preparation and care in seed planting results in the biology of each seedling pushing the shoot up and root down. A cooperative effort by all results in a uniform emergence.
The first appearance of corn seedling tissue emerging from the imbibed seed is the new root. This young seedling primary root can be considered as three regions: meristem, elongation and mature. The cell elongation area provides the main initial force for pushing through the kernel pericarp. Cell elongation and maturation involves production of many new molecules composing the cell walls as they grow. The pectins, hemicelluloses and celluloses that compose the new cell walls are composed of several sugar-related molecules joined together through specific reactions, assisted by enzymes, heat energy and chemical energy such as from ATP. Energy and components for the biosynthesis of these cell wall components comes mostly from the endosperm. Starch in the endosperm is broken down with enzymes into sucrose molecules, moved to root cell cytoplasm where other enzymes break down the sucrose into glucose and fructose. With other enzymatic action, the fructose is made into more glucose. Modification of these sugars allows other new carbohydrate based molecules that become linked to form the more complex polysaccharides such as pectin, hemicellulose and cellulose for the new cell walls. We see what looks like a rather simple process- seed swells, root protrude and a few days later the stem emerges from the seed. What we don’t see is a complex utilization of stored energy, production of complex proteins some of which act as enzymes assisting in linking molecules together and thus giving outer strength to cells. Also, unseen is production of anti-microbial compounds to ward off the many organisms attracted to the very molecules stored and manufactured in the seed. We don’t witness the genetics that programs for these processes. Humans successfully selected for these features from a wild plant species, adapting it to worldwide growing environments. The complexity of corn seed germination still can be challenged in a cold, wet spring such as being experienced in the central U.S.A. spring, but it is amazing that these processes work even under tough environments |
About Corn JournalThe 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.
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