Sugar is the product of photosynthesis, a process at which corn is especially good. The sucrose form of sugar is moved (translocated) from the photosynthetically-active leaf (source) to sinks such as growing leaves, roots and, eventually, seeds. Hormones, mostly cytokinins, direct direction of the flow. Translocation occurs through the phloem portion of vascular bundles through cell membranes at the cost of some energy. Cytokinins are mostly produced by the newly developing cells at growing points such as tips of root branches, leaf buds, growing leaf tips and embryos in newly formed kernels. We humans selected from the Teosinte ancestor, plants that not only met the minimal needs of producing seeds to assure a future generation but also those with extra storage of carbohydrate in the fruit (grain) for our own consumption. To do this we selected for excessive photosynthesis, temporary storage of excess carbohydrates in the pith of the stalk and eventual movement of it to the grain. This was not done cheaply. We had to get more leaf area and more root tissue to not only support the plants but also to uptake the water and nutrients to grow the bigger plant and to initiate the larger grains. All of this required more energy. After pollination, the newly formed embryo in each kernel begins to produce the cytokinins directing the flow of sugar towards it. This is occurring at the same time that root tips are not as prolific and consequently producing less cytokinin.
It takes about 10 days after pollination for the flow to each kernel to gain full speed. Varieties, and environments, differ in the flow rate per kernel but from day 11 to about day 40 the flow per kernel appears to be constant. Production of sugars per day may be affected by cloudy days, or leaf damage but the power of the individual kernel sinks remains strong during that time. Any shortage of new sugar is replaced by sugars stored in the stalk pith tissue. After the 50thday, the draw per day is reduced until finally an abscission layer is formed at the base of the kernel in which the phloem tissue no longer can move the sugars. However during that 60-day period the root is competing with the kernels for sugars and our attempt to capture the maximum carbohydrate in the grain.
Nutrition and moisture in corn silks allow the fast movement of the pollen tube towards the ovule and contribution of the male genetics to the next generation. Those same favorable silk characteristics also can be used by invading fungi. Rapid deterioration of the silk tissue after pollen tube growth offers protection within a few days after pollination, but environments and genetics can have a drastic effect on the time of silk vulnerability and the biology of potential invaders. Aspergillus flavusgets much attention because of is dangerous toxin produced on infected corn. Fusarium verticilloidesis another common invader of corn kernels through silk infection that can produce a mycotoxin (i.e. fumonisin). Others such as Diplodia maydisand Gibberella zeaealso can utilize the silks and initial entry into the ear.
These fungi are mostly saprophytic feeders on plant debris and intensity of their spore production is greatly dependent of corn debris from the previous season near the new crop plants. Their biology also is influenced by the environment affecting competition with other saprophytes feeding on debris and production of spores when the silks are exposed.
Duration of silk vulnerability is also associated with environment. Cool, moist weather a few weeks before normal pollination may cause silks to be exposed before pollen is produced- and may favor Diplodia(Stenocarpella) maydis. Extended dry, warm periods during the pre-pollination time, may cause pollen production before silk elongation and exposure but favor Aspergillus flavussporulation and distribution by the time the un-pollinated silks do emerge. Fusarium species (including Gibberella zeae) produce massive numbers of spores under most environments.
Plant pathologist have shown that one can induce ear infection by directly spraying the silks with the spores of each of these pathogens. These studies have shown evidence of resistance variance among genotypes but usually only on a scale and not of absolute absence of disease. Evaluation for resistance from natural infection is not easy. One can record occurrence of infection within plots, but each genotype may not be exposed to the same environments, including time of silk exposure. One does need to use care before drawing conclusions about ear rot susceptibility based upon single location observations.
Ear rots are prime examples of the complex biology of host and pathogens interacting with environments. Ear rot may not be noticed until harvest, but the problem involved the dynamics occurring at pollination time of the season.
It is ideal when freshly emerged silks are exposed to corn pollen for more than just assuring a full set of kernels on the developing ear. Growth of the pollen down the silk channel is followed by rapid dehydration of the silk, inhibiting or at least reducing the infection of the silk by fungi such as Aspergillus species (A. flavus and A parisiticus). Drought stress tends to delay elongation of silk but have little effect on pollen production, resulting in silks emerging from the ear after most pollen has been distributed. Excellent soil moisture conditions can cause silk emergence before tassels have produced viable pollen for distribution. Rain during silk and pollen emergence can inhibit pollen distribution and viability.
Even pollinated silks are vulnerable to infection by fungi, at least for a short time before the complete ‘browning’ of the silk. Completely yellow silks and very brown silks appear to be more resistant to the invasion of fungi. It seems probable that the new bright yellow silks have resistance factors that are reduced as the silk deteriorates. Perhaps the brown silks are too dry for fungal growth. Those conditions between these stages are most easily invaded by Aspergillus species according to this report: (Phytopathology 74:1284-1289). It was also shown that the fungus reached the ovule within 8 days, depending upon environment, after applying the fungus to the silk. After reaching the ear, the fungus spreads to other deteriorating silk and eventually to the kernel surfaces. Without physical injury to the developing kernels, such as from insect feeding, the fungus remains on the pericarp surfaces. However, as the kernels mature, then it does start infecting endosperm tissue as well.
Resistance to Aspergillus species is believed to involve many common corn fungal resistance genes. Part of the interaction in the kernel involves the starch synthesis. The fungus thrives best on glucose as an energy source but there is some resistance metabolism that could be interfering with the synthesis of starch from glucose molecules. Regardless of the cause of the interactions, Aspergillus infected kernels have less hard starch. Review of resistance studies of Aspergillus flavus in corn can be found at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4117183/
Millions of pollen grains in a hybrid field, and yet only a small percentage land on the stigma of the female flower. The sticky hairs on the silk assist in holding the pollen in place. Attached pollen grain rehydrates from the moisture in the silk. A germ tube emerges through the pollen grain pore, invading the silk usually through the silk hair. It continues to be dependent upon silk moisture as the germ tube grows down the center of the silk, probably with the assistance of hormones to guide it. A nucleus at the tip of the tube is active in providing the DNA codes for the growth, while two other nuclei are carried along the tube. Although a few pollen grains may simultaneously be germinating on an individual silk, usually only one succeeds in progressing all the way. The silk begins to collapse as the winning pollen tube grows, shutting off water to later pollen grains- and to potential fungal invaders. The winning pollen tube reaches the ovule in up to 24 hours, depending upon the distance from where it penetrates the silk. Nutrition for pollen tube growth and metabolism is supplied by the silk.
After reaching the ovule, the other two haploid nuclei in the pollen tube are released into the cytoplasm of the ovule. One nucleus migrates towards and joins the egg cell nucleus to form a diploid embryo. The other nucleus migrates to the cell that already has two haploid nuclei to form the triploid nucleus of the endosperm.
The genetics of the next generation is set for this seed. Hopefully, the winning pollen grain was the one intended by the humans involved.
Maize male and female flowers are on separate branches of the corn plant, thus the species is called monoecious, as opposed to the dioecious flowers of soybeans. Both the ear-forming branch and the terminal tassel is composed of multiple flowers. Each kernel that forms in the ear traces to a single flower with a single ovule within the fruit wall, the ovary. Both male and female flowers of corn begin as dioecious but the male portion in the ear and the female flower in the tassel are aborted very early in the development of each. A mutation or an environmental factor can overcome the abortion, resulting in tassel seed or terminal tassel on and an ear.
A corn tassel may include up to 1000 spikelets, each one including 2 florets. These individual flowers are enclosed in the modified leaves called glumes. Each of the florets have three stamens, consisting of filaments and anthers. Each anther includes multiple cells called microspore mother cells or microsporangia. Meiosis occurs in these diploid cells resulting in 4 haploid microspores per mother cell. This occurs over a period of 3 days. Microspores become free of each other as they grow for a few more days. The individual haploid nucleus in each microspore undergoes mitosis, resulting in two haploid cells within the individual pollen grain. The pollen grain secretes a pollen wall within another 7 days. Starch crystals accumulate within the pollen grain during that wall formation period as the cytoplasm of the pollen grain dehydrates. A small pore is formed in the pollen wall.
Pressure from the growing pollen grains and dehydration of anther walls causes the split that allows the release of pollen grains. One thousand spikelets each with 2 florets with three anthers each with hundreds of pollen grains easily produces a cloud pollen. Production of the spikelets over a period of days results in daily release. Pollen longevity may only be a few hours in high heat but the release over consecutive days in a field of corn usually assures viable pollen reaching most viable female stigma.
The remarkable human selection and development of maize adapted to multiple environments because of available genetic diversity is largely due to the separation of male and female flowers.
An interesting review of the maize pollen development can be found at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC160181/pdf/040879.pdf
The ovary is formed from the diploid tissue of the mother plant. Like other flowering plants the female sex organ is called the pistil, consisting of the ovary, a style and stigma. The style, like in other flowering plants allows the movement of the pollen sperm to be transmitted to the ovule. In corn, this style is exceptionally long and is known as the silk. Towards the outer end of the silk is a portion that has many hairs (trichomes) that aid in capturing pollen and encourage them to germinate. This is known, botanically, as the stigma. Each silk is part of a single flower of the female plant and thus leading to a single ovary with its enclosed ovule. Cells making up the silk elongate basically due to osmotic pressure as water is transported to the cells as well as photosynthetic sugars for energy. Environmental conditions including soil moisture, leaf disease and light intensity interact with genetics to influence the movement of essential elements to the growing silk cells. The oldest ovaries at the base of the forming ear are the first to develop and elongate, but they also have the furthest to go before emerging from the surrounding leaves. First to emerge often is those a short distance from the base of the ear.
Corn silk emergence may occur over a 10-day period as those at the tip of the developing ear eventually emerge. Without pollination or stresses, an individual silk remains viable for about 10 days. A viable pollen grain germinates within minutes of adherence to the silk. Growth of the pollen germ tube into the silk initiates the halt to that silk’s elongation. As the pollen tube progresses down the silk channel towards the ovule, silk cells dehydrate and collapse, effectively inhibiting infection by fungi. Timing of the pollination and silk emergence is essential to successful fertilization of the ovule cells. Water pressure being more essential to silk emergence than the production of pollen, makes corn seed production very dependent on field conditions. Genetics vary for vulnerability to stress related silk extension. Inbreds and hybrids vary in root growth patterns for absorption of water from soil as well as the tendency to move water to the developing silks. Duration of silk emergence without pollination also influences the vulnerability to ear mold fungi. Aspergillus infection, often causing aflatoxin, is related to drought delaying silk emergence and thus poor pollination. Diplodia ear rot is often related to long silk emergence periods without pollination when rain inhibits movement of viable pollen to the silk, adding to the vulnerability of the silk to infection by this fungus. Insect feeding of fresh silk also is linked to fungus infection.
Environment and genetics greatly influence the biology of flowering in corn.
Most hybrid corn fields in Northern Illinois are pollinating this week. Within the past two weeks, at least one lateral meristem emerged from one of the stem nodes. This meristem produces a series of modified leaves but eventually produces 500-1000 special meristems. Each of these modified meristems includes one ovule cell with a nucleus that includes one set of the 10 chromosomes inherited from each of that plants parents. These diploid ovule cells undergo meiosis resulting in 4 cells, each with only one representative of each of the 10 chromosomes, a random mix of some from each of the two original parents. Only one of the 4 haploid cells resulting from meiosis develops into a megaspore cell, as the other three degenerates. These megaspore cells not only have the haploid nucleus but also other cell components such as mitochondria (and their DNA), plasmids and ribosomes.
The single megaspore cell undergoes three successive mitotic divisions, resulting in 8 nuclei, all haploid. This megaspore cell thus becomes an embryo sac containing 7 cells, as two nuclei are included in one of the cells. That cell within the embryo sac is destined to become the endosperm after pollination. One cell at the base of the embryo sac becomes the egg cell. Other cells within the embryo sac apparently participate in pollination only by producing hormones to attract the sperm cell from the pollen tube to the egg cell.
Elongation of special tissue at the outer tip of each embryo sac begins to elongate, to emerge from the surrounding leaves. These structures known as silk become the channel for eventual entrance of the pollen nuclei.
Most of the carbohydrates in a corn plant are found in cellulose and related polysaccharides such as pectin and lignin as cell wall components. There are multiple genes involved in the synthesis of these compounds that are composed linking glucose molecules together as cells complete the expansion stage of plant growth. Individual tissues differ in the amount and thickness of the cell walls. Leaf epidermal cells have thicker cell walls than the parenchyma cells within the leaves. The complex components and toughness of the leaf epidermal cell wall allows an initial defense against micro-organisms and insects. Lignin components of these outer cells are especially important because of the difficulty of digestion by insects such as the corn borer (Ostrinia nubilalis).
Parenchyma cells within the leaves have thinner cell walls allowing these living cells to function as main sites of photosynthesis in the leaves and as storage units in the corn stalk. Cells surrounding the phloem and xylem cells of the vascular bundles become thicker as the leaves expand. Much of that comes as the lignin component increases. Xylem cell walls thicken with more lignin as well.
Corn stalks increase in strength as cells forming the outer rind increase in lignin components. Most of the lignification in stalk rind occurs after completion of internode elongation. Lignification strengthens the stalk making it less vulnerable to insect invasion and lodging. The process is not always uniform, being influenced by environment factors such as temperature, water and minerals. Hybrids differ in vulnerability to ‘brittle snap’ in which the plants a few weeks before flowering can break at a node during wind storms. Some aspect of lignification of is related to this problem.
Mature plant stalk strength is related to the combination of cell wall strength of the outer stalk cells and the attachment of the inner parenchyma cells to those rind cells. These form the mechanical strength of a rod versus that of a tube if desiccation of the parenchyma occurs if the plant wilts because of root rot later in the season.
Corn fields in northern Illinois are close to peaking in growth, with tassels showing beginning to emerge. It is remarkable that these field showed only small seedlings a few months ago. We are observing the result of light energy being transformed into chemical energy in the chloroplasts within leaf cells. That energy bonding carbon dioxide molecules to hydrogen atoms derived from water molecules formed glucose, some of which was moved to mitochondria within the cells. Respiration in the mitochondria transformed the bonding energy within glucose to form ATP molecules as an energy source for other cellular activity and growth. New cells, originating from the cell division at growing points, elongated due to turgor pressure caused by movement of water molecules and osmosis.
Glucose molecules eventually were chemically hooked together as specific enzymes changed their structures, causing hydrogen bonding to be shared by the molecules. Hundreds of them formed a solid wall of cellulose surrounding the cells. Cellulose is the most common polysaccharide in cell walls but other modifications of the linking the carbon, hydrogen and oxygen atoms form other strengthening compounds such as hemicellulose, lignin and pectin. The ratio of these molecules varies among tissues affecting their function. Each varies in ultimate strength and digestibility. All contribute to the rapid growth that we witness in a corn plant at this time of the season.
Each step of synthesis, beginning with photosynthesis, is dependent upon specific enzymes and energy. Enzymes are proteins, strings of amino acids attached in the ribosomes of cells. Twenty amino acids have been identified, based on their arrangement of hydrogen, carbon, oxygen and nitrogen atoms. The arrangement is dictated by the DNA code of the cell nucleus. Each of the 4 nucleic acids in the chromosome, when transcribed into RNA at the appropriate time, is carried to the ribosome where the appropriate amino acid is attached to the next one according to the arrangement of the code. That arrangement of amino acids affects the specific enzymatic action of the protein participating in synthetic reactions within cells, including formation of cellulose chains.
That rapid growth of corn plants is the result of external factors such as light, water and fertilizer but it is also interactions within each cell as influenced by the genetics. One summary of cellulose synthesis and regulation can be found on this online publication by the American Society of Plant Biologists https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3894906/
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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.