Multiple copies of chlorophyll molecules are located on the folded membranes (thylakoids) within the chloroplasts in corn leaves. The complex structures of these molecules allow the absorption of the blue and red wavelengths of daylight, reflecting the green portion of light that we see in green leaves.
Two major types of chlorophyll molecules are found in corn leaves. Chlorophyll a includes 57 atoms of carbon, 72 atoms of hydrogen, 5 of oxygen, 4 nitrogen and one magnesium. Absorption of a photon of light energy frees up an electron from the chlorophyll a molecule that gets transmitted to chlorophyll b molecules that has only 2 less atoms of hydrogen and one more oxygen atom. This energy ultimately is used to split the water molecule by freeing the hydrogen ion to be combined with carbon dioxide to form glucose. Oxygen molecules are thus freed as a gas and moved through the plant stomates into air.
Construction of the complex components of chlorophyll molecules can occur in the dark but the final step requires light. Seedlings grown in complete darkness are yellow but shortly after exposure to light they become green. Nitrogen or magnesium deficiencies in the plant will affect the chlorophyll construction but also other minor elements such as iron are needed in the biosynthesis process of the enzymes leading to the construction of chlorophyll. Although much of the genetics controlling final construction and function of chlorophyll is within the chloroplast DNA, the chromosomal DNA within the cell nucleus affects the supply of essential chemicals.
Growth of plant tissue needed to supply those chemical components for chlorophyll initially supplied by carbohydrate energy in the seed, becomes dependent on that supplied by chlorophyll as glucose energy is transmitted to mitochondria where the molecules are broken apart supplying the energy in the form of ATP to be used to drive construction of more corn structures. This cycle continues until excess energy is deposited in grain and the plant begins senescence.
Plant cell cytoplasm not only include mitochondria, those symbiotic organelles with their own DNA, that function to transform energy stored in glucose into ATP, but also another group of organelles called plastids. Like mitochondria, these microscopic organelles have their own DNA organized in a circle for similar to bacteria. This along with other structural aspects has led to the speculation that a couple billion years ago a bacterial species developed a symbiotic relationship with a primitive single celled life form in which both organisms benefited. The major contribution of plant plastid probably came from a cyanobacterium (blue-green algae) that became the chloroplast in plants.
Plastids, like mitochondria, are transmitted only through the egg cell and not the pollen of the plant. Therefore, the plastid DNA is that of these organelles in the female parent of a hybrid. Plastid DNA may include only about 100 genes but they do include the ability to duplicate the plastid within a cell. Mutations within that DNA have been identified. They remain dependent on the other plant genetics to provide the components for their structure and function. For example, albino plants are affected by cell nuclear chromosome mutations.
Chloroplasts are the plastids in which chlorophyll is formed and photosynthesis occurs. These plastids have two outer layers of membranes as well as several folds of membranes on the inside. Chloroplasts accumulate mostly in mesophyll cells which are those between the epidermal outer layer of cells and the vascular bundles of a corn leaf. Each of these cells have many chloroplasts often organized on the side of the cell most exposed to the light. Epidermal cells forming the stomata also have chloroplasts with the function of affecting the opening of the stomata, allowing transmission of CO2 into the leaf, and evaporation of water from the plant. Bundle cells surrounding the vascular tissue in a corn leaf also includes chloroplasts, contributing to the efficiency of this C4 photosynthetic system of corn.
There are plastids that do not have chlorophyll but function to store starch (leucoplasts) and other pigments (cyanoplasts). Plastids are initially formed in a juvenile form in the embryo meristem of a corn seed. These pro-plastids duplicate in meristem cells as the hypocotyl approaches the soil surface. They have the molecular components of chlorophyll the final complex chlorophyll molecules are not formed until exposed to light as they emerge from the soil.
Much is happening within the new leaves emerging from the soil in the spring season.
With its original investment in a seed, and some outside investment in tillage, fertilizer, insect control and weed control, the single seed provides the energy to emerge from the soil. For the next 40-60 days the young plant uses free light energy to multiply its size. Light energy is absorbed in chloroplasts with a portion of the energy tied up in bonding carbon dioxide and water molecules into carbohydrates. Some of this molecule bonding energy is released in mitochondria in the form of ATP molecules, that assist in production of more complex carbohydrates such as cellulose for more cell walls and more proteins as cells grow. The new proteins are dictated by DNA, as it’s nucleic acid code is translated to RNA that is moved to cell ribosomes, where the code attracts amino acids to be attached in an order required for a specific protein.
These processes within that plant results in leaf growth, therefore accelerating the absorption of light. Root growth allows more mineral and water, providing and increasing the supply of some raw materials for manufacture cell components. Increasing leaf size increases movement of CO2 through stomata, although with the cost of some water loss during the day. The cost is minimized by stomata closing at night.
Most of the energy absorbed by chloroplasts is used to create new leaf tissue until the hormone changes within the plant stimulates the growing point initially very near the soil surface to begin producing new cells and elongating the cells in place. Plant energy is now shifted to elongation of the stem as it becomes a temporary carbohydrate storage location. After expending energy to produce pollen and ovules in the ear, the new energy production in leaves and stored energy from stalks is moved to the growing kernels for another 50-60 days.
The result of self-investment from that one kernel and outside investment of the farmer, with utilization of free light energy, the return is 500-700 kernels. This is a superior pyramid scheme!
Of course, humans assisted along the way with breeding and selecting the right genetics, seed producers for growing and caring for the original seed and farmers in choosing and cultivating the hybrid seed. Corn is a remarkable plant for conversion of free energy into multiplication of itself.
Seedlings in Northern Illinois today are mostly about 2-3 inches (5-7 cm) above soil level. One month from now these plants will stand 24-36 inches (60-92cm) high. All of that above ground structure will be due to cell elongation in leaf tissue, as the ‘stem’ remains close to the ground within the tight wraps of the leaf sheaths.
Much of the above-ground growth for the first 30-40 days after seedling emergence is due to cell elongation within leaves. This not only allows the expansion of leaf blades to increase the mass of leaf tissue, but also elongation of the leaf sheaths, pushing up the plant height. Cell elongation is not only occurring in the outer tissue, but internal cells also grow in size during this time.
Cell elongation is driven by energy allowing production of cell components such as cell wall cellulose and lignin but also increase in the membranes, ribosomes, mitochondria and chloroplasts needed to drive the growth. Immature cells, before tightly constricted by deposits of solid cell walls, expand with water pressure during this growth pressure. Consequently, soil water and root development become major factors affecting the size of the corn plant during this pre-flowering stage. Root development not only affects the absorption and movement of water into these growing leaf cells but also uptake minerals needed for the general metabolism. Expansion of leaf blades during this time also increases the absorption of light driving photosynthesis, providing more energy for cell function and growth.
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 easy to be amazed during this time of the year to watch the rapid growth within a corn field, but we are only seeing a tiny fraction of what is happening within those plants.
Corn fields in Northern Illinois are showing corn seedlings this past week. Planting was a little late this year because of wet soils. Seeds imbibed water nearly immediately after planting. Heat provided the energy for initiating the activation of metabolism within the cells. Activation of enzymes already present in the cells assisted in the respiration process in which glucose was used to produce ATP (adenosine triphosphate). This energy, combined with components of the glucose molecules, nitrogen and phosphorus molecules produced more enzymes, some of which triggered production of RNA (ribonucleic acid) coded by the DNA (Deoxyribonucleic acid) in specific genes of the chromosomes. RNA molecules were transferred to the ribosome organelles in the cells where the nucleic acid codes attracted specific amino acids hooked together in long chains as new proteins. Some of these proteins were specifically coded to become enzymatically active in other metabolism, including production of more cell growth.
Cell growth, largely by cell elongation, pushed the root tissue downwards and the shoot tissue upwards. Initial shoot elongation occurred in the hypocotyl but as the apical meristem reached about ½ inch of the surface absorption of light stimulated that elongation to stop, and the rapid growth of the cells in leaf tissues near the apical meristem. The first of these leaves, the coleoptile, wrapped tightly around enclosed 5 or 6 leaf primordia, continued pushing through the soil as cell elongation continued. Growth to this stage was the result of water absorption providing pressure for cell elongation and energy derived from stored starch in endosperm, translation of DNA into enzymes and production of new cellular parts and cell walls.
Further exposure to light after emergence resulted in some plastids within the cells of leaves, including that small coleoptile, to produce chlorophyll. Photosynthesis in these chloroplasts is the beginning of dependence on energy from the seed endosperm. Chloroplasts, influenced by their own DNA, inherited through the female parent of the corn plant, and the DNA of the cell nucleus react to environment including light intensity, heat, minerals in soil and chemical additives. Chloroplast function contribute to the cell growth, as more leaves emerge from the soil with growing momentum.
We are accustomed to seeing the result of corn breeders’ selections of genetics that work in expected environment. Most of us do not see the results of genetics in which the above process fails. We also hope to not witness the cases where the environments failed to support this process, with insufficient oxygen in water-logged soils, misapplied herbicides or poor fertilizer application. Usually we only see the remarkable result of coordination of cellular activity when a corn seed germinates, and a seedling emerges.
Corn seedlings respond to water stress by reduced leaf growth but increased root growth. The physiology involved in that root growth as it ‘searches’ for water has a complex interaction with plant hormones, cell development, osmosis dynamics and genetics.
Primary root in a corn seedling was initiated with germination at the point where the scutellum is at node connecting the radical and the shoot parts of the seedling. It is the main root structure until about 2 leaves are expanded above ground and the secondary nodal roots are initiated about 1 inch below soil surface.
Root growth occurs with cell division at the root tip and cell expansion a short distance from the tip. Whereas leaf expansion of water stressed seedlings appears to be reduced, primary root growth increases under these conditions. The plant hormone abscisic acid (ABA) increases in the tip. This is associated with an increase in osmotic pressure, and thus more water, into the newly formed root cells. Also increasing ABA is linked to an increase in the amino acid proline, perhaps delaying the finishing of cell walls. As a consequence, the cells near the root tip become longer than in roots not facing water deficient conditions. ABA also appears to interfere with production of ethylene in the cells, a compound associated with inhibiting cell growth.
Four genes are associated with the ABA interaction in water-stressed primary root growth. There is evidence that as many as 1779 genes are expressed in root cells of water stressed primary roots as opposed to 1297 genes expressed in roots with no water stress. Corn plants react to environments in complex ways that we usually are not aware. We select genetics for the preferred gross reaction, but a lot is happening within the plant.
Detailed summary of this complex interaction can be found on internet at https://www.ncbi.nlm.nih.gov/pubmed/15448181.
Current and future students of corn have multiple avenues to follow their interests. This species, because of its efficiency in transferring light energy into products useful to humans, its unique historic developmental interaction with people, and its basic biology encourages study at sub-cellular level as well as crop production.
Advances in molecular genetics increasingly attempts to understand what those 30-40000 genes are actually doing in the cells. What determines the timing of a particular piece of DNA to be activated. A single gene consisting of a string of multiple, often more than 100, nucleic acids codes for production of a specific protein. A mutation that may only result in one of the nucleic acids, can result in significant change in the protein structure and function. Many of the proteins are biologically active as enzymes influencing the chemical pathways to production of important organelles such as mitochondria and chloroplasts, both of which have their own DNA, ultimately influencing the transforming light energy into energy for growth of the plant.
Variability in the DNA code among corn varieties also allows for creation of morphological differences in corn plants. This is expressed from the time of germination until completion of the life cycle of the plant. Plants that have identical DNA, such as inbreds and single cross hybrids, will have identical morphology if grown in uniform environments. Careful study of the morphology leads to the realization that each inbred and each single cross hybrid expresses structural characters unique to that genetics. These are expressed at each growth stage from germination to mature plant. The huge number of genes in corn results in expression, whether related to grain production or not, but some simply codes for unique leaf shape, root growth direction, ear height, husk coverage and tassel branches. There are also multiple characters that we do not necessarily see, such as resistance to a disease that is not occurring where the crop is grown.
Corn is grown in multiple environments within a field and from tropical to very temperate countries. Students of the agronomy of corn research for best methods to increase efficient production from corn in each environment. Theoretically, each genotype has a unique requirement for best production but the variability in soils and weather contributes to difficulty of matching the genetics with the crop production methods.
There is room for studies of nearly all aspects of the biology of corn, from the lab to the field. We still have a lot to learn from the specifics of a physiological process that occurs within cells to the expression of genetics at the whole plant level. Everyone involved with this crop becomes a student of the corn.
It initially surprises people accustomed to manufacturing inanimate objects, that seed are living organisms that change after production. Furthermore, despite the fact that all of the seed within a seed lot may be genetically identical, each has its own physiological state at any specific time. Some seed within that lot may be vigorous at a specific time, while others may be dead, and others may be weakened to the point that they germinate but only emerge from the soil at a later time. Seeds do age but not all at the same rate.
Genetics influence the rate of ageing in corn seed. Chromosomal DNA affects structural aspects of seed, affecting reactions to important environmental pressures such as drying rate, pathogen resistance and vulnerability to handling damage. DNA of mitochondria influence the respiration process needed for the energy to grow new tissue during germination. Despite all seed of a single cross hybrid have the same chromosomal DNA and probably the same mitochondrial DNA if harvest from the same parent, each seed’s environment can be sufficiently different to affect the rate of ageing.
It is well established that seed production environments can include stresses that affect the rate of ageing in the field. It may be drought stress after pollination or delay in harvest because of rain allowing the seed to remain at a higher moisture for a prolonged time. A week delay in harvest may have a detrimental effect on seed’s ageing process.
Seed’s produced on a single ear do not have exactly the same environment. Pollination within that ear in the seed field occurred within 4-5 days, as the first silks to emerge are towards the base of the ear. They are not all equally exposed to pathogens. Artificial drying rate is a major factor affecting the delay of ageing in corn seed. Seed shape and location on the ear must have some interaction with the drying rate. Further handling of the seed during shelling, seed treatment and bagging is not equal for each individual ear. Seed producers attempt to handle seed gently and carefully to avoid damage, but some individual seeds inevitably are more affected than others.
Our company evaluates germination and emergence of corn seed samples. Nearly always, if a sample has a low percent of dead seed, the emergence from our artificial soil mix is uniform. As the seed percent of germinating seed decreases, the uniformity decreases as more seedlings do finally emerge, some only showing the ‘spike’ (coleoptile) while others have third leaf unfolded. It is rare to samples in which the seed are only dead or vigorously alive. It is much more common to see intermediate stages of ageing in samples with a few dead or samples in which nearly all seedlings are growing vigorously.
Seed producers attempt to manage genetics, production environments and testing to provide high performing seed for crop producers. Ageing rate of seed is not always manageable and sometimes seed and crop producers get surprised.
Corn embryo’s being planted on May 1 are about ¼ inch (0.6 cm) in length. The future shoot portion of the embryo is half the size of the embryo. Two to three months later that shoot length has been multiplied by 800-1000. Within the embryo are cells with organelles such as mitochondria, plastids, ribosomes and other membranous structures needed to carry out this remarkable growth rate. Within the nuclei of these cells are the 10 pairs of chromosomes with the 30-40000 genes, coded by long strings of nucleic acids. Within a few hours of water imbibition, the few genes in the mitochondria are activated. Appropriate codes within their DNA produce RNA strings of nucleic acid, that are moved to ribosomes, producing proteins appropriate to enzymatically remove the energy binding the carbon and oxygen molecules in glucose and moving that energy into adenosine triphosphate (ATP). This energy is utilized in manufacturing the other structures for rapid cell elongation and cell duplication, pushing the seedling shoot to the soil surface.
With exposure to light, some cellular plastids with guidance from their own DNA and supplies from the other cell components, produce chlorophyll. This pigment allows absorption of light frequencies providing energy to drive the capture of carbon, oxygen and hydrogen molecules in the process of photosynthesis. Resulting glucose molecules are moved to the growing cells that utilize the new molecules in manufacture of structural complex molecules such as fatty acids and proteins used for cell metabolism and cell wall structures such as cellulose and lignin.
It is easy to be amazed when we seed the rapid growth of young corn plants and even more impressed to know that we are only seeing the result of remarkable interactions occurring at the cellular level.
Soon after exposed to moist soil, water moves through the corn kernel pericarp and thin layer of seed coat cells. It is purely a physical phenomenon of water moving from a higher concentration towards a lower concentration. Hydration of organelles in the cells of the scutellum results in activation of enzymes to digest the stored carbohydrates in the scutellum and endosperm and movement to the cells in embryo growing points. Mitochondria are activated with the increased moisture, their membranes beginning the process of releasing the energy in glucose molecules into adenosine triphosphate (ATP), the molecule that drives the production of enzymes needed for the construction of structure of new and expanded cells. Water continues to participate in the metabolism, including the release of the energy from ATP. The released energy was holding one of the phosphate molecules to the triphosphate portion of the ATP molecule during construction of a complex compound. As this bonding energy is released, leaving behind adenosine diphosphate (ADP). Cellular respiration in mitochondria uses energy from glucose molecules to add a phosphate molecule back to ADP to form new ATP, allowing continual growth of the embryo.
Appropriate portions of DNA are ‘read’, forming RNA that is moved from the nucleus to ribosomes within cell cytoplasm. The nucleic acid codes are translated into hooking appropriate amino acids into long strings, producing specific proteins used as enzymes for other metabolic activity or construction of cell products.
Corn seed becomes a dynamic place when the water moves in.
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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.