Heat, water and corn
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.
Photosynthesis in Corn
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.
Water and Plant height
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.
Seedling anthracnose no problem
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.
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.