Cell morphology and water
Corn cell structures and functions are affected by environment. Water supply to the cells influences plant function throughout its life. Imbibition of the seed initiating the cell elongation of cells and their functions occurs only when water concentration surrounding the seed is sufficient. Cell elongation driven by turgor pressure pushes the roots down and shoot up through the soil.
At about the V5 (5 visible leaf collars) the growing point differentiates into a tassel. It is all cell growth now. Cell elongation in the stem cells as well as in the leaf cells is greatly affected by water pressure within the cells. Warm wet conditions consequently result in taller plants and larger leaves. Kernel row number is set by V6 but the number of kernel ovules per ear is affected by the water pressure. Dry conditions resulting in few kernels and smaller tassels. Drought also can reduce the opening of the stomata and consequently less CO2 intake and reduced photosynthesis. Energy for the cell growth is provided by photosynthesis, the sugars guided to the various sinks in roots, leaf tips, shoots and tassel. The uppermost leaves get the direct sunlight allowing the highest photosynthetic rate. As the canopy closes in, the lowest leaves receive only a fraction of the light, sometimes not producing enough carbohydrate to meet respiratory needs for normal metabolism. These lowest leaves often become susceptible to relatively weak pathogens, develop yellow spots and drop off. By V6 stage the corn plant growing point is not putting out new cells, differentiation is over and now it is up to the dynamics of cell elongation to determine final plant development.
Genetics influence the root growth direction. Hybrids with a tendency to be highly branched near the soil surface are adapted to absorb and transport more water from organic material near the soil surface but may have trouble reaching deep into soil during drought conditions. Hybrids that tend to have fewer branches near the surface, but extremely deep roots may excel in drier conditions but can be more vulnerable to lodging in strong wind conditions.
Larger leaves, expanding due to leaf cell water supply, may have potential for more photosynthesis per plant. Larger leaves also have potential for more stomata increasing the supply of CO2 but also the loss of water via transpiration.
The corn plant’s interaction with water occurs at the cellular level. The cell’s response is affected by genetics and other environmental influences such as minerals, light intensity and pathogens. It is complicated!
Among the specialized cells in corn leaves are the stomata. Each stomate consists of two guard cells and two companion cells. Corn, being a monocot, has these specialized cells organized parallel to the length of the leaf at a rate of about 36000 stomata per square inch on the top of the leaf and about 50000 per square inch on the bottom to the leaf.
Blue light wavelengths detected by a carotenoid activates a process in which potassium ions flow into the guard cells. Thus, the water concentration in the cell drops, resulting in osmotic pressure for water to enter the guard cells. The shape of the guard cells allows uneven swelling and a pore opening up between the two guard cells. Photosynthesis produces sucrose that then contributes to the osmotic pressure in the cells. At the end of the daylight, starch is synthesized from the sucrose and potassium ion concentration is reduced and the opening between the two guard cells closes. Other compounds within these cells also contribute to this phenomenon. https://academic.oup.com/jxb/article/57/2/381/489968.
This is essential, of course, to allow diffusion of CO2 into leaves for photosynthesis in other leaf cells. Open stomates allows O2 to be released to the atmosphere but also water loss. Water evaporation through stomates (transpiration) is affected by the relative humidity in surrounding atmosphere as the water concentration within the leaf spaces is nearly 100%. Cohesiveness of water molecules ‘pulls’ water up to the leaves so that essentially every molecule of water that goes out the stomata is replaced by one from the root tissue.
During the day, stomata are open, carbon dioxide moves into the leaf, oxygen moves out and so does water. At night, photosynthesis in the guard cells stops, water moves out of the guard cells causing the swelling to be reduced and the pore is closed. We will discuss more about this phenomenon later as the plants grow.
Stomata are essential to corn and have a dramatic affect on productivity as import of carbon dioxide and export of water occurs through the stomata. This topic is covered in Corn Journal 5/10/16.
Corn tassel cells
When the plant apical meristem cells interpret the combination of daylight length and accumulated heat, as determined by genetics, the terminal shoot meristem and at least one of the meristems located at a stem nodes, begins to produce cells for the tassel and shoot. Resulting modified stems and leaves now produce the male and female flowers of the corn plant.
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 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. (Corn Journal 7/17/18)
Corn shoot apical meristem
New above-ground cells are produced with cell division in the apical meristem. Genetics and environment control the eventual size and function of those new cells. This was addressed in the Corn Journal Blog on 8/8/2019:
Corn shoot apical meristem is genetically controlled to switch from producing new leaf and cells to the terminal male flowers of the tassel. The main environmental factor influencing this switch in temperate zone corn is heat energy. Earlier maturing corn requires less heat to trigger this change in apical meristem products, allowing corn to mature in short seasons far from the tropical environments of corn’s origin.
Plant height is determined by the number of cells produced by cell division at the apical meristem before switching to producing the cells that becomes the tassel and the elongation of the cells. Elongation of the stem cells is enhanced by water pressure applied to the young cells before maturing with less flexible cell walls. Thus, water availability to the roots, root volume and transport of water to the expanding cells in upper plant also affects the eventual plant height.
Corn planted later than normal in temperate zones, accumulating heat units quicker than usual, produce fewer stalk cells because apical meristem is induced to produce tassel cells quicker. If water availability for cell expansion is less than optimum, the result of these two factors will be shorter plants than usual for a hybrid.
Corn stalk cells
Corn stalk cells of the rind have thick walls with lignin, hemicellulose and cellulose, all carbon-based compounds formed after carbohydrates were shipped to the stem locations. Rind cells are major barriers to pathogens and insects and contribute to the withstanding of lodging pressures. Hybrids vary in the thickness of the rind with strength measured with special penetration equipment.
Stalk pith tissue is composed of parenchyma cells with thinner walls allowing import of sugar molecules. Cytoplasmic activity in these cell plasmids converts the glucose into starch molecules. This functions as an energy storage for future use in roots and grain, as hormones directs the movement. As grain begins to form, sugars are moved at a steady daily pace. When stresses, such as leaf disease, or hail damage to leaves or cloudy weather reduce photosynthesis, the reserve from the stalk cells is pulled to the grain.
Vascular tissue in the stalk becomes the vehicle for the movement to the grain and root, while the xylem supplies moisture to the stalk cells from the root.
Stalk pith cells connect to the rind cells, essentially a solid rod of the stalk and thereby adding to the stalk total strength. Some have estimated that this is about 33% of the total stalk strength.
Movement of sugars to the grain can result in deprivation of sugar needs for root tissue, resulting in early death of root tissue. This reduces the uptake of water by roots eventually causing the leaves to wilt and the stalk parenchyma cells to collapse. The latter results in pith cells to pull away from the rind, essentially changing the rod structure to a tube. Death of these cells allows the advance of fungi as active cell metabolic resistance is no longer effective. Consequently, the stalk easily lodges. We often refer to these as stalk rot as fungi present are identified. The real problem, however, was the starving of roots.
Photosynthetic stresses combined with the draw of sugars to the grain reduced the available of chemical energy for root cells. Death of the root resulted in wilting of plant and redaction of stalk pith cells. Withdrawal of the pith cells away from the rind cells weakened the strength of the stalk.
Corn Cell communication
Cells of the corn plant are the source of the plant structure and function. Converting light energy into chemical energy allows for growth and production of cell structures and chemical compounds allowing for all of living cell function. The totality of construction and function of the plant is determined by its DNA and interaction with environment. Ultimately, however, the movement within the plant of different cell products requires communications among the cells.
Movement of water within the plant is mostly made by simple physical principle of diffusion and osmosis. I recall a plant physiologist professor many, many years ago illustrating by sharing a story of being on a mountain road in a discussion with wife whether they were going uphill or not. He claimed he got out of car, poured water on the road and declared that water runs downhill! Water moves from a high concentration to a low concentration, such when diluted by sugar. That principle applies to movement of minerals and compounds as well, if they can make it through barriers such as cell membranes.
Hormones are actively involved with movement of cell products. Four notable groups of hormones are auxins, gibberellins, cytokinins and abscisic acid. Each is associated with different functions in the plant.
Auxins, such as indole acetic acid (IAA), affect cell elongation. Produced in the apical meristem it is associated with initial elongation of the stem tip. Absence in the lateral buds at base of each corn leaf, prevents branching. Removal of the apical meristem allows production of more auxin in lateral buds and consequential branching of the corn plant.
Gibberellins, such as gibberellic acid, promotes elongation of the cells in the stem but not in the apical meristem.
Cytokinins promote cell division in the meristems such as in the newly pollinated embryos. Zeatin is a cytokinin in corn embryos.
Abscisic acid inhibits cell growth. It is most active in developing the restriction of flow of more carbohydrates into the mature grain, causing the cells at base of kernel to form a ‘black layer’.
It is not known completely how these hormones cause these affects. It is mostly assumed that they are acting with cell DNA but some reactions, such as auxin causing roots of a germinating seed to turn to go downwards because cells on one side elongate more than those on the other side, seems too quick to only be reacting with DNA. Regardless mechanism, plant hormones are essential participants in the communication among corn cells.
Corn root epidermal cells
Although many cells of the corn roots have the basic structures similar to other plant parts, there are significant differences appropriate to the root functions. Root cell production originates from active root tips, producing epidermal cells, intermediate parenchyma cells and vascular bundles complete with xylem and phloem tissue.
The important function of uptake of water and minerals occurs mostly through the youngest and newest cells near the root tips. This is enhanced by extensions from some epidermal cells called root hairs, vastly expanding the exposure to the root surface for absorption that occurs mostly by osmotic pressure. This is enhanced by diluting the water in root hair cells with sugars supplied ultimately by leaves and transferred through the phloem.
Root hair cell walls block large items, such as fungal mycelium, but smaller molecules pass through to the cell membrane that is selective in allowing entrance. Root hair extensions of the epidermal cells live for only a few days but as the root tip produces more new cells that produce new root hairs, the enhanced absorption of water and minerals continues. Auxin hormones are involved in the initiation of the root hairs. The short life is probably useful as the permeability enhances the potential for invasion by pathogens. After the root hairs disintegrate root cells increase the function of transporting to the xylem.
Of course, genetics influences the branching of roots, and production of root hairs. In addition to the multiple genes probably involved, a single root hairless mutant gene rth3 gene has been identified and in trials has shown to be associated with significant losses in grain yield. https://www.ncbi.nlm.nih.gov/pubmed/18298667/.
Lot of things happening in the corn plant that we don’t see.
Xylem cells in corn
Xylem cells in the vascular system from the roots to the leaves is the main tube that allows water and minerals to be moved from roots to leaves. A major portion of the xylem, tracheids, are dead cells at maturity that have strong thick walls providing the strength to withstand strong water pressures but have small pores to allow flow into surrounding cells. Also, part of the xylem tissue, are living parenchyma that allow movement of water and minerals to other cells. These cells accumulate potassium ions to be distributed into mesophyll of leaves. Xylem parenchyma cells accumulate starches in the corn stalk pith areas, to be later moved thru the phloem vessels to the developing kernels after pollination.
Tracheid cells in the xylem do not have end walls allowing them to form a tube. Water pressure from water entering the root cells to the movement of water to leaves where a portion of it evaporates through open stomata openings, causing transpiration pull. This push and pull causes a constant flow of water through the tubes of the xylem.
Phloem cells provide the transport of sugar and protein molecules up and down the plants. Xylem cells provide the flow of water and minerals up the plant. Movement in the phloem occurs via living membranes and requires energy. Movement of the water from roots upward does not require energy but simply removal of water through stomata and tendency of water molecules to adhere to other water molecules (cohesion). Thick xylem tracheid walls form the tube to contain the force.
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.