Annual plants such as corn undergo physiological changes after flowering, especially in corn that is genetically selected to maximize capture of products of photosynthates in the grain. Flow of carbohydrates within the plant are directed by hormones produced in meristems. Before flowering that flow went to growing leaves and roots near meristems. Excess carbs were stored in parenchyma cells in stalk tissues. After flowering, hormones direct the flow towards the developing kernels.
Genetics and environments influence the intensity of the flow. Hybrids that tend to have more total starch in the ear either because of more kernels or larger kernels are favored by humans but risk early death of roots and leaf tissue that still require the energy provided by carbohydrates for cellular metabolism. Environments that reduce optimum photosynthesis during the grain fill period accelerate the depletion of carbohydrate reserves stored in the stalk tissue. In some hybrids, perhaps all, the depletion becomes most evident in the stalk tissue near the flag leaf, eventually resulting in an abscission layer to form at the base of the flag leaf, cutting off water to that leaf and eventual wilting of the leaf. Fungi such as Colletotrichum graminicolaare able to invade the outer rind of that small stalk tissue with typical anthracnose symptoms. This loss of productive photosynthetic tissue in the small leaf is insignificant and could be indicating good grain fill. Loss of significant root tissue is more important. The challenge of the corn breeder is to select hybrids that have the balance of maximum grain production capturing all carbohydrates available without causing too much damage to needed life functions in the plant. The challenge of the grower is to provide environments that maximize this 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 50th day, 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. Movement of materials within a plant is called translocation. Minerals are translocated from the roots to leaves through the xylem portion of the plant vascular system. Soluble sugars produced in chloroplasts in leaf cells moves through cytoplasmic connections into the living phloem cells of vascular tissue. The flow direction is guided by hormones such as cytokinins. After pollination, much of the translocation of sugar is directed to the kernels. Tracing the direction and timing of flow is often done by using radioactivity such as radioactive N in nitrate uptake in roots (Plant physiology 100(3):1251-8 · December 1992).
A study published nearly 42 years ago (Plant Physiol. (1978) 52, 436-439) compared the movement of sugar movement from leaves to developing kernels when the plants were under drought stress. The section of leaf tissue was fed radioactive CO2 in plants either stressed by root pruning or restriction of water to the plant. Radioactive carbon was incorporated in sugar by photosynthesis at about the same rate in stressed and non-stressed plants. This indicated that rate of photosynthesis was not being affected by the drought stress. Sugars were translocated to the kernels, stalk near the ear node and the ear shank and husk. Two hours after treatment with the radioactive carbon dioxide, 7.6% of the radioactive carbon remained in the treated leaf area of the non- stressed plants but 20.5% remained in the water stressed leaf tissue. 18.6% of the carbon was moved to the kernels in the non-stressed plants but only 11.0% in the stressed plants. Similar differences were found for the stalk and ear shank distribution. This study indicated that the effect of drought stress after pollination was greater on the movement of the sugars from leaf tissue than reduction in photosynthesis. Maintenance of living tissue in stalks by translocation to that tissue during drought stress is probably significant as well. 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 to 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 50th day, 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. A few weeks after pollination, dynamics affecting resistance to leaf pathogens changes. Cytokinins are increasingly concentrated in the developing grain embryos, causing more translocation of sugars from leaf tissue to the ear, reducing availability for cellular metabolism in the leaf tissue. Leaves lower in the canopy, in the shadow of upper leaves have reduced photosynthetic rates due to receiving less than 5% of the light intensity as those exposed to full sunlight. Not having sufficient energy to maintain its cells, senescence of these leaves begins. Among those cell functions is the production of anti-pathogen biochemical that limit leaf pathogens.
Disease pressure increases in lower leaves with the higher humidity and longer dew periods that favor leaf pathogens. Cool, cloudy and wet weather in those 50 days of grain fill after pollination further favors the fungal leaf pathogens. This increased disease pressure on as leaf tissues occurs at the time in which they are losing the ability to react to invading organisms. Lowest leaves senesce first as the lower photosynthetic rate and increased disease kills tissue. Even weak pathogens, such as Fusarium species, invade the vascular tissue of such leaves causing the leaf to wilt, while the upper canopy leaves remain green and fully functional. The senescence pattern progresses up the plant as it gets closer to meeting the active translocation period of 50 days after pollination. If there was exposure to pathogens such as Exserohilum turcicum, the cause of northern corn leaf blight, earlier in the season, the disease appears to move up the plant. This can cause difficulty in comparing resistance levels among hybrids varying in maturity such as in research plots. Those with earlier pollination dates may appear to be more susceptible, especially if the environment favored the disease, simply because the leaf senescence was more advanced than the later hybrids. This can be misleading not only in determining differences in innate resistance levels but also in predicting potential grain yield or increased stalk damage from the disease. One of the obstacles to evaluating and presenting precise resistance ratings for a disease is relative leaf senescence among corn hybrids. Add this to environmental factors and pathogen intensity pressures and races and one should only expect that disease resistance ratings are not precise predictors of damage from a leaf disease. Corn Journal (8/17/2017) Corn plant growth is greatly affected by a broad class of complex chemicals called hormones. Two of the kinds of hormones related to grain fill are the cytokinins and abscisic acid (ABA). These two hormones have opposing functions in plants, including the development of corn kernels. Cytokinins function is to increase cell division and delay senescense of tissue. They are produced in roots and transported via the xylem to meristems such as in each kernel. They also may be produced in seed embryos also but evidence for that is elusive. Regardless, cytokinins accumulate in developing seeds where they are responsible for stimulating cell division. Cytokinins are also linked with the transportation or at least the attraction of sugar to the developing kernels.
Abscisic acid, on the other hand, is associated with cutting off of translocation to tissue basically by causing a layer of thick-walled cells impervious to movement of materials. Abscisic acid production increases when the plant is stressed. The black layer at the base of mature corn kernels and at the base of husk leaves in a mature corn ear are stimulated by abscisic acid. Freshly pollinated ovules have a balance of these two hormones. A non-stressed corn plant normally has a balance favoring the cytokinins stimulating more cell division and, consequently, flow of sugars to the individual kernels. However, if the plant is under heat or drought stress the balance tends to favor abscisic acid. The affect can be abortion of those kernels. Corn kernels within the first 10 days after pollination are most vulnerable, perhaps because the accumulation of cytokinin is too great to be overcome by a short-term increase in abscisic acid. Genetics and environments influence the production of these two critical hormones affecting grain yield in a corn field. Pollination was successful and hormones in the new growing points of pollinated ovules begin the process of moving sugars to each new embryo. Shortage of energy movement during the first 10 days is critical. Plant stresses that cut off this movement, having a change in balance of hormones for each kernel.
Two major hormones in corn are cytokinins and auxins. Cytokinins affect cell division and auxins affect cell elongation. Cytokinins are produced primarily in root tip meristems and transported via xylem to other meristems such as those developing in each pollinated ovule. As these embryo meristem cells divide, the attraction of cytokinins increases. Concentration of cytokinins in these meristems also affects translocation of glucose molecules to each developing embryo, as this carbohydrate moves through the phloem from leaves and stem pith tissue to the new cells. Excessive stress affecting water for xylem transport, or, reducing sugar production can reduce the constant flow of cytokinins to developing kernels. If this occurs during the first 10 days after pollination, another hormone, abscisic acid (ABA) accumulates at the base of the ovule. This hormone causes development of thick-walled cells, blocking transport of cytokinins and sugar into the kernel. As a result, the kernel does not develop further. Genetics and environment have a great effect on the balance of hormones during corn grain development. The meristem in at least one of the lateral buds of a corn plant develops into an ear. This meristem includes 500-1000 lateral meristems with mother cells with diploid sets of chromosomes, 10 chromosomes from each of that plant’s parents. Meiosis occurs in this diploid cell, resulting in 4 haploid cells, each cell having only a single set of 10 chromosomes consisting of a random mix of the two parent’s chromosomes. Three of the 4 haploid cells degenerate, leaving a single megaspore. This megaspore nucleus undergoes mitosis three times, resulting in 8 cells within the megaspore structure now called the embryo sac. One cell at the bottom of the embryo sac becomes the egg cell while two of the haploid cells fuse in the center of the embryo sac.
The embryo sac (ovule) is enclosed in an ovary, at part of the female part of the parent plant. Part of this female flower is the silk., extending from a single ovary and attached to its ovule. The male flower also produces pollen via meiosis followed by a single mitosis, resulting in two haploid nuclei. A pollen grain adhering to the silk, germinates and extends down the silk to the ovule. Upon entrance of the ovule, one nucleus fuses with the haploid egg cell forming a diploid nucleus to become the seed embryo. The other pollen haploid nucleus fuses with the two ovule nuclei in the center of the ovule resulting in a triploid nucleus, having two sets of chromosomes from the female parent and one from the male. This triploid nucleus undergoes mitosis to become the endosperm of the seed. Whereas the inheritance of the embryo, and its resulting mature plant, is determined equally by the genetics of the male and female parents, characteristics of the endosperm is slanted towards the genetics of the female parent. If the female parent has the recessive Y1 gene, and thus a white endosperm, but the pollen is from a parent with dominant gene and thus has yellow endosperm pigment, the resulting endosperm will be lemon white in color. The female genetics has the major affect on endosperm function in the maize seed because it contributes two of the three sets of chromosomes in endosperm cells. |
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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