Black layer timing
It not easy to correlate all environmental and genetic factors to predict time between pollination and formation of the abscission layer (black layer) at base of kernels when the season is as abnormal as 2019 in the USA corn belt. It is established that heat determines the differentiation of growing point into flowering structures instead of leaves in temperate corn hybrids. We mostly classify relative corn maturities among hybrids either by days to black layer or by total heat units, somewhat assuming that the difference between total heat units to black layer and heat units to flowering is the heat units needed to fill the grain.
But are heat units as important for that fill period as it was for determining differentiation of apical meristem? Photosynthesis rate is mostly constant and optimum when temperatures are between 72°F-82°F, slower at 55° and above 85° F, according to one study (Journal of Experimental Botany, Volume 28, Issue 3, June 1977, Pages 519-524).
General metabolism in the corn plant is affected by temperature but it generalized that the optimum is between 55° and 85° as calculated by the GDU’s. However, does that apply to the metabolism directly involved in translocation of carbohydrates from leaves and stem to the grain? Or is it simply about 55 days from pollination to end of grain fill even if cooler than usual for corn even when pollination was later in season in 2019>
Temperatures affecting grain fill during this late flowering seasons probably will give us new information concerning factors affecting black layer formation in corn.
Roots after pollination
Prior to pollination corn plants are dependent upon roots to absorb minerals and water to be transferred to the developing above-ground growth. Size and efficiency of roots of each plant are major in supporting the growth. Genetics and environments are major factors in successful leaf and stalk development. Photosynthates produced in the leaves are transported to the roots through living phloem cells to supply energy for growth of the roots and active uptake of minerals. Living cells of root cells, including the root hairs that are active, uptake of mineral ions requires the energy to move them from a lower concentration in the soil to a higher concentration in the root cells.
Water uptake from the soil is moved to the xylem cells, that are essentially non-living tubes, in which the water tension character allows water to be pulled into the root and up to the stem and leaves. As a molecule of water either transpires through leaf stomata or used in new upper cell development, a molecule of water is pulled into the root. Water absorption and transport to above plant parts is a physical process mostly carried out in dead cell tissue but is dependent upon continuation of the tubes.
Mineral transport into roots, and transport of energy for this process, is directed by plant hormones produced in meristems. After pollination, the hormones produced in the seed embryos, direct sugar transport to the developing kernels. As this redirection increases with growth of kernels, transport of energy to the root cells decreases. Water transport system continues but mineral absorption from the soil is reduced. Root expansion stops. One of the functions of the living cells is to ward off potential fungal invaders by actively producing anti-fungal chemicals. As the energy for these cells is reduced, the resistance to microbes also is reduced, increasing vulnerability of xylem being plugged by fungal mycelium.
Now it becomes a race to the finish. Did the plant produce and transport sufficient carbohydrate to root cells before pollination to sustain enough living functions in roots to avoid destruction by fungi?
Corn grain production
Every year, fields of corn are different from other years. This year, in the USA, it is more different than usual. Most of it is related to extreme delays in planting time and some by soil moisture extremes. After the first 10 days after pollination, kernels begin to absorb carbohydrates at about 2% of their final total per day. This rate continues for the next 40 days, drawing on the new photosynthetic production plus reserves stored in the pith cells of the stalk. Genetics and environments before and after pollination contribute to the amount transferred each day to the kernels. The number of kernels on the plant affect the total being transferred.
Uneven environments associated with late planting and extreme wet soils results in wide differences among individual plants, resulting in uneven numbers of pollinated ovules, and plant to plant differences in numbers of kernels per plant within a field. Individual plants that emerged later than adjacent plants may have same number of kernels as adjacent plants but will not have same photosynthesis rate because of shading from adjacent plants. Competition for uptake of minerals will probably also be inferior. Hybrids that have high number of pollinated ovules, despite this early stress, may have a larger draw of carbohydrates to the ear but the delayed emergence plants of these hybrids may deplete supply stored in stalks, developing early death of roots, leading to stalk rot.
2019 corn production in the USA will be different. Biology is the same, the environment is different.
Photosynthesis in corn
Energy for corn growth comes from carbohydrates soon after seedling emergence. Multiple physiological, structural and environmental factors affect the production of those carbohydrates during the development of each mature plant. Size and directions of the leaves affects penetration of light into the canopy. Minerals affect the conversion of light energy into chemical energy. After pollination these factors influence ability of the plant to produce sufficient carbohydrates to each developing kernel with minimal transfer of carbohydrates stored in the stem tissue. This Corn Journal blog of August 16, 2016 addresses the relationship between light intensity and photosynthesis in corn.
Our eyes don’t allow us to be aware of differences in light intensity, but the photosynthesis rate in corn leaves is very sensitive to this factor. Having a C4 photosynthesis system, the rate of photosynthesis in an exposed chloroplast increases directly with light intensity up to full sunlight. C3 plants, such as soybeans, can only use 3/10ths of full sunlight hitting the leaves. Several years ago, I carried a light meter to measure foot candles in many corn environments to help me understand light intensity differences. Measuring in foot candles is not quite as good as PAR (Photosynthetically Active Radiation) measurements as done with more recent instruments but it did demonstrate to me the differences in light intensity within corn environments.
Full, unobstructed sunlight has intensity of 10000 fc (foot- candles) when measured in Ohio, Illinois and in Colorado. However, the measurement in the shadow of a single corn leaf is only 1000 fc. That portion of the leaf in the shade of the single leaf is only producing 1/10th the carbohydrate as the same area of the leaf fully exposed to direct sunlight. The lowest leaf in a canopy receives only about 50fc, hardly enough to sustain its own tissue and is one of the reasons that these leaves disintegrate. As shown in the graph above, cloudy days also reduce the photosynthetic rate in corn plants.
It is a precarious balance between sugar demands established in the ear after pollination and factors affecting photosynthesis in fields. Growers are in control of some factors such as hybrid selection, fertilizers and plant density but others such as rain affecting kernel numbers, or leaf diseases potentially removing effective leaf area and cloudy weather can drastically affect whether the corn plant stays alive until the completion of grainfill.
Kernel fill time
The biology of the corn plant after the initial ten days of movement of glucose to the developing kernels is consistent with nearly all hybrids. This Corn Journal blog of August 2016 applies even in the USA Midwest wild summer of 2019.
As the corn embryo develops, and the cytokinins accumulate in the pollinated ovule after the first 10 days, there is a constant translocation of sugars to each kernel for each day. The total daily movement continues for about the next 40 days, almost regardless of daily variable rates of photosynthesis due to cloudy weather or leaf damage from disease. Sugars are drawn from all leaves and even those stored in the stalk pith tissue. The total draw to the ear is determined by genetics of the variety, environmental factors including minerals and the number of kernels. The number of kernels is also determined by genetics and environment factors such as minerals and especially water available during ovule formation and pollination.
The daily transfer of sugars during days 50 to 60 of grain fill is greatly reduced until the abscisic acid affect causes thick cell walls to form at the base of the kernel, cutting off the sugars transfer into the kernel and the movement of water from the kernel. This is known as the black layer.
Sugars translocated to the ear are sugars not available to other living tissue in the plant. Roots are especially dependent on the same sugars to support metabolism functions, including warding off the potential microbe invaders. Starving roots, as they rot, eventually reduce water uptake and, if insufficient water to meet the transpiration rates from leaves, a permanent wilt will occur. With the wilt, movement of sugars to the kernels is stopped, abscisic acid takes over, causing the black layer to form a base of kernel. The consequence is light grain weight on the affected ear.
Hormones after pollination
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.
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.
Time from germination to production of pollen in temperate zone corn is determined by amount of heat per day after germination. The shoot apical meristem produces leaf and stem cells until it gains the hormonal signals to switch to producing the tassel cells. This occurs while the growing point is surrounded by the growing leaves usually at about the V6 stage. The 2019 rain during April and May delayed planting much beyond normal. Pollination in most fields in Northern Illinois is at least two weeks later than normal.
It will be interesting to see if warmer temperatures after planting this season will cause earlier initiation of the shoot apical meristem to tassel cells and consequently fewer leaves. We attempt to characterize hybrid maturities by daily heat units, with heat units to flowering and/or heat units to abscission layer formation in kernels but actually it is mostly determined by heat for apical meristem differentiation. Time from pollination to completion of grain fill as the abscission layer cuts off translocation of carbohydrates to the grain is mostly a time factor of about 55 days. Consequently, the heat from planting to shoot apical meristem differentiation is the most critical factor on determining when these late planted corn fields will have completed grain fill.
The 2019 corn season in much of the northern USA corn belt will be remembered as distinct from previous season.
Spread of new corn diseases
We are often surprised when a new (to us) race of an established corn pathogen is found or when a pathogen is found in a new area. It becomes reported as new, but it is highly probable that it had occurred in past seasons but was not found and identified by someone to realize that it is distinct.
Having inoculated disease nurseries with common pathogens near other nursery plants for multiple years, it was always notable to me that the fungal blight was usually not seen elsewhere in the same field. The northern leaf blight fungus, Exserohilum turcicum, produces spores in 2-3 weeks after first infecting the leaf. Those spores from those lesions blown to damp leaf tissue to allow new infection will not produce more spores for another 2-3 weeks. Most corn environments do not provide the needed moisture for continual new infections when the beginning one started at the V8 stage of corn growth. The discovery of the new race in a seed field in Indiana probably was aided by frequent movement in the field by machinery and people plus a susceptible inbred. The fact that after first identified it was found by several specialists scattered across many areas of USA the same year suggest that it had existed for a several seasons slowly building in intensity.
I recall seeing a field in Southern Minnesota that was heavily infected with the fungus Kabatiella zeae causing eyespot that was mostly limited to the area of the field that was in corn the previous year. Apparently, the minimal tillage for the previous season had resulted in large loads of inoculum to infect the new crop but spores had not spread sufficiently to adjacent fields to be noticeable.
A major factor in spread of new fungal pathogens is initial infection, probably not noticed by humans for a few years but eventual increase with minimal tillage and continuous corn growing in the same field. Continuous crop growing is associated with root worm vectored Maize Chlorotic Mottle Virus, one of the components of the corn lethal necrosis in Nebraska and Kansas.
Some pathogens are spread over long distances by wind. Rust fungi, Puccinia sorghiand Puccinia polysoraspread north into the central corn belt by winds. Vectors of viruses are spread by wind as well.
Genetic diversity available within Zea mayshas always provided adequate resistance available to corn breeders within a few years after a new pathogen is identified. Early detection is important to preventing significant damage to the crop and altering the cropping system can be a significant immediate control.
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.