If a corn plant draws more carbohydrates to the ear during those critical 60 days after pollination than it can supply with current photosynthesis and storage in the stalk, it depletes the supply needed to keep root cell’s metabolism. The deterioration of root cell metabolism allows invasion of organisms in the soil and inability of the roots to transport water to the plant parts above the soil. Transpiration from the leaves continues until all available water is gone. Then the plant wilts. Symptoms of the wilt become slightly visible for a few days before all leaves turn gray and droop. We call this premature death. These wilted plants occur as individuals, often surrounded by green plants that did not overdraw on its carbohydrate supply to fill its kernels. Often these individual wilted plants have more kernels than those adjacent green plants or have excess photosynthetic stresses.
The wilted plants initially have green outer rind to the lower stalk, but the pith tissue inside the stalk has puled away from the outer rind, as part of the wilting process. This weakens the stalk strength by one third. The outer rind slowly turns from a green color to yellow and shows symptoms of invasion of fungi as they digest the remaining cellulose and proteins in the stalk cells, further weakening the stalk strength.
The critical stresses leading to stalk rot occur during the kernel fill period, those 60 days after pollination. If a plant makes it through that time without wilting, it probably will not get premature root ands stalk rot by normal harvest time. Inspection of corn plants about 60 days after pollination allows the grower to access the to predict probability of stalk rot and lodging in the field that season. Individual gray plants with yellow and brown color to outer rind of the lowest 2-3 above-ground nodes are most likely to lodge with slight wind pressure. These plants easily fall with slight pressure. One can also easily pull the plants up from the soil as the dead roots offer little resistance.
Why did that plant commit more to kernel fill that its photosynthesis could supply? Was it shaded by adjacent plants because of inadequate spacing of seed, did it have excess damage to leaves from pathogens or insects. Did the environment of the roots cause less root mass or destruction from pathogens or insects? Perhaps the genetics of the hybrid encouraged a higher kernel number in that environment than could be supported under the photosynthetic stress of the season.
Having a few early wilted plants in a field can be a sign that the hybrid maximized its ability to produce grain yield for that season’s environment. One can learn a lot with inspection of the field shortly after the 60-day kernel fill period.
Sugars are transported to each kernel for days 10-50 after pollination if most corn plants if field conditions are favorable. Then the transport slows for another 10 days.
Each kernel in a corn ear is a fruit. As with most other fruits, sugars are transported to the kernel through the vascular system from the leaves and the stem. Plant hormones like auxins and gibberellins produced in the seed embryo meristems actively guide the sugars to the seed within the fruit. Corn kernels have only one seed. Although much of the sugar is moved outside of the embryo to the endosperm, sugar also provides energy for growth and development of the embryo. As the embryo matures, auxin production is reduced. Consequently, physiological demand for sucrose supply to the kernel is reduced.
Reduction of sucrose in the cells at the base of the kernel causes the balance of ethylene and auxin to change. Ethylene increase causes the layer of parenchyma cells closest to the kernel base to lose cell wall contents, while those adjacent cells away from the kernel gain wall thickness. Eventually an abscission layer forms cutting off all movement of sugar into the kernel and water movement away from the kernel thru the stem tissue (the cob).
Research by J.J. Afuakwa, Crookston and R.J Jones (Crop. Sci. 24. 285-288) showed that reduction of sucrose available to the kernel was a major factor in induction of the abscission layer (black layer). Although it is mostly related to maturation of the embryo, it could be induced by other factors reducing sucrose supply to kernels. Reduction of photosynthesis by leaf disease or frost damaged leaves could result in shortened time to black layer. Early plant death, perhaps from root rot, causing leaves to wilt and thus removing sugar supplies induce black layer within a few days.
Plants with green leaves 60 days after pollination now will undergo slow maturation with abscission layers forming at the base of each leaf. Those abscission layers cut off the transport of sugars from the leaves and the transport of water to the leaves and removal of water from the plant via transpiration. Reduced competition with kernels for sugars stored in the stalk pith tissue allows roots to slowly age.
Plants that did not manage to make it the full 60 days with green leaves likely had photosynthetic stress reducing the ability to meet the demand by kernels for sugars. As a result, root tissue on that plant lost its ability to adequately battle the multiple potential pathogens. Loss of living root tissue during the kernel fill period results in less water uptake and consequently early wilting of the entire corn plant. Kernel filling stops at that time.
Competition for carbohydrates between kernels in developing ear and plant metabolism sites especially in the roots of a corn plant is established 10 days after pollination. This is a battle being fought individually by each plant in a corn field, as each plant can have slightly different environmental factors influencing its ability to produce carbohydrates and different numbers of kernels.
Net supply of carbs is influenced by environments, like light intensity, in which the rate of photosynthesis drops directly with less light. Cloudy days result in lass carbs produced. Leaf disease, reducing photosynthesis due to less leaf area, as does shade from adjacent plants. We have selected genetics programed to move carbs to non-photosynthesizing tissue like the roots with excess stored in stalk. During the grain fill time of the corn plant, newly produced carbs are transported to the kernels as well as those stored in the stalk pith tissue. The draw to kernels is constant, influenced by genetics and minerals, regardless of daily photosynthesis factors. Competition between kernel development and root cell metabolism for carbohydrates reserves stored in the stalk tissue becomes more intense if daily photosynthesis is reduced during the 40-day period of maximum draw by developing kernels.
Energy supplied by the carbohydrates pulled to the root tissue is used in its cells’ metabolism for normal function including producing the compounds needed to ward off the multiple organisms in the soil attempting to devour the root tissue. Defense of the living, functioning root tissue is essential to the rest of the corn plant, as the minerals and water absorbed by roots and transferred upwards are essential to function.
It is a battle essentially between roots and kernels for carbs that is fought by each individual plant in a corn plant especially between day 10 and day 50 after pollination. If carb supply is not sufficient to meet both needs, kernels win the race. The roots degenerate prematurely and are unable to supply the water to leaves to match the loss of leaf water due to transpiration. As a result, the plant wilts. Kernels’ win is temporary, as the wilted plant no longer can transport carbs to kernels.
Photosynthesis is the engine driving corn from the seedling to maturity. Previous generation's photosynthesis provided the energy for the seedling to emerge from the soil. Current generation photosynthesis provides more than sufficient energy for the metabolism to build tissue to construct a plant with expansive roots, large leaves, and 6-9 feet of stalk within a few months of seedling emergence. The corn plant not only provides the energy for the building materials for its new structures and daily metabolism but also has excess carbohydrates that are stored in the stalk pith cells.
Then, at midseason, it produces flowers. After pollination for the female flowers, hormones in the plant shifts the direction of flow of the carbohydrates from leaves and stalk pith cells to the the new embryos and its storage compartment, the endosperm. Environment and genetics determine the rate of flow of carbohydrates to the new fruit of the corn plant. The number of fruits (kernels) also becomes a big influence on the total draw from the carbohydrate supply.
Flow for first 10 days after pollination is relatively slow but then it speeds to a faster, relentless pace for the next 40 days. That daily pace of movement of carbs to the ear on the plant continues regardless of daily variation in photosynthetic rates due to environmental variables. If cloudy weather slows photosynthesis, the reserves in the stalk provide the difference. If leave disease reduces leaf area available to light energy conversion to carbohydrates, storage from early days is called upon for movement to the new kernels.
This high rate of flow of carbs to the new kernels slows after day 50 for about 10 days until it is cut off with the formation of an abscission layer at the base of each kernel. If the other plant parts such as the root tissue survived the competition for stored carbohydrates in the stalk, slow senescence occurs in the plant cells. This is the outcome desired by all corn growers but environmental stresses can interfere with the completion of maximum deposit of carbs in the kernel on living plants.
About 5-6 weeks after pollination is a good time to evaluate corn hybrids for resistance to leaf disease. This Corn Journal blog from 2017 discusses some of the dynamics in resistance to leaf diseases.
Leaf epidermal cell’s walls and the waxy leaf surface provide the first line of defense against microbes. Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf. This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader. Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth. Much initial reaction is limited to host cells adjacent to the infection site.
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight)
Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen. These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another. On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
The system described above is referred to as general or horizontal resistance. It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen. Horizontal resistance is expressed in corn plants by fewer leaf disease lesions. Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure. Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.
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.