Corn for Grain
Insight 05


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Agronomists widely use the corn kernel “black layer” as an indicator of physiological maturity. It is also generally known that visible factors, such as green leaf loss or defoliation due to hail, frost, or disease, can cause the black layer to form earlier than with the normal maturation process. It is less recognized that periods of very cool weather (without frost) during grain fill can also cause the black layer to form early. Little background information is readily available on the anatomical and physiological processes surrounding black layer formation. In this STRIKEPOINT edition, these aspects of corn development will be highlighted from a historical perspective on how the science behind this knowledge evolved.

Early Anatomical Observations

Figure 1. Anatomy of a corn kernel showing key structures involved in black layer formation near physiological maturity. The black layer forms in a region of cells several layers thick between the endosperm base of the kernel and the vascular area of the pedicel.

In early seed development, a black layer forms in a region of cells several layers thick between the endosperm base of the kernel and the vascular area of the pedicel (see Figures 1-4). Near physiological maturity, these cells compress or collapse into a dense layer, which appears visibly black. Concurrently, the cells at the base of the endosperm also become crushed. These are specialized vascular cells, which absorb and transfer to the kernel plant nutrients plus sucrose and other sugars produced by the plant in photosynthesis. This stops their capability for movement of sugars and nutrients from within the plant into the kernel. A suberized barrier forms around the seed tip when the black layer connects with the kernel pericarp (outer wall) and testa (seed coat).

Figure 2. Close view of progression in color changes in the placental region of the corn kernel as cells compress or collapse into a dense layer, which eventually appears visibly black.

Within the ear, the black layer usually forms first in the tip kernels with progression a few days later to the large kernels at the base. Canadian researchers (Daynard and Duncan, 1969) proposed that as a survival mechanism when food (sugars produced in photosynthesis and other nutrients) supply to the ear is limited from the rest of the plant, these resources are apportioned within the ear so that some kernels can develop fully while others abort early or are “shut off” from the translocation pathway by formation of the black layer. These limits would likely be greatest for the tip kernels, which are last to be pollinated and farthest from the food sources within the plant. This led to the hypothesis that black layer forms whenever movement of sugars and other plant nutrients to the kernel is decreased to a threshold level, either due to plant stresses, which reduce supply of sugars produced by photosynthesis for the plant, or due to plant maturity when the plant stops photosynthesis and soil nutrient uptake under favorable growing conditions.
In the late 1960s and early 1970s, researchers reported that black layer formation occurred after an extended period of cool weather – before either leaf disease or frost had reduced green leaf area or before plant maturity. Raymond Baker, the first Pioneer corn breeder, stated “An extended period of cool weather in the fall when the daily average temperatures stay below 55 °F  (12.7 °C) for a week will usually stop growth without an actual freeze” (Baker, 1970).

Figure 3. Progression of black abscission layer formation.

Minnesota Physiology Studies Explore Black Layer Causes
These observations led Minnesota researchers to evaluate the cause of corn black layer formation by conducting both field defoliation and lab experiments. In the lab experiment, both temperature and sucrose movement into developing kernels could be varied (Afuakwa et al., 1984). Defoliation limits sucrose supply by reducing the plant’s photosynthetic capacity.

Figure 4. Kernels from a R6 plant showing embryo (germ), endosperm (starch), and black layer.

Previous research had shown that cold weather greatly slows or stops translocation, or movement, of sucrose within the plant, which would reduce availability to the kernels. Sucrose supply could be directly evaluated by culturing kernels in a lab with or without sucrose.
Field defoliation experiments showed that black layer development occurred at a range of grain moistures, kernel sizes, and calendar days or heat units (Figures 5 and 6). Early loss of leaves caused black layer to form at higher grain moistures, lower kernel weight, and with reduced days or heat units than normal.


Figure 5. Adapted from Afuakwa et al., 1984. Top – Percent kernel moisture at corn black layer formation following defoliation at three growth stages. Bottom – Effect of defoliation at three growth stages on corn kernel weight at black layer. Values are averages of two years and two hybrids for each Relative Maturity (RM).

Figure 6. Adapted from Afuakwa et al., 1984. Number of Growing Degree Days (GDD) (top) and number of calendar days (bottom) by which defoliation advanced corn black layer formation. Values are averages of two years and two hybrids for each Relative Maturity (RM).


Kernel moisture when black layer formed ranged from 32% for plants grown in the field to 76% for kernels developing under controlled lab conditions at 86 °F (30°C) without sucrose (Figure 7). Calendar days from pollination to black layer appearance ranged from 29 days at 86 °F (30°C) in the lab without sucrose to 65 days under cool temperatures (50 °F and 59 °F/ 10°C and 15°C). Black layer formed when kernel weight averaged 45 mg when cultured at 86 °F (30°C) without sucrose to 270 mg for field-grown plants. Kernels from plants grown in the field or in the lab with both higher temperatures and high sucrose supply had dented, and kernels were without visible endosperm liquid when the black layer developed. However, when the black layer appeared for lab-cultured kernels without sucrose, there was no denting or clear milk line. Contents were becoming firm but still were moist throughout the endosperm.


Figure 7. Adapted from Afuakwa et al., 1984. Effect of temperatures and sucrose availability on percent corn kernel moisture of in vitro (lab) grown corn kernels.
Percent kernel moisture of field-grown kernels is included for comparison (maroon line with triangles). Measurements stopped once kernel black layer had formed in more than half of the kernels sampled. Vertical bars are shown only for the last sampling period and show one standard error of the mean.

Sucrose Supply is Key Factor
These results confirmed that black layer formation is more related to continuous sucrose supply to the developing kernel than any specific environmental sequence or physical aspect of the kernel. The researchers concluded that conditions that reduce this supply could also impact flow to kernels of other metabolism products or hormones, but sucrose supply to the developing kernel appears to be a key factor.

Monitor Both Milk line and Black Layer
While disappearance of milky kernel contents can be an indicator of physiological maturity (Afwaukwa and Crookston, 1984; Figure 8) in northern regions with cool weather periods during grain-fill or when other factors, such as major leaf loss or stalk breakage, cause reduced photosynthesis or plant death, black layer may appear in kernels that still have visible fluid in the endosperm. In these instances, the milk line may disappear, and the entire kernel tends to become soft or doughy. Grain drying will occur without the usual milk line progression (Figure 9).

Figure 8. Progression of milk line in corn kernels from R5, or early dent, (left) to R6, or physiological maturity, (right). Photo courtesy of Steve Butzen, DuPont Pioneer.

Figure 9. Plant death due to stalk breakage causes corn milk line to disappear and black layer to form without the usual progression of milk line to the base of the kernel. Similar responses can occur with major leaf loss or extended periods of cool temperatures. Photo courtesy of Dr. R.L. Nielsen, Purdue University.


  • The corn kernel “black layer” is widely used as an indicator of physiological maturity. Knowledge of the anatomical and physiological processes surrounding black layer develop-ment is useful to understand conditions that cause its formation.
  • The black layer forms when a layer of cells compress and turn dark where the kernel attaches to the cob. Specialized nutrient transfer cells at the base of the kernel also collapse, and this barrier stops movement of sugars into the kernel.
  • Several field and lab experiments confirmed that black layer forms whenever sucrose supply to the developing kernel is decreased to a threshold level.
  • Factors that stop this flow include plant maturity – but also leaf loss due to hail, frost, and disease, plus periods of very cool temperatures (without frost) during grain fill.
  • Under these conditions, black layer may form when kernels still have visible fluid in the endosperm. Therefore, both kernel milk line progression and black layer should be considered when monitoring late-season corn development.

Glossary of Terms
Endosperm – Tissue which surrounds the developing seed embryo and provides food for seed growth
Pedicel – Structure that attaches the kernel to the cob
Pericarp – Outer wall of the kernel (seed)
Physiological Maturity – When the crop has reached maximum possible grain yield and kernel growth is complete
Placenta – Part of the ear where the developing kernels (or ovules) are attached to the cob
Suberized – Deposition of suberin on the walls of plant cells; suberin is a waxy, waterproof substance
Testa – Seed coat
Translocation – Conduction or movement of soluble food from one part of the plant to another
Vascular Area – Plant tissues specialized for moving water, dissolved nutrients, and food from one part of a plant to another

Abendroth, L.J., R.W. Elmore, M. J. Boyer, and S. K. Marlay. 2011. Corn growth and development. PMR 1009. Iowa State University Extension, Ames, Iowa.
Afuakwa, J.J., and R. Kent Crookston. 1984. Using the kernel milk line to visually monitor grain maturity in maize. Crop Sci. 24: 687-691.
Afuakwa, J.J., R. Kent Crookston, R.J. Jones. 1984. Effect of temperature and sucrose availability on kernel black layer development in maize. Crop Sci. 24: 285-288.
Baker, R. 1970. Black layer development. One way to tell when your corn is mature. Crops Soils 24 (1): 8-9.
Butzen, S. 2014. Managing for delayed corn crop development. Crop Insights 24:10. DuPont Pioneer Agronomy Sciences.
Daynard, T.B. 1972. Relationships among black layer formation, grain moisture percentage, and heat unit accumulation in corn. Agron. J. 64: 716-719.
Daynard, T.B., and W.G. Duncan. 1969. The black layer and grain maturity in corn. Crop Sci. 9: 474-476.
Endicott et al. 2015. Corn growth and development. DuPont Pioneer. Johnston, Iowa.
Johann, H. 1935. Histology of the caryopsis of yellow dent corn, with reference to resistance and susceptibility to kernel rots. J. Ag. Res. 51: 855-883.
Kiesselbach, T. A., and E. R. Walker. 1952. Structure of certain specialized tissues in the kernel of corn. J. Bot. 39: 561-569.
Nielsen, R.L. 2013. Effects of stress during grain filling in corn. Purdue Univ. [online]


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Revised: March 2017
Expires: March 2018