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eturn to table of

eturn to table of contents 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 DuPont Pioneer corn breeder and author of an early popular press article on black layer development, stated “An extended period of cool weather in the fall when the daily average temperatures stay below 55 ºF for a week will usually stop growth without an actual freeze” (Baker, 1970). In Ontario in 1969 and 1970, premature black layer formation developed 1 to 4 days after a week with daily maximum average temperatures of 55 ºF or less (Daynard, 1972). 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. 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. 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 ºF without sucrose (Figure 7). Calendar days from pollination to black layer appearance ranged from 29 days at 86 ºF in the lab without sucrose to 65 days under cool temperatures (50 ºF and 59 ºF). Black layer formed when kernel weight averaged 45 mg when cultured at 86 ºF without sucrose to 270 mg for fieldgrown plants. Percent Kernel Moisture at Black Layer Formation Kernel Weight (mg) at Black Layer Formation 36 34 32 30 28 26 24 22 20 305 285 265 245 225 205 185 80 Day RM 105 Day RM Soft Dough Dented Half-Milk Control Soft Dough Dented Half-Milk Control Stages of Defoliation Figure 5. Adapted from Afuakwa et al. (1984). Percent kernel moisture at corn black layer formation following defoliation at three growth stages (top). Effect of defoliation at three growth stages on corn kernel weight at black layer (bottom). Values are averages of two years and two hybrids for each Relative Maturity (RM). GDD by Which Defoliation Advanced Black Layer Formation Days by Which Defoliation Advanced Black Layer Formation 180 150 120 90 60 30 0 50 40 30 20 10 0 80 Day RM 105 Day RM Soft Dough Dented Half-Milk Soft Dough Dented Half-Milk Stages of Defoliation 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). 26

eturn to table of contents 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. Percent Kernel Moisture 100 80 60 40 20 Field Grown 86 °F (+ sucrose) 86 °F ( - sucrose) 59 °F 50 °F 13 23 33 43 53 63 73 Days After Pollination 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 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 (yellow 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. 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. 27

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