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eturn to table of contents KERNEL BLACK LAYER FORMATION IN CORN: ANATOMY, PHYSIOLOGY, & CAUSES by Paul R. Carter, Ph.D., Senior Agronomy Sciences Manager • The corn kernel “black layer” is widely used as an indicator of physiological maturity. Knowledge of the anatomical and physiological processes surrounding black layer development 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 24

eturn to table of contents INTRODUCTION 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 during 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 article, these aspects of corn development will be highlighted from a historical perspective on how the science behind this knowledge evolved. EARLY ANATOMICAL OBSERVATIONS One of the first reviews of the black layer concept was in a paper on corn susceptibility to kernel rots in the 1930s in which the formation of a black “closing” layer was described in the placental region of maturing corn kernels (Johann, 1935). The structure of the black layer was detailed in the 1950s by Nebraska scientists Kiesselbach and Walker (1952). Pericarp Endosperm Embryo Black Layer Pedicel 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. 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 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 3. Progression of black abscission layer formation. Cells compress into thin, black layer 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. Figure 4. Kernels from a R6 plant showing embryo (germ), endosperm (starch), and black layer. 25

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