Kernel abscission formation

​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.  
 
Early formation of black layer before normal time for full transport of sugars to the kernels results in reduced grain weight.  Shortened time to black layer also reduces some of the water replacement in the grain normally occurring as water moves out through the vascular system.  Consequently, black layered kernels may have higher moisture percentage than usual. Some of these biological factors contributed to the grain production differences in USA Midwest in 2019.