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.
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.