Grain fill after pollination was at a rate of about 1% per day for the first 10 days, then about 2% per day for the next 40 days and about 1% per day for the last 10 days. As the seed embryo matures, the ratio of abscisic acid to other hormones tips towards the abscisic acid. This causes the cells at the base of the kernel to form hard walls eventually resulting in the abscission layer (black layer), cutting off the movement of carbohydrates and nutrients into the grain and therefore the displacement of moisture from the grain.
Lack of attraction of hormones to the ear causes similar balances with abscisic acid in the husk leaves resulting in abscission layers to form at their bases, causing the husks to lose the supply of water and nutrients from the roots. Genetics and environments influence these processes.
If the plant wilts early, because of root death, the abscission layer forms almost immediately, blocking the flow of carbs to the grain, resulting in light grain weight.
Water loss from the grain, after black layer is due to evaporation. Looseness and size of the husk leaves, composition of the kernel and moisture retention in the cob are the genetic factors. There are some observations that husk leaves on plants that die early do not separate from the ear at the normal rate expected for the hybrid, thus slowing down the grain moisture loss. When all goes well, the plants remain green at black layer, the husks turn white and loose as the moisture level in the grain drops rapidly from the 35% at black layer to an economically feasible level for harvesting.
Remaining leaves on the maturing corn plants slowly form abscission layers at their bases as photosynthesis rates slow. Color changes to a yellow and eventually brown. The outer rind of the stalk, however, remains green and the pith tissue solid until freeze if the plant made it too black layer without wilting.
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.