Abscission layers in corn

Nearly all plant species shed plant parts with the function of allowing continuation of the species.  Temperate zone deciduous tree leaves reduce the supply of nutrients to the chloroplasts as summer ends.  Nitrogen and other nutrients are transported from the chloroplasts and other leaf cell contents to plant parts that will survive the winter.  This physiological change reduces movement of auxin hormones to the cells at the base of the leaf where it is connected to the stem.  This stimulates the production of lignin and suberin in cells immediately below a layer of cells called the abscission layer. This thickening of cell walls essentially cuts off all translocation of water and nutrients from the stem to the leaves.  It also causes the leaves to break from the stem at the point of the abscission layer while preserving the moisture in the stem. 
 
The same process occurs in all plants, including in the tropics, although not as remarkably simultaneously colorful as with trees in the temperate zone at this season in north America.  Similar physiological phenomena occur as fruit are released from the base plant.  This was also true for the Teosinte species selected 10,000 years ago by someone because, contrary to most Teosinte plants, this genotype formed abscission layer at the base of the fruit (kernel) but did not quickly allow the kernel away from the parent plant.  Maintaining the attachment of the kernel to the rachis (cob) even after abscission layer (black layer) formed allowed more efficient harvest for those folks 10,000 years ago, as it does for the modern farmer.  Selection of this character by corn breeders since that time have attempted to maintain appropriate attachment of the kernel (fruit) to the cob thru harvest.  
 
Leaves surrounding the corn ear, the husks, undergo similar abscission layers after the black layer is formed in the kernels.  This cuts off the water movement into the husk leaves, usually causing them to withdraw from being tightly wrapped around the ear.  This is associated with faster evaporation of water from the kernels.  Likewise, abscission occurs in cells at the base of the cob. This further removes water movement into the cob, allowing for further drying of cob and kernels.  Corn breeders challenge is to select genetics that allow rapid kernel drying while maintaining attachment of the ear to the corn plant until harvest.  Stress conditions may cause premature formation of abscission layers in kernels and husk leaves.  This has been associated with ears dislodging from the plant.  Pests such as European Corn Borer have been associated with premature dropping of ears in the field. 
 
The balance of timely formation of abscission layers to allow drying of grain, maintaining adequate attachment of kernels to cob and ears to the plant when grown in multiple environments continues to be a complex genetic feat.  Combining these characters with maximum translocation of sugar to the grain before formation of the abscission layer at the base of the kernel adds to the corn breeders’ accomplishments for the past 10,000 years.