Pericarp invasion resistance

​Carbohydrates stored in the endosperm of a corn kernel is a potential source of nutrition for fungi and insects.  The pericarp can be a major barrier to attack to these potential invaders.  Studies concerning the tropical corn storage insect, Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect.  Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).

Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi.  Most enter the ovary through the silk channel immediately before pollination.  This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen.  After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells.  Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.

The phenomenon known as silk-cut can expose the seed to fungal infection.  After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary.  Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow.  These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444).  This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi.  Genetics and environments influence the occurrence of silk-cut.  Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.