The wild relatives of maize, namely teosinte and Tripsacum, have a smaller, thinner pericarp (two cells at 35µ thickness) than that known in maize (4 to 18 cells totaling 70 to 250µ. In the wild relatives, the strength of the pericarp is supplemented by physical reinforcement from the enclosing cupulate fruit case; the kernels of these relatives will pop on heating, when within the fruit case, but not when they are naked by removal from their enclosure.
With the emergence of the kernel from the fruit case during the evolution of maize, apparently there was selection for a thicker pericarp that provided protection against disease, allowed popping and gave the evolving maize kernel the potential to expand into gigantic sizes.
When man carried primitive maize to South America, where it gained freedom from backcrossing to the thin pericarp of teosinte, it followed a pathway toward greater productivity by an increase in kernel size. The ultimate product, the giant-kerneled Cuzco flour maize, has a pericarp of about 18 cells at 200µ.
In Mexico, introgression by certain genetic components from teosinte into maize is now limited in part by teosinte's thin pericarp and the upward inclination of its outer glume, positioned to puncture the pericarp should an expanding maize-type kernel press against it. As a result the pathway to high productivity per ear in Mexico was usually one of high condensation and increases in kernel row number. Apparently the thick pericarp of the best popcorns became protected from teosinte introgression by certain gametophyte (Ga) genes.
The thin-pericarp barrier is especially important with sweet corn, which has kernels that balloon out in the milk stage at a time when they are often handled roughly during harvesting. Both machine picking and cutting from the cob require some kernel strength. The extra-thin pericarps tend to split in the milk stage, resulting in either the direct destruction of the kernels or their eventual loss through pathological infections.
Ordinarily maize and its relatives, as well as most other grasses (except barley), have only a single-celled aleurone. But the variation in maize from some South American races (Coroico) includes a multi-layered aleurone. Experiments are now underway to determine if several aleurone layers can help contain the turgor pressure of the developing endosperm and thereby permit a maize pericarp only two cells thick. The substitution of a thick aleurone for a thick pericarp would be nutritively ideal; the pericarp is largely indigestible cellulose, while the aleurone layer is highly digestible and rich in proteins.
W. C. Galinat and P. Chandravadana
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