More on pericarp and aleurone thickness in maize and its relatives
The pericarp of maize and its relatives was found to range from 2 to 20 cell layers and 25 to 200 microns. A number of crosses were made between thick and thin pericarp inbreds of maize. Both of these types were also crossed to teosinte, which has an extremely thin pericarp measuring 25 microns. The F1 and F2 progenies of these crosses were examined.
The method used to measure the pericarp is simple:
1) Select mature kernels from the middle of the ear.
2) Soak kernels for 24 hours.
3) Using a single-edged razor, remove the crown of the kernel.
4) Take free hand sections on the abgerminal side of the kernel.
5) Stain sections with dilute safranine.
6) Mount sections in glycerine.
7) Measure sections under high power (250X) using an ocular micrometer.
8) Take an average of 3 readings per kernel and 3 kernels per ear.
The relatives of maize have the thinnest pericarp (Table II). There was no selective pressure to evolve a tough protective pericarp in the presence of their hard fruit cases. A sample of sweet corn types shows a wide range, from the thin pericarp inbred 677a at 57 microns, to Spancross, a tough hybrid 140 microns thick. The thickest pericarp observed was found in Redenbachers Gourmet Popping Corn at 180 microns.
The inheritance of popcorn pericarp is believed to be controlled by one dominant gene for thinness and many modifying genes for thickness. The results of the sweet corn-teosinte crosses seem to agree with this observation (Table III). Here the F1 pericarp is nearly the same thickness as that of the teosinte parent. The F1 of the different crosses all have nearly the same pericarp thickness independent of the thickness in the sweet corn parents. Thin pericarp, a feature seen in teosinte, is dominant as expected for a wild type trait.
A number of popcorn varieties were examined for aleurone thickness as well as pericarp thickness. Six popcorn varieties were examined, two of which appear in Table II. All of the varieties examined were found to have a very thin aleurone layer about the same size as that of teosinte, and at most, one-half the size of sweet corn aleurone. The popcorns, with their very thick pericarp, have small aleurone cells and teosinte, with its protective fruit case, also has small aleurone cells. But in the sweet corns, while man's selection has made the pericarp thinner, the aleurone cells have become larger. This would seem to indicate that thick aleurone compensates to some degree for thin pericarp in forming a protective layer.
W. F. Tracy, P. Chandravadana, and W. C. Galinat
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