The interspace (is) factor exposes a bare face of rachis between the apex (upper lip) of the cupule and the glume cushion of the diverging spikelet(s) above. Botanically, the interspace represents the backside of the internode from the alternate rachis segment (phytomer). When condensation is sufficiently low for the length of the interspace to become equal to the length of the cupule below, then there is enough extra space to interlock the pedicellate member from an adjacent pair while the sessile member stays in line with its cupule row. The result is a 50% reduction in the kernel-row number. The most common kernel row number in Coroico is nine, on long slender ears with enlarged butts that are 18-rowed.
The exposure of the kernel from the
teosinte fruitcase required that it adapt by evolving two new systems of
kernel protection. An inner system of either a thickened pericarp or a
thickened aleurone that would guard against "self-popping" of mature kernels
due to weather and, secondly, a new outside system of husk leaves for varmint
protection. The Coroico kernels have the primitive thin pericarp caused
by the tpe gene of only three to four cell thickness. When expressed
in a teosinte background, where apparently the tpe of Coroico originated,
the pericarp is only two to three cells thick and resistant to self-popping.
Viability of teosinte seed is, thereby, insured by the physical support
derived from containment within the fruitcase. Protection of the exposed
kernel from self-popping in Coroico under the direction of the Guarany
Indians in South America has taken the aleurone pathway while in Central
America the pathway was by the pericarp. Sweet corn improvement based on
a tender or thin pericarp may be best achieved by use of the Coroico complex
(see my item here on multi-layered expression). The tpe gene is
incompletely dominant and in some backgrounds it may be inviable due to
pericarp splitting and pathogen infection. The multiple aleurone factor
may be inherited as a single dominant gene (Wolf et al., Crop Sci. 12:440,
1972).
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