The phenotype and environmental interaction of the bu (leaf burn) gene that occurs in the 5125 inbred of sweet corn suggest the following explanation. The bu gene may retard development or functioning of the plasmodesm connections passing through fine perforations in the bundle sheath wall in relationship to the transport demands of growth and photosynthesis. The supporting evidence is that when the growth rate is slowed down by environmental factors such as drought, low temperature or low light intensity, the burn-blotch phenotype is poorly expressed as if transport into the bundle sheath under these conditions is able to keep pace with slower growth. Under conditions favoring rapid growth, pale green blotches develop in which there is an accumulation of starch in the mesophyll with little or no starch in the bundle sheath. Necrosis sets into these paler areas, especially in younger rapidly growing tissue.

Although the thin-walled, loosely packed cells of the mesophyll may be specialized for photosynthetic activity, they are not adapted for starch storage. Thus, an accumulation of starch in such a C4 type of mesophyll could result in the death of the tissue. It seems that the bu gene represents a retrogressive step back toward the more primitive C3 system that can store starch in the mesophyll and lacks a bundle sheath specialized for such storage. In last year's Maize News Letter (52:58-59, 1978) we described the leaf-burning in 5125 as a monogenic trait bearing on C3-C4 photosynthesis.

Walton C. Galinat

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