Several days of high temperatures (ca. 100 F) in early July in Massachusetts accentuated the leaf-burning and stunting that tends to occur in the sweet corn inbred, Iowa5125. Three self-pollinated families of recovered derivatives of 5125 had F2 ratios totaling 72 non-burn to 21 burn type plants and one sib-pollinated family had 19 non-burn to 18 burn plants. These good fits to the 3:1 and 1:1 Mendelian ratios clearly indicate that a single gene, hereby designated bu (burn), is involved. Seed from self-pollinations on a non-burn family is available as an improved form of 5125, hereafter to be designated as MA5125.
Sometimes leaf-burning in 5125 will occur in horizontal bands on the leaf, apparently corresponding to high day and low night temperatures. Evidence supporting temperature as a factor in leaf burning and area of starch deposition came from experiments in growth chambers, although no effect was found on "Krantz anatomy." Cross-sections in the green areas of burn type leaves that were stained with I-KI solution showed that, contrary to normal leaves, they failed to store starch in the bundle sheath plastids and, instead, starch was stored in the mesophyll plastids. There are a number of possible explanations, one of which is that the 5125 burn type plants have difficulty in the transfer of photosynthate from the mesophyll to the bundle sheath where the decarboxylation occurs. As the photosynthate backs up in the mesophyll of 5125, it seems to be converted and temporarily stored as starch in this area such as under the C3 system of photosynthesis at higher temperatures. Thus, the burn (bu) mutant gene may involve the loading and transfer system that is most efficient in plants with C4 anatomy.
While 5125 is less inclined to burn at lower temperatures, there is no evidence that it grows better at lower temperatures than other sweet corn inbreds.
The various races of corn and all of its known relatives that we have examined have only the "Krantz-type" of leaf anatomy that characterizes the C4 type of photosynthesis, as is summarized in Table I.
The derivatives of inbred 5125 showing the burn and non-burn phenotypes were examined cytologically. The meiotic behavior of the non-burn plants was normal. In contrast, about 10% of the cells from burn type plants showed meiotic irregularities such as asynapsis, fragmented chromosomes, mis-division of univalents, random distribution of univalents, etc. Examination of the pollen revealed about 4 to 6% abortion.
The C4 system of photosynthesis has evolved over the millenia in grasses adapted to high temperature conditions. A C3 photosynthesis type of corn might be better able to cope with the low temperatures that occur in the early part of the growing season or at high altitudes and latitudes.
W. C. Galinat, P. Chandravadana, and J. Starbuck
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