Wall, J. R. and C. F. Andrus.
In 1951 a most unusual phenotype appeared in some of our segregating breeding lines which has been termed brittle stem (TGC 5, "Brittle stem, an apparently new sub-lethal gene in tomato" by C. F. Andrus.). At that time it was reported that the presumably mutant gene expresses itself only under field conditions. Plants homozygous for the gene grown to maturity in the greenhouse are perfectly healthy and produce a good number of fruit, but plants transferred to the field before flowering produce no fruit, the rapidly developing growing points suddenly stop growth at the onset of flowering and all flowers abscise. There is gradual deterioration of the older normal foliage until all that remains is a plant with no expanded foliage, only bare brittle stems and petioles. This state of near death has been maintained by plants in the field at Charleston for over ten weeks (June 25-Sept. 15). However, with the re-establishment of a more favorable temperature environment, latent buds will grow into normal fruiting branches. This fact has been established by transplanting sub-lethal plants from the field in september and placing them in the greenhouse where new growth developed with the return of cooler weather.
From F2 and testcross data, it has been established that the brittle stem condition is inherited as a single gene recessive. The normal allele of this gene does not have complete penetrance tince heterozygous plants are sometimes detectable in the field.
It has been found recently that the puzzling behavior of this mutant in the greenhouse as compared to the field is determined by temperature. The greenhouse tomato crop is grown during the winter and develops under lower temperatures than does the summer grown field crop. The temperature threshold for appearance can only be approximated at this time. Preliminary results from a temperature controlled growth chamber indicate that night temperature of 66-68 deg. F. and day temperature of 85-87 deg. F. are conducive to full phenotypic expression provided such temperatures occur for at least three successive weeks. There may well be a higher temperature threshold for expression in younger plants since it is known that young tomato plants have a higher optimum night temperature than do older plants.
It is suggested that this sub-lethal gene be called brittle stem with the gene symbol being btl*. There is a good possibility that this gene will prove to be a temperature sensitive biochemical mutant such as is known in Neurospora, though a definite statement on this must await future experimentation.
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* Suggested by C. M. Rick.