| The observations that substitution lines Timstein 2A and Timstein 2D both
react positively suggest that Timstein probably carried contrasting alleles
to Chinese Spring at loci (possibly homoeoalleles) on these chromosomes.
It can be hypothesized that the gene on Timstein 2D is allelic with the
weaker allele observed on Chinese Spring 2D, and if so, then it would appear
that the chromosome arms 2AL and 2DAlpha are homoeologous. To test the above hypothesis that Timstein carries two alleles relative to Chinese Spring involved in phenol colour reaction, we tested 51 F2 plants from the cross Chinese Spring x Timstein. Thirty three plants reacted positively and 18 plants reacted negatively. This segregation is not significantly different from that expected for a single factor difference (P>0.05), but the result is difficult to reconcile in relation to the observations on the chromosome substitution lines. Phenol reaction of glumes A similar series of experiments to those listed above might be expected to elucidate the genetics of the glume phenol reaction, since exploratory studies have shown that glumes of Chinese Spring give much paler coloration with phenol, than glumes of at least some of the donor varieties of the substitution lines. However insufficient glume samples were available in the present study for any definite conclusions to be established. Biochemistry of the phenol reaction Some evidence that a diphenol is an intermediate in the phenol reaction comes from the observation that the rate of oxidation of DOPA (dihydroxyphenyl alanine) by bran extracts corresponds directly to grain phenol reaction (ABROL et al. 1971). This observation, confirmed in the present studies, has been the basis for using DOPA as substrate when staining electrophoretic gels for "tyrosinase" activity (TIKOO et al. 1973). The methods of these workers were used to examine the possibility that different isoenzymes are involved in the various intensities of grain coloration, or are associated with the two different chromosomes involved. Up to five isoenzymes were observed for bran extracts, but their number and intensities were not related to the grain phenol reactions of a wide range of samples. Furthermore, isoenzymes were obtained for extracts of Duramba bran, and even of flour, neither of which shows a phenol reaction. The presence of such isoenzymes in flour has also been reported by KOBREHEL et al. (1972). A possible explanation of the inconsistency between the results of gel electrophoresis and of photometric assay of the extracts is that the various colours of the phenol reaction are due to the presence of inhibitors which suppress coloration in Duramba bran and in flour, for example, but which are separated from the enzymes during electrophoresis. However, JOSHI et al. (1969) have advanced evidence against such a possibility. There is also the further possibility that the phenol test is non-enzymic (CSALA 1972), since a reasonably normal phenol reaction can be obtained for grains that have been heated at 100o for a number of hours, without added water. However, we found that the phenol reaction is quite sensitive to moist heat, suggesting at least partial involvement of enzymic activity. Clearly the biochemistry of the phenol reaction is not as simple as has sometimes been supposed, and caution is needed in interpreting the results of DOPA oxidation in relation to the phenol reaction. Until the steps involved are further elucidated, the term phenolase complex (MASON 1955) should be used, and specific names such as tyrosinase should be avoided as synonyms for the phenol reaction. |
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