As indicated in an earlier report (MNL 50:84-86, 1976), immature embryos can respond in either of two ways after excision: one, embryonic growth persists, causing the embryo to flip over, and callus proliferates directly from the developing embryo; two, embryonic growth aborts and, with the embryonic axis remaining in contact with the medium, callus proliferates directly from the scutellum. The origin of the callus is critical since the experiments of Green and Phillips (Crop Sci. 15:417-421, 1975) indicate that only scutellar callus is capable of regeneration.
Several conclusions may be drawn from the data presented in Table 1, which includes experiments conducted since the earlier report. First, if one is not concerned with the origin of the callus and simply observes whether or not callus is produced, then it appears that the auxin concentration has little influence on callus initiation. However, if one does consider the origin of the callus, then it appears that there is a definite relationship among four factors: the age, size, and genotype of the embryo, and the concentration of hormone. Scutellar callus was produced at all three concentrations of 2,4-D. In each case, the genotype was the "genetic stock" (see earlier report), the age was 12 days, and the size was 2-3 mm. All other samples from this line were either older and/or larger and produced only embryonic callus.
The effect of hormone concentration is even more evident after Tables 2 and 3 are examined. In Table 2, the percentages are calculated from the total number of embryos isolated while in Table 3 they are calculated only from the total number of embryos producing callus. Apparently, scutellar callus production is favored by lower hormone concentrations. As the concentration increases, the growth response is somehow shifted in favor of embryonic callus production.
Only embryonic callus has been initiated from the other genotypes tested. Clearly, the indication is that for every genotype there is a different combination of size, age, and hormone concentration necessary for the development of scutellar callus.
Unfortunately, attempts to regenerate plants from either type of callus have been in vain. Experiments are still being continued in this area.
William S. Rafaill
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