| 4. Type IV erect-type mutants The mutants of this type directly arose as heterozygotes or chimeral plants in X2 generation of X-ray treated normal plants, (0.42%) but they occur red sporadically in the next generation of Type III heterozygotes (0.84%). The heterozygotes had shortest spikes, thin straws and narrow leaves in all types of the erect-type mutants, but they possessed dense and erect spikes, stiff short straws and erect leaves. The seed germination rate and seed fertility were as low as 71.5% and 65.2% respectively in the heterozygotes. The heterozygotes gave quasi-normals, quasi-Type III erect-type heterozygotes, Type III erect-type homozygotes, Type IV heterozygotes, Type IV homozygotes and lax-eared plants in a ratio 1 : 5.4 : 2.5 : 0 : 0.6 : 0 in the following generation. The segregated normals and Type III erect-type heterozygotes had each a slightly shorter height and spike-1ength. Therefore, the writer gave them a head sign "quasi" to distinguish from real type plants The homozygotes showed more intensified characters of the heterozygotes, but they included two forms of plants of which A-form being more vigorous and larger in plant appearance than B-form, which segregated in the next generation of the A-form. These two forms were also different in chromosomal constitutions. The Type IV heterozygotes possessed 41 chromosomes including one isochromosome. In meiosis of PMC's, chromosome associations 20II + 1I (69.5%) and 19II + 3I (28.80%) were observed, besides a few of 1III + 18II + 2I (0.68%) and 1IV + 18II+1I (1.07%). And these associations of chromosomes included without exception one isochromosome as a heteromorphic pair or two unequal univalents. The isochromosome of this type was closely similar to that of Type III heterozygotes both in size and form. The A-form homozygotes, which segregates B-form in the following generation, had 41 chromosomes including two isochromosomes, and in meiosis of PMC's, chromosome associations 20II+1I (68.33%) and 19II+3I (30.24%) mainly were seen, besides a few of 1III + 18II +2I (0.62%) and 1IV + 18II + 1I (0.8%). These associations were usually included a pair of or two equal isochromosomes. The B-form homozygotes, which are more slender and shorter plants than the A-form, had 2n=40 chromosomes, and they formed 20II including one homomorphic isochromosome pair (67.85%) or 19II + 2 isochromosome univalents (25.66%), besides a few of 18II+4I (4.8%), 1III+17II+3I (0.41%) and 1IV+17II+2I (1.22%) in meiosis of PMC's. The other segregates quasi-normals, quasi-Type III erect-type heterozygotes, Type III erect-type homozygotes and lax-eared plants had the chromosome constitutions 2n=42, 2n=41+1 iso., 2n=40+2 iso. and 2n=41 respectively. |
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