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Intergenotypic Competition Between Tall and Dwarf Wheats. II. In Hybrid Bulks¹

Intergenotypic competition was studied in populations developed from a hybrid of two wheats (Triticum aestivum L. em. Thell.), ‘D6301’ (short-statured) and ‘Ramona 50’ (standard-height). The F₂ through F₆ generations were grown in the field in two replicate subpopulations without artificial selection. Heights of individual plants, taken from the F₃, F₄, F₅, and F₆ bulk populations, were studied in the same environment. Mean height increased and frequency of short-statured plants decreased markedly from F₃ to F₆. Height of 62 random lines isolated from each of the F₂, F₄, and F₆ generations showed a decrease in phenotypic variance from F₂ to F₆. Grain yields of the F₂, F₄, and F₆ bulk populations and the means of 62 random lines (in F₃, F₅, and F₇) indicated a significant increase from the F₄ to F₆ and from the F₅ to F₇ generations. At F₇ the yield of the bulk population equalled the yield of the lower-yielding parent, Ramona 50. Variance in yield among lines increased from F₃ to F₇, but extremely short genotypes did not appear in the F₇. Extremely tall genotypes yielded less than those of intermediate height. Thus, in the bulk population directional and stabilizing selection affected both yield and plant height. Some lines in the F₇ had higher yields than either parent. Thus, in spite of natural selection against the agronomically desirable short plants, the bulk populations were useful as source materials in selection for increased grain yield.

Key Words: Triticum aestivum L. em. Thell. • Directional selection • Stabilizing selection • Bulk population breeding

² Rockefeller Foundation Scholar, now senior plant breeder, Agric. Research Corp., Gezira Research Stn., Wad Medani, Sudan; and professor of agronomy.

Received for publication November 12, 1974.