Intergenotypic Competition Between Tall and Dwarf Wheats. I. In Mechanical Mixtures1

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Abstract
Intergenotypic competition was examined in Triticum aestivum L. em. Thell. populations composed of ‘Ramona 50’ (R50), a tall variety, and ‘D6301,’ a short‐statured variety. Their respective relative heights were about 100 and 75% and relative grain yields in pure stands were about 80 and 100%. Replicate equal‐frequency mixture‐bulk populations were synthesized and maintained without artificial selection or reconstruction for four generations. D6301 decreased in frequency over time with a concurrent decrease in the population yield level. Yield and endpoint yield components were examined in three environments for each genotype in a frequency study where the genotypes appeared in all combinations at 0.125 frequency increments. Population yields were predictable on the basis of pure‐stand performance, but the contribution of D6301 to total yield was significantly below expectations at all frequencies. Complementary competition was indicated because R50 showed a corresponding increase in yield over expectations from pure‐stand performance. Reproductive values were strongly frequency‐dependent for both genotypes, with the competition effect becoming increasingly larger either with decreasing frequency of D6301 or increased frequency of R50. The poor competitive ability of D6301 was expressed as a large decrease in number of kernels per spike and possibly decreased spike number, while kernel weight was not affected. R50 attained its advantage by increased kernel weight at lower frequencies and possibly increased spike number; kernel number per spike remained constant over all frequencies.

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