The effect of selection for two-seeded pods in diploid and tetraploid populations and the effect of chromosome doubling on two-seededness were studied. The material obtained was examined to find out how important two-seeded pods are for seed set, grain weight and seed yield.
In a mixoploid plant the percentage two-seeded pods in the tetraploid heads was far smaller than that in the diploid heads. Only when the character had developed relatively strongly in the diploid, did it exceed a very low value.
By selection of individual plants in the best families of a diploid group of plants the percentage two-seeded pods could be raised from on average 7.4 to 30.6% as early as the following generation. Similar selection in tetraploid material with 2.5% two-seeded pods did not increase the percentage.
The reduction of grain weight of seed in two-seeded pods compared with that in one-seeded pods was greater in the tetraploid plants than in the diploids.
No effect of two-seededness on seed yield was found in our material. A marked effect on seed-set was observed in a diploid population (286 plants) when the percentages two-seeded pods rose about 16%.
As the correlation between the calculated number of heads and the seed yield per plant is high, it may be adopted that in our material the number of heads per plant was decisive of seed yield. This will contribute to the small effect of two-seededness on the seed yield of the plant.
The results of the present study showed that the production of tetraploids with a high percentage two-seeded pods will take more advantage of selection of diploid material followed by doubling of chromosomes than of direct selection in the tetraploids.
Protrusion of pods in heads indicates a very high percentage two-seeded pods. This will enable relatively rapid selection for two-seeded pods.
A quite small percentage two-seeded pods also occurred in two tetraploid populations that had not been selected for two-seeded pods.