Seed set was analyzed in three amaranth species (A. cruentus, A. caudatus, and A. lividus) under conditions of self- and cross-pollination during four vegetative periods in 1995–1998. It was demonstrated, that the populations of these species are polymorphic for the breeding system and that they consist of self-sterile and self-fertile plants. The populations were composed mostly of self-sterile plants and the percentages of self-fertile plants were only 34.7 %, 18.7 %, and 12.5 % for A. cruentus, A. caudatus, and A. lividus respectively. The level of self-fertility was high in the first year of isolation, it sharply decreased during the subsequent years of selfing, due to inbreeding effect. Seed set under self- and cross-pollination is determined by species-specificity of the plants, influence of conditions of their growth, and inbreeding depression. Conditions of growth had the strongest influence on seed set. Index of its influence varied from 0.53 to 0.87 for the cross-pollinated plants and from 0.13 to 0.68 for the self-pollinated ones. Analysis of variance demonstrated a weak effect of the interaction of these factors. Spearman's rank correlation demonstrated a highly significant agreement between the seed set values for the self- and cross-pollinated plants of A. cruentus and A. lividus. The lines derived from the three amaranth species varied widely in the seed set values under both self- and cross-pollination conditions.
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