Single crosses and backcrosses from established lines are importeat parental sources for developing new inbred lines in hybrid maize (Zea mays L.) breeding. The objectives of this study were (i) compare, both theoretically and experimentally, the testcross properties of F2 vs. first backcross populations; and (ii) to outline criteria for choosing between these two types of base populations in recycling breeding programs. Two homozygous lines (P1, P2) and individual plants from their F2 and first backcross (BC1, BC2) generations were mated to an unrelated single‐cross tester. A total of 186 testcross progenies were evaluated for grain and forage traits at three and two locations, respectively. The five generations (P1, P2, F2, BC1, BC2) differed significantly (P < 0.05) in their testcross means (X̄) for grain (GY) and forage (FY) yield. A nonepistatic model accounted for 92 to 99% of the variation among testcross generation means for five of the six traits examined, yet the presence of epistasis was indicated in several instances. In accordance with theory, the ratio of BC (the pooled estimate of BC1 and BC2) to F2 genotypic variance among testcross progenies (σ2g) was close to 0.50 for all traits but one. Heterogeneous σ2g estimates in BC1 and BC2 were found for GY, FY, and grain dry matter content, suggesting the presence of epistasis. Predicted selection response for testcross performance (ΔG) was greatest in the F2. Testcross progenies of the F2, BC1, and BC2 generations displayed similar phenotypic and genotypic correlations as expected by theory. The “usefulness” criterion U = x̄ + ΔG was employed to assess the breeding prospects of each generation. With weak selection, the better backcross had superior usefulness for four traits, whereas with strong selection the F2 had highest usefulness for all traits except GY.

The paper summarizes the results of a series of experiments on enumeration of N2‐fixing bacteria (diazotrophs) and hormonal effects of Azospirillum on root development. Numbers of N2‐fixing and N‐heterotrophic bacteria were determined on the root (rhizoplane plus “inner” root surface) and in the rhizosphere soil (0–3 mm from the root surface) of Arrhenatherum elatius, other forage grasses and some herbaceous plant species. Pot experiments involved freshly collected soil from an unfertilized grassland area containing its natural population of N2‐fixing bacteria. The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil, suggesting that diazotrophs were not selectively advantaged at the root surface. Supply of mineral nitrogen (NH4NO3) decreased the proportion of N2‐fixing bacteria at the rhizoplane as well as in the rhizosphere soil. Similar results were obtained when N was supplied via the leaves. The data suggest that N2‐fixing bacteria in the rhizosphere are poor competitors once they loose their competitive advantage of binding dinitrogen. Correspondingly, the increase in the MPN of the diazotrophs found during plant development was interpreted as a result of decreased available combined N in the rhizosphere. The proportion of N2‐fixing bacteria relative to the total number of bacteria was generally below 1%. Considering the potential amount of substrate released from the roots and the substrate requirement of the bacterial population, N2‐fixation was considered insignificant for plant growth under the given conditions. For the investigations on possible beneficial effects on plant development by bacterial hormones, Azospirillum brasilense was chosen because evidence suggests that amongst the soil bacteria releasing hormones, especially IAA, certain strains of this species are more important than other bacteria. Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs. Similar results were obtained after application of IAA. This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense. The increase in root surface may improve acquisition of nutrients and enhance growth of plants. Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar. Again pure IAA resulted in a similar increase in nodule number. Increases in nodule number were only in part associated with a change in root morphology. Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely.

Factors influencing indoleacetic acid release from Azospirillum brasilense Cd (ATCC 29729) were investigated under batch culture conditions. In the presence of ammonium in the liquid culture solution, indoleacetic acid release from bacterial cells was considerably higher than in the corresponding N-free medium that was slightly solidified with agar to provide conditions for N2 fixation. In the solution culture, the concentration of indoleacetic acid was low during the logarithmic growth phase and increased rapidly with the beginning of the stationary phase. A similar time course was found for the indoleacetic acid content (dry weight basis) of the bacterial cells. Addition of tryptophan strongly stimulated the release of indoleacetic acid, which again showed a steep rise in the early stationary phase. The pH of the nutrient solution increased from 6.8 to 9.3 during incubation but this was not responsible for the steep rise in indoleacetic acid levels. The sole carbon source in the substrate (DL-malic acid) was identified as the limiting growth factor. This suggests that the high increase in indoleacetic acid production in the stationary phase is the expression of an overall change in cell metabolism when the carbon source is exhausted. At this developmental stage the high production of indoleacetic acid seems to have the character of a secondary product. Press sap from wheat roots did not promote indoleacetic acid production by bacterial cells.Key words: Azospirillum, indoleacetic acid, tryptophan, pH solution.

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