N₂‐fixing bacteria in the rhizosphere: Quantification and hormonal effects on root development

Abstract

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.

The Role of Soil Beneficial Bacteria in Wheat Production: A Review

“…Inoculation with A. brasilense Cd and the application of pure IAA to the roots both increased root length, number of lateral roots and number of root hairs in wheat as observed by earlier workers [67,68]. IAA-producing Azospirillum sp.…”

Effects of thiamine and nitrogen fertilizer form on the number of N2−fixing and total bacteria in the rhizosphere of maize plants

“…Irrespectively of this enhancement effect of N, the ratios of diazotrophs amongst the total bacteria population were less than 1 O/ o in Mascherode soil, and less than 0.05% in Niger soil. This low proportion of diazotrophs is in agreement with results of Martin et al, (1989) in different grass species of temperate climates. Many taxa of diazotrophic bacteria such as Azotobacter sp., Bacillus sp., Clostridium sp., Enterobacter sp., and Klebsiella sp.…”

Comparative study of impact of Azotobacter and Trichoderma with other fertilizers on maize growth

“…The increase observed in FYM (T8) was 3.6% only. (2017) The enhancement in leaves growth as a result of biofertilizers inoculation may be due to the production of phytohormones by the biofertilizers and/or improving the availability of nutrients (Martin et al, 1989;Jagnow et al, 1991). Number of leaves seen in T7 and T5 might be due to more availability of nitrogen which is consistent with Okajina et al (1983); Woldesenbet and Haileyesus (2016).…”