Currently, bacterial inoculant technology focuses on improving long-term storage conditions to ensure adequate rhizobia numbers and their effectiveness as plant growth promoters. This study aimed to investigate whether storage at 4 °C for four years of alginate beads immobilizing Bradyrhizobium sp. SEMIA6144 maintains bacterial vitality, efficacy in growth promotion, and ability to establish early interactions with Arachis hypogaea L. The recovery of viable SEMIA6144 cells decreased over time (10% at six months, 1% at one year, and 0.01% at four years), while cell vitality remained high at 94.1%, 90.2%, and 93.4%, respectively. The unsaturated/saturated fatty acid ratio declined during storage, reducing membrane fluidity and metabolic activity. Mobility and root adhesion of SEMIA6144 decreased after one and four years. However, growth promotion in peanuts inoculated with SEMIA6144 beads was observed through increased biomass, total chlorophyll, leaf number, leaf area, and decreased chlorophyll fluorescence compared to non-inoculated plants. Although nodulation was low in plants inoculated with four-year-old beads, leghemoglobin levels were maintained. These results demonstrate that Bradyrhizobium sp. SEMIA6144 can be stored for four years in alginate beads at 4 °C, maintaining its vitality and ability to establish a symbiosis that stimulates early peanut growth. Understanding these physiological changes could be valuable for the future improvement of long-lasting inoculants.

Peanut (Arachis hypogaea L.) is one of the most important oilseeds crops worldwide. Through symbiosis with the bacterium Bradyrhizobium sp., peanuts can assimilate atmospheric nitrogen, reducing the need for chemical fertilizers. However, this nitrogen fixation process is highly sensitive to environmental factors that can inhibit the early stages of symbiotic interaction. In this study, we propose the encapsulation of Bradyrhizobium sp. SEMIA6144 and the flavonoid naringin (Nar) in alginate beads to improve flavonoid stability and promote nodulation kinetics in peanuts. Three types of beads were synthesized: A (control, SEMIA6144 only); B (SEMIA6144 induced with 10 µM Nar); and C (SEMIA6144 co-entrapped with 1 mM Nar). Although Nar increased cell mortality (2-fold compared to control) and reduced metabolic activity—particularly at 1 mM—cells in beads B and C responded by altering their membrane fatty acid profile (30% and 55.5% of 18:1, respectively) leading to a reduction in saturated fatty acids (5.8% and 13.1% for 16:0 and 18:0 in B; 11.8% and 21.2% in C). Bacterial release kinetics followed a primarily Fickian diffusion model, with minor matrix–bacteria interactions in Nar-treated beads. Notably, bacterial release in peanut root exudates was 6%, 10%, and 11% higher for beads A, B, and C, respectively, compared to release in physiological solutions. Nar-beads enhanced the formation of curved root hairs, promoted bacterial colonization in root hair zones, and stimulated the appearance of rosette-like structures associated with nodule initiation. In conclusion, encapsulating Bradyrhizobium sp. SEMIA6144 with Nar in beads represents a promising strategy to improve symbiotic nitrogen fixation in peanuts.