Long-Life Inoculant: Bradyrhizobium Stored in Biodegradable Beads for Four Years Shows Optimal Cell Vitality, Interacts with Peanut Roots, and Promotes Early Growth

Abstract

Currently, bacterial inoculant technology focuses on improving long-term storage conditionsto ensure adequate rhizobia numbers and their effectiveness as plant growth promoters. Thisstudy aimed to investigate whether storage at 4 ◦C for four years of alginate beads immobilizingBradyrhizobium sp. SEMIA6144 maintains bacterial vitality, efficacy in growth promotion, and abilityto establish early interactions with Arachis hypogaea L. The recovery of viable SEMIA6144 cellsdecreased over time (10% at six months, 1% at one year, and 0.01% at four years), while cell vitalityremained high at 94.1%, 90.2%, and 93.4%, respectively. The unsaturated/saturated fatty acid ratiodeclined during storage, reducing membrane fluidity and metabolic activity. Mobility and rootadhesion of SEMIA6144 decreased after one and four years. However, growth promotion in peanutsinoculated with SEMIA6144 beads was observed through increased biomass, total chlorophyll, leafnumber, leaf area, and decreased chlorophyll fluorescence compared to non-inoculated plants. Althoughnodulation was low in plants inoculated with four-year-old beads, leghemoglobin levelswere maintained. These results demonstrate that Bradyrhizobium sp. SEMIA6144 can be stored forfour years in alginate beads at 4 ◦C, maintaining its vitality and ability to establish a symbiosis thatstimulates early peanut growth. Understanding these physiological changes could be valuable forthe future improvement of long-lasting inoculants.