Selection of metal resistant plant growth promoting rhizobacteria for the growth and metal accumulation of energy maize in a mine soil - effect of the inoculum size

Heavy metals, such as Cd and Zn, are spilled in soils by several anthropogenic sources, including mining activities. Their toxic effects can be minimized using plants especially when paired with plant growth promoting rhizobacteria (PGPR), under phytomanagement strategies. Several factors can contribute to the failure of rhizobacterial inoculation, such as bacteria selection and the inoculum size. In this work five metal resistant PGPR (Ralstonia eutropha 1C2, Chryseobacterium humi ECP37, Pseudomonas fluorescens S3X, Rhizobium radiobacter EC1B and Pseudomonas reactans EDP28) were investigated for their in vitro growth promoting traits and for their ability to induce growth of maize seedlings exposed to Zn and Cd. PGPR inoculum size (10 and 20 mL) and inoculation effectiveness was assessed in energy maize sowed in a mine soil. The results showed that some bacteria only exhibited or enhanced PGP traits when exposed to metals. The bacterial strains ECP37 and EDP28 were the most efficient in improving seedling growth with increasing metal concentrations, followed by S3X. When inoculated in energy maize grown in mine soil, these same strains also outperformed the others by increasing shoot biomass and elongation, metal accumulation, and by decreasing it in roots. The most evident effect of doubling the inoculum size was the increase in Cd accumulation, which was of 17% and 31% in roots and shoots, respectively. Other effects included a slight reduction in shoots' biomass (13%) and a general decrease in P tissue content. The results obtained suggest that PGPR selection prior to inoculation in the target soils should be primarily based in seedling growth promotion under metal exposure. Additionally, the size of the inoculum applied in the soil rhizosphere appears to be important in remediation processes and should be taken into account when planning phytomanagement strategies, especially when the biomass of plants is an important demand.