No abstract

The potential of rhizobacteria in assisting plants used in the phytostabilization or re-vegetation of soils contaminated by heavy metals is gaining interest all around the world. In this context, six rhizobacterial strains isolated from highly heavy metal-contaminated soils situated in abandoned mining sites around the Oujda region (Morocco) were tested with Sulla spinosissima (L.), a native leguminous plant expanding in this area. The strains used were multi-resistant to heavy metals and possessed multiple plant growth-promoting traits. Potential beneficial effects of the strains were also evaluated in planta by measuring various growth and physiological parameters of inoculated Sulla plants grown in sterilized sand. Inoculation with the Rhodococcus qingshengii strain LMR340 boosted plant biomass (39% to 83% increase compared to uninoculated plants), chlorophyll and carotenoid content (up to 29%), and antioxidant enzyme activities (15% to 80% increase). Based on these interesting findings, selected strains were inoculated into plants growing in a heavy metal, multi-polluted, and poor soil. Under these conditions, non-inoculated plants and those inoculated with the strain LMR250 were unable to grow, while the other five bacterial inoculants restored plant growth. The best performing strain, Pseudarthrobacter oxydans LMR291, could be considered as a good biofertilizer and/or biostimulant candidate to be used for promoting the growth of selected plants in re-vegetation and/or phytostabilization programs of degraded and contaminated soils.

Most of the soils are N and P-deficient and require high applications of chemical fertilizers to ensure optimal crop production. However, this practice poses in most cases serious environmental issues. In recent years, inoculation with beneficial bacteria has emerged as a safe and sustainable alternative to chemical fertilizers. In this context, we investigated the effect of two symbiotic Agrobacterium radiobacter strains (LMR670 and LMR676) and two plant growth-promoting rhizobacteria Bacillus sp. (M131) and Enterobacter sp. (P1S6), as single or combined inoculants, on common bean growth and yield under N and P-deficient conditions. In a first trial, Agrobacterium strains’ symbiotic efficiency with common bean was evaluated in a low phosphorus and nitrogen soil under greenhouse conditions. Strain LMR670 recorded the highest nodules number (53 nodules per plant) and shoot dry weight (0.553 g plant-1). This strain was then used in combination with the PGP rhizobacteria in a common bean co-inoculation assay under sufficient and deficient P levels (80 kg ha-1of P and No P added). Single inoculation with LMR670 recorded the highest shoot dry weight (82% increase compared to non-fertilized control) compared to combined inoculants (46-47% increase). To corroborate the obtained results, a field experiment was conducted using the same treatments. LMR670 as a single inoculant or mixed with M131 was consistently effective leading to common bean yields comparable to N and P fertilized plants (13.07 t ha-1 and 12.35 t ha-1 respectively). In addition, single inoculation with the multi-PGP strain M131 showed positive effects on all common bean growth parameters and yield value (14 t ha-1); exceeding even N and P fertilized control plants. These results suggest that the strains LMR670 and M131 can be used, in a single or combined inoculation, as effective biological fertilizers for common bean cultivation to replace phosphorus and nitrogen fertilizers. Globally our results highlight the potential of native phytobeneficial strains for successful nodulation, growth and yield of common bean under N and P-deficient conditions.