Synergistic Effect of Plant-Growth-Promoting Rhizobacteria Improves Strawberry Growth and Flowering with Soil Salinization and Increased Atmospheric CO₂ Levels and Temperature Conditions

Biofertilization with plant-growth-promoting rhizobacteria (PGPR) can positively affect the growth and health of host plants and reinforce their tolerance of stressors. Here, we investigate the use of isolated PGPR consortia from halophytes to improve strawberry growth and flowering performance under saline and elevated CO₂ and temperature conditions. Growth, flower bud production, and the photosynthetic apparatus response were determined in strawberry plants grown at 0 and 85 mmol L⁻¹ NaCl and in two atmospheric CO₂-temperature combinations (400/700 ppm and 25/+4 °C, respectively). Biofertilization improved strawberry plant growth and flower bud production, independently of salinity conditions, at ambient CO₂ and 25 °C, while bacterial inoculation only had a positive effect on plant growth in the presence of salt in high CO₂ and at +4 °C. Biofertilizers 1 and 3 generated the largest biomass of strawberries at 400 ppm CO₂ and 0 and 85 mmol L⁻¹ NaCl, respectively, while biofertilizer 1 did so in the presence of salt and in an atmosphere enriched with CO₂ and at +4 °C. The effect of the consortia was mediated by bacterial strain PGP properties, rather than by an improvement in the photosynthetic rate of the plants. Furthermore, biofertilizers 1 and 2 increased the number of flower buds in the absence of salt, while biofertilizers 3 and 4 did so for salt-inoculated plants at 400 ppm CO₂ and at 25 °C. There was no effect of inoculation on flower bud production of plants grown at high CO₂ and at +4 °C. Finally, we concluded that the effect of bacterial inoculation on strawberry growth and flowering depended on the type of bacterial strain and growth conditions. This highlights the importance of developing studies considering stress interaction to assess the real potential of biofertilizers.